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GSA HoG Division Activities -   Boston GSA Meeting 2001

        Pardee Symposium  Session 34, K4
        Discipline & Posters Sessions 96 & 97
        Field Trips

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Pardee Symposium
Ophiolites as Problem and Solution in the Evolution of Geological Thinking
Salley [Sarah E.] Newcomb and Yildirin Delek, Presiding

SESSION NO. 34, 1 :30 PM Monday, November 5, 2001, GSA,  K4. GSA Historyof Geology Division, GSA Structural Geology & Tectonics Division; International Geology Division; Society of Economic Geologists; History of Earth Sciences Society) Hynes Convention Center, Ballroom B

[ 1]  From Brongniart to Boston; a history of the ophiollte concept from 1813 to the present

[ 2]  N. l. Bowen and the origin of ultramafic rocks

[ 3]  Ophiolites and the oceanic upper crust; two-way traffic in scientific concepts

[ 4]  Identifying modern analogues for ophiolite complexes

[ 5]  Ophiolites: extension in a convergent setting, with particular reference to the western Hellenic ophiolites

[ 6]  Development of the ideas on the ophiolites of the Alpine-Apennine and the Dinaride-Hellenide orogenic systems

[ 7]  Precambrian ophiolites in collisional and accretionary orogens: increasing evidence to constrain an evolving concept

[ 8]  Evolution of thought concerning high- and ultrahigh-pressure metamorphic belts: Subduction, recrystallization, and exhumation

[ 9]   Paleomagnetism of ophiolites and ocean-island basalts in the Tethys and Iapetus oceans

[10]  Mantle and deep crustal dynamics and oceanic spreading centers as deduced from ophiolites

[11]  Geology of supra-subduction zone systems - their relevance to the origin of ophiolites

[12]  Supra-subduction zone ophiolite complexes

[13]  Ophiolites and greenstones in the Japanese orogenic belts: multiplicity and diversity of the accreted oceanic lithosphere

1 :30 PM Moores, Eldridge M.

FROM BRONGNIART TO BOSTON; A HISTORY OF THE OPHIOllTE CONCEPT FROM 1813 TO THE PRESENT

Since Brongniart's (1813) definition of ophiolite until the mid-1960's, two contrasting views of ophiolites prevailed. Continental European geologists thought that there was a close association of serpentines, pillow lavas, and radiolarites ("Steinmann's Trinity") which resulted from outpourings of mafic/ultramafic magma in the deep troughs of "eugeosynclines". English-speaking geologists argued that orogenic belt serpentines represented intrusions, either solid or magmatic, into "eugeosynclines", and they had no genetic connection to other parts of the "Trinity" Studies of pseudostratiform ophiolitic sequences in Italy, northern Greece, and

Papua in the mid 1960's led to Hess's (1965) acceptance of ophiolites as fragments of the oceanic crust. Studies in the late 1960's on the Troodos complex, Cyprus, with its magnificantly exposed and preserved sheeted dike complex, (Gass, 1968, Moores and Vine, 1971 ) led to acceptance of ophiolites as fragments of oceanic crust formed at spreading centers. Ophioiites thus formed became a key ingredient in integration of land geology with "The New Global Tectonics". Since 1970, questions of the tectonic environment of ophiolite formation and emplacement have dominated the discussion. Chemical arguments that ophiolites formed mainiy in a back-arc, or supra-subduction zone, setting have been highly popular, but new data from the oceans, the mantle, and the concept of "historical contingency" suggest that the tectonic environment cannot be determined by chemistry alone, but must also take into account the geology of each occurrence. The internal structure of many ophiolities can be compared with the structure of oceanic crust determined from ODP and other marine geological/geophysical studies It is now possible to infer oceanic processes from ophiolites for times prior to 200 Ma. The tectonics of emplacement or "obduction" (Coleman. 1971) of ophiolites now seems clearly to be primarily the result of aborted subduction of continental marginal or island arc crust. The presence of ophiolitic rocks, included sheeted dikes, in the Proterozoic and Archean indicates sea floor spreading throughout Earth's history. The Proterozoic may have begun when oceanic crust thinned to a subductable thickness.               [TOC]

1:45 PM Young, Davis A.

N. L. BOWEN AND THE ORIGIN OF ULTRAMAFIC ROCKS

In the first half of the twentieth century, petrologists commonly accepted the concept of ultramafic magmas. Harker and Vogt suggested that ultramafic liquids formed by the re-melting of accumulated olivine and pyroxene. Harker and Hess proposed that the liquid us temperatures of ultramafic melts were significantly lowered by dissolved volatiles Vogt maintained that the temperatures of olivine-rich melts were lowered substantially by addition of iron. Bowen, however, consistently rejected the concept of ultramafic magma. Although Bowen appealed to the lack of ultramafic lavas and the general paucity of contact metamorphism around ultramafic plutons as evidence against ultramafic magmas. experimental data clinched the matter for him. In 1914, he found that forsterite melts at 1890°C, a temperature far too high for dunite magma to exist in the crust. In the 1930s, Bowen and Schairer determined a significant portion of the olivine solid solution loop, and. contrary to the opinion of Vogt, they showed that olivine with a substantial amount of iron still required unreasonably high temperatures to exist as melt in the crust. As a result, Bowen maintained that ultramafic plutons represented accumulations of olivine and pyroxene that were intruded as mushes of solid crystals lubricated by thin films of fluid. In his final study on ultramafic rocks with Tuttle in 1949, Bowen again showed that no liquid exists in the system MgO-SiO2-H2O at crustal temperatures He also argued that serpentinites could not crystallize directly from a magma of serpentine composition but formed by absorption of water from wall rocks and alteration of olivine.               [TOC]

2:00 PM Cann, Johnson R.

OPHIOLITES AND THE OCEANIC UPPER CRUST; TWO-WAY TRAFFIC IN SCIENTIFIC CONCEPTS

Current understanding of the processes that construct and modify the oceanic upper crust has been the result of nearly 50 years interplay between concepts derived from ophiolites and those derived from active mid-ocean ridges. The Troodos ophiolite in Cyprus has played a central role in this exchange, since there the upper ophiolitic crust is preserved for over 100 km across strike, and has scarcely been deformed or heated more than a few tens of degrees since it formed in an arc environment over 90 million years ago. Early in the platetectonic debate, the presence of the sheeted dyke complex in Troodos, demonstrating 70 km of 100% extension, was crucial in allowing acceptance of the new ideas. Later, the recognition of the exhalative nature of the ore deposits within the extrusive unit prepared the way for the discovery of black smokers, and then the oceanic observations in turn brought new insights into the structure and activity of the hydrothermal systems that had fed the ophiolitic deposits. These insights are now being returned to the oceans. Demonstration of the narrowness of the zone of crustal construction in the oceans, and the structure of upper crust at spreading axes, led to a reassessment of the structure of the upper crust in Troodos, in turn generating new approaches to the construction of the upper crust in the oceans. This two-way traffic seems set to continue, fuelled by the contrasting types of information that can be gained from each environment. In the oceans, rates of processes can be measured, and the spatial distribution of current activity determined. In ophiolites one can walk around on cross-sections of the fully-formed crust, unhampered by the limited exposure available on the ocean floor. Because of chemical differences between even the most oceanic of ophiolitic crust and that of open ocean basins, parallels must be drawn with care; it is naturally safer to compare physical processes than petrological ones. But creative interplay between the two environments, so crucial early in the plate tectonic revolution, shows no sign of abating. This topic is not just history; it is also active science in the making.               [TOC]

2:15 PM Pearce, Julian A.

IDENTIFYING MODERN ANALOGUES FOR OPHIOLITE COMPLEXES

Interpreting ophiolites in terms of their original tectonic settings of formation has evolved through the interplay of marine and on-Iand research. Before 1960, the common occurrence of 'ophiolitic rock types' had been recognised and linked to submarine processes and thereby to the geosyncline concept. The 1960s was marked by the recognition that the best-preserved ophiolites had the structure and rock types of oceanic crust and underlying mantle, and this culminated, in 1972, with the Penrose Conference definition of ophiolites which is still used today. In the 1970s, the assumption that ophiolites formed at mid-ocean ridges began to be seriously tested when geochemical fingerprinting started to indicate a range of tectonic settings. This, and the rapid expansion of marine geology and ocean drilling, emphasised that spreading ridges in marginal ocean basins were, in many cases, better analogues than midocean ridges for ophiolite formation and led to the use of the term 'supra-subduction zone ophiolite' for ophiolites with subduction components in their geochemical signatures. Development of the subduction zone-ophiolite link was a major theme of the 1980s, supported by many new cruises and a second major phase of ocean drilling in the Western Pacific. This enabled the petrological and geochemical features of marginal basin lithosphere to be properly defined. The science of the 1990s extended the likely range of modern analogues of ophiolite complexes. MARGINS science highlighted the relationship between many ophiolites and the lithosphere formed by a combination of tectonic and magmatic processes at rifted continental margins. A combination of ocean drilling and on-Iand studies revealed that ridge subduction could give a distinctive ophiolite-like sequence in a fore-arc setting. Ongoing work on Western Pacific drill-core emphasised the potential importance of subduction initiation for producing ophiolites, especially those containing boninitic magma types, and demonstrated that many marginal basins are floored by thinned arc crust, which resembles some ophiolitic sequences. The RIDGE campaign of the 1990s was also instrumental in highlighting differences between ultra-slow, slow- and fast-spreading ridges and so enabling still more detailed refinement of original tectonic setting.              [TOC]

2:30 PM Smith, Alan G.

OPHIOLITES: EXTENSION IN A CONVERGENT SETTING, WITH PARTICULAR REFERENCE TO THE WESTERN HELLENIC OPHIOLITES

Reference to some of the western Hellenic ophiolites (Vourinos, Pindos and Othris) as serpentine and dolerite eruptions by Boue dates from at least 1840; the observation of tuffs and pillow lavas(?), possibly not ophiolitic, in Othris in 1880 led Neumayr to suggest an origin as large submarine eruptions. A submarine selling wes reinforced by Brunn's mapping of the Pindos and Vourinos ophiolites in the 1950s. He concluded that they were in situ; formed an entity; had been empiaced in late Jurassic time along feeders concealed under the Meso-Hellenic trough and had flowed outward to the NE (Vourinos) and SW (Pindos). Sheared serpentinite at the base of the ophiolites was attributed to post-consolidation effects related to later tectonics, with the required water possibly being derived from the underlying sediments. The amphibolites and garnet-mica schists at the base of the ophiolites were interpreted as slivers of the underlying Paleozoic metamorphic basement. In 1960 Brunn made the first analogy between these ophiolites and the rocks of the mid-Atlantic ridge. Moores (1969) recognized the fundamentai difference between magmatic and tectonized ultramafics in the Vourinos and suggested that many of the ultramafic rocks had been emplaced by solid flow about a NE-trending symmetry axis. All of these interpretations were made in an essentially fixist framework

Subsequent interpretations assume a plate tectonic selling. Like many other ophiolites, the main problems have been the 'root zone' of the ophiolites; the location and dip direction of associated subduction zones; the tectonic selling(s) indicated by their marked variations in composition; the relationship between extension and convergence; how they were emplaced onto a continent; and significance of the metamorphic sole; and the spatial and temporal relations of the Hellenic, ophiolites to contiguous ophlolites in Albania and Iormer Yugoslevia. The Hellenic ophiolites are a critical link between the Tethyan ophiolites in the eastern Mediterranean and the Alpine-Apennine ophiolites to the W. Detailed age dating and geochemical analyses will probably give significant insights into how extension has taken place in this particular convergent setting.               [TOC]

2:45 PM Bortolotti, Valerio

DEVELOPMENT OF THE IDEAS ON THE OPHIOLITES OF THE ALPINE-APENNINE AND THE DINARIDE-HELLENIDE OROGENIC SYSTEMS

The term "ophiolites" was first defined in 1813 by Brogniart studying serpentinites, gabbros and diabases from the Aipine-Apennine orogenic system. Subsequently, these ophiolites, as well as those of the Dinaride-Hellenide belt, were studied by Steinmann, Brunn and Aubouin, who recognized their allocthonous nature and Jurassic age. Alter acceptance of the concept that ophiolites are fragments of oceanic lithosphere, studies on the ophiolites from Northern Apennines, Western Alps and Corsica considerably increased in order to assess their tectono-magmatic significance. The peculiar MOR-type ophiolitic stratigraphy of these sequences was defined early in the 1970s by several Itaiian and French researchers. In accordance with Modern oceanic analogues, some authors suggested that the AlpineApennine ophiolites were generated in a true oceanic environment. By contra&!. other authors postulated a denudation of sub-continental Iherzolitic mantle during a passive, asymmetric oceanic opening. In this model no genetic relationships between mantle and magmatic rocks are postulated. Ophiolites from the Dinaride-Albanide-Hellenide belt have been studied since the beginning of the XX century, mostly by French, Austrian and American researchers. However, the idea that two types of ophiolitic sequences, showing different lithostratigraphical, petrological, geochemical, and metallogenic features occur in this orogenic systems only took place from the end of the '70s. These ophiolitic types generally identify two sub-parallel NNWSSE trending belts delimited by a high-angle thrust, and are usually defined as the Eastern (EOB) and Western (WOB) ophiolitic belts. The EOB shows low- and very low-Ti geochemical character and has been interpreted as originating in a supra-subduction zone (SSZ) selling. The WOB mainly shows high-Ti geochemical affinity, and has been interpreted as originating in a mid-ocean ridge selling probably characterized by low spreading rate. Recent geological and petrological studies indicate that the EOB and WOB probably originated in a single oceanic basin, and that some ophiolitic sequences from the WOB also record an interaction between magmatic processes related to both MOR and SSZ selllngs, suggesting a more complex tectono-magmatic scenario than that previously depicted.               [TOC]

3:30 PM Windley, Brian F.

PRECAMBRIAN OPHIOLITES IN COLLISIONAL AND ACCRETIONARY OROGENS: INCREASING EVIDENCE TO CONSTRAIN AN EVOLVING CONCEPT

When the term ophiolite was defined by Anonymous in 1972, ophiolites were considered to be an important component in collisional orogens. However, now it is realised that ophiolites also occur in accretionary orogens, and in their modern tectonic-equivalent environment, the accretionary wedges in circum-Pacific orogens. Today many examples are known of Precambrian ophiolites in both collisional and accretionary orogens, ranging from ca. 3.80 Ga to ca. 540 Ma, providing us with key information, not only of plate tectonic processes in the Precambrian, but also of early crustai growth processes. Study of Phanerozoic and Precambrian ophiolites is closely connected with a) the recognition of oceanic plateaux and thus of plume tectonics in the geological record, and b) the occurrence of ultrahigh-pressure metamorphic rocks, which provide information on subduction and exhumation of material to/from mantle depths. Thus, study of Precambrian ophiolites reveals a wealth of information that is essential lor understanding crust-mantle growth processes, and plate tectonic versus plume tectonic processes with time

To help understand Precambrian ophiolites, we will discuss several aspects of Cenozoic ophiolites, as well as the following factors: the presence or absence of associated high-pressure and ultrahigh-pressure rocks, geochemical parameters such as Dupal-type ophiolites, the trace element and REE geochemistry of Archean and Early Proterozoic ophiolites, the mode of occurrence of ophiolites in Phanerozoic and Archean orogens, the differences between modem and Archean ocean-ridge metamorphism, the recognition of plateaus in ophiolites with implications lor plume and superplume origins, the style of accretionary processes during the Archean early crustal growth stage, changes in stratigraphy and crustal thickness with time, and the supra-subduction zone origin of ophiolites. Finally, we emphasize the importance of unravelling late-stage overprinting by consuming plate boundary processes, before it is possible to understand the origin and emplacement of ophiolites, past and present.               [TOC]

3:45 PM Ernst, W. Gary

EVOLUTION OF THOUGHT CONCERNING HIGH- AND ULTRAHIGH-PRESSURE METAMORPHIC BELTS: SUBDUCTION, RECRYSTALLIZATION, AND EXHUMATION

Alpine-type collisional belts are produced by subduction of an ocean basin and the underflow of sialic crust. Old, competent continental units typify these complexes; ophiolites are common in a few, but are rare in most such belts. Metamorphism of profoundly subducted parts of the orogen-60 to 125 km-ranges from high-pressure {HP) to ultrahigh-pressure {UHP). Coeval calcalkaline arc volcanics + plutonics are rare. In contrast, paired Pacific-type orogenic belts consist of an outboard, oceanic trench + accretionary prism deposited on oceanic crust, and an inboard continental margin or island arc, developed within and landward from longlived subduction zones. The outboard graywacke + shaley melanges, minor deep-water cherts % carbonates, and ubiquitous disaggregated ophiolites recrystallized under HP conditions at depths of 15-35 km. A massive, coeval calcalkaline arc dominates the subparallel, inboard terrane; associated metamorphism is typified by high temperatures. In both Alpineand outboard Pacific-terranes, structural vergence is seaward, reflecting similar convergent plate-tectonic settings Because of their structural integrity, some downgoing Alpine-type microcontinents, island arcs, or continental promontories descend to great depths before decoupling from the sinking plate; in contrast, incompetent, ductile, Pacific-type graywacke + shale terranes break loose from the descending oceanic crust-capped lithosphere at shallow depths. In both, the rapid ascent {0.5-1.0 cm/yr) of portions of subducted material as thin aspect-ratio sheets (-1 km thick), combined with extension above and subduction below, promotes lateral heat conduction and cooling of decompressing HP and UHP metamorphic complexes. Exhumation to mid-crustallevels is due to buoyancy. worldwide, blueschist-eclogite terranes consist of small, high-density mafic and/or peridotitic lenses surrounded by voluminous, lowdensity quartzofeldspathic % serpentinitic material, thus aggregate terrane density is less than that of unaltered mantle. Exposure of rising HP and UHP belts is a consequence of erosional decapitation and gravitational collapse of the subduction complex.               [TOC]

4:00 PM Van der Voo, Rob

PALEOMAGNETISM OF OPHIOLITES AND OCEAN-ISLAND BASALTS IN THE TETHYS AND IAPETUS OCEANS

Abundant paleomagnetic data exist for ophiolites, but they are not without problems. Structural corrections are sometimes difficult to ascertain, but more importantly, the age of the magnetization can be uncertain. For example, typical in-situ ocean-floor rocks have abundant titanomagnetltes, whereas the magnetic minerals in ophiolites are usually Ti-poor. This suggests alteration at some stage after formation and before or during obduction. The magnetizations of ocean-island basalts, on the other hand, seem to survive the accretion/obduction process better than those of ophiolites, as will be illustrated with examples from the northern Appalachians. Still, well-known ophiolite complexes in the former Tethys (Oman, Xigaze, Troodos) have yielded important information about paleolatitudinal displacements Collectively these data suggest that ophiolites do not appear to be typical ocean.fioor basalts formed at a ridge near the middle of an ocean. Their paleolatitudes in the Tethys and lapetus reflect origins near the continental margins, possibly all in back-arc basin settings. It can be concluded that typical mid-oceanic lithosphere seems to subduct without leaving much of a trace at the surface. Ocean-island basalts, in contrast, have a better chance of preservation near the surface. Whereas fully subducted oceanic lithosphere is no longer of use to paleomagnetists, it can nevertheless be used by paleogeographers In that its presence in the mantle can be recognized by tomographic imaging techniques. Examples will be briefly discussed of the paleogeographic Inferences made from mantle P-wave velocity anomalies under Asia.               [TOC]

4:15 PM Nicolas, Adolphe

MANTLE AND DEEP CRUSTAL DYNAMICS AND OCEANIC SPREADING CENTERS AS DEDUCED FROM OPHIOLITES

Structural mapping of mantle and crustal structures in ophiolites, mainly the Oman ophiolite, reveals a contrasted behavior between center and tips of ridge segments. Large deformation ascribed to homogeneous HT flow beneath the ridge of origin is always recorded in segments. It corresponds either to signature of mantle upwelling or mantle drifting away from the upwelling areas. In ophiolites thought to represent fast spreading situations as Oman, the large plastic deformation in the horizontally drifting mantle peridotites is coupled with a similarly large magmatic deformation in the overlying an flat laying layered gabbros. In contrast, tips of propagating segments are intensely deformed, displaying vertical mohos underlined by shear zones in the mantle and low crust. This crust is also injected by mafic dikes and brecciated on a large scale HT thrusts affecting the mantle section about 1 Ma after accretion and associated with fast rotations (30°/Ma) deduced from paleomagnetic studies are related to microplate activity.               [TOC]

4:30 PM Hawkins, James W.

GEOLOGY OF SUPRA-SUBDUCTION ZONE SYSTEMS - THEIR RELEVANCE TO THE ORIGIN OF OPHIOLITES

Ophiolites, interpreted as relict fragments of oceanic crust and upper mantle, provide insights and constraints for oceanic petrologic layering inferred from seismic velocities, and for the evolution of lithosphere formed at mid-ocean ridges {MOR). Thus "ophiolite=seafloor=ophiolite." Ophiolite assemblages within accreted terranes {e.g., North American Cordillera) support an oceanic origin for many terranes. But many ophiolites have crustal sections thinner or thicker than seafloor. Silicic plutons and extrusives are common. Both mafic and silicic rocks may have isotopic and trace element chemistry {particularly high field strength elements) different from MOR. These ophiolites are fragments of oceanic lithosphere but probably NOT from deep sea MOR unless we postulate curious crustal tectonic-petrologic schemes to explain their accretion. From where do they come? Oceanic lithosphere is formed not only at MOR but off axis on leaky transforms and seamounts as well as in supra-subduction zone (SSZ) settings of convergent margins. Modern SSZ systems offer insights to the origin of ophiolites as well as the volcanic arc-derived material commonly found with them In particular, occurrences of nearly coeval volcanic/plutonic/arc clastic rocks, granitoids, silicic tuffs, "cherty argillite", and boninites with ophiolite series rocks require a geologic setting like SSZ but unlike MOR. The geology of the Tonga-Kermadec, Vanuatu, Mariana-Izu-Bonin SSZ systems (immature intra-oceanic arcs) and Luzon, Fiji, Palau-Kyushu Ridge (more evolved systems), resembles many Cordilleran ophiolites and their tectonically juxtaposed arc complexes. SSZ forearc and volcanic arc magmas include boninite, arc tholeiite and calc-alkaline series. Mature backarc basins are dominated by MORB but may include transitions to island arc basalt and have seamounts of ocean island and arc series basalt. Forearc and backarc basins may have thick accumulations of arc-derived tuffs and siliciclastics. Parts of backarc basins, starved of clastics, have metalliferous deposits, "umbers", sedimentary protoliths for cherty argillite, and pelagic sediments. Mature island arcs may have intrusive granitoids (e.g.. tonalite, plagiogranite) complementary to silicic tuffs and pyroclastics.               [TOC]

4:45 PM Dewey, John F.

SUPRA-SUBDUCTION ZONE OPHIOLITE COMPLEXES

Large ophiolite nappes (Oman, Newfoundland, Papua) clearly have a supra-subduction zone (SSZ) origin in extensional or transtensional arcs from their petrology, structure and chemistry. However, largely ultramafic-free ophiolite fragments in accretionary complexes such as the Annieopsquotch of Newfoundland and the South Connemara terrane of western Ireland are probably slices of clipped-off seamounts, oceanic plateaux, and transform-related upper oceanic crust. The Ordovician Bay of Island and Coastal Complexes in western Newfoundland and the related ophiolite complexes of Baie Verte, Mings Bight, Tilt Cove and the Notre Dame Bay Zone exhibit extremely complicated structural/magmatic relationship with a mid-Cambrian through early Ordovician primitive arc complex. The Sleepy Cove pillow lavas have an oblique transtensional constrictional fabric cut by the late Cambrian Twillingate Granodiorite cut in turn by the Mortons Harbour mafic dike complexes locally forming a sheeted complex, witnessing the progressive transtensional disruption of an oceanic island arc leading to the intra-arc development of oceanic crust and mantle, the whole supra-subduction zone package emplaced during the Ordovician as a high-Ievel ophiolite sheet as the Laurentian margin collided with the arc The Coastal Complex is probably a frontal arc assemblage nucleated in an intra-arc or fore-arc transform.

SSZ ophiolites occur through the Appalachian/Caledonian orogen where they appear to be the highest level nappes above metamorphic complexes, where sediments were deposited in rift complexes at the Laurentian margin and where deformation and metamorphism resulted from continental margin/arc collision Two major unique periods of SSZ ophiolite obduction and blueschist metamorphism, following immediately-preceding ophiolite generation in arc settings, occurred during the Ordovician and the Cretaceous where they are associated with periods of high sea-Ievel and appear to represent times of high intraplate compressional stress resulting from ridge push during continental dispersal.               [TOC]

5:00 PM Ishiwatari, Akira

OPHIOLITES AND GREENSTONES IN THE JAPANESE OROGENIC BELTS: MULTIPLICITY AND DIVERSITY OF THE ACCRETED OCEANIC LITHOSPHERE

Ophiolites in the circum-Pacific orogenic belts are called "cordilleran-type" with negative adjectives such as "incomplete", "dismembered" and "metamorphosed", contrasting to the typical "Tethyan-type" (Coleman, 1986). However, the circum-Pacific ophiolites have some positive significance in the globai tectonics in view of their unique features. (1) Multiple tectonic superposition of ophiolites of widely varying ages. (2) Wider petrologic diversity of ophiolites with highly depleted mantle peridotite. (3) Occurrence of ophiolites with thick crustal sections. These features suggest their repeated generations and emplacements in the supra-subduction zone environments The Japanese ophiolites are commonly soled with younger blueschist rocks and are tectonically underlain by much younger accretionary complexes For example, in southwestern Japan, the Ordovician Oeyama ophiolite (>450 Ma) is underlain by the 320 Ma Renge biueschist and the late Permian (250 Ma) Akiyoshi accretionary complex. This relation suggests tectonic erosion or non-accretion during the intervening Siluro-Devonian time. Similar gap exists between the early Permian (280 Ma) Yakuno ophiolite and the underlying Jurassic Tamba accretionary complex (150 Ma) An accretionary complex is characterized by the "oceanic plate stratigraphy" composed of greenstone, chert, limestone, mudstone and sandstone in younging order The basal greenstone commonly includes 018 with high Ti and Nb concentrations, but the ophiolite is almost devoid of 018. In the present western Pacific, Izu-Mariana and Tonga areas are characterized by the existence of ophiolite outcrops in trench-slope, the absence of accretionary complexes, and the occurrence of submarine blueschist rocks, whereas the areas off northeastern Honshu and Hokkaido are characterized by the vast development of present-day accretionary complexes These contrasting two stages may have been repeated in any segment of the Japanese orogenic belt throughout the Phanerozoic. The periods of oceanic island arc and marginal basin development (ophiolite formation) and tectonic erosion (blueschist metamorphism) might have alternated with the accretion periods, during which accretionary complexes were developed through off-scraping of oceanic sediments and seamounts.              [TOC]

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T 70  Ophiolites as Problem and Solution in the Evolution of Geological Thinking  II
Yildirin Delek and Sarah E. Newcomb, Presiding.

SESSION NO. 96, 1:30 PM,  Tuesday, November 6, 2001,  GSA.   Abstracts with Programs, Volume 33, No.6, 2001 GSA Annual Meeting, ESSN 0016-7592 [A-225 to A-227] (History of Geology Division, GSA Structural Geology and Tectonics Division; International Geology Division, the History of Earth Sciences Society) Hynes Convention Center, 302

[ 1]  Ophiolite studies and global geochemical cycles

[ 2]  Ophiolites as faithful records of the oxygen isotope ratio of ancient seawater

[ 3]  The role of ophiolite studies in constraining seawater circulation and alteration of modern oceanic crust

[ 4]  The effect of increased slow spreading extension on the ocean crust dike/basalt hydrothermal sulfur anomaly (Macquarie Island, Southern Ocean)

[ 5]  Bio-interaction with basaltic glass and its importance in mapping the depth of oceanic biosphere

[ 6]  Recent developments in understanding the evolution of ophiolites: an overview of new geochemical and petrogenetic models for ophiolites of the SW Pacific from the Poya terrane, new caledonia and the Tangihua complex, New Zealand

[ 7]  The Coast Range Ophiolite, California: multistage origin of a suprasubduction zone ophiolite

[ 8]  What constitutes "emplacement" of an ophiolite?

[ 9]   A comparative study of two ophiolitic melanges in the California Coast Ranges and Tibet, with implications on dismemberment processes of oceanic crust

[10]  The discovery of the Ankara Melange and its unsung hero Oguz Erol Sengor, A.M.C.

[11]  Ophiolite obduction and plate margin evolution: evidence from the Oman mountains

[12]  Spinel compositions as clues to the origin and tectonic significance of metadunite bodies in the Blue Ridge belt of the Southern Appalachian Orogen

SESSION NO. 97 (Posters), 1:30 PM,  Tuesday, November 6, 2001,  GSA.  Abstracts with Programs, Volume 33, No.6, 2001 GSA Annual Meeting, ESSN 0016-7592 [A-225 to A-229] (History of Geology Division, GSA Structural Geology and Tectonics Division; International Geology Division, the History of Earth Sciences Society) Hynes Convention Center, Hall D

[13]  The Choate Mine: A representative history of chromite mining from an ophiolite in Maryland

[14]   Pre-obduction structures of the Thetford Mines Ophiolitic Complex (TMOC), Quebec: Implications for oceanic crust formation and PGE mineralisation

[15]   History of development of the ideas on the origin of the Albanian Ophiolites and its contribution to the ophiolite concept

[16]  Structural and microstructural analysis of a paleo-transform fault zone in the Neyriz Ophiolite, Iran

[17]  Tectonostratigraphic significance of new biostratigraphic, chronostratigraphic, and geochronologic data (40Ar-39Ar) from the Khoy ophiolite, northwestern Iran

[18]  New biostratigraphic data and geochemical data from the Sabzevar Ophiolite, north central Iran [15]

[19]  Assessing the stability of chromium at contaminated sites by proxy of chromium geochemistry in serpentine soils

[20]  Soldiers Delight Natural Environment Area, Maryland, USA: toward preservation of a rare, serpentinite-based ecosystem

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1 :30 PM Gregory, R. T.

OPHIOLITE STUDIES AND GLOBAL GEOCHEMICAL CYCLES

Geochemical profiles through ophiolite complexes provided the necessary link between the study of global geochemical cycles and plate tectonics. The hydrothermal circulation that occurs beneath the seafloor is the primary mechanism for exchange between the mantle of the Earth and the hydrosphere. The subduction of hydrothermally altered crust and overlying sediments is the primary mechanism for crustal recycling. Oxygen and strontium isotopes of seawater track the competition between continental weathering and midocean ridge hydrothermal exchange to control the composition of the oceans. Mixing between river water and end-member black smoker fluid extends this analysis to the major cation and anion composition of seawater. Information derived from ophiolite studies on the elemental fluxes and the depth of seawater penetration into the oceanic crust provides constraints on the important rate constants associated with these competing processes. The isotopes of strontium, a trace element in seawater, provide proxy information on spreading rates and sea level throughout geologic time. The same tectonic rates that account for the Sr isotope evolution of seawater indicate that the oxygen isotopic composition of the ocean is constrained to vary within narrow limits (per mil level). Isotopic analysis of dredge samples and ophiolite complexes demonstrated that seawater-ocean crust interactions result in a oxygen isotopic zonation of the oceanic crust with complementary enriched and depleted masses (concentration times volume) centered on the initial isotopic composition of the crust. This requires that the oxygen isotopic composition of the ocean resides at near steady-state conditions over Earth history; critical for the application of oxygen isotopes for paleoclimate analysis. The inferences from ophiolite complexes contrast strongly with the results of measurements on carbonates from epicontinental seaways. Ophiolites and greenstone belts track exchange processes between the ocean and the igneous crust whereas most carbonate measurements track the surface ocean on continental shelves. For oxygen iso- topes, the volume of epicontinental seaways and the rates of meteoric water input suggest a resolution to the controversy that accounts for both data sets.            [TOC]

1 :45 PM Muehlenbachs, Karlis

OPHIOLITES AS FAITHFUL RECORDS OF THE OXYGEN ISOTOPE RATIO OF ANCIENT SEAWATER

Fragments of ancient seafloor rocks can serve as proxies for the delta 18-O of seawater Alteration of the seafloor by seawater imparts a very characteristic oxygen isotope fingerprint on the seafloor. The pillow basalt section becomes enriched in 18-O in proportion to the amount of secondary, high-18-O, low temperature mineral growth. In contrast, the sheeted dykes and gabbros generally are depleted of 18-O reflecting the modal proportion of secondary, low-18-O, chlorite and epidote. These trends simply reflect the high water to rock ratio of the seafloor alteration and the temperature dependence of the 18-O partitioning between minerals and water. Superposrtion of high delta 18-O pillows wrth low delta 18-O dykes and gabbros is unique and is easily recognized in well preserved ophiolites and has been described from numerous locations. Older, dismembered and highly metamorphosed segments of the oceanic crust may still retain the original seawater imprint because their subsequent obduction and metamorphism was relatively closed to external fluids Even suites of diamond bearing eclogites from kimberlites still have contrasting high and low delta 180 eclogites proving that subduction into the mantle is not sufficient to erase the seawater fingerprint. Inspection of all the seafloor, ophiolite and eclogite data reveals no secular trend in delta 18-O indicating that the delta 18-O of seawater has not changed wrth geological age. Since the delta 18-O of seawater itself is fixed by seafloor/seawater exchange, the constancy of delta 18-O of seawater implies that the scale and style of sea water/seafloor interactions has not changed over time.             [TOC]

2:00 PM Stakes, Debra So

THE ROLE OF OPHIOLITE STUDIES IN CONSTRAINING SEAWATER CIRCULATION AND ALTERATION OF MODERN OCEANIC CRUST

Ophiolites provide a crrtical three-dimensional view into oceanic crust to address questions of the depth and distribution of seawater penetration and the cumulative mineralogical and geochemical impact of seawater alteration. Systematic mapping of the lower crustal sections of ophiolites (e.g. Oman and Cyprus) and lower crustal sections of modern oceanic crust (e.g. Atlantis Bank (ODP Site 735B) and Hess Deep) document the presence of high-temperature seawater interactions. Large variations in oxygen isotopic variations within complete crustal sections (3 to 12 permil d18O) and secondary mineral chemistry reveal water-rock interactions to variable depths within the lower dikes and uppermost gabbros. Studies from Oman, Atlantis Bank, and the Hess Deep provide evidence for high temperature reactions (above 500C), seawater penetration early in cooling history, shift of magnetic signal from extrusives to intrusives by formation of secondary magnetite Secondary mineral assemblages typical of dikes and gabbros from Ophiolrtes (e.g. chlorite and epidote after plagioclase and pyroxene, titanite after magnetite; talc after olivine) can explain the chemistry of the hydrothermal fluids

Ophiolrte studies may still prove to be powerful analogues in looking at more complex processes in modern oceanic crustal formation. Multiple hydrothermal systems are created by diachronous intrusive events. The presence of high temperature shear zones and late magmatic fluids, examined in detail at the ultra-slow spreading Atlantis Bank, has extended models for seafloor structures. The paradigm of propagating rifts and overlapping spreading centers has been built primarily upon arguments from geophysics and basalt chemistry. Yet the lower crust formed in such an environment would show much complexity as a new magmatic system is superimposed upon slightly older intact crust. Late gabbro dikes crosscutting deep deformed gabbros with extensive high temperature alteration indicate that this complexity may be evident in the northern portion of the Oman ophiolite. The "non-ideality" of this ophiolite may in fact be a window into the subsurface processes related to nonlinear behavior of mid-ocean ridges.             [TOC]

2:15 PM Davidson, Garry J,

THE EFFECT OF INCREASED SLOW SPREADING EXTENSION ON THE OCEAN CRUST DIKE/BASALT HYDROTHERMAL SULFUR ANOMALY (MACQUARIE ISLAND, SOUTHERN OCEAN)

Sulfur isotopic enrichment above typical igneous values occurs within sulfide-rich Transrtion Zone rocks of oceanic crust in ODP Hole 504B, and on the island of Troodos. This has been crted as evidence for the transfer of seawater sulfate into ocean crust during off-axis hydrothermal alteration. The process likely involves mixing of high-temperature reduced fluids (sourced from low permeability sheeted dikes) with lower temperature sulfate-bearing waters sourced from the overlying basalts. However, the homogeneity of the process is far from established, or the influence of other factors such as degree of syn-mixing extension.

At Macquarie Island, extensive exposures of typical slow spread ocean crust (812 Ma) contain a significant disseminated and vein-controlled sulfur anomaly on the dike-basalt contact. We have studied this phenomenon in a 7.5 km long, paleo-ridge-parallel section, where the initiating edge of a sheeted dike swarm (Sandell Bay Sheeted Dikes; SBSD) is exposed, as well as its upper basalt contact.

Background pyrrte S contents are high across the SBSD-basalt contact for 550 m. However, there is also a strong structural control that produces significant alongstrike varia tions in sulfide abundances, and an association wrth quar1zcemented sphalerite-bearing fault breccias, with evidence of turbulent flow regimes. These differences in the size of the sulfur anomaly are attributed to the active extensional slow spreading setting. With increasing extension, diffuse mixing across the dyke-basalt contact was replaced by channelized flow and dynamic mixing in extensional fault arrays. The most intense alteration occurred in fault splays sub-parallel to the ridge axis. The size of the Transition Zone sulfur-sink must be reassessed to take account of this heterogeneity.            [TOC]

2:30 PM Furnes, Harald

BIO-INTERACTION WITH BASALTIC GLASS AND ITS IMPORTANCE IN MAPPING THE DEPTH OF OCEANIC BIOSPHERE

During the last decade much attention has been focused on the existence of microbes contributing to the alteration of glassy rim of pillow lavas in upper oceanic crust The first suggestion of microbial pitting of volcanic glass was made two decades ago, without any convincing mechanism explaining how microbes facilitate dissolution of glass. Later, in a study of Icelandic hyaloclastites the presence of bacteria was observed, hosted within alteration textures of basaltic glass. Based on this finding it was suggested that colonising microbes may cause local variations on pH and/or ligands that allow them to chemically "drill" into a silicate substrate, a proposal that was experimentally verified. Subsequently, similar textures were found in the fresh glassy part of pillow lavas from several DSDP/ODP drill sites into the in-situ volcanic basement with ages from Quaternary to 170 Ma, and in various ophiolites (90 Ma Troodos-Cyprus, 160 Ma Mirdita ophiolite-Albania, 440 Ma Solund-Stavfjord ophiolite-W. Norway) The microbial influence on the alteration process of basaltic glass has also been confirmed by documenting the presence of DNA and ribosomal RNA within the structures of anticipated biological origin. Further evidence of microbial fingerprints within altered glass has been demonstrated by low and high d13C values, as well as the element distribution of C and N. The upper crust displays a large variability In the relative importance of biotic to abiotic alteration, and the degree of bioalteration decreases with depth Thus, the fraction of bioalteration of the total alteration of glass ranges from 20-90% in the upper 300 m down to a maxi mum of 10% at - 500 m depth. This might be due to a natural variability in the abundance of bioaltered glass or to biased sampling from low drilling recovery in young crust. The proportion of bioaltered to abiotically altered glass does not show any systematic variation with the age of crust. Thus bioalteration lasts as long as abiotic alteration does.             [TOC]

3:15 PM Nicholson, Kirsten Ngaire

RECENT DEVELOPMENTS IN UNDERSTANDING THE EVOLUTION OF OPHIOLITES : AN OVERVIEW OF NEW GEOCHEMICAL AND PETROGENETIC MODELS FOR OPHIOLITES OF THE SW PACIFIC FROM THE POYA TERRANE, NEW CALEDONIA AND THE TANGIHUA COMPLEX, NEW ZEALAND

Cretaceous ophiolites obducted during the early mid Tertiary onto Paleozoic-Mesozoic Gondwanan margin crust can be traced from Papua New Guinea through New Caledonia to New Zealand. Two of these ophiolites, the Tangihua Complex, New Zeaiand and the Poya terrane, New Caledonia, have recently been studied leading to new theories regarding their for mation and emplacement.

The Tangihua Complex and Poya terrane appear very similar. They have roughly the same ages of formation (Cretaceous), rock types (predominate~ basalt) and stratigraphy although the Poya terrane was emplaced in the Eocene-Oligocene compared with Oligocene-Miocene for the Tangihua Complex. Early workers attributed the formation of both complexes to a large spreading centre lying eastward of New Zealand and New Caledonia. However, recent work has concluded that they could not have been formed by the same large-scale rift system as the ophioklites are geochemically distinct, and have experienced different metamorphic PT paths.

The Poya terrene basalts are predominantly P- and N-MORB tholeiites generated during the opening of a small marginal basin northeast of New Caledonia. Alteration and metamor phism in the Poya terrane shows four distinct phases, the highest of which is blueschist. The Tangihua basalts range from back arc tholeiites to calcalkaline arc basalts suggesting formation in an arc/backarc setting. The Tangihua Complex has three stages of alteration, the high est of which is greenschist, and contains well-preserved zeolite assemblages.

During the break up of the eastern Gondwana margin, c. 80-100 Ma, there is much uncertainty regarding specific plate movement It is believed that in New Zealand the situation was complicated by the presence of the Phoenix microplate, which stalled subduction, preventing the development of a mature arc system and initiating spreading. The Poya terrane was most likely formed before the Tangihua Complex, northeastward of New Caledonia in the opening of a small marginal basin. Hence the same overall tectonic regime resulted in two very different local settings, the history of which has taken years to unravel.            [TOC]

3:30 PM Shervais, John W.

THE COAST RANGE OPHIOLITE, CALIFORNIA: MULTISTAGE ORIGIN OF A SUPRASUBDUCTION ZONE OPHIOLITE

The middle Jurassic Coast Range ophiolite (CRO) of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from northern California to the southern Transverse Ranges. The CRO represents a major but short-lived episode of oceanic crust formation that affected much of western North America just prior to the Nevadan orogeny. Formation of the CRO was associated with convergent plate tectonics, as shown by its tectonic position overlying the Franciscan subduction complex, its stratigraphic position beneath forearc sediments of the Great Valley Series, and the chemical composition of lavas and plutons which comprise the ophiolite. Geochemical studies over the last 20 years have shown that the overwhelming bulk of all volcanic rocks erupted within the CRO formed by arc related volcanism. Chemical characteristics common to all suites include low TiO2 (<1.3 wt%), low Nb (<3 ppm), Ti/V <22, Zr/Y <4, a range in SiO2 from basalt to rhyolite, and slightly depleted to enriched LREE. Volcanic rock associations and crosscutting relationships in the plutonic section demonstrate three major igneous episodes: (1) eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex, (2) extensional deformation of older plutonics, intrusion of refractory wehrlite-cpxites, and eruption of refractory lavas, (3) eruption of silicic lavas and intrusion of hornblende-quartz diorite. This sequence represents the progressive stages of ophiolite formation In a rifted forearc setting. The last magmatic event to effect the CRO (stage 4) is "oceanic" in character, typically represented by basaltic dikes with MORB geochemistry (TiO2> 1.4 wt%, Ti/V=25-30, Nb=6-14 ppm) or by OIB seamounts that overlie the ophiolite (Stonyford). We correlate this stage with a ridge collision event that resulted in a major reorganization of plate motions and the Nevadan orogeny

This sequence of events is observed in many suprasubduction zone (SSZ) ophiolites, and suggests that all SSZ ophiolites form in response to a common process. The existence of this consistent progression implies that ophiolite formation is not a stochastic event, but is a natural consequence of the SSZ tectonic setting.             [TOC]

3:45 PM Wakabayashi, John

WHAT CONSTITUTES "EMPLACEMENT" OF AN OPHIOLITE?

Ophiolites have come to be recognized as on-land fragments of oceanic crust. Oceanic crust becomes an ophiolite as a result of a process known as "emplacement". For Tethyan ophiolites, emplacement has been defined as the thrusting of an ophlolite over a continental mar gin, an event that took place within the first several million years alter intraoceanic thrusting or subduction initiated beneath the ophiolite. This emplacement process is driven by the collision of a leading edge of a continental mass with a subduction zone, above which the future ophiolite is situated. In contrast, emplacement for other types of ophiolites is either poorly defined, or differs markedly from the definition of emplacement for Tethyan ophiolites. For example. the Macquarie Island ophiolite has been emplaced over oceanic crust (instead of a continental margin) due to transpression as a result of oblique convergence along the plate boundarry between the Australian and Pacific plates. Many ophiolites in the North American Cordillera were tectonically emplaced onto a subduction complex. rather than onto a continental mass. The process of subduction and accretion beneath many of the Cordilleran ophiolites was progressive with no uniquely defined emplacement episode, except for the initiation of subduc- tion. For example, the Franciscan subduction complex formed beneath the Coast Range ophioiite of California as a result of over 140 million years of continuous subduction. No specific accretionary event in the history of the Franciscan complex can be singled out as the emplacement of the Coast Range ophiolite, with the exception of the inception of the Franciscan subduction. Thus, for the Macquarie Island and many Cordilleran ophiolites, emplacement may be best defined as the inception of intraoceanic thrusting or subduction and subsequent accretionary uplift with continued subduction, whereas for Tethyan ophiolites emplacement can be construed as collision-driven thrusting of fossil oceanic crust over the leading edge of a continental margin, following the initiation of intraoceanic thrusting.             [TOC]

4:00 PM Huot, Francois

A COMPARATIVE STUDY OF TWO OPHIOLITIC MELANGES IN THE CALIFORNIA COAST RANGES AND TIBET, WITH IMPLICATIONS ON DISMEMBERMENT PROCESSES OF OCEANIC CRUST

Ophiolitic melanges are important components of orogenic belts in that they contain trag- ments of partially disrupted or even disappeared terranes whose understanding is essential in reconstructing paleo-tectonic settings. Both the Round Mountain Serpentinite Melange (RMSM) in California and the Beimarang Melange in Tibet are the products of dismemberment of an overlying arc-related ophiolite and underlying units related with oceanic domains of MORB or BABB affinity. Similarities and differences between these chaotic units have implications in understanding the complex processes of ophiolitic melange formation and collisional belts in general.

Both melanges are dominated by highly sheared serpentinized peridotites derived from disruption of the juxtaposed ophiolitic upper mantle peridotites. Mafic components enclosed in the melanges are characterized by low pressure secondary mineral assemblages typical of oceanic metamorphism retrograded from amphibolite down to pumpellyite-prehnite facies. These mafic lithologies have contrasting origins since the RMSM includes composite siivers of basalts and cherts derived from the structurally underlying Franciscan Complex whereas the Beimarang Melange contains diabase and gabbros which intruded the peridotites prior to the melange for- mation In the latter locality no mafic component derived from the structurally underlying Tethyan units has been accreted to the serpentinite belt despite the fact that in other areas along the suture zone basaltic fragments of Tethyan origin are associated with similar melanges.

Accretion of basaltic fragments from the oceanic domain to an ophiolitic melange in a trench setting is not a ubiquitous feature and may require specific conditions to be achieved. These conditions include physical and chemical properties of the subducting lithosphere such as its age, permeability, topography, composition and fracturing. We suggest that different original settings of the paleo-subducting lithospheres, and hence different compositions and ages, might have been responsible for such contrasted behaviors. Basaltic fragments of back- arc composition in the RMSM might be more easily tectonically accreted than MORB-like Tethyan fragments which were probably older, denser and less permeable.            [TOC]

4:15 PM Sengor, A.M.C.

THE DISCOVERY OF THE ANKARA MELANGE AND ITS UNSUNG HERO OGUZ EROL SENGOR, A.M.C., ITU Maden Fakultesi, Jeoloji Bolumu, Ayazaga, Istanbul 80626 Turkey, sengor@itu.edu.tr

Ankara Melange was discovered by James McCallien and his only doctoral student in the U. of Ankara, Department of Geography, Oguz Erol. Erol was sent out to the Elmadag region to map and McCallien, advised him to get the stratigraphy established first. Erol obtained - not entirely legally - a set southeast of Ankara through a military friend and began mapping. Soon he was frustrated. No sensible sequence seemed present in the region where late Palaeozoic to late Mesozoic sedimentary and mafic and ultramafic rock seemed distributed haphazardly. McCallien could hardly believe his reports and urged him to try harder. Finally he accompanied Erol on a weekend excursion to see his problem. He became similarly baffled! What Erol had reported was true. There was no sequence to be mapped. McCallien then did what to him seemed to be the most sensible course. He advised Erol to go on mapping. Soon Erol began separating blocks and matrix! He noted that Ernst Chaput's previously mapped "Old Elmadagi Series" contained Permo-Carboniferous limestones that did not occur as interbedded layes. Instead, Erol noted, they were in a sheared matrix. He and McCallien began talking about the sheared matrix forming a "plastic medium" in which the blocks had been "churned" They started using terms such as "Elmadagi Boulder Beds" and "mixed series" They realised that it could only be dated by dating its matrix. McCallien later invited his old friend Bailey to see the area. They did so under Erol's guidance and using his unpublished maps. They published their results acknowledging Erol's 'help" but not sharing the credit of discovery with him. Although Erol's thesis was later published in Turikey, the world at large never learned his true contribution. This is a social issue. However, that the melange concept was rediscovered by Erol and McCallien independently of Greenly and that it was to be independently rediscovered twice more later (by Noble in 1941 and by Hsu in 1968) shows that geological discovery is an objective thing independent of the social hangups of the discoverers Although the efficient market theory does not work in the scientific world owing to limited biological and technological capabilities of the scientists, it is nonsense to claim that social factors have a controlling influence of the development of science within a scientific society.             [TOC]

4:30 PM Gray, Do Ro

OPHIOLITE OBDUCTION AND PLATE MARGIN EVOLUTION: EVIDENCE FROM THE OMAN MOUNTAINS

Three windows into the Arabian margin beneath the Samail ophiolite complex provide insights into the behavior of the continental margin during ophiolite obduction. The windows progres sively sample deeper levels within the Arabian margin going from the Hawasina window in the northwest to the Jabal Akhdar window and the Saih Hatat window to the southeast. The Saih Hatat window reveals the development of at least two plates within the formerly autochtho nous Arabian margin successions. The lowermost exposed plate consists of a composite plate of strongly deformed Arabian platform sediments and malic schist associated with the original opening of the Hawasina ocean basin Locally. these rocks preserve evidence of eclogite facies metamorphism. Overlying the lower plate is an upper plate antiformal nappe whose upper limb is pinned to the undeformed Arabian margin successions to the south and whose core consists of strongly deformed Hatat Schist. The lower limb of the nappe is trun cated by the fault between the upper and lower plates. Structures within both plates exhibit the northnortheast lineation associated with latest extension of the Samail ophlolite and its emplacement. The Jabal Akhdar window exposes a large anticline developed in the post Permian shelf carbonates underlain by cleaved and more tightly foided Proterozoic sediments. The Hawasina window is the largest exposure of the Hawasina nappes and parautochthonous shelf rocks that were clearly overridden by the ophiolite. The structural style in this northernmost window has more in common with the deformation observed in the Saih Hatat window and contrasts with the structural style of the frontal Hawasina nappes exposed south and west of the mountains. These relationships suggest that the Arabian mar gin was not the passive recipient of the obducted Samail ophiolite.             [TOC]

4:45 PM Raymond, Loren A.

SPINEL COMPOSITIONS AS CLUES TO THE ORIGIN AND TECTONIC SIGNIFICANCE OF METADUNITE BODIES IN THE BLUE RIDGE BELT OF THE SOUTHERN APPALACHIAN OROGEN

More than 200 ultramafic bodies occur as isolated pods wi1hin peli1ic schist and amphibolite in the eastern part (the Gossan Lead Thrust Block = GLTB) of the Blue Ridge Belt (BRB) of the Southern Appalachian Orogen. Important petrogenetic and petrotectonic issues remain unresolved for most bodies. Most BRB ultramafic bodies are metadunites and metaharzburgites. with mineral assemblages representing anhydrous upper amphibolite facies conditions, or chlori1e (+ amphibole + talc) schists that reflect hydrated assemblages of the middle to lower amphibolite facies. The metamorphic textures and mineral assemblages represent probable Taconic, Acadian and Alleghenian recrystallization events. Unknown are the nature of the protolithic bodies (ophiolite, alaska-type, A-type intrusion) and the reasons for the abundance of relatively homogeneous metadunite and metaharzburgite bodies Eastern BRB rocks are considered to have oceanic affinities and recent discoveries of eclogite and retrograded eclogite associated with ultramafic rocks at the structural base of the GLTB support the view that these rocks mark a Taconic suture. A few of the bodies (e.g., the Buck Creek body) likely are fragments of oceanic crust (ophiolites). All may be. In the metadunites and metaharzburites, most chrome spinels have low Mg Meetings/01boston.htm#s (< 40) and high Cr Meetings/01boston.htm#s (> 80) characteristic of spinels of alpine metamorphic bodies and the lower parts of ophiolites. Variable Ti components (wt. % = 0.02-1.95) suggest a dual history of depletion and enrichment consistent with petrologic data and spinel chemistries from Hess Deep and Oman Ophiolite rocks. We suggest that the bodies of metadunite (and some metaharzburgite) represent melt channels within the uppermost sub-MOR mantle that locally experienced melt induced enrichment (forming high TiO2 values). These bodies were structurally resistant to fragmentation during melange-forming processes at the Taconic convergent margin. Associated mafic rocks are preserved as amphibolites. whereas associated ultramafic rocks may have been serpentinized. hydrothermally altered. metamorphosed. and fragmented to the point that the only remaining remnants are various chlorite schists.            [TOC]

5:00 PM Johnsson, Harald B.

THE CHOATE MINE: A REPRESENTATIVE HISTORY OF CHROMITE MINING FROM AN OPHIOLITE IN MARYLAND

The Choate Chromite Mine is located wi1hin the Soldiers Delight Ultramafite, a fragment of a large central Appalachian ophiolite in the Maryland Piedmont. Of the historic underground metal mines in the state. it is probably the best preserved and most accessible. The history of the Choate Mine is representative of chromite mining in Maryland in the nineteenth and early twentieth centuries. Worked intermittently for almost a century. the mine was an important source of chromite for the renowned Tyson mining family of Baltimore and later experienced a brief resurgence during World War I. Using narrative and slides. we will take a glimpse of the story of this mine as it has been revealed and interpreted through the study of an assortment of primary and secondary sources.            [TOC]

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SESSION NO.97 , 1 :30 PM, Tuesday, November 6, 2001, Posters Abstracts with Programs, Volume 33, No.6, 2001 GSA Annual Meeting, ESSN 0016-7592 [A-228 to A-229]

BTH 59 Pagé, Philippe

PRE-OBDUCTION STRUCTURES OF THE THETFORD MINES OPHIOLITIC COMPLEX (TMOC), QUEBEC: IMPLICATIONS FOR OCEANIC CRUST FORMATION AND PGE MINERALISATION

The TMOC is divided into two different petrologic and structural units, a NW mantle dominated massif (Thetford Mines unit) and a SE crustal-dominated massif (Mont Adstock unit). Three types of tectonic fabrics can be identified, pre-, syn- and post-obduction. Post-obduction structural features consist of NE-trending Acadian open folds and reverse faults, Late Silurian Early Devonian backfolds and normal faults such as the Saint Joseph fault (SJF).The TMOC occurs in the hanging wall of the SJF. Structures attributed to Ophiolite obduction onto the Laurentian Margin consist mainly of a metamorphic sole and related structures at the base of the mantle section. Pre-obduction structures show two different sub-vertical orientations, NS and NW-trending. There are underlined by sheared dunite (mylonite), and the NS-trending set, appears to control the emplacement of mafic-ultramafic bodies into the cumulates. In the upper part of the crust, NS structures are associated with doleritic dykes, fault breccias, upwardly decreasing throws suggest that they are syno-ceanic grow faults. A Pt-Pd survey shows that chromite-bearing as well as pyroxene-bearing rocks are good prospects for PGE exploration The greatest PGE concentrations are encountered in discordant chromitite bodies but non-negligible grades have been found in the pyroxenite, near the contact with gabbroic units, and in the bedded chromitite-bearing dunite.             [TOC]

BTH 60 Shallo, Minella

HISTORY OF DEVELOPMENT OF THE IDEAS ON THE ORIGIN OF THE ALBANIAN OPHIOLITES AND ITS CONTRIBUTION TO THE OPHIOLITE CONCEPT

The Albanian ophiolites are part of the DinarideHellenide segment of the AlpineHimalayan orogenic system and occur in two subparallel belts between the Pelagonian and Apulian platforms. The western belt consists of lherzolite, gabbro, and extrusive rocks in a <5 km-thick sequence, whereas the eastern belt includes harzburgite-dunite, gabbro, diorite, plagiogranite, sheeted dikes, and extrusive rocks in a 10km-thick sequence. Nowack (1929) expressed the idea that peridotites in Albania might represent an exhumed 'sima' at the surface Russian geologists (19511961) considered all plutonic rocks and peridotites as Jurassic intrusions within Triassic volcanic rocks. Aubouin & Ndojaj (1964) used the term 'ophiolite' and interpreted this assemblage as a differentiation product of basaltic magma. Dede et al. (1966) based on petrological and metallogenic data and Tashko (1974) based on geochemical data distinguished the western and eastern belts of ultrabasic rocks, respectively. Shallo et al. (1980) and Shallo (1980) suggested a seafloor spreading origin of the ophiolites in a marginal basin west of the Pelagonian platform and recognized in the eastern belt the existence of a sheeted dike complex. Turku (1981) separated in the Mirdita region a calci-basalto-dacitic series of an island arc affinity and a diabasic series of tholeiitic affinity within the volcanic sequence. In 1985, 1987, Shallo et al. distinguished the eastern vs. western ophiolites as characterized by their disparate pseudostratigraphy, showing different petrological, geochemi cal, and metallogenic features; their alternative tectonic model suggested a seafloor spreading origin of the western belt followed by an intra-oceanic subduction to produce the eastern belt Subsequent studies (i.e., Beccaluva et al., 1994; Bebien et al., 1995, 1998; Bortolotti et al., 1996; Dilek et al., 2001) confirmed the existence of two geochemically different ophiolite types. Interpretations derived from the studies of the Albanian ophiolites have closely followed the evolving concepts in ophiolite research over the years and may yet provide significant clues for the formation and tectonic emplacement of "oceanic crust" in subduction-accretion cycles in Tethyan-type orogenic belts.            [TOC]

BTH 61 Sarkarinejad, Khalil

STRUCTURAL AND MICROSTRUCTURAL ANALYSIS OF A PALEO-TRANSFORM FAULT ZONE IN THE NEYRIZ OPHIOLITE, IRAN

On-land exposures of fracture zone related ophiolites provide an excellent opportunity to compare rocks and structures in these paleo-transform fault systems with the observations from the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP). This comparison is essential to obtain new information on the kinematics and mechanisms of deformation within oceanic fracture zones. An ENE-trending and steeply dipping possible fossil oceanic fracture zone, consisting mainly of harzburgite and gabbros, occurs in the Neyriz ophiolite in SW Iran. Scattered exposures of pillow lavas, sheeted dykes and malic plutonic rocks crop out in the eastem side of the fracture zone. Mafic rocks within the fossil fracture zone display C-, C'- and S-bands foliations consisting of amphibole- and feldspar-rich bands. Deformed hornblende por phyrociasts are observed to show symmetrical tails or F-type porphyroclast systems in XY thin sections. Some mantled porphyroclasts also show anticlockwise stair-stepping rotation and oblique foliation with asymmetrical complex stripes and wings, forming *-type porphyroclasts systems Hornblende c-axis of highgrade S-C mylonites exhibit strong lattice preferred orientation (LPO) with M-and G-type origin The mean orientation of harzburgite tectonite foliations in the peridotites and the measured mean trends of the LPO patterns of hornblende caxis in mylonitic mafic rocks are generally ENE. The LPO pattems of hornblende c-axis show weakly to moderately developed asymmetrical girdles with respect to foliation. These micro-structural observations collectively indicate a sinistral sense of shearing that affected the mafic rocks within the fracture zone. Regional tectonics of the Neyriz ophiolite suggests that this fracture zone acted as a left-lateral transform fault, which was part of a slow-spreading (4.25 cm/y) system. The active spreading center segment was located to the NW of the current exposure of the ENE-trending fossil oceanic fracture zone The trend of the inferred fossil oceanic fracture zone in the Neyriz ophiolite mimics the general orientation of the fracture zones of the Carlsberg Ridge system in the Indian Ocean.            [TOC]

BTH 62 Kariminia, Mohsen

TECTONOSTRATIGRAPHIC SIGNIFICANCE OF NEW BIOSTRATIGRAPHIC, CHRONOSTRATIGRAPHIC, AND GEOCHRONOLOGIC DATA (40AR-39AR) FROM THE KHOY OPHIOLITE, NORTHWESTERN IRAN

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo Tethys. This northwest-southeast trending ophiolite complex consists from bottom to top (northeast to southwest) of a welldefined basal zone of metamorphic rocks, peridotites (mainly harzburgites and dunites) and serpentinized peridotites, gabbros, sheeted dikes (?), pillow and massive lava flows. Associated sedimentary rocks include a variety of Lower and Upper Cretaceous pelagic limestone which occurs within basaltic andesite as interlayers or exotic blocks, ranging in size from 10 to 100 meters. Campanian to Maastrichtian planktonic foraminifera have been recovered from the pelagic limestone at a number of localities. Thin-section analysis of the limestone revealed the presence of Globotruncanita calcarata (uppermost Campanian) at one of these localities and Globotruncanita conica (upper Maastrichtian) at another locality The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. Two 40Ar39Ar plateau ages of 158.6 ±1.4 Ma and 154.9 ±1.0 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic. However, four 40Ar39Ar plateau ages for hornblendes from amphibolites from the basal metamorphic zone give values of 104.6 ± 0.3 Ma, 105.7 ± 0.2 Ma, 106.7 ± 0.2 Ma, 1098 ± 0.7 Ma, suggesting an emplacement age of about Mid-Albian Pelagic limestone and radiolarian chert samples from the volcanic member produced identifiable radiolaria (e.g., Novixitus spp., Pseudodiclyomitra pseudomacrocephala, Archaeodictyomitra sliteri, Thanarla praeveneta, Ouinquecapsu/aria splnosa ) and planktonic foraminifera (e.g., Hedbergella planispira, Hedbergellea delrioensis, Thalmaninella evoluta) which are indicative of the upper Albian and lowermost Cenomanian. The 1999 GSA time scale places the Cenomanian-Albian boundary at 99 ±Ma and the Cenomanian-Turonian boundary at 93.5 ± 4 Ma. Therefore, the lower Cenomanian to upper Albian chronostratigraphic assignment of pelagic limestone samples are relatively compatible with the 40Ar-39Ar ages of emplacement.           [TOC]

BTH 63 Ghazi, A. Mohamad

NEW BIOSTRATIGRAPHIC DATA AND GEOCHEMICAL DATA FROM THE SABZEVAR OPHIOLITE, NORTH CENTRAL IRAN

The Sabzevar ophiolite is one of the largest ophiolite complexes of Iran located along the northern boundary of the central Iranian micro-continent. The igneous rocks of this complex consist of peridotites (harzburgite, dunite and lherzolite), serpentinite, minor pyroxenite, gab bros, and a volcanic sequence that exhibits a wide range of composition from basalts and basaltic andesites to rhyodacite-dacites, rhyolites and basanites. Sedimentary rocks include a variety of Upper Triassic to Upper Cretaceous marine strata. These include pelagic fossiliferous carbonates that are mixed with the pillow basalt and basaltic andesite as interlayers or exotic blocks ranging in size from 10 to 100 meters. Also present are extensive units of radiolarian chert which are interbedded within the basalts and basaltic andesites The result from geochemical analyses indicates the presence of at least four different types of extrusive rocks in the Sabzevar ophiolite. The geochemical data clearly identifies some of the extrusive rocks to have formed from three distinct types of basaltic melts; i) the group-1 basaltic rocks, which formed from an initial melt with N-MORB-like (LREE depleted) chemical signatures and are petrogenetically related to the gabbros, ii) group-2 basaltic rocks which have E-MORB chemi cal signatures, and iii) group-3 basaltic rocks with LREEenriched signatures and incompati ble trace element patterns that suggest an island arc affinity. Pelagic limestone samples from the volcanic member produced a number of identifiable planktonic foraminifera. The most significant of the taxa recovered include Globotruncanita calcarata (Zone 2B; uppermost Campanian) and Globotruncana contusa (Zone 1 B, Subzone 1 B1 to 1 A; upper Maastrichtian) The Sabzevar ophiolite contains other volcanic rocks that have non-MORB like geochemical signatures (e.g., withinplate, island arc).            [TOC]

BTH 64 Oze, Christopher J.

ASSESSING THE STABILITY OF CHROMIUM AT CONTAMINATED SITES BY PROXY OF CHROMIUM GEOCHEMISTRY IN SERPENTINE SOILS

Weathering of ultramafic rocks particularly rich in serpentine at Jasper Ridge Biological Preserve (JRBP), Stanford, CA, produces soils with high concentrations of chromium and other biologically toxic elements. Chromium, an inorganic, carcinogenic contaminant, persists in the environment as either Cr(III), a mild toxin, and/or Cr(VI), a known human carcinogen causing both chronic and acute toxic effects. Chromium in serpentine soils is initially released in the form of Cr(III) and is considered chemically immobile; however, Cr(III) may oxidize to form Cr(VI) in the presence of high valence manganese oxides. By evaluating the sources of chromium and the processes governing its release, transport, and attenuation, these soils can be used as a proxy for assessing the long-term behavior of chromium in Cr-contaminated sites.

Analyses of total chromium within JRBP soils demonstrate that chromium concentrations are comparatively enriched with respect to the bedrock and range from 1725 ppm to 4760 ppm. Variable concentrations of chromium with depth provide evidence that the chromium is mobile and a maximum typically occurs near the boundary of the AB horizon. Total organic carbon and organic acids present in each soil horizon may increase the solubility of chromium-bearing phases accounting for the maximum at the AB horizon interface. Although high concentrations of chromium and manganese are present, Cr(VI) was not detected in any of the samples. Using SEM, XRD, and XAS, the overall abundance and depth distribution of chromium reflects detrital chromite/quartz present in sand size fractions, chromium sorption onto mineral surfaces, and chromium precipitated as (oxy)hydroxides and clay minerals.            [TOC]

BTH 65 Flanagan-Brown, Riley E.

SOLDIERS DELIGHT NATURAL ENVIRONMENT AREA, MARYLAND, USA: TOWARD PRESERVATION OF A RARE, SERPENTINITE-BASED ECOSYSTEM

The Maryland Department of Natural Resources, in cooperation with the private, nonprofit organization, Soldiers Delight Conservation, Inc., has set aside over 2000 acres of land north west of the city of Baltimore with the purpose of preserving its natural and cultural heritage As a part of the state forest and park service and Patapsco Valley State Park, Soldiers Delight Natural Environment Area (SDNEA) owes its special designation to the presence of chromite-rich serpentinite bedrock. With time, this serpentinized fragment of uppermost mantle, part of an ophiolite obducted onto North America during the Taconic Orogeny, yields serpentine soils that support over 35 rare and endangered plant species These dry, nutrient-poor, and relatively toxic soils support only those plants that have adapted to these extreme conditions SDNEA is also home to the Choate mine, one of the first chromite mines in the country.

Native Americans, who burned this and other areas within the Appalachian Piedmont to hunt deer. kept the area free of most trees, maintaining the prairie-like character of the barrens. European settlement of the area brought both fire suppression and development. Since grazing and mining activities came to an end in the early 1900's. encroachment upon the already decimated serpentinite grasslands by Virginia Pine, Eastern Red Cedar, and thorny Greenbriar threatens their destruction. In addition, exotic and invasive plant species pose a serious threat, spreading throughout SDNEA at alarming rates. Current, successful efforts to restore the grasslands include clearing and controlled burning each year (as weather conditions permit) along with well-planned eradication of invasives. With these efforts, rare and endangered wild-flowers, including Sandplain Gerardia (Agslinis scuta), Serpentine Aster (Aster depauperstus), Fameflower (Tslinum teretifolium), and the Fringed Gentian (Gentisnopsis crinits) will have a chance to thrive amidst a sea of Indian Grass (Sorghsstrum nutsns) and Little Bluestem (Schizschyrium scopsrium). The paleoecology, Native American and European experiences, and mining history of the area serve as an important educational resource. This combination of natural and cultural heritage, derived from the presence of serpentinite, makes SDNEA an important place to preserve.             [TOC]