Ophiolitic Material Research Articles

Arc-Related Origin of Volcaniclastic Sequences in the Misis Complex, Southern Turkey

The Miocene ophiolitic mélange-flysch association of the Misis Complex in southern Turkey contains three main units of clastic sediments that exhibit abundant acidic, mafic, and ultramafic clasts of magmatic origin. These are: (a) mixed volcaniclastics (vitric, lithic, and crystal tuffs) from the "matrix" of the mélange; (b) volcaniclastics (vitric tuffs) from large bedded sedimentary olistoliths; and (c) turbiditic flysch sediments with an admixture of magmatic, metamorphic and sedimentary clasts. Chemical and petrographie data suggest that magmatic clasts within the volcaniclastic sediments (units a and b) were derived from two main sources: an acidic calc-alkali volcanic arc and a back-arc ophiolite complex, now represented by basalt pillow lavas and ultramafic blocks within the mélange. The turbiditic flysch sediments (unit c) represent the accumulated debris of the denuded acidic arc, together with ophiolitic material derived via dismemberment and tectonic erosion of the back-arc basin crust. Chemical data indicate that the ophiolitic source, however, did not come into prominence until the late Middle Miocene.

The origin and tectonic significance of mélanges in Eastern Sabah, Malaysia

Extensive chaotically disrupted rock bodies are found across Sabah in diverse tectonic settings. These units, which include broken formations, a dismembered arc-ophiolite complex and mud-matrix mélanges were formed by a combination of sedimentary, tectonic and diapiric processes. The mélanges of the eastern part of Sabah are considered to be related to basin initiation in an intra-arc setting, and are not related directly to subduction-accretion processes. The East Sabah mélanges are composed predominantly of deep water clastic sediments, deposited in the Central Sabah Basin, mixed with varying amounts of exotic ophiolitic material. The sedimentary clasts have a variety of well-developed “soft-sediment” deformation structures. These are in part inherited into the mélange from syn-sedimentary deformation, and in part generated during the mélange-forming events. The mélanges are not metamorphosed, and vitrinite reflectance data suggest that except for a few localities, a maximum temperature of less than 120°C was attained in the matrix. The ophiolitic blocks commonly have tectonic shearing fabrics associated with low temperature metamorphism. These metamorphic fabrics are incompatible with the deformation mechanisms in the sedimentary clasts and the mud matrix, and are interpreted as being inherited from shear in the ocean floor environment (e.g. fracture zones) or associated with ophiolite obduction. The similarity in clast type, deformation history and general structure also suggest that all the mélanges of the east of Sabah were formed in a series of related events during late Early Miocene and earliest Middle Miocene time. It is postulated that these events, which triggered sedimentary, diapiric and tectonic mélange-forming processes, are related to the coeval onset of extension and sea floor spreading in the southeastern Sulu Sea.

Discussion of ophiolites in Northeast and East Africa: implications for Proterozoic crustal growth

W. R. Church writes: In discussing the Pan-African geology of Northeast and East Africa, Berhe (1990) affirms that ophiolite decorated lineaments in the Arabian—Nubian Shield represent suture zones. The difference of opinion between Berhe (1990) and Stern et al . (1989) concerning the location of the supposed suture zones of northern Sudan and southern Egypt, is a clear illustration of the ambiguity that may arise through the uncritical use of this paradigm. Whereas Berhe considers the Sol Hamed—Wadi Onib ophiolite belt to represent in situ oceanic material located along a north—south trending extension of the Saudi Arabian Yanbu—Sol Hamed ‘suture’, Stern et al. 1989 link the latter with the east—west trending Allaqi—Heiani ophiolite belt, which they consider to be a major east—west suture ‘extending well into the interior of North Africa. This important difference in opinion is a clear indication of the arbitrary nature of suture selection based solely on the distribution of ophiolitic rocks. Furthermore, neither explanation may be correct. The distribution of ophiolitic rocks in the southern part of the Eastern Desert and northern Sudan is more likely controlled by the southern Eastern Desert domal culmina­tion, out-of-sequence faulting, and the development of zones of intense ductile strain. Within the domal culmination, ophiolitic material exposed at lower structural levels near Abu Swayel, Gebel Nagy, Um Krush and perhaps Gebel Gerf, overlie highly deformed and metamorphosed pelitic metasediments containing a ‘con­tinental’ Nd isotope signature ( Harris et al. 1984 ), quartzo-feldspathic gneisses (Umm Tundeiba), hornblende—cummin

THE HEZAN UNITS: A FRAGMENT OF THE SOUTH NEO‐TETHYAN PASSIVE CONTINENTAL MARGIN IN SE TURKEY

Recent progress in structural and stratigraphic analysis of the allochthonous units situated north of Hazro in the Border Fold zone of SE turkey has shed light on the nature and signijicance of the Gondwanan margin there. Allochthonous material consists of several superimposed units ranging in age from the Early Trias to the Late Senonian (Hezan units), in which the transition between neritic carbonates and the pelagic and detrital facies is well indicated by an Ammonitico‐rosso facies of mid Jurassic age. Above it, ophiolitic material and radiolarites represent the former Tethyan crust and cover. The Hezan units thus belong to the former Gondwanan margin thrust southwards in Lute Cretaceous times, and a tentative reconstruction of its Mesozoic evolution there is facilitated by the absence of later major tectonic events.This reconstruction shows marked diferences compared with the more complex nearby margins described in the Hellenides or Zagrides, where parallel ridges and troughs reflect crustal extension. By contrast, the simple “staircase”geornetry of the present margin is best interpreted as the result of the fracturing of the Gondwanan continent, separating the Apulian and African Plates as early as the Trias.

Geological history of the Cretaceous ophiolitic complexes of northwestern South America (Colombian Andes)

The Western Cordillera of Colombia was formed by intense alpine-type nappe-forming folding and thrusting. The Cretaceous (80–120 Ma B.P.) tholeiitic material of the Western Cordilleran nappes has been obducted onto the Paleozoic and Precambrian polymetamorphic micaschists and gneiss of the Central Cordillera. Near Yarumal, the Antioquia batholith (60–80 Ma B.P.) intrudes both obducted Cretaceous oceanic material and the polymetamorphic basement rock of the Central Cordillera. Therefore, nappe emplacement and obduction onto the Central Cordillera occurred during Late Senonian to Early Paleocene.The nappes travelled from northwest to southeast so that the highest unit, the Rio Calima nappe therefore has the most northwestern source, whereas the lowest units originated from a more southeastward direction. Sedimentological analysis of the volcanoclastic and sandy turbidite material from each unit suggests a marginal marine environment. During Cretaceous times the opening of this marginal sea, from now on called the “Colombia marginal basin”, probably originated by detachment of a block from the South American continent related to the Farallon-South America plate convergence.In the Popayan area (southern Colombia), the Central Cordilleran basement exhibits glaucophane schist facies metamorphism. This high pressure low temperature metamorphism is of Early Cretaceous (125 Ma B.P.) age and is related to an undated metaophiolitic complex. The ophiolitic material originating from the Western Cordilleran is thrust over both the blueschist belt and the metaophiolitic complex. These data suggest that the “Occidente Colombiano” suffered at least two phases of ophiolitic obduction during Mesozoic time.

Middle miocene deduction and late miocene beginning of collision registered in the hengchun peninsula: Geodynamic implications for the evolution of Taiwan

Taiwan is located in the axis of the Manila Trench. It results from an oblique collision between the northernmost part of the Luzon arc and the Chinese passive margin. This active collision follows the subduction of the Oligocene-Miocene oceanic crust of the South China Sea along the Manila Trench. The tectonized Chinese margin emerged in the Hengchun peninsula (South Taiwan). Gentle folds which are delineated by the Quaternary reefal limestones demonstrate Recent deformations. These folds deformed a thick detrital sequence of Miocene age (Ssuchung Chi series) which was previously strongly folded and thrust westward (axis NS-N20) upon the Renting mélange of Latest Miocene age. These main deformations, sealed by the Middle Pliocene, are the evidence for the onset of collision in this part of Taiwan at the end of the Miocene. Because of its obliquity, the collision started already in the northern part of Taiwan during the Late Miocene (6-7-8 Ma ?). The Ssuchung Chi series, a sequence of proximal turbidites, has contained, since the Middle Miocene (NN 6~13 Ma), fragments of an Oligocene to Lower Miocene oceanic crust. This ophiolitic material is very similar to the East Taiwan Ophiolite of the Coastal Range. It originated most probably from a slice of South China Sea crust obducted in Middle Miocene times (13–14 Ma) upon the Chinese margin (North of the Hengchun peninsula). This obduction occurred 7 to 8 Ma before the beginning of collision. These results make it possible to propose an evolutionary model for Taiwan from the Oligocene to the Recent, with the different phases of a collision between a volcanic arc and a passive margin.

Tectonic evolution of southwestern Newfoundland as indicated by granitoid petrogenesis

Four granitoid suites are recognized in the region of the Cape Ray Fault Zone of southwestern Newfoundland. The two oldest (Ordovician–Silurian (?)) suites represent partial melts of their enclosing host rocks. The Port aux Basques granite is modelled as a partial melt of the gneissic component of its host, Port aux Basques Complex. The Cape Ray granite forms a dominantly tonalitic terrane derived by partial melting of ophiolitic material. The Red Rocks granite and a megacrystic phase of the Cape Ray granite form coherent lines of geochemical descent from the parental tonalite but show evidence of some continental crust contamination.The Late Devonian Windowglass Hill granite is a subvolcanic equivalent of felsic volcanic rocks in the Windsor Point Group. Both units were derived as partial melts of continental crust.The post-tectonic, Late Devonian to Early Carboniferous Strawberry and Isle aux Morts Brook granites constitute the youngest granitoid suite in the region. These A-type granitoids were derived as partial melts of an underlying depleted granulitic (felsic) crust. The depleted nature of the source may have resulted from previous generation of the Windowglass Hill granite and Windsor Point Group. The only possible protolith for the granulitic source is Precambrian Grenvillian gneiss. The presence of this gneiss beneath the Cape Ray Fault Zone of southwestern Newfoundland implies that the complete series of lithologies is allochthonous.

Variety of subduction and accretion processes in cretaceous to recent plate boundaries around Southwest and central Japan

The Shimanto and Miura-Boso terranes in southwest and central Japan are analyzed for the understanding of variation of the subduction-accretion process in convergent plate boundaries. The Shimanto is a Cretaceous to Recent Honshu accretionary prism of 200 km width in the Honshu arc side and is made up of melange, flysch and olistostrome units. These are repeated by many thrust faults and local broad and tight folds to make Schuppen structures. Shallow marine and olistostromal bodies overlie part of the Schuppen structures unconformably or are included within the structures. Normal subduction tectonics prevail for the formation of the accretionary prism but the effects of probable transform tectonics during late Oligocene to early Miocene, are considerable.The Miura-Boso region is a terrane newly accreted from the Izu arc to the Honshu arc. This terrane is made up of Miocene and younger volcaniclastic sedimentary rocks, dismembered ophiolitic materials and some of the rocks from the Shimanto Supergroup. The Miura-Boso terrane is characterized by transpressional tectonics associated with eduction during the sedimentation on the highly oblique subduction zone between the Eurasia (later North America) and Philippine Sea plates.Several cases for the emplacement of the basaltic and other oceanic rocks to the landward side of the accretionary prism are discussed to account for the basic problem of the subduction-accretion tectonics.

Petrography and geochemistry of feldspathic and mafic sediments of the northeastern Pacific margin

ABSTRACTThis study attempts to ascertain whether the differing provenance sources and plate tectonic settings of deposition of clastic sediments and rocks can be identified by chemical means, thus opening the increased use of these rocks and their metamorphosed derivatives in plate tectonic modelling. Mineralogically immature feldspathic and mafic sands, muds, sandstones and shales from Mesozoic, Cenozoic and Holocene clastic deposits in California, Oregon, Alaska and Colorado have been both modally and chemically analysed providing a valuable data base (217 samples; 216 samples chemically analysed, many for 28 elements).There is significant upward chemical variation in the Great Valley sequence of California which mimics previously described petrographic variation and in turn reflects provenance changes with igneous episodes and erosional stripping of the Sierra Nevada in late Mesozoic time. Differing sandstone petrofacies result in varying chemical signatures and while the provenances of monomict sediments are easiest to identify, polymict sources involving granitoid or ophiolitic material can often be identified by potassium feldspar or Cr contents. The distinction of K which is derived from detrital potassium feldspar from K derived from detrital illite, micas or other sheet minerals, is best made by a Niggli al–alk plot against k. Mafic sandstones derived from mafic volcanic or plutonic rocks preserve essentially mafic igneous chemistry and could be possible parent sources of some amphibolites which grade into metasediments. The lithified erosion products of the Sierra Nevada calc-alkaline igneous rocks have higher Niggli al–alk and higher average Niggli si at any given mg value than the fresh igneous rocks enabling meta-arkoses to be distinguished from meta-igneous rocks.Applying previously published chemical criteria gives the actual plate tectonic setting of deposition of most of the sandstones studied. This suggests that the chemical composition of sandstones can yield much more information about the provenance and plate tectonic setting than hitherto recognised.

Glaucophane schists and ophiolites of the northern California Coast Ranges: Isotopic ages and their tectonic implications

The geology of a strip of 15′ quadrangles across the Franciscan Complex in the Coast Ranges of northern California is the basis for interpretation of the significance of potassium-argon and fossil ages of glaucophane schists, ophiolitic materials, and associated sedimentary rocks. Whole-rock and mineral separates of Franciscan rocks were analyzed, and 69 potassium-argon ages were obtained. White micas and actinolites from blueschist blocks in melange give apparent ages clustering about 142 and 153 m.y., but they probably represent a continuum. Blue amphibole ages from blocks range from 98 to 151 m.y.; the younger ages are from the eastern portion of the central melange belt. Published U-Pb ages from the ophiolite belt give crystallization ages from 153 to 165 m.y.; K-Ar ages extend from 143 to 166 m.y. Whole-rock ages from the South Fork Mountain Schist range from 113 to 158 m.y. but cluster around 124 m.y. The association of older K-Ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m.y. ago. Fossil evidence indicates that the basal sediments of the Great Valley Sequence were deposited upon the disrupted Coast Range Ophiolite within 5 to 10 m.y. of the oldest crystallization ages for ophiolite. Fossils within melange units, broken formations, and unsubducted trench-slope deposits provide evidence for melange development by the Early Cretaceous, with youngest melange units generally lying to the west. Available chronologic evidence is compatible with a tectonic model involving melange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex. Upflow of melange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot. The viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature-sensitive geochronologic tools.

Rifting, Drifting, and Crustal Accretion in Taiwan: ABSTRACT

Permian and younger rocks in Taiwan record the effects of rifting plus passive margin deposition, sea-floor spreading, convergence and/or oblique closure, volcanic plus plutonic arc construction, arrival of ophiolitic materials, and suturing of exotic trench-argillite and andesitic arc assemblages. Large portions of the sialic crust formed essentially in situ, then were deformed and thrust landward during subsequent tectonic events; however, far-traveled terranes and oceanic fragments played a substantial role in the accretionary process. Recognized and suspected exotics include: (1) lower or middle Mesozoic amphibolites plus serpentinites (now high-rank metaophiolites), situated anomalously in the upper Mesozoic Tailuko metamorphosed miogeoclinal and/or continental slope belt; (2) upper Mesozoic Yuli high-pressure, low-temperature metamorphic trench-argillite melange complex; (3) Mio-Pliocene tectonic blocks of blueschistic ophiolite emplaced in the east-central part of the Yuli terrane; (4) ophiolitic olistostromal debris supplied in the Pliocene to the Lichi Melange of the Coastal Range from Miocene oceanic crust of the South China Sea; and (5) the Neogene calc-alkaline Luzon arc which collided with the sialic crust-capped Asiatic plate in the Plio-Pleistocene. The Cenozoic slate series is an additional, largely fault-bounded parautochthonous terrane deposited as the Tertiary miogeoclinal cover along the Asiatic continental margin and thrust westward during the Plio-Pleistocene arc collision. Taiwan thus represents an intricate Phanerozoic collage of l thotectonic belts, produced by various accretionary processes. End_of_Article - Last_Page 474------------

The northwest portion of the Gander Zone — a geophysical interpretation

Geophysical data from the northwestern portion of the Gander Zone of Newfoundland are used to test existing geological models and to propose amended ones for the upper crustal structure of the region. The eastern geophysical boundary of in situ Paleozoic oceanic crustal material associated with the Iapetus Ocean is interpreted to be the GRUB line. An enigmatic anomaly belt in the Gander Group east of the GRUB line is identified and modelled. Three hypotheses are advanced for its origin, none of which can be eliminated on simple geophysical grounds. This anomaly belt is interpreted to present obducted ophiolitic materials emplaced in the continental rise prism represented by the Gander Group sediments.

Direction of ophiolite emplacement inferred from Cretaceous and Tertiary sediments of an adjacent autochthon, the Bey Dağlari, southwest Turkey

The late Mesozoic to mid-Tertiary depositional history of an autochthonous block in southwest Turkey is outlined in relation to the direction of emplacement of adjacent allochthonous units. In this area, a lively debate persists as to whether the Antalya Complex, located along the Mediterranean coast, should be correlated with the Lycian Nappes, sited to the northwest in central western Anatolia. Both allochthonous bodies include Mesozoic ophiolites and similar shallow- to deep-water sedimentary facies. Correlation of the Antalya Complex and the Lycian Nappes requires transport over the adjacent autochthon, the Bey Daglari. Here field data from the southern part of the Bey Daglari are used to trace the depositional history of the autochthon from Late Cretaceous to late Miocene time. A pattern of late Mesozoic pelagic carbonate deposition giving way to more heterogeneous sedimentation in the early Tertiary is documented. By early Miocene, the autochthon was blanketed by a thick sequence of Miocene elastics, mostly derived from an ophiolite. Evidence from sedimentary structures shows that transport was from the east and northeast. Depositional models are advanced which involve an early Miocene delta-submarine fan giving way in later Miocene times to an alluvial fan. Demonstration that in this area the ophiolite material was derived from the east provides additional important evidence of the existence of a southerly Mesozoic oceanic basin, distinct in origin from the Lycian nappes to the north.

Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite

The Precambrian Coal Creek serpentinite, Llano Uplift, Texas, occurs within the upper part of the Packsaddle Schist, which appears to represent a thick sequence of shelf-edge volcaniclastic arc-flank metasediments. The serpentinite has a foliation defined by lizardite pseudomorphs after original foliated harzburgite tectonite, which deviates as much as 15° from the regional country-rock foliation. A few samples contain the relict assemblage olivine + orthopyroxene + anthophyllite reflecting re-equilibration to regional metamorphic conditions near 710 °C at ∼3.5 kb. Oxygen and deuterium isotopic data indicate the lizardite could have formed in equilibrium with magmatic water at about 300 to 400 °C during regional uplift. A likely petrogenetic model involves the olistostromal emplacement of a fragment of ophiolitic material, the Coal Creek ultramafic body, into the volcaniclastic arc-flank sediments. The island-arc model suggested for the Llano Uplift and the existence of ophiolitic material imply that brittle plate collisions were locally important 1,200 m.y. ago and that the orogenic event that affected the Llano terrane (that is, the Grenvillian orogeny) involved island-arc-continent interactions. The Llano Uplift contains the only exposed ophiolitic material along the entire “Grenvillian” orogenic belt.

Kaweah serpentinite mélange, southwest Sierra Nevada foothills, California

Prebatholithic rocks of the southwest Sierra Nevada foothills contain a 125-km-long northwest-trending disrupted and metamorphosed ophiolite belt. Much of the belt consists of a serpentinite-matrix melange in which the ophiolitic material is dispersed. The melange matrix consists of schistose serpentinite derived from tectonitic peridotite and sedimentary serpentinite. The tectonic blocks consist mainly of peridotite, gabbro, basalt, chert, ophicalcite, and silica-carbonate rock and are as long as several kilometres. Within the tectonic blocks relict primary features such as bedding, pillows, dikes, and cumulate layering remain. The melange represents disrupted and internally mixed oceanic lithosphere of latest Paleozoic to possibly earliest Mesozoic age. Outcrop mapping of the melange reveals clustering of blocks into several lithologic associations. The associations are defined as melange units, which appear to represent the vestiges of once-intact ocean-floor sections. Three types of melange units have been recognized: (1) peridotite-gabbro units; (2) gabbro-basalt units; and (3) peridotite-chert-basalt units. The first two units represent crust and upper mantle whose deformation and metamorphic history began at the site of ocean-floor genesis. The third unit represents abnormal crust composed of ultramafic protrusions and interbedded sedimentary serpentinite, ophicalcite, chert, and pillow lava. Protrusive activity also started at the site of ocean-floor genesis and continued for an extended time after sea-floor–spreading transport of the ophiolite belt away from the genesis site. Genesis was at an oceanic spreading center that was cut by a transverse fracture zone. Ocean-floor melange developed along the fracture zone by the combined effect of protrusive acitivity and wrench faulting. Emplacement of the fracture-zone complex (ophiolite belt) resulted from large-scale wrench faulting that truncated the ancient continental margin and juxtaposed the complex against the modified margin. During transport to the continental margin, a chert-argillite oliostostrome complex was shed across the ophiolite belt. The olistostromes carried limestone blocks with fauna exotic to North America. Once in the vicinity of the continental margin, the ophiolite belt served as basement for continent-derived submarine-fan deposits and island-arc volcanic rocks, both of Late Triassic to Middle Jurassic age. Deformation of these strata along with their ophiolitic basement continued along the older fracture zone trends. The strata now exist as highly deformed depositional remnants above serpentinite melange.