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1.
The Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper” Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical features, low Ti/V ratios (< 20) and depletion in high field strength elements and light rare earth elements.The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites. The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin. 相似文献
2.
A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran) 总被引：3，自引：0，他引：3
Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj–Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj–Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic–Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran, the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene. 相似文献
3.
The studied serpentinites occur as isolated masses, imbricate slices of variable thicknesses and as small blocks or lenses incorporated in the sedimentary matrix of the mélange. They are thrusted over the associated island arc calc-alkaline metavolcanics and replaced by talc-carbonates along shear zones. Lack of thermal effect of the serpentinites upon the enveloping country rocks, as well as their association with thrust faults indicates their tectonic emplacement as solid bodies. Petrographically, they are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of carbonates, chromite, magnetite, magnesite, talc, tremolite and chlorite. Chrysotile occurs as cross-fiber veinlets traversing the antigorite matrix, which indicate a late crystallization under static conditions. The predominance of antigorite over other serpentine minerals indicates that the serpentinites have undergone prograde metamorphism or the parent ultramafic rocks were serpentinized under higher pressure. The parent rocks of the studied serpentinites are mainly harzburgite and less commonly dunite and wehrlite due to the prevalence of mesh and bastite textures. The serpentinites have suffered regional metamorphism up to the greenschist facies, which occurred during the collisional stage or back-arc basin closure, followed by thrusting over a continental margin. The microprobe analyses of the serpentine minerals show wide variation in SiO2, MgO, Al2O3, FeO and Cr2O3 due to different generations of serpentinization. The clinopyroxene relicts, from the partly serpentinized peridotite, are augite and similar to clinopyroxene in mantle-derived peridotites. The chromitite lenses associated with the serpentinites show common textures and structures typical of magmatic crystallization and podiform chromitites. The present data suggest that the serpentinites and associated chromitite lenses represent an ophiolitic mantle sequence from a supra-subduction zone, which were thrust over the continental margins during the collisional stage of back-arc basin. 相似文献
4.
《地学前缘(英文版)》2020,11(6):2347-2364
The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ± chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 wt.%), P2O5 (<0.05 wt.%), K2O (0.01–0.51 wt.%) and total alkali contents (0.12–3.04 wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu1 = 1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N = 1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N = 0.10–0.27 and (La/Yb)N = 0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting. 相似文献
5.
Plate kinematics, origin and tectonic emplacement of supra-subduction ophiolites in SE Asia 总被引：2，自引：0，他引：2
A unique feature of the Circum Pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes, most of which display petrological and geochemical characteristics typical of supra-subduction zone oceanic crust. In SE Asia, a majority of the ophiolites appear to have originated at convergent margins, and specifically in backarc or island arc settings, which evolved either along the edge of the Sunda (Eurasia) and Australian cratons, or within the Philippine Sea Plate. These ophiolites were later accreted to continental margins during the Tertiary. Because of fast relative plate velocities, tectonic regimes at the active margins of these three plates also changed rapidly. Strain partitioning associated with oblique convergence caused arc-trench systems to move further away from the locus of their accretion. We distinguish “relatively autochthonous ophiolites” resulting from the shortening of marginal basins such as the present-day South China Sea or the Coral Sea, and “highly displaced ophiolites” developed in oblique convergent margins, where they were dismantled, transported and locally severely sheared during final docking. In peri-cratonic mobile belts (i.e. the Philippine Mobile Belt) we find a series of oceanic basins which have been slightly deformed and uplifted. Varying lithologies and geochemical compositions of tectonic units in these basins, as well as their age discrepancies, suggest important displacements along major wrench faults.We have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro-geochemical data to their original location and their former tectonic settings. Some of the ophiolites occurring in front of the Sunda plate represent supra-subduction zone basins formed along the Australian Craton margin during the Mesozoic. The Philippine Sea Basin, the Huatung basin south of Taiwan, and composite ophiolitic basements of the Philippines and Halmahera may represent remnants of such marginal basins. The portion of the Philippine Sea Plate carrying the Taiwan–Philippine arc and its composite ophiolitic/continental crustal basement might have actually originated in a different setting, closer to that of the Papua New Guinea Ophiolite, and then have been displaced rapidly as a result of shearing associated with fast oblique convergence. 相似文献
6.
Iran is a mosaic of Ediacaran–Cambrian (Cadomian; 520–600 Ma) blocks, stitched together by Paleozoic and Mesozoic ophiolites. In this paper we summarize the Paleozoic ophiolites of Iran for the international geoscientific audience including field, chemical and geochronological data from the literature and our own unpublished data. We focus on the five best known examples of Middle to Late Paleozoic ophiolites which are remnants of Paleotethys, aligned in two main zones in northern Iran: Aghdarband, Mashhad and Rasht in the north and Jandagh–Anarak and Takab ophiolites to the south. Paleozoic ophiolites were emplaced when N-directed subduction resulted in collision of Gondwana fragment “Cimmeria” with Eurasia in Permo-Triassic time. Paleozoic ophiolites show both SSZ- and MORB-type mineralogical and geochemical signatures, perhaps reflecting formation in a marginal basin. Paleozoic ophiolites of Iran suggest a progression from oceanic crust formation above a subduction zone in Devonian time to accretionary convergence in Permian time. The Iranian Paleozoic ophiolites along with those of the Caucausus and Turkey in the west and Afghanistan, Turkmenistan and Tibet to the east, define a series of diachronous subduction-related marginal basins active from at least Early Devonian to Late Permian time. 相似文献
7.
The ophiolitic extrusive sequence, exposed in an area north of Sabzevar, has three major parts: a lower part, with abundant breccia, hyaloclastic tuff, and sheet flow, a middle part with vesicular, aphyric pillow lava, and an upper part with a sequence of lava and volcanic-sedimentary rocks. Pelagic limestone interlayers contain Late Cretaceous (Maastrichtian–Late Maastrichtian) microfauna. The supra-ophiolitic series includes a sequence of turbidititic and volcanic-sedimentary rocks with lava flow, aphyric and phyric lava, and interlayers of pelagic limestone and radiolarian chert. Paleontological investigation of the pelagic limestone and radiolarite interlayers in this series gives a Late Cretaceous age, supporting the idea that the supra-ophiolitic series formed in a trough, synchronous with the Sabzevar oceanic crust during the Late Cretaceous. Geochemical data indicate a relationship between lava in the upper part of the extrusive sequence and lava in the supra-ophiolitic series. These lavas have a calc-alkaline to almost alkaline characteristic, and show a clear depletion in Nb and definite depletions in Zr and Ti in spider diagrams. Data from these rocks plot in the subduction zone field in tectonomagmatic diagrams. The concentration and position of the heavy rare earth elements in the spider diagrams, and their slight variation, can be attributed to partial melting of the depleted mantle wedge above the subducted slab, and enrichment in the LILE can be attributed to subduction components (fluid, melt) released from the subducting slab. In comparison, the sheet flow and pillow lava of the lower and middle parts of the extrusive sequence show OIB characteristics and high potassium magmatic and shoshonitic trends, and their spider diagram patterns show Nb, Zr, and Ti depletions. The enrichment in the LILE in the spider diagram patterns suggest a low rate of partial melting of an enriched, garnet-bearing mantle. It seems that the marginal arc basin, in which the Sabzevar ophiolite was forming, experienced lithospheric extension in response to slab rollback. This process, which formed a backarc basin, may have aborted the embryonic arc, stopped arc magmatism, and led to the rise of mantle diapirs. The extrusive ophiolite sequence, north of Sabzevar probably formed during the transition from a marginal arc basin to a backarc basin during the Late Cretaceous. 相似文献
8.
C. Dupuis R. Hbert V. Dubois-Ct C. Guilmette C.S. Wang Y.L. Li Z.J. Li 《Journal of Asian Earth Sciences》2005,25(6):293
The southern contact of the Yarlung-Zangbo Suture Zone ophiolitic belt is marked by a highly sheared serpentinite mélange containing ultramafic blocks. These peridotites can be divided into three main groups. (1) Lherzolites and Cpx-harzburgites contain brownish spinel with Mg# of 0.7–0.75 and Cr# of 0.15–0.27. They resemble fertile abyssal peridotites with generally smooth LREE-depleted and fairly flat MREE–HREE profiles. (2) Transitional harzburgites contain reddish spinels with Mg# of 0.57–0.66 and Cr# of 0.35–0.46. They resemble depleted abyssal or supra-subduction zone peridotites in that MREE–HREE profiles have positive slopes indicative of high degrees of partial melting. LREE profiles vary from depleted to slightly enriched, consistent with some interacting melt. (3) Harzburgites and dunites contain dark reddish spinels with Mg# of 0.47–0.68 and Cr# of 0.40–0.63. They have U-shaped profiles characteristics of interaction between LREE-enriched melt and REE-depleted mantle residues. Fractional melting modelling indicates that Cpx-harburgites may be the residues from 5 to 15% melting, transitional harzburgites from 15 to 23% melting, and harzburgites and dunites from 22 to 29% melting. The South Sandwich arc-basin system is considered a modern analog of the initial geodynamic setting. 相似文献
9.
The Piranshahr metaperidotites in the northwestern end of the Zagros orogen were emplaced following the closure of the Neotethys ocean. The ophiolitic rocks were emplaced onto the passive margin of the northern edge of the Arabian plate as a result of northeastward subduction and subsequent accretion of the continental fragments. The metaperidotites have compositions ranging from low-clinopyroxene lherzolite to harzburgite and dunite. They are mantle residues with distinct geochemical signatures of both mid-ocean ridge and supra subduction zone (SSZ) affinities. The abyssal peridotites are characterized by high Al2O3 and Cr2O3 contents and low Mg-number in pyroxenes. The Cr-number in the coexisting spinel is also low. The SSZ mantle peridotites are characterized by low Al2O3 contents in pyroxenes as well as low Al2O3 and high Cr-number in spinel. Mineral chemical data indicate that the MOR- and SSZ-type peridotites are the residues from ∼15–20% and ∼30–35% of mantle melting, respectively. Considering petrography, mineralogy and textural evidence, the petrological history of the Piranshahr metaperidotites can be interpreted in three stages: mantle stable stage, serpentinization and metamorphism. The temperature conditions in the mantle are estimated using the Ca-in-orthopyroxene thermometer as 1210 ± 26 °C. The rocks have experienced serpentinization. Based on the textural observations, olivine and pyroxene transformed into lizardite and/or chrysotile with pseudomorphic textures at temperatures below 300 °C during the initial stage of serpentinization. Subsequent orogenic metamorphism affected the rocks at temperatures lower than 600 °C under lower-amphibolite facies metamorphism. 相似文献
10.
《China Geology》2020,3(2):299-313
Swarms of orthopyroxenite and websterite veins are found within Egiingol residual SSZ peridotite massif of Dzhida terrain (Central Asian Orogenic Belt, Northern Mongolia). The process of Egiingol pyroxenite veins formation is investigated using new major and trace element analyses of pyroxenite minerals, calculations of closure temperatures and composition of equilibrium melt. The pyroxenites show abundant petrographic and geochemical evidence for replacement of the residual peridotite minerals by ortho- and clinopyroxene due to melt-rock interaction. Relics of peridotite olivines are found in pyroxenites, Cr# of spinel increases from peridotites to pyroxenites, and compositions of ortho- and clinopyroxene change from peridotite to pyroxenite. The authors show that calculated equilibrium melts for investigated pyroxenites are very similar to compositions of boninite lavas from the Dzhida terrain. Therefore, formation of pyroxenite veins most likely resulted from percolation of boninite melts through the Egiingol peridotites. Orthopyroxenite veins formed at first, followed by websterite veins. Thus, the authors assume that pyroxenite veins represent the channels for boninitic melts migration in supra-subduction environment. 相似文献