Ophiolites along the indus Suture Zone (ISZ) are important not only for the information they provide about the India\|Eurasia collision,but may also contain economic mineral deposits (chrome,noble metals,and Ni\|sulfi...Ophiolites along the indus Suture Zone (ISZ) are important not only for the information they provide about the India\|Eurasia collision,but may also contain economic mineral deposits (chrome,noble metals,and Ni\|sulfides).Several ophiolites occur in the ISZ,and the Nidar ophiolite is one that occurs on the eastern Ladakh.This paper presents the geochemistry of ultramafic rocks from the Nidar ophiolite to understand tectonic environment and economic potential.Nidar ophiolite consists of pillow lavas,radiolarian cherts,rodingites,peridotites and chromitites.The lower part of the ophiolite consists of tectonized harzburgite,hosting dunite enveloped podiform chromitites.The transition zone between upper mantle and lower oceanic crust is characterized by serpentinized dunite with disseminations of chromites.Peridotites in Nidar have low whole\|rock values of w (TiO\-2)(0 02%～0 03%), w (Al\-2O\-3)(2%～3%), w (CaO)(0.5%), w (Zr)(40×10 -6 ),and w (Y)(30×10 -6 ),and enriched in refractory elements such as w (Cr)((3000～5000)×10 -6 )and w (Ni)((3000～4000)×10 -6 ).Thus,the Nidar peridotites are more depleted in elements partitioned into the liquid,such as Al and Ca and the moderately refractory element Ti.The peridotites have fractionated chondrite\|normalized PGE patterns.These geochemical characteristics suggests their formation by high degrees of partial melting in a supra\|subduction zone environment,where melting was enhanced by addition of volatiles.The Cr # of 0.82 in the chromite of the chromitites also suggests a depleted mantle source and a supra\|subduction zone environment for the host rocks.Ophiolites formed in supra\|subduction zone setting are very conducive for many metallic and non\|metallic mineral deposits.展开更多
he 2500km long Indus\|Tsangpo Suture has been recognized as one of the best examples of continent to continent collisional Suture Zone. It has come into existence as a result of subduction followed by continental coll...he 2500km long Indus\|Tsangpo Suture has been recognized as one of the best examples of continent to continent collisional Suture Zone. It has come into existence as a result of subduction followed by continental collision (55～60Ma) between Indian (Sinha, 1989, 1997; Sinha et al., 1999) and Eurasian plates. While considering the recent palaeogeographic reconstruction of Pangea during late Palaeozoic it appears that a southern belt of Asian microcontinents stretching from Iran and Afghanistan through southern Tibet to western Thailand, Malaysia and Sumatra, comprise several continental blocks and numerous fragments that have coalesced since the Mid\|Palaeozoic along with the closure of Tethys. The origin, migration, assembly and timing of accretion of all these blocks to their present geotectonic position is not well known and there is no Permo—Triassic crust left in the present day Indian Ocean. The oldest ocean crust adjacent to the west African and Antarctic margin is of early or middle Cretaceous age (approximately 140～100Ma) (Searle, 1991). The Karakoram\|Hindukush microplate in the west and the Qiangtang\|Lhasa block in the central and eastern segment of South Asia margin are among those blocks already welded with Asian plates around 120～130Ma ago, before the collision of India (55～60Ma) with the collage of plates forming Peri\|Gondwanian microcontinents. But the reconstruction of palaeogeographic configuration remain incomplete due to paucity of authentic geologic information available from Karakoram, Pamir and Western Tibet. Prior to our discovery no early Permian plant remains and palynomorphs were ever reported from Karakoram terrane. Our discovery of Early Permian remains and late Asselian (about 280～275Ma) palynomorphs provides crucial clue regarding the palaeogeographic reconstruction of the Karakoram\|Himalayan block in the Permian time.展开更多
Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the maj... Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.……展开更多
High-pressure(HP)veins were extensively developed in western Tianshan high-pressure(HP)metamorphic belt.The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids.Anal...High-pressure(HP)veins were extensively developed in western Tianshan high-pressure(HP)metamorphic belt.The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids.Analytical data show that the immediately adjacent host-rocks of the studied HP vein are eclogites and gradually turned into blueschist as the distance from the veins increases,which indicates that the vein-forming fluid was derived from adjacent host-rocks;the boundaries between the vein and the host-rocks are sharp,which indicates that the fracture of the host-rocks is brittle during the vein-forming process.It is suggested that this type of HP veins is precipitated from the liquid formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies,which results in hydrofracturing of the rocks and provides the space for the vein to precipitate.The width of the eclogite-facies host-rocks is usually 1-2 cm,which provides the direct evidence that the fluid flow is on centimeter-scale.展开更多
Previous studies on ophiolite and granite revealed that a paleo\|ocean which was referred to as “Proto\|Tethys" developed in north part of western Kunlun in Pt 3～Pz 1.The geotectonic unit in Kuda, western Kunlu...Previous studies on ophiolite and granite revealed that a paleo\|ocean which was referred to as “Proto\|Tethys" developed in north part of western Kunlun in Pt 3～Pz 1.The geotectonic unit in Kuda, western Kunlun is composed of early Paleozoic accretionary complexes and the metamorphic equivalents due to subduction of Proto\|Tethys: Along the Xinzang highway (from Yecheng, Xinjiang to Shiquanhe, Tibet), about 30km north of Kuda, a schistose granodiorite intrudes a thick unit of deformed mafic rocks; Non to weakly metamorphosed early Paleozoic fore\|arc turbidite exposed in Yixiekegou; A set of metamorphic rocks about 20km wide in Kuda which was thought of the Precambrian basement is a large\|scale ductile shear zone determined by detailed field observation and laboratory research; About 10km north of Kuda, there is an undeformed potassic granite batholith intrudes the shear zone.The shear zone mainly consists of parametamorphic rocks and orthometamorphte rocks. Moreover, it contains various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone and dismembered ophiolite suite, such as dunite, peridotite, gabbro and anorthisite with high number up to 91. The dismembered ophiolite suite may probably represents a remnant oceanic crust that related to the breakup of Rodinia. The parametamorphite rocks is composed of mica\|schist, quartzite and gamete\|mica schist,etc., of which protoliths are sedimentary rocks such as mudstone, sandstone, siliciclastic rocks and impure carbonates that most likely accumulated along the passive continental margin of pelagic sedimentary materials. The orthometamorphic rocks is composed of granitic gneiss and amphibolite,etc. The protoliths of amphibolite is oceanic island basalt of MORB basalt that was accretioned in accretionary complex. The accretionary wedge complex suffers intense ductile strain and metamorphism due to the subduction of Proto\|Tethys with the metamorphic grade reaching the greenschist facies to amphibolite facies. The 40 Ar/ 39 Ar age 451Ma of metamorphic homoblende in amphibolite is interpreted to closely date attainment of maximum metamorphic conditions representing the age of initial subduction at the latest, while the 40 Ar/ 39 Ar age 425Ma of biotite is interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon representing the cease of subduction. These data provide significant information on the timing of accretion and the age of the subducted Proto\|Tethys. The whole rock Sm\|Nd isochronal age of amphibolite is problematic 737Ma that maybe represent the rifting age of the Rodinia.展开更多
The East Kunlun terrain is located on the northern Qinghai—Tibet plateau, composed of the East Kunlun Mountain and the Qaidam Basin and bounded by the Qilian terrain on the north and Bayanhar—Songpan Ganze terrain o...The East Kunlun terrain is located on the northern Qinghai—Tibet plateau, composed of the East Kunlun Mountain and the Qaidam Basin and bounded by the Qilian terrain on the north and Bayanhar—Songpan Ganze terrain on the south. It is regarded as a composite orogenic belt characterized by having developed superimposed ductile tectonic regimes reflecting the collision orogeny during Early Paleozoic and Triassic periods. It has also experienced transformation from ductile to brittle deformation caused by the post orogeny, since Jurassic after the formation of the East Kunlun Mountain and the Qaidam Basin. A Paleozoic subduction complex zone was recently recognized along the north border of the East Kunlun terrain from Da Qaidam to Dulan (Xu,et al, 1 999). It is composed of ophiolite (of Early Paleozoic age?), tectonic melange and very high\|pressure metamorphic rocks with eclogite (\%p\%=2 2GPa, \%t\%=720℃) (Yang,et al.,1998) and garnet\|peridotite (\%p\%=2 5GPa, t =837℃). The Anyemaqin Triassic subduction complex zone trending in NWW\|SEE was developed along the eastern segment of the south border of the East Kunlun terrain. It is mainly composed of ultramafic and mafic rocks, pillow basalt, radiolaria\|bearing clastic rocks, tectonic melange and mylonite. The subduction complex zone contains a series of the southward overthrusting imbricated slices. Instead of this subduction complex zone, a 2 km\|wide sinistral strike\|slip ductile shear zone trending in E\|W was developed along the western segment. On the basis of macroscopic and microscopic studies on a series of structural sections, we divide the East Kunlun Mountain into four tectonic units as follows: (1) North Proterozoic Metamorphic Basement Zone; (2) South Early Paleozoic—Triassic Superimposed Fold Zone; (3) South Triassic Transpression Zone; (4) Anyemaqin Back\|Arc Decollement\|Thrust Zone.展开更多
A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the E...A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.展开更多
The Alpine zone of Central Pamir is elongated in sublatitudinal direction between the Hercynians of Northern Pamir and the Cimmerians of Southern Pamir south of the Vanch\|Akbaital thrust. Its western continuation is ...The Alpine zone of Central Pamir is elongated in sublatitudinal direction between the Hercynians of Northern Pamir and the Cimmerians of Southern Pamir south of the Vanch\|Akbaital thrust. Its western continuation is overthrusted by the Herat fault and its eastern continuation is cut by the Karakoram strike\|slip fault.. The Central Pamir is a mainly S\|vergent (at the southern part N\|vergent) Alpine nappe stack then folding in antiform. It comprises deposits from Vendian to Neogene which have a thickness of 10km. Paleozoic and Mesozoic tectonic activity was poorly displaied in its limits. Rifting took place in Early and probably Upper Paleozoic. Pre\|Upper Cretaceous unconformity is known only in southern (autochthonous) part of the Zone as a result of closing of Bangong\|Nu Jiang ocean. In northern (allochthonous) part of the zone the sequence of Mesozoic and Paleogene rocks has no unconformities. Alpine endogenous processes were developed very intensively. They implied nappes and imbricate structures, linear folding, different igneous activity, zonal metamorphism. Slices of pyroxenites and gabbroids occured. Calc\|alkaline lavas and tuffs constitutes the major part of Paleocene to Miocene sequence (andesites\|ryolites\|in Paleogene, alkaline basalts in Oligocene—Miocene). Oligocene—Miocene zonal metamorphic belt of the intermediate type of high pressure including series of granitegneiss domes can be traced along the Central Pamir. Cores of domes include migmatites and remobilized bodies of the Early Paleozoic gneissic granites. The decompression took place at a later stage and rocks were overprinted by the andalusite\|sillimanite type metamorphism.. Syenite and leucogranite bodies, pegmatite and aplite veins were emplaced.展开更多
The 26 December 2004 earthquake off Sumatra coast focused world attention on the Sunda arc subduction zone.Bangladesh is along the strike of and within a rupture-distance from that enormous M-9.3 earthquake. This coun...The 26 December 2004 earthquake off Sumatra coast focused world attention on the Sunda arc subduction zone.Bangladesh is along the strike of and within a rupture-distance from that enormous M-9.3 earthquake. This country is situated where the India-Sunda subduction zone rises from oceanic depths to subaerial exposure as a result of incipient continent collision where the trench meets the huge sediment展开更多
Kinematic models compute the temperature distribution by prescribing a constant convergent velocity for the subducting slab,resulting in an artificial velocity discontinuity,which may accelerate the heating of subduct...Kinematic models compute the temperature distribution by prescribing a constant convergent velocity for the subducting slab,resulting in an artificial velocity discontinuity,which may accelerate the heating of subducting slabs.For the purpose to moderate the influence of such artificial discontinuity, we construct a 2D thermal model for subduction zones with a velocity boundary layer,within which the velocities decrease linearly with the distance from the interfaces of slabs.Temperatures are calculated展开更多
Changes in oceanic O-Sr-C isotopic compositions, global cooling and Asian continental aridification beginning in the Middle-Late Eocene(47-34 Ma) are considered to have been caused by the uplift of the Tibetan plateau...Changes in oceanic O-Sr-C isotopic compositions, global cooling and Asian continental aridification beginning in the Middle-Late Eocene(47-34 Ma) are considered to have been caused by the uplift of the Tibetan plateau.The specific timing and uplift mechanism,however,have long been subjects of debate.We investigated the Duogecuoren lavas of the central-western Qiangtang block,which form the largest outcrops among Cenozoic lavas in north-展开更多
Cenozoic volcanic rocks, forming a series of volcanic rocks platform or highland on different sub\|volcanic rocks belt, had distributed widely in northern Qinghai—Tibet plateau, and characterized by potassinm\|rich l...Cenozoic volcanic rocks, forming a series of volcanic rocks platform or highland on different sub\|volcanic rocks belt, had distributed widely in northern Qinghai—Tibet plateau, and characterized by potassinm\|rich lavas which main petrologic types are latidacite and latite. The petrogenesis of volcanic rocks in northern Tibetan plateau had been well\|studied, and thought they play an important role to uplift of Tibetan plateau. Based on the study of lithochemistry and isotopic age determination of the rocks in Qiangtang area, which is the most southern and earliest volcanic rocks belt of Cenozoic volcanic rocks, I contribute another view here to the early uplift of Tibetan plateau. Cenozoic volcanic rocks in Qiangtang region, thickness of the rocks are from 200 to 1000m in different area, belong to shoshonite and high\|K calc\|alkaline series, highly enrich alkaline specially enrich potassinm, large\|ion lithophytes elements (Rb,Sr,Ba,and Th), LREE and deplete high\|field strength elements (Ta,Er,Hf,and Ti). Chondrite\|normalized REE pattern of volcanic rocks show an absence of significant Eu anomaly, and indicate that the volcanic source was plagioclase\|free and probably resulted from melting in lower crust or upper mantle. Cenozoic volcanic rocks in Qiangtang region are similar petrologic and geochemical characteristics to that have been reported in more northern Hoh Xil and Kunlun area in Tibetan plateau. The age scope of volcanic rocks in Qiangtang area were from 44.1～32.6Ma, among which the age mainly were 44.1～42.5Ma, which belong to mid\|Eocene epoch to primary Oligocene epoch, and could be classified into four out pouring periods, according to the potassium\|argon age dating. The study on lithochemistry and age of the rocks in Qiangtang area suggest that Intracontinental subduction southward Qiangtang tectonic belt plays an important role to Cenozoic volcanism, and imply that the convective removal of the thicked lithosphere, which formed at the collision mountain\|building processes, are responsible for the formation of Cenozoic volcanic rocks, the earliest uplift of Tibetan plateau. and regional extension in Qiangtang area.展开更多
Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km ...Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km and 2800～3000km in the western and eastern syntaxes, respectively, since the late 45Ma. According to seafloor\|spreading reconstruction, a total shortening of 3000～500km was estimated after the initial contact of the two plates at 55～50Ma (Molnar and Tapponier, 1975 ; Molnar et al., 1988 ; Replumaz, 1999). Since 40Ma, the part of shortening only accommodated by the Himalayan belt was estimated around 470km in the western part (Coward and Butler, 1985) and 550 to 630km to the east (Ratsbacher et al., 1994 ; Replumaz, 1999). In contrast, global plate reconstructions suggest that the shortening in the Himalaya is of about 1250～250km (Achache et al., 1984 ; Powell et al., 1988 ; Dewey et al.,1989 ; Klootwijk et al., 1992 ; Matte et al., 1997). This discrepancy between the amount of shortening estimated by balancing the Himalayan belt and by plate reconstruction favour the existence of a greater India buried up to 1000km north of the present\|day Indus suture zone and subducted before Middle Eocene time (Klootwijk et al., 1979 ; Patriat and Achache, 1984).展开更多
The Alpine\|Himalayan system is considered as the type of continent\|continent collision orogen, the result of which is the amalgamation of Eurasia, India and Africa into a quasi\|continuous landmass. The Grenville Pr...The Alpine\|Himalayan system is considered as the type of continent\|continent collision orogen, the result of which is the amalgamation of Eurasia, India and Africa into a quasi\|continuous landmass. The Grenville Province of Canada, the youngest tectonic province of the Canadian shield, results from the collision of Laurentia with continental masses including Baltica, Amazonia, Congo and Kalahari (Hoffman, 1991), a collision that formed, between 1.2 Ga and 1 0Ga, the Rodinia supercontinent. The Grenville Province is the largest exposed section of the resulting Grenville belt, the length of which compares with that of the Alpine—Himalayan system whereas its width may reach that of Himalaya—Tibet. Unfortunately, due to the depth of erosion and to the dismembering of the belt in post\|grenvillian times, the Grenville Province lacks some of the diagnostic features of collision belts such as ophiolites and the SE\|vergent counterpart of the ca. 1 0Ga Grenville Front. However, the level of exposure is such that it provides direct access to the deep crustal parts of the orogen which are only accessible through geophysical methods in modern, little eroded mountains. Recent advances in the understanding of collision belts (e.g. LITHOPROBE and INDEPTH projects) now allow to identify the type of features that are common to the Grenvillian and the Himalayan belts in order to test the analogue model.展开更多
文摘Ophiolites along the indus Suture Zone (ISZ) are important not only for the information they provide about the India\|Eurasia collision,but may also contain economic mineral deposits (chrome,noble metals,and Ni\|sulfides).Several ophiolites occur in the ISZ,and the Nidar ophiolite is one that occurs on the eastern Ladakh.This paper presents the geochemistry of ultramafic rocks from the Nidar ophiolite to understand tectonic environment and economic potential.Nidar ophiolite consists of pillow lavas,radiolarian cherts,rodingites,peridotites and chromitites.The lower part of the ophiolite consists of tectonized harzburgite,hosting dunite enveloped podiform chromitites.The transition zone between upper mantle and lower oceanic crust is characterized by serpentinized dunite with disseminations of chromites.Peridotites in Nidar have low whole\|rock values of w (TiO\-2)(0 02%～0 03%), w (Al\-2O\-3)(2%～3%), w (CaO)(0.5%), w (Zr)(40×10 -6 ),and w (Y)(30×10 -6 ),and enriched in refractory elements such as w (Cr)((3000～5000)×10 -6 )and w (Ni)((3000～4000)×10 -6 ).Thus,the Nidar peridotites are more depleted in elements partitioned into the liquid,such as Al and Ca and the moderately refractory element Ti.The peridotites have fractionated chondrite\|normalized PGE patterns.These geochemical characteristics suggests their formation by high degrees of partial melting in a supra\|subduction zone environment,where melting was enhanced by addition of volatiles.The Cr # of 0.82 in the chromite of the chromitites also suggests a depleted mantle source and a supra\|subduction zone environment for the host rocks.Ophiolites formed in supra\|subduction zone setting are very conducive for many metallic and non\|metallic mineral deposits.
文摘he 2500km long Indus\|Tsangpo Suture has been recognized as one of the best examples of continent to continent collisional Suture Zone. It has come into existence as a result of subduction followed by continental collision (55～60Ma) between Indian (Sinha, 1989, 1997; Sinha et al., 1999) and Eurasian plates. While considering the recent palaeogeographic reconstruction of Pangea during late Palaeozoic it appears that a southern belt of Asian microcontinents stretching from Iran and Afghanistan through southern Tibet to western Thailand, Malaysia and Sumatra, comprise several continental blocks and numerous fragments that have coalesced since the Mid\|Palaeozoic along with the closure of Tethys. The origin, migration, assembly and timing of accretion of all these blocks to their present geotectonic position is not well known and there is no Permo—Triassic crust left in the present day Indian Ocean. The oldest ocean crust adjacent to the west African and Antarctic margin is of early or middle Cretaceous age (approximately 140～100Ma) (Searle, 1991). The Karakoram\|Hindukush microplate in the west and the Qiangtang\|Lhasa block in the central and eastern segment of South Asia margin are among those blocks already welded with Asian plates around 120～130Ma ago, before the collision of India (55～60Ma) with the collage of plates forming Peri\|Gondwanian microcontinents. But the reconstruction of palaeogeographic configuration remain incomplete due to paucity of authentic geologic information available from Karakoram, Pamir and Western Tibet. Prior to our discovery no early Permian plant remains and palynomorphs were ever reported from Karakoram terrane. Our discovery of Early Permian remains and late Asselian (about 280～275Ma) palynomorphs provides crucial clue regarding the palaeogeographic reconstruction of the Karakoram\|Himalayan block in the Permian time.
文摘 Compared with the oceanic crust, knowledge about the formation of the continental crust (CC) is relatively poor. Although melting of subducted slabs in the early history of the Earth has been considered as the major way that shaped the chemical characteristics of the CC by most geologists, as the CC shares many characteristics with modern adakites, some geologists argued that Archean TTG was formed in the same way as modern arcs rather than slab melting, whereas others proposed that melting at the bottom of the thickened oceanic crust was more important. Recently,the debate is mainly focused on the unique subchondritic Nb/Ta value of the CC, and particularly, how Nb and Ta fractionated from each other and consequently how, in detail, the CC was built.……
基金Project(2001CB409803)supported by the Major State Basic Research Development Program of China
文摘High-pressure(HP)veins were extensively developed in western Tianshan high-pressure(HP)metamorphic belt.The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids.Analytical data show that the immediately adjacent host-rocks of the studied HP vein are eclogites and gradually turned into blueschist as the distance from the veins increases,which indicates that the vein-forming fluid was derived from adjacent host-rocks;the boundaries between the vein and the host-rocks are sharp,which indicates that the fracture of the host-rocks is brittle during the vein-forming process.It is suggested that this type of HP veins is precipitated from the liquid formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies,which results in hydrofracturing of the rocks and provides the space for the vein to precipitate.The width of the eclogite-facies host-rocks is usually 1-2 cm,which provides the direct evidence that the fluid flow is on centimeter-scale.
文摘Previous studies on ophiolite and granite revealed that a paleo\|ocean which was referred to as “Proto\|Tethys" developed in north part of western Kunlun in Pt 3～Pz 1.The geotectonic unit in Kuda, western Kunlun is composed of early Paleozoic accretionary complexes and the metamorphic equivalents due to subduction of Proto\|Tethys: Along the Xinzang highway (from Yecheng, Xinjiang to Shiquanhe, Tibet), about 30km north of Kuda, a schistose granodiorite intrudes a thick unit of deformed mafic rocks; Non to weakly metamorphosed early Paleozoic fore\|arc turbidite exposed in Yixiekegou; A set of metamorphic rocks about 20km wide in Kuda which was thought of the Precambrian basement is a large\|scale ductile shear zone determined by detailed field observation and laboratory research; About 10km north of Kuda, there is an undeformed potassic granite batholith intrudes the shear zone.The shear zone mainly consists of parametamorphic rocks and orthometamorphte rocks. Moreover, it contains various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone and dismembered ophiolite suite, such as dunite, peridotite, gabbro and anorthisite with high number up to 91. The dismembered ophiolite suite may probably represents a remnant oceanic crust that related to the breakup of Rodinia. The parametamorphite rocks is composed of mica\|schist, quartzite and gamete\|mica schist,etc., of which protoliths are sedimentary rocks such as mudstone, sandstone, siliciclastic rocks and impure carbonates that most likely accumulated along the passive continental margin of pelagic sedimentary materials. The orthometamorphic rocks is composed of granitic gneiss and amphibolite,etc. The protoliths of amphibolite is oceanic island basalt of MORB basalt that was accretioned in accretionary complex. The accretionary wedge complex suffers intense ductile strain and metamorphism due to the subduction of Proto\|Tethys with the metamorphic grade reaching the greenschist facies to amphibolite facies. The 40 Ar/ 39 Ar age 451Ma of metamorphic homoblende in amphibolite is interpreted to closely date attainment of maximum metamorphic conditions representing the age of initial subduction at the latest, while the 40 Ar/ 39 Ar age 425Ma of biotite is interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon representing the cease of subduction. These data provide significant information on the timing of accretion and the age of the subducted Proto\|Tethys. The whole rock Sm\|Nd isochronal age of amphibolite is problematic 737Ma that maybe represent the rifting age of the Rodinia.
文摘The East Kunlun terrain is located on the northern Qinghai—Tibet plateau, composed of the East Kunlun Mountain and the Qaidam Basin and bounded by the Qilian terrain on the north and Bayanhar—Songpan Ganze terrain on the south. It is regarded as a composite orogenic belt characterized by having developed superimposed ductile tectonic regimes reflecting the collision orogeny during Early Paleozoic and Triassic periods. It has also experienced transformation from ductile to brittle deformation caused by the post orogeny, since Jurassic after the formation of the East Kunlun Mountain and the Qaidam Basin. A Paleozoic subduction complex zone was recently recognized along the north border of the East Kunlun terrain from Da Qaidam to Dulan (Xu,et al, 1 999). It is composed of ophiolite (of Early Paleozoic age?), tectonic melange and very high\|pressure metamorphic rocks with eclogite (\%p\%=2 2GPa, \%t\%=720℃) (Yang,et al.,1998) and garnet\|peridotite (\%p\%=2 5GPa, t =837℃). The Anyemaqin Triassic subduction complex zone trending in NWW\|SEE was developed along the eastern segment of the south border of the East Kunlun terrain. It is mainly composed of ultramafic and mafic rocks, pillow basalt, radiolaria\|bearing clastic rocks, tectonic melange and mylonite. The subduction complex zone contains a series of the southward overthrusting imbricated slices. Instead of this subduction complex zone, a 2 km\|wide sinistral strike\|slip ductile shear zone trending in E\|W was developed along the western segment. On the basis of macroscopic and microscopic studies on a series of structural sections, we divide the East Kunlun Mountain into four tectonic units as follows: (1) North Proterozoic Metamorphic Basement Zone; (2) South Early Paleozoic—Triassic Superimposed Fold Zone; (3) South Triassic Transpression Zone; (4) Anyemaqin Back\|Arc Decollement\|Thrust Zone.
文摘A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.
文摘The Alpine zone of Central Pamir is elongated in sublatitudinal direction between the Hercynians of Northern Pamir and the Cimmerians of Southern Pamir south of the Vanch\|Akbaital thrust. Its western continuation is overthrusted by the Herat fault and its eastern continuation is cut by the Karakoram strike\|slip fault.. The Central Pamir is a mainly S\|vergent (at the southern part N\|vergent) Alpine nappe stack then folding in antiform. It comprises deposits from Vendian to Neogene which have a thickness of 10km. Paleozoic and Mesozoic tectonic activity was poorly displaied in its limits. Rifting took place in Early and probably Upper Paleozoic. Pre\|Upper Cretaceous unconformity is known only in southern (autochthonous) part of the Zone as a result of closing of Bangong\|Nu Jiang ocean. In northern (allochthonous) part of the zone the sequence of Mesozoic and Paleogene rocks has no unconformities. Alpine endogenous processes were developed very intensively. They implied nappes and imbricate structures, linear folding, different igneous activity, zonal metamorphism. Slices of pyroxenites and gabbroids occured. Calc\|alkaline lavas and tuffs constitutes the major part of Paleocene to Miocene sequence (andesites\|ryolites\|in Paleogene, alkaline basalts in Oligocene—Miocene). Oligocene—Miocene zonal metamorphic belt of the intermediate type of high pressure including series of granitegneiss domes can be traced along the Central Pamir. Cores of domes include migmatites and remobilized bodies of the Early Paleozoic gneissic granites. The decompression took place at a later stage and rocks were overprinted by the andalusite\|sillimanite type metamorphism.. Syenite and leucogranite bodies, pegmatite and aplite veins were emplaced.
文摘The 26 December 2004 earthquake off Sumatra coast focused world attention on the Sunda arc subduction zone.Bangladesh is along the strike of and within a rupture-distance from that enormous M-9.3 earthquake. This country is situated where the India-Sunda subduction zone rises from oceanic depths to subaerial exposure as a result of incipient continent collision where the trench meets the huge sediment
文摘Kinematic models compute the temperature distribution by prescribing a constant convergent velocity for the subducting slab,resulting in an artificial velocity discontinuity,which may accelerate the heating of subducting slabs.For the purpose to moderate the influence of such artificial discontinuity, we construct a 2D thermal model for subduction zones with a velocity boundary layer,within which the velocities decrease linearly with the distance from the interfaces of slabs.Temperatures are calculated
文摘Changes in oceanic O-Sr-C isotopic compositions, global cooling and Asian continental aridification beginning in the Middle-Late Eocene(47-34 Ma) are considered to have been caused by the uplift of the Tibetan plateau.The specific timing and uplift mechanism,however,have long been subjects of debate.We investigated the Duogecuoren lavas of the central-western Qiangtang block,which form the largest outcrops among Cenozoic lavas in north-
文摘Cenozoic volcanic rocks, forming a series of volcanic rocks platform or highland on different sub\|volcanic rocks belt, had distributed widely in northern Qinghai—Tibet plateau, and characterized by potassinm\|rich lavas which main petrologic types are latidacite and latite. The petrogenesis of volcanic rocks in northern Tibetan plateau had been well\|studied, and thought they play an important role to uplift of Tibetan plateau. Based on the study of lithochemistry and isotopic age determination of the rocks in Qiangtang area, which is the most southern and earliest volcanic rocks belt of Cenozoic volcanic rocks, I contribute another view here to the early uplift of Tibetan plateau. Cenozoic volcanic rocks in Qiangtang region, thickness of the rocks are from 200 to 1000m in different area, belong to shoshonite and high\|K calc\|alkaline series, highly enrich alkaline specially enrich potassinm, large\|ion lithophytes elements (Rb,Sr,Ba,and Th), LREE and deplete high\|field strength elements (Ta,Er,Hf,and Ti). Chondrite\|normalized REE pattern of volcanic rocks show an absence of significant Eu anomaly, and indicate that the volcanic source was plagioclase\|free and probably resulted from melting in lower crust or upper mantle. Cenozoic volcanic rocks in Qiangtang region are similar petrologic and geochemical characteristics to that have been reported in more northern Hoh Xil and Kunlun area in Tibetan plateau. The age scope of volcanic rocks in Qiangtang area were from 44.1～32.6Ma, among which the age mainly were 44.1～42.5Ma, which belong to mid\|Eocene epoch to primary Oligocene epoch, and could be classified into four out pouring periods, according to the potassium\|argon age dating. The study on lithochemistry and age of the rocks in Qiangtang area suggest that Intracontinental subduction southward Qiangtang tectonic belt plays an important role to Cenozoic volcanism, and imply that the convective removal of the thicked lithosphere, which formed at the collision mountain\|building processes, are responsible for the formation of Cenozoic volcanic rocks, the earliest uplift of Tibetan plateau. and regional extension in Qiangtang area.
文摘Different attempts have been done to deduce the shortening of the Himalayan belt during the India\|Asia convergence. Dewey et al. (1989) and Le Pichon et al.(1992) calculated an India\|Asia shortening of 2300～2150km and 2800～3000km in the western and eastern syntaxes, respectively, since the late 45Ma. According to seafloor\|spreading reconstruction, a total shortening of 3000～500km was estimated after the initial contact of the two plates at 55～50Ma (Molnar and Tapponier, 1975 ; Molnar et al., 1988 ; Replumaz, 1999). Since 40Ma, the part of shortening only accommodated by the Himalayan belt was estimated around 470km in the western part (Coward and Butler, 1985) and 550 to 630km to the east (Ratsbacher et al., 1994 ; Replumaz, 1999). In contrast, global plate reconstructions suggest that the shortening in the Himalaya is of about 1250～250km (Achache et al., 1984 ; Powell et al., 1988 ; Dewey et al.,1989 ; Klootwijk et al., 1992 ; Matte et al., 1997). This discrepancy between the amount of shortening estimated by balancing the Himalayan belt and by plate reconstruction favour the existence of a greater India buried up to 1000km north of the present\|day Indus suture zone and subducted before Middle Eocene time (Klootwijk et al., 1979 ; Patriat and Achache, 1984).
文摘The Alpine\|Himalayan system is considered as the type of continent\|continent collision orogen, the result of which is the amalgamation of Eurasia, India and Africa into a quasi\|continuous landmass. The Grenville Province of Canada, the youngest tectonic province of the Canadian shield, results from the collision of Laurentia with continental masses including Baltica, Amazonia, Congo and Kalahari (Hoffman, 1991), a collision that formed, between 1.2 Ga and 1 0Ga, the Rodinia supercontinent. The Grenville Province is the largest exposed section of the resulting Grenville belt, the length of which compares with that of the Alpine—Himalayan system whereas its width may reach that of Himalaya—Tibet. Unfortunately, due to the depth of erosion and to the dismembering of the belt in post\|grenvillian times, the Grenville Province lacks some of the diagnostic features of collision belts such as ophiolites and the SE\|vergent counterpart of the ca. 1 0Ga Grenville Front. However, the level of exposure is such that it provides direct access to the deep crustal parts of the orogen which are only accessible through geophysical methods in modern, little eroded mountains. Recent advances in the understanding of collision belts (e.g. LITHOPROBE and INDEPTH projects) now allow to identify the type of features that are common to the Grenvillian and the Himalayan belts in order to test the analogue model.
