ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00569.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00570.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe High Himalayan Crystallines (HHC) of SE Zanskar consist of biotite paragneisses, of orthogneisses that derive from early‐Palaeozoic granitoids, of minor metabasics and of post‐metamorphic leucogranites of Miocene age.Two main metamorphic events have been documented in the HHC. The first event occurred atP= 12.0 ± 0.5 kbar andT= 750 ± 50° C in rare metabasics intruded by early‐Palaeozoic granitoids. In the biotite paragneisses, thermobarometric estimates of the first event point to comparableTatP4–5 kbar lower. The first event is followed by a pervasive syn‐tectonic crystallization characterized by lowerPandT. On the basis of the cooling ages of the metamorphic minerals and on the geological evidence, the second event is referred to the Tertiary Himalayan crystallization. Further petrological and geochronological studies are necessary to prove whether a few mineral relics ascribed to the first event define a polyphase Himalayan evolution or if they record the incomplete obliteration of an older history during the Himalayan event.The HHC of SE Zanskar show a decrease in metamorphic grade from the middle structural levels upward, close to the Kade unit, and downward, close to the Lesser Himalaya (from sillimanite‐K‐feldspar‐biotite‐bearing assemblages to kyanite‐staurolite‐muscovite‐bearing assemblages). This metamorphic zonation is probably a consequence of the polyphase history of intracontinental thrusts and of the tectonic emplacement of hot crustal slabs within shallower and colder thrust sheets at relatively late stages of the continental collision

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00571.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Main Central Thrust (MCT) south of Mt Everest in eastern Nepal is a 3 to 5km thick shear zone separating chlorite‐bearing schist in the lower plate from sillimanite‐bearing migmatitic gneiss in the overlying Tibetan Slab. The metamorphic grade increases through the MCT zone toward structurally higher levels. Previous workers have suggested that either post‐ or synmetamorphic thrust movement has caused this inversion of metamorphic isograds. In an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault, four well‐calibrated thermobarometers were applied to pelitic samples collected along two cross‐strike transects through the MCT zone and Tibetan Slab. Results show an increase in apparent temperature up‐section in the MCT zone from 778 K to 990 K and a decrease in temperature to ∼850 K in the lower Tibetan Slab, which is consistent with synmetamorphic thrust movement. A trend in calculated pressures across this section is less well‐defined but, on average, decreases up‐section with a gradient of ∼28MPa/km, resembling a lithostatic gradient. Pressure‐temperature paths for zoned garnets from samples within the MCT zone, modelled using the Gibbs' Method, show a significant decrease in temperature and a slight decrease in pressure from core to rim, which might be expected for upper plate rocks during synmetamorphic thrust movement. Samples from the uppermost Tibetan Slab yield higher temperatures and pressures than those from the lower Tibetan Slab, which may be evidence for later‘resetting’ of thermobarometers by intrusion of the large amounts of leucogranite

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00572.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTAll along the Himalayan chain an axis of crystalline rocks has been preserved, made of the Higher Himalaya crystalline and the crystalline nappes of the Lesser Himalaya. The salient points of the metamorphism, as deduced from data collected in central Himalaya (central Nepal and Kumaun), are:1The Higher Himalaya crystalline, also called the Tibetan Slab, displays a polymetamorphic history with a first stage of Barrovian type overprinted by a lower pressure and/or higher temperature type metamorphism. The metamorphism is due to quick and quasi‐adiabatic uplift of the Tibetan Slab by transport along an MCT ramp, accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover. The resulting metamorphic pattern is anapparent(diachronic) inverse zonation, with the sillimanite zone above the kyanite zone.2Conversely, the famous inverted zonation of the Lesser Himalaya is basically aprimarypattern, acquired during a one‐stage prograde metamorphism. Its origin must be related to the thrusting along the MCT, with heat supplied from the overlying hot Tibetan Slab, as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust.3Thermal equilibrium is reached between units above and below the MCT. Far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the Tibetan Slab during the second metamorphic stage; but closer to the MCT front, the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the Tibetan S

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00573.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTIn the main Himalayan range in the Ladakh‐Zanskar area, domal structures have been observed at structurally deeper levels in the tectonic unit of the Higher Himalayan Crystalline. Their formation occurred during a second, temperature‐dominated phase (M2) of high‐grade regional metamorphism, characterized by the semipelitic paragenesis of sillimanite‐K‐feldspar and incipient anatexis. The doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast‐southwest trending compression. In the main Himalayan range the development of the dominant S2foliation is related to deformation during the doming phase, which started early in the M2event. The deformation propagated continuously north‐east and south‐west with time. In the north‐east, on the northern slopes of the main Himalayan range, this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late‐ to postmetamorphic normal fault system of the Zanskar shear zone. Towards the south‐west, deformation is expressed by compressional movements, e.g. at the Main Central Thrust (MCT) in the Kishtwar window area. The observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic Kishtwar window and of the whole

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00574.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTBlueschists occur along the Indus Suture Zone in Ladakh as tectonic thrust slices, as isolated blocks within mélange units and as pebbles within continental detrital series. In the Shergol‐Baltikar section high‐pressure rocks within the Mélange unit lie between the Dras‐Naktul‐Nindam nappes in the north and the Lamayuru units in the south. The blueschists are imbricated with mélange formation of probably upper Cretaceous age. They are overlain discordantly by the Shergol conglomerate of post Eocene (Oligo‐Miocene ?) age. Blueschist lithologies are dominated by volcanoclastic rock sequences of basic material with subordinate interbedding of cherts and minor carbonates. Mineral assemblages in metabasic rocks are characterized by lawsonite‐glaucophane/crossite‐Na‐pyroxene‐chlorite‐phengite‐titanite ± albite ± stilpnomelane. In the quartz bearing assemblages garnet is present but omphacite absent.P‐Testimates indicate temperatures of 350 to 420°c and pressures around 9–11 kbar. Geochemical investigations show the primary alkaline character of the blueschist, which suggests an oceanic island or a transitional MORB type primary geotectonic setting. K/Ar isotopic investigations yield middle Cretaceous ages for both whole rocks and minerals. Subduction related HP‐metamorphism affecting the Mesozoic Tethyan oceanic crust developed contemporaneously with magmatism in the Dras volcanic are and the Ladakh batholith. Subsequent collision of India with Asia obducted relics of subduction zone material which later became involved in nappe emplacement durin

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00575.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTAlong a cross‐section through the Lesser and Higher Himalayan units at the Kishtwar window area (north‐west India), a polyphase, Barrovian‐type metamorphism has been delineated in relation to the development of the Main Central Thrust (MCT). In the metapelitic mineral assemblages, three metamorphic phases have been distinguished:(a)conditions up to amphibolite grade at moderate to high pressures (alm + rut + ilm + kya + qtz) characterize the M1phase;(b)pressure release and/or temperature increase as a result of movement along the MCT and the formation of gneiss domes in the Higher Himalaya, as expressed by oriented (N70°‐100° E) fibrolite, defines the M2phase; and,(c)finally during uplift of the Kishtwar window area, a retrogressive M3phase is characterized by the assemblage quartz‐muscovite‐chlorite.Both optically zoned and single‐stage garnets have been examined with the electron microprobe to determine their element partitioning. Normal zoning has been found in samples below the MCT in the Lesser Himalaya, indicating prograde growth during the M2phase, whereas tectonically above, in the Higher Himalaya unit, the garnets reveal double‐stage growth with a complex zoning pattern due to reaction‐partitioning during M1and M2and reverse‐zoning at their rims during the retrogressive M3phase. Geothermometry on metapelites along a cross‐section through the MCT zone and the Higher Himalaya imply distinct readjustments of garnet‐biotite exchange equilibria and indicate isothermal conditions (500‐600° C) throughout the section during the M3retrogression. Pressure calculations (gro‐an‐kya‐qtz and alm‐rut‐ilm‐kya‐qtz) suggest a decrease in pressure towards the top of the section (6‐7.5 to 4.5‐5 kbar), as corroborated by fibrolite replacing kyanite. The spatially inverse metamorphism exposed within the Lesser Himalaya of the Kishtwar window is regarded as a product of polyphase metamorphism combined with ongoing thrusting and shearing and is reflected by c

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00576.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTThe Darjeeling‐Sikkim region provides a classic example of inverted Himalayan metamorphism. The different parageneses of pelitic rocks containing chlorite, biotite, garnet, staurolite, kyanite, sillimanite, plagioclase and K‐feldspar are documented by a variety of textures resulting from continuous and discontinuous reactions in the different zones. Microprobe data of coexisting minerals show thatXMgvaries in the order: garnet

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00577.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY

摘要:

ABSTRACTSouth of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north‐dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post‐Himalayan upl

ISSN:0263-4929    

DOI:10.1111/j.1525-1314.1989.tb00578.x

出版商:Blackwell Publishing Ltd    

年代:1989

数据来源: WILEY