Polyphase metamorphism and the development of the Main Central Thrust

ABSTRACT Along 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 M₁ phase;
• (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 M₂ phase; and,
• (c) finally during uplift of the Kishtwar window area, a retrogressive M₃ phase 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 M₂ phase, whereas tectonically above, in the Higher Himalaya unit, the garnets reveal double-stage growth with a complex zoning pattern due to reaction-partitioning during M₁ and M₂ and reverse-zoning at their rims during the retrogressive M₃ phase. 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 M₃ retrogression. 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 condensed M₂ isograds around the Kishtwar window.     

The metamorphism in the Central Himalaya

ABSTRACT All 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:

• 1 The 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 an apparent (diachronic) inverse zonation, with the sillimanite zone above the kyanite zone.
• 2 Conversely, the famous inverted zonation of the Lesser Himalaya is basically a primary pattern, 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.
• 3 Thermal 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 Slab.

     

Low-thermal-gradient Hercynian metamorphism in the eastern Pyrenees

Abstract Fluids with compositions in the system CO₂-H₂O-NaCl were trapped in quartz veins enclosed in low-grade metamorphic rocks (chlorite zone) on the southern flank of the Canigó Massif, eastern Pyrenees. The veins, which also contain arsenopyrite crystals, were formed contemporaneously with the main Hercynian foliation and metamorphism. Volumetric properties of the fluid and the results of arsenopyrite geothermometry suggest P-T trapping conditions of 4.6–6 kbar and 450–530° C. This implies that an episode of metamorphism with an average geothermal gradient of 25° C km⁻¹ occurred during the main deformation event. This episode preceded the low- P /high- T metamorphism described around domes and to date considered as characteristic of the Hercynian orogeny in the Pyrenees.     

High-precision geothermobarometry across the High Himalayan metamorphic sequence, Langtang Valley, Nepal

One of the long recognized features of Himalayan geology is the apparent inversion of metamorphic sequences, as evidenced in both metamorphic parageneses and thermobarometric data. With the aid of an extended thermobarometric dataset from the Langtang Valley section of the Higher Himalayan Crystallines, it can be demonstrated that the relatively large uncertainties associated with traditional thermobarometric techniques severely limit the tectonic interpretation of metamorphic gradients across the Himalayas. We apply the recently developed Δ PT  approach, which significantly improves the precision to which pressure and temperature differences between samples may be calculated. High-precision thermobarometric data reveal an isothermal, rather than inverted, temperature array at Langtang, while the pressure data suggest significant structural complexity, with the Higher Himalayan Crystallines in the Langtang section comprising two distinct, possibly duplicated sequences, each having experienced considerable structural attenuation following metamorphism.     

Timing of Himalayan ultrahigh-pressure metamorphism: sinking rate and subduction angle of the Indian continental crust beneath Asia

Coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from Higher Himalayan Crystalline rocks in the upper Kaghan Valley, north‐west Himalaya. The metamorphic peak conditions of the coesite‐bearing eclogites are estimated to be 27–32 kbar and 700–770 °C, using garnet–pyroxene–phengite geobarometry and garnet–pyroxene geothermometry, respectively. Cathodoluminescence (CL) and backscattered electron (BSE) imaging distinguished three different domains in zircon: inner detrital core, widely spaced euhedral oscillatory zones, and thin, broadly zoned outermost rims. Each zircon domain contains a characteristic suite of micrometre‐sized mineral inclusions which were identified by in situ laser Raman microspectroscopy. Core and mantle domains contain quartz, apatite, plagioclase, muscovite and rutile. In contrast, the rim domains contain coesite and minor muscovite. Quartz inclusions were identified in all coesite‐bearing zircon grains, but not coexisting with coesite in the same growth domain (rim domain). ²⁰⁶Pb/²³⁸U zircon ages reveal that the quartz‐bearing mantle domains and the coesite‐bearing rim were formed at c. 50 Ma and 46.2 ± 0.7 Ma, respectively. These facts demonstrate that the continental materials were buried to 100 km within 7–9 Myr after initiation of the India–Asia collision (palaeomagnetic data from the Indian oceanic floor supports an initial India‐Asia contact at 55–53 Ma). Combination of the sinking rate of 1.1–1.4 cm year^?1 with Indian plate velocity of 4.5 cm year^?1 suggests that the Indian continent subducted to about 100 km depth at an average subduction angle of 14–19°.     

喜马拉雅造山带东段错那洞片麻岩穹窿的变质作用及构造意义

错那洞穹窿是喜马拉雅造山带北部发育的一系列片麻岩穹窿之一,因其赋存有超大型稀有金属矿床而倍受关注。本文对错那洞穹窿核部产出的石榴石十字石蓝晶石白云母片岩进行了岩石学、相平衡模拟和锆石U-Pb年代学研究,为揭示穹窿的成因和成矿作用提供了重要限定。岩石学研究表明,石榴石蓝晶石十字石白云母片岩的共生矿物组合是石榴石+蓝晶石+十字石+白云母+斜长石+石英+钛铁矿+金红石,为典型的中压角闪岩相变质岩。相平衡模拟表明岩石的变质温压条件为670℃和9. 0kbar,并未经历部分熔融。锆石U-Pb定年结果表明,片岩的变质作用发生在47～29Ma,即经历了一个较长期(～20Myr)的变质演化过程。结合现有研究成果,我们认为错那洞片麻岩穹窿具有与喜马拉雅造山带北部发育的其它片麻岩穹窿相同的成因,穹窿核部的中级变质岩为高喜马拉雅结晶岩系的上部构造层位,其变质作用发生在印度大陆向拉萨地体之下低角度俯冲过程中;穹窿核部淡色花岗岩是高分异的异地花岗岩,是高喜马拉雅结晶岩系下部高温高压麻粒岩部分熔融所形成的熔体经历高程度分离结晶产物。此外,本文研究成果为印度与亚洲大陆的碰撞时间和性质提供了进一步约束。     

Contact metamorphism around the Stawell granite, Victoria, Australia

The contact metamorphosed metapelitic and metapsammitic rocks surrounding the Stawell granite, western Victoria, Australia, are divided into three zones: the low-grade zone, the medium-grade zone and the high-grade zone. Detailed petrological study shows consistency of element distributions, implying that equilibrium was widely attained in the rocks, although equilibrium volumes are generally small (millimetre scale) and considerable mineral chemical variations exist between adjacent domains. The metamorphic mineral assemblages are generally of high variance (KFMASH variance ≤ 2). Consequently, the chemical evolution of assemblages is controlled largely by bulk composition and metamorphic temperature, the former factor being more important in most rocks. The chemographic relations of mineral assemblages in low- and medium- to high-grade zones are presented in compatibility diagrams projected from biotite, quartz and H₂O, and biotite, K-feldspar and H₂O, respectively. These compatibility diagrams have the advantage of showing both quartz-bearing and quartz-absent assemblages. The metamorphic reactions are modelled successfully by a calculated petrogenetic grid that combines both KFASH and KMASH equilibria. Based on petrographic observations and with constraints from the calculated petrogenetic grid, the following KFMASH reactions, in the order of increasing metamorphic grade, are responsible for producing the various mineral assemblages in the Stawell rocks: chl + mu + q = bi + cd + V, chl + q + cd = g + V, mu + bi + q = ksp + cd + V, mu + q = ksp + and + cd + V (or KASH mu + q = ksp + and + V), mu + cd = ksp + and + bi + V, mu + bi + and = ksp + sp + V, and + bi = ksp + sp + cd + V, mu + bi = ksp + cor + sp + V, mu = ksp + cor + and + sp + V (or KASH mu = ksp + cor + V), bi + cd + q = g + ksp + V. The combined KFASH and KMASH grid provides constraints on reaction coefficients in the above sequence of reactions and on temperature and pressure of metamorphism.     

Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist

Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent-Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic-calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1^IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and Na^M4 to (A1^VI+ Fe³⁺+ Ti). These variations imply that structurally high rocks belong to a lower-pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.
Amphibole composition is dependent not only on structural position (i.e. P-T ), but also upon bulk-rock composition. The important controls are whole-rock Mg/(Mg + Fe²⁺+ Mn) and Fe³⁺/Fe²⁺. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.
Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic-calcic amphibole. Sodic-calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.     

Inverted metamorphism and the Main Central Thrust: field relations and thermobarometric constraints from the Kishtwar Window, NW Indian Himalaya

Following the early Eocene collision of the Indian and Asian plates, intracontinental subduction occurred along the Main Central Thrust (MCT) zone in the High Himalaya. In the Kishtwar–Zanskar Himalaya, the MCT is a 2 km thick shear zone of high strain, distributed ductile deformation which emplaces the amphibolite facies High Himalayan Crystalline (HHC) unit south‐westwards over the lower greenschist facies Lesser Himalaya. An inverted metamorphic field gradient, mapped from the first appearance of garnet, staurolite and kyanite index minerals, is coincident with the high strain zone. Petrography and garnet zoning profiles indicate that rocks in the lower MCT zone preserve a prograde assemblage, whereas rocks in the HHC unit show retrograde equilibration. Thermobarometric results derived using THERMOCALC indicate a P–T increase of c. 180 °C and c. 400 MPa across the base of the MCT zone, which is a consequence of the syn‐ to postmetamorphic juxtaposition of M1 kyanite grade rocks of the HHC unit on a cooling path over biotite grade footwall rocks, which subsequently attain their peak (M2) during thrusting. Inclusion thermobarometry from the lower MCT zone reveals that M2 was accompanied by loading, and peak conditions of 537±38 °C and 860±120 MPa were attained. M1 kyanite assemblages in the HHC unit, which have not been overprinted by M2 fibrolitic sillimanite, were not significantly affected by M2, and conditions of equilibration are estimated as 742±53 °C and 960±180 MPa. There is no evidence for dissipative or downward conductive heating in the MCT zone. Instead, the primary control on the distribution of peak assemblages, represented by the index minerals, is postmetamorphic ductile thrusting in a downward propagating shear zone. Polymetamorphism and diachroneity of equilibration are also important controls on the thermal profile through the MCT zone and HHC unit.     

Petrology of a non-classical Barrovian inverted metamorphic sequence from the western Arunachal Himalaya, India

In this study, we reconstruct the inverted metamorphic sequence in the western Arunachal Himalaya using combined structural and metamorphic analyses of rocks of the Lesser and Greater Himalayan Sequences. Four thrust-bounded stratigraphic units, which from the lower to higher structural heights are (a) the Gondwana rocks and relatively weakly deformed metasediments of the Bomdila Group, (b) the tectonically interleaved sequence of Bomdila gneiss and Bomdila Group, (c) the Dirang Formation and (d) the Se La Group are exposed along the transect, Jira–Rupa–Bomdila–Dirang–Se La Pass. The Main Central thrust, which coincides with intense strain localization and the first appearance of kyanite-grade partial melt is placed at the base of the Se La Group.Five metamorphic zones from garnet through kyanite, kyanite migmatite, kyanite-sillimanite migmatite to K-feldspar-kyanite-sillimanite migmatites are sequentially developed in the metamorphosed low-alumina pelites of Dirang and Se La Group, with increasing structural heights. Three phases of deformation, D₁–D₂–D₃ and two groups of planar structures, S₁ and S₂ are recognized, and S₂ is the most pervasive one. Mineral growths in all these zones are dominantly late-to post-D₂, excepting in some garnet-zone rocks, where syn-D₁ garnet growths are documented. Metamorphic isograds, which are aligned parallel to S₂ were subsequently folded during D₃. The deformation produced plane-non-cylindrical fold along NW–SE axis.In the garnet-zone, peak metamorphism is marked by garnet growth through the reaction biotite + plagioclase → garnet + muscovite. An even earlier phase of syn-D₁ garnet growth occurred in the chlorite stability field with or without epidote. In the kyanite-zone metapelites, kyanite appeared via the pressure-sensitive reaction, garnet + muscovite → kyanite + biotite + quartz. Staurolite was produced in the same rock by retrograde replacement of kyanite following the reaction, garnet + kyanite + H₂O → staurolite + quartz. These reactions depart from the classical kyanite- and staurolite-isograd reactions in low-alumina pelites, encountered in other segments of eastern Himalaya. In the metapelites, just above the kyanite-zone, melting begins in the kyanite field, through water-saturated and water-undersaturated melting of paragonite component in white mica. Leucosomes formed through these reactions are characteristically free of K-feldspar, with sodic plagioclase and quartz as the dominant constituents. With increasing structural height, the melting shifts to water-undersaturated melting of muscovite component of white mica, producing an early K-feldspar + kyanite and later K-feldspar + sillimanite assemblages and granitic leucosomes.Applications of conventional geothermobarometry and average P–T method reveal near isobaric (at P  8 kbar) increase in peak metamorphic temperatures from 550 °C in the garnet-zone to >700 °C for K-feldspar-kyanite-sillimanite-zone rocks. The findings of near isobaric metamorphic field gradient and by the reconstruction of the reaction history, reveal that the described inverted metamorphic sequence in the western Arunachal Himalaya, deviates from the classical Barrovian-type metamorphism. The tectonic implication of such a metamorphic evolution is discussed.     

Five generations of monazite in Langtang gneisses: implications for chronology of the Himalayan metamorphic core

Monazite grains from Greater Himalayan Sequence gneisses, Langtang valley, Nepal, were chemically mapped and then dated in situ via Th–Pb ion‐microprobe analysis. Correlation of ages and chemistry reveals at least five different generations of monazite, ranging from c. 9 to >300 Ma. Petrological models of monazite chemistry provide a link between these generations and the thermal evolution of these rocks, yielding an age for the melting of Greater Himalayan rocks within the Main Central Thrust sheet (c. 16 Ma), and for the timing of thrust sheet emplacement that are younger than commonly viewed. Chemical characterization of monazite is vital prior to chronological microanalysis, and many ages previously reported for monazite from the Greater Himalayan Sequence are interpretationally ambiguous.     

喜马拉雅造山带东段错那地区高喜马拉雅结晶岩系的变质作用与构造意义

位于造山带核部的高喜马拉雅结晶岩系是由印度大陆俯冲到亚洲大陆之下经历变质作用的产物,是研究喜马拉雅造山带形成与演化过程的理想载体。本文对造山带东段错那地区高喜马拉雅结晶岩系上部构造层位的正片麻岩进行了岩石学、相平衡模拟,锆石与独居石U-Pb年代学研究。研究结果表明这些岩石的峰期矿物组合为石榴石+斜长石+钾长石+黑云母+白云母+石英+钛铁矿,保留有深熔作用的结构特征。岩石中的石榴石具有生长成分环带。相平衡模拟表明,岩石的峰期变质条件为710~750℃和9.0~10.5kbar,具有一个顺时针型变质作用P-T轨迹,其进变质过程以升温、升压和部分熔融为特征,退变质作用为降温、降压过程。锆石与独居石U-Pb定年表明,这些正片麻岩具有510~490Ma的原岩年龄,和27~11Ma的退变质时间。本研究表明高喜马拉雅结晶岩系的上部构造层位经历了高角闪岩相变质作用与部分熔融,为造山带的构造演化提供了重要信息。     

Tectonothermal evolution of the High Himalayan Crystalline Sequence, Langtang Valley, northern Nepal

The High Himalayan Crystalline Sequence in north-central Nepal is a 15-km-thick pile of metasediments that is bound by the Main Central Thrust to the south and a normal fault to the north. The Langtang section through the metasediments shows an apparent inversion of metamorphic isograds with high-P, kyanite-grade rocks exposed beneath low-P, sillimanite-grade rocks. Textural evidence confirms that the observed inversion is a result of a polyphase metamorphic history and phase equilibria studies indicate that thermal decoupling has occurred within a mechanically coherent section of crust. Rocks now exposed at the base of the High Himalayan thrust sheet underwent Barrovian regional metamorphism (M1) prior to 34 Ma in the early stages of the Himalayan orogeny, recording metamorphic conditions of T= 710 ± 30° C, P= 9 ± 1 kbar. After the activation of the Main Central Thrust, which emplaced these metapelites southwards onto the lower grade Lesser Himalayan formations, the upper part of the thrust sheet was overprinted by a second heating event (M2), resulting in sillimanite-grade metamorphism and anatexis of metapelites at T= 760 ± 30° C, P= 5.8 ± 0.4 kbar between 17 and 20 Ma. Crustally derived, leucogranite magmas have been emplaced into low-grade Tethyan sediments on the hangingwall of the normal fault that bounds the northern limit of the metapelitic sequence. The cause of the selective heating of the upper section of the metasediments during M2 cannot be reconciled with either post-thrusting thermal relaxation or advection models. The cause of M2 remains problematical but it is suggested that heat focusing has occurred at the top of the High Himalayan Crystalline Sequence as a result of movement on the normal fault blanketing metapelites of high heat productivity with low-grade sediments of low thermal conductivity. This model implies that the normal fault was active before M2, consistent with decompression textures that formed during, or shortly after, sillimanite-grade metamorphism.     

Domal structures and high-grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north-west Himalaya, India

ABSTRACT In 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 S₂ foliation is related to deformation during the doming phase, which started early in the M₂ event. 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 main Himalayan range.     

Campaign-style U-Pb titanite petrochronology:Along-strike variations in timing of metamorphism in the Himalayan metamorphic core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core(HMC), in order to document and investigate the causes of along-strike variations in its Oligocene-Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya.This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ～30 Ma to 12 Ma, recording(re-)crystallization between 625 ℃ and 815 ℃. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen.Disparities in the duration and timing of metamorphism within the HMC are best explained by alongstrike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes.These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger,thicker crystalline core in the eastern Himalaya.     

Timing of metamorphism in the Tauern Window, Eastern Alps: Rb-Sr ages and fabric formation

The Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post-Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature-time-deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose P-T evolution and structural setting are already well understood. The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28-30 Ma. The Peripheral Schieferhülle of the south-east Tauern Window yields Rb-Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre-date 31 Ma, and were reworked by the formation of the large north-east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post-dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples. These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.     

Tectonometamorphic evolution of the Himalayan metamorphic core between the Annapurna and Dhaulagiri, central Nepal

The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma ⁴⁰Ar/³⁹Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.     

喜马拉雅造山带的高压超高压变质作用与印度-亚洲大陆碰撞

喜马拉雅造山带是印度与亚洲大陆碰撞作用的产物,正在进行造山作用,是研究板块构造的天然实验室.高压和超高压变质岩分布在喜马拉雅造山带的核部.这些变质岩具有不同的形成条件、形成时间和形成过程,为印度与亚洲碰撞带的几何学、运动学和动力学提供了重要的限定.含柯石英的超高压变质岩产出在喜马拉雅造山带的西段,它们形成在古新世与始新世之间(53～46Ma),为印度大陆西北边缘高角度超深俯冲作用的产物,并经历了快速俯冲与快速折返过程.在约5 Myr内,超高压变质岩从＞100km的地幔深度折返到了中地壳深度,且仅仅叠加角闪岩相退变质作用.高压榴辉岩产出在喜马拉雅造山带中段,形成时间约为45Ma,为印度大陆低角度深俯冲作用的产物,经历了至少20Myr的长期折返过程,叠加麻粒岩相退变质作用和部分熔融.高压麻粒岩产出在喜马拉雅造山带的东端,是印度大陆东北缘近平俯冲作用的产物,峰期变质作用时间约为35Ma,经历了约20Myr的长期折返过程,叠加了麻粒岩相和角闪岩相退变质作用,并伴随有多期部分熔融.因此,喜马拉雅造山带的变质作用具有明显的时间与空间变化,显示出大陆深俯冲与折返过程的差异性,以及大陆碰撞造山带形成机制的多样性.     

东特提斯喜玛拉雅侏罗纪菊石年代地层学研究新进展

西藏南部拉弄拉地区的侏罗纪菊石资料研究表明 ,该地有特提斯喜玛拉雅地区最完整的侏罗纪菊石层序 ,据此可与西北欧标准菊石带以及其他地区菊石层序对比。     

印度与欧亚板块碰撞以来东喜马拉雅构造结的演化

在野外填图,构造观察及前人研究的基础上,本文识别并描述了东喜马拉雅构造结中的推覆断裂、正断裂及走滑断裂、背斜(形)和向斜(形)等构造类型,讨论了这些构造位置及与印度板块挤入,印支地块旋转的关系,还探讨了东喜马拉雅构造结对印度板块持续向北推挤下的特殊应变调节方式。在印度大陆部分,东喜马拉雅构造结由3个向外逐渐变新的构造结组成,即北东向的南迦巴瓦峰复式背斜、北西向的桑复式向斜及北东向的阿萨母复式向斜。上述3个构造结是协调印度板块的挤入、喜马拉雅弧的扩展及印支地块的旋转的构造。在欧亚大陆内部的冈底斯岛弧,在派区及阿尼桥走滑断裂协调下,高喜马拉雅结晶岩的基底挤入冈底斯岛弧内部,在大拐弯顶端形成向上的挤出构造。在南迦巴瓦峰构造结的北西侧,由于掀斜式抬升及重力滑动,使得冈底斯盖层与结晶基底脱耦,上盘盖层沿东久向北西方向滑动。在南迦巴瓦峰构造结北东侧,由于印支地块的挤出和旋转,形成一系列的北西向走滑断裂,如实皆断裂、嘉黎—高黎贡断裂、澜沧江断裂及红河断裂等。