Neoproterozoic collisional metamorphism in overthrust terranes of the Trans-Angarian Yenisey Ridge,Siberia

Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).     

Petrological and geochronological constraints on regional metamorphism along the northern border of the Bitterroot batholith

Quantitative thermobarometry in pelites and garnet amphibolites from the Bitterroot metamorphic core complex, combined with U–Pb dating of metamorphic monazite and zircon from footwall rocks, provide new constraints on the P – T  – t evolution of footwall rocks. The thermobarometric and geochronological results, when correlated with observations from other regions bordering the Bitterroot batholith, define a regional metamorphic history for the northern margin of the Bitterroot batholith consisting of three distinct events beginning with early prograde metamorphism (M1) coincident with arc-related magmatism and crustal shortening at c .  100–80 Ma. Magmatism and crustal thickening led to regional upper-amphibolite facies metamorphism (M2) and anatectic melting between 64 and 56 Ma. Mineral textures related to high-temperature isothermal decompression (M3), coincident with late stages of magmatism in the Bitterroot complex footwall (56–48 Ma), are only preserved in areas adjacent to extensional structures. The close temporal relationship between peak metamorphism and the onset of footwall decompression indicates that thermal weakening was an important factor in the initiation of Early Eocene regional extension and tectonic denudation of the Bitterroot complex and possibly the Boehls Butte metamorphic terrane. The morphology of the decompressional P – T  – t path derived for Bitterroot footwall rocks is similar to other trajectories reported for Cordilleran core complexes and may represent a transition in the deformational style of core-bunding detachments responsible for exhumation.     

北祁连东段米家山地区碰撞变质作用研究

:米家山地区位于北祁连东段 ,区内下奥陶统阴沟群为一套海相火山—碎屑岩建造。在加里东末期 ,米家山岩体最终定位。通过填图发现 ,岩体与阴沟群变质地层呈“互层状”产出。在岩体的中心部位发育两条角闪岩相变质带 ,其变质条件由早期中—高压中温向晚期低压高温演化 ,变质作用为多期碰撞变质作用。这一现象不仅反映了造山作用是一个连续过程 ,同时也说明了与其相伴的变质作用必然是一个变化的过程。在与花岗岩相伴的地区 ,严格地用区域变质作用和接触变质作用来解释变质成因是与板块构造观点不相符合的 ,它们可能是一个起因的不同结果 ,是相互联系的     

STRUCTURAL AND THERMAL EVOLUTION OF THE SOUTH ASIAN CONTINENTAL MARGIN ALONG THE KARAKORAM AND HINDU KUSH RANGES,NORTH PAKISTAN

STRUCTURAL AND THERMAL EVOLUTION OF THE SOUTH ASIAN CONTINENTAL MARGIN ALONG THE KARAKORAM AND HINDU KUSH RANGES,NORTH PAKISTAN     

A reappraisal of episodic burial metamorphism in the Andes of central Chile

The Jurassic to Miocene sequences of the central Andes, east of Santiago, reputedly show repeated cycles of episodic sub-greenschist facies, burial metamorphism that are identified by sharp breaks in metamorphic grade at major stratigraphic boundaries. This paper presents the first detailed petrochemical analysis of these low-grade metamorphic sequences by examining the progressive development of secondary minerals, reaction progress in mafic phyllosilicates, and topological variations in the low-grade assemblages as a means of testing this model. The results indicate a progressive increase from zeolite facies through to close to the onset of greenschist facies from Miocene to Jurassic rocks. Combined analysis of reaction progress in mafic phyllosilicates and petrochemical relationships of chlorite–pumpellyite–actinolite in metabasites provides no evidence for sharp metamorphic breaks at major stratigraphic boundaries. Integrating the results presented here with the most recent models of stratigraphic/tectonic development of the central Andes shows that the metamorphism took place in two episodes, and was not episodic on a 40-million-year cycle. An absence of sharp breaks in metamorphic grade in any part of the succession, as demonstrated here, shows that the original petrographic establishment of low-grade facies provided insufficient resolution of changes in metamorphic conditions to establish definitive evidence of such breaks. Accordingly, this study suggests that re-assessment of metamorphic breaks reputedly developed in other areas of the Andean Cordillera is imperative in order to resolve the questions raised here about the origin of the low-grade metamorphism.Editorial responsibility: B. Collins     

Low-grade regional metamorphism in the Mesozoic-Cenozoic volcanic sequences of the Central Andes

ABSTRACT All the Mesozoic and Cenozoic volcanic rocks of the Central Andes (from southern Ecuador to central Chile), except Recent ones, have been affected by episodes of regional metamorphism, without change in texture and structure. The metamorphism, which ranges from low zeolite to greenschist facies, can be classified as burial metamorphism because there is an overall increase in metamorphic grade with stratigraphic depth in the individual volcanic sequences separated by regional unconformities. Some sequences display metamorphic patterns transitional to ocean-floor and to geothermal field types, reflecting variations along and across the Andes in tectonic setting and thermal gradients. Volcanism was closely followed by metamorphism during each cycle characterizing the geological history of the Central Andes. The episodic nature of the metamorphism has led to breaks in metamorphic grade at regional unconformities and repetition of facies series, where strata of higher grade may even overlie those of lower grade. The existence of permeability-controlled distribution patterns of secondary minerals within individual flows shows that gradients of chemical activity, rate of reaction and P_fluid were acting, in addition to temperature and P,_tot overall gradients, during the regional metamorphism. The alteration is accompanied by chemical changes and disturbances of the K-Ar and Rb-Sr isotope systems. Similarities between Mesozoic facies series in the western and eastern flanks of the Andes are consistent with a mechanism of ensialic spreading-subsidence.     

Two-sided Variscan thrust tectonics in the Vosges Mountains,northeastern France

Abstract

Variscan convergence produced two-sided (bivergent) crustal-scale thrusting in the Vosges Mountains. In the northern Vosges the central polymetamorphic crystallines were thrust to the NW over Cambrian to Silurian low-grade and very low-grade metamorphic clastics. Synorogenic upper Devonian - lower Carboniferous turbidites and volcanics were folded into NW-vergent structures which display SE-dipping slaty cleavage. The entire sequence shows increasing metamorphism and deformation from NW to SE. Late right-lateral strike-slip faulting along the Lalaye-Lubine fault zone outlasted thrusting. In the southern Vosges a lower Carboniferous turbiditic basin that was fringed on the south by a volcanic arc was tectonically shortened by south-directed tectonic imbrication of slivers of varied rocks including ultramafics, gneissic basement, and synorogenic elastics. The increasing degree of deformation and metamorphism towards the north suggests a thrust contact with the polymetamorphic gneisses of the central Vosges. The final stages of Variscan convergence were accompanied by voluminous granitic plutonism and by faulting along NNE-SSW and E-W-trending strike-slip faults. The tectonic evolution reflects progressive Variscan closure of a previously extended basinal crust in a high-temperature regime.     

SHRIMP monazite and zircon geochronology of high-grade metamorphism in New Zealand

Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360–370 Ma, penecontemporaneously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110–125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.     

大别山北淮阳带东段石榴斜长角闪岩石炭纪变质作用的测定

北淮阳带位于大别碰撞造山带北部,相对于南部三叠纪超高压变质带来说,通常被认为是一个相对低级变质的构造岩石单位.以商(城)-麻(城)为界,分为西段和东段.其中,东段未发现与西段相对应的古生代大洋俯冲记录,也缺乏相关的古生代岩浆作用和变质作用方面报道.因而严重地阻碍了对大别造山带形成和演化的深刻理解和准确认识.北淮阳带东段岩石类型丰富,主要包括3类(套)岩石:(1)变质岩,如变质花岗岩(花岗片麻岩)、(石榴)斜长角闪岩、大理岩、云母石英片岩和变质复理石等;(2)岩浆岩,主要为中生代花岗岩、正长岩和火山岩等;(3)盆地沉积岩,主要为中新生代沉积及少量石炭纪沉积.然而,该区一直未发现古生代变质作用的岩石学记录.为此,对北淮阳带东段金寨县铁冲一带与大理岩相共生的石榴斜长角闪岩开展了锆石SHRIMP U-Pb定年和初步的岩石学研究.结果表明,该区石榴斜长角闪岩经历了355±5 Ma变质作用,因而首次发现研究区经历了石炭纪变质作用,为秦岭-桐柏造山带的东延以及大别山碰撞造山带的古生代构造演化过程提供了新的制约.      

Miocene Tectonic Evolution from Dextral-Slip Thrusting to Extension in the Nyainqentanglha Region of the Tibetan Plateau

Dextral-slip in the Nyainqentangiha region of Tibet resulted in oblique underthrusting and granite generation in the Early to Middle Miocene, but by the end of the epoch uplift and extensional faulting dominated. The east-west dextral-slip Gangdise fault system merges eastward into the northeast-trending, southeast-dipping Nyainqentangiha thrust system that swings eastward farther north into the dextral-slip North Damxung shear zone and Jiali faults. These faults were took shape by the Early Miocene, and the large Nyainqentangiha granitic batholith formed along the thrust system in 18.3-11.0 Ma as the western block drove under the eastern one. The dextral-slip movement ended at -11 Ma and the batholith rose, as marked by gravitational shearing at 8.6-8.3 Ma, and a new fault system developed. Northwest-trending dextral-slip faults formed to the northwest of the raisen batholith, whereas the northeast-trending South Damxung thrust faults with some sinistral-slip formed to the southeast. The latter are replaced farther to the east by the west-northwest-trending Lhunzhub thrust faults with dextral-slip. This relatively local uplift that left adjacent Eocene and Miocene deposits preserved was followed by a regional uplift and the initiation of a system of generally north-south grabens in the Late Miocene at -6.5 Ma. The regional uplift of the southern Tibetan Plateau thus appears to have occurred between 8.3 Ma and 6.5 Ma. The Gulu, Damxung-Yangbajain and Angan graben systems that pass east of the Nyainqentangiha Mountains are locally controlled by the earlier northeast-trending faults. These grabens dominate the subsequent tectonic movement and are still very active as northwest-trending dextral-slip faults northwest of the mountains. The Miocene is a time of great tectonic change that ushered in the modern tectonic regime.     

The metamorphic aureole of the Nisa-Alburquerque batholith (SW Iberia): implications for deep structure and emplacement mode

The Nisa-Alburquerque granitic batholith (southern Variscan Belt, Iberian Peninsula) has been studied by petrological, structural and geophysical approaches, obtaining contrasting models for its deep structure and emplacement sequence. In order to test these models and gain knowledge on the thermal increase induced by the intrusion, we have studied its contact aureole, which was developed in similar country rock lithologies (mica schists alternating with metasandstones and feldespatic schists) all along the northern external contact of the batholith. Our results indicate no change in metamorphic grade and some variations in aureole width, which narrows toward the western sectors of the batholith. Cordierite is the only contact metamorphic mineral developed together with a high temperature biotite probably related to the granite thermal input. By considering these new data, together with zircon saturation temperatures within the granite and previous petrological and geophysical studies, we propose a model in which the feeder zones of the granitic magmas were an eastern main one and a western secondary one. We have also made comparisons of the metamorphic grade in the country rocks and the xenoliths within the granite. Most of the xenoliths have the same metamorphic facies as the country rocks (Crd-zone), though some of them contain slightly different assemblages (And + Crd), which could be explained in different ways: (1) differences in the primary schist compositions, (2) increased time-span of xenoliths in contact with the melt and (3) xenolith incorporation at slightly higher depths during final granite ascent.     

Quantifying the P–T–t conditions of north–south Lhasa terrane accretion: new insight into the pre‐Himalayan architecture of the Tibetan plateau

An integrated field, petrological and geochronological study of the Basong Tso region of south‐eastern Tibet has constrained the timing and P–T conditions of north–south Lhasa terrane accretion and provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high‐grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U–Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25 °C at c. 204–201 Ma, whereas the Zhala complex experienced peak metamorphic conditions of 5.0 ± 1.0 kbar and 740 ± 40 °C at c. 198–192 Ma. Microstructural analysis suggests that the two belts share a common early prograde history, after which the Basong Tso complex attained peak conditions following rapid burial, and the Zhala complex approached peak conditions along an isobaric path. Overall it is inferred that the Basong Tso and Zhala complexes represent the lower and upper structural levels of an evolving orogen that underwent Barrovian‐type metamorphism following collision (M1), followed by Buchan‐style overprinting at higher structural levels due to heat advection by syn‐tectonic granites (M2). Mylonitization (sensu lato) of the Basong Tso complex and juxtaposition of the two units occurred after attainment of peak conditions. The dominance of Mesozoic regional metamorphism across most of the Tibetan plateau indicates that Cenozoic crustal thickening processes, where present, are only manifested at depth.     

右江盆地三叠纪岩层极低级变质作用及地球动力学意义

右江盆地三叠纪槽盆相浊流沉积岩系遭受过区域极低级变质作用。依据地质观察和伊利石结晶度、绿泥石-云母堆垛集合体、标志性粘土矿物及白云母(伊利石)b0参数测定资料,阐述了泥质岩石的成岩变质作用经浅层(近)变质作用到浅变质作用的转换特征。变质温度区间为150-350℃,低压类型,具高地温梯度(40-43℃/km).变质级及亚带总体与地层时代及岩层在地层柱中的位置有耦合关系,而与区域变形强度无关。最后指出区域极低级变质作用是印支-燕山构造旋回早期及区域变形前的地质事件,属于地壳伸展构造背景下右江边缘型盆地内部的埋藏型变质作用。     

Accretion and post-accretion metamorphism in subduction complex terranes of the New England Fold Belt, eastern Australia

Abstract Regional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite-pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate- and low-pressure type, probably in a compressive arc or back-arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1-M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out-stepping of tectonic elements has occurred at long-lived convergent margins elsewhere, similar overprinting of high- by lower-pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out-stepping are followed by the subduction of a major heat source.     

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet,far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.     

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.     

右江盆地三叠纪岩层极低级变质作用及地球动力学意义

 右江盆地三叠纪槽盆相浊流沉积岩系遭受过区域极低级变质作用。依据地质观察和伊利石结晶度、绿泥石-云母堆垛集合体、标志性粘土矿物及白云母(伊利石)b0参数测定资料,阐述了泥质岩石的成岩变质作用经浅层(近)变质作用到浅变质作用的转换特征。变质温度区间为150-350℃,低压类型,具高地温梯度(40-43℃/km).变质级及亚带总体与地层时代及岩层在地层柱中的位置有耦合关系,而与区域变形强度无关。最后指出区域极低级变质作用是印支-燕山构造旋回早期及区域变形前的地质事件,属于地壳伸展构造背景下右江边缘型盆地内部的埋藏型变质作用。     

Evidence for an inverted metamorphic gradient associated with a Precambrian suture, southern Wyoming

Abstract An inverted metamorphic gradient associated with the northern mylonite zone of the Cheyenne belt, a deeply eroded Precambrian suture in southern Wyoming, has been documented within metasedimentary rocks of the Early Proterozoic Snowy Pass Supergroup. Metamorphic grade in the steeply dipping supracrustal sequence increases from the chlorite through the biotite, garnet, and staurolite zones both stratigraphically and structurally upward toward the northern mylonite zone. A minimum temperature increase of approximately 100° C over a km-wide zone is required for this transition. Parallelism of inverted isograds with the trace of the northern mylonite zone implies a genetic relationship between deformation associated with that zone and the inverted metamorphic gradient within the Snowy Pass Supergroup. Field evidence together with microstructural and petrofabric analysis indicate northward thrusting of amphibolite-grade rocks over rocks of the Snowy Pass Supergroup along the northern mylonite zone. Mineral equilibria and garnet-biotite geothermometry on synkinematic mineral assemblages within the Snowy Pass metasedimentary rocks indicate deformation at minimum temperatures of 480° C and pressures of 350–400 MPa (3°5–4°0 kbar). This implies tectonic burial or upper plate thickness of 13–15 km. The narrow character of metamorphic zonation and microtextures within the Snowy Pass Supergroup which indicate late synkine-matic growth of garnet and staurolite, preclude rotation of pre-existing isograds by folding as a mechanism for development of the inverted gradient. Conductive transport of heat from the upper into the lower plate across the originally low-angle thrust is insufficient to produce the necessary temperatures in the lower plate. Shear heating is considered insufficient to produce the observed metamorphic transition unless high shear stresses are postulated. Up-dip advection of metamorphic fluids is a feasible, but unproven, mechanism for heat transport. The possibility that rapid uplift due to stacking of several thrust sheets may have played a role in preserving the inverted metamorphic gradient cannot be evaluated at present.     

Tectonic history of the ophiolitic rocks of the highland border fracture zone, Scotland: Stable isotope evidence from rock-fluid interactions during obduction

The mafic and ultramafic rocks of the Highland Border Fracture Zone are ophiolitic remnants of a pre-Grampian marginal basin that opened either within, or to the north of, the Dalradian sedimentary pile. Closure of the basin was achieved through a combination of northerly-directed subduction, and obduction of ophiolitic thrust-slices onto the basin's southern margin. During the early stages of obduction, young hot peridotite slabs were thrust over the cold upper surfaces of lower thrust sheets, producing a dynamothermal metamorphic sole. Serpentinisation of these peridotites, whilst they were still cooling, occurred in a near-surface position through the interaction of meteoric waters. Subsequently, the ophiolitic thrust-sheets, which comprise lizardite serpentinites, spilitic pillow lavas, and aureole rocks, were thrust over the uppermost Dalradian nappes which were themselves being expelled southwards, thereby accommodating basement shortening. Grampian regional metamorphism of the nappe pile and overlying Highland Border Suite ophiolitic thrust sheets, produced greenschist metaspilites from the spilitic pillow lavas, induced minor retrogression in the aureole rocks, and caused the lizardite in the serpentinites to be recrystallised and replaced by antigorite. The Highland Border Suite greenschist facies metamorphic fluids were D-enriched compared with low-grade Dalradian metamorphic waters, and may have been mixtures of the latter and D-rich dehydration fluids released from the mafic rocks during dynamothermal metamorphism. Brittle fracturing and shearing in the serpentinites were responses to late deformation at different crustal levels during the final stages of emplacement, which involved gravity-sliding as well as downbending of the Dalradian nappes and ophiolitic thrust-sheets against the elevated Midland Valley block.     

Geologic and U–Pb geochronologic evidence for early Paleozoic tectonism in the Dadeldhura thrust sheet, far-west Nepal Himalaya

The Dadeldhura thrust sheet inm western Nepal consists of Proterozoic–Lower Paleozoic sedimentary and plutonic rocks, and their metamorphic equivalents, that rest structurally on Proterozoic strata of the Lesser Himalayan sequence. Although regional metamorphism and ductile deformation were widespread during Tertiary thrust emplacement, relicts of early Paleozoic tectonism are preserved locally. New field and geochronologic studies, together with the findings of previous workers, indicate that this early Paleozoic tectonism included: (1) regional metamorphism to at least garnet grade, (2) regional folding of a thick metamorphic sequence into a broad east–west trending syncline, (3) outcrop-scale folding of metasedimentary rocks, (4) emplacement of Cambro–Ordovician granitic bodies during and after the metamorphism and deformation, (5) uplift and erosion of the metamorphic sequence, with garnet-grade rocks locally exposed at the surface, and (6) derivation of Ordovician conglomeratic sandstones from the early Paleozoic orogen. Similar records of metamorphism, deformation, and uplift/erosion have been found in other regions of the Himalaya, indicating that rocks of the Dadeldhura thrust sheet were originally involved in a regionally extensive orogenic system. Future tectonic models of Himalayan orogenesis must accommodate this early Paleozoic event.