Quantitative P-T paths from zoned minerals: Theory and tectonic applications

An analytical approach to the analysis of zoning profiles in minerals is presented that simultaneously accounts for all of the possible continuous reactions that may be operative in a given assemblage. The method involves deriving a system of simultaneous linear differential equations consisting of a Gibbs-Duhem equation for each phase, a set of linearly independent stoichiometric relations among the chemical potentials of phase components in the assemblage, and a set of equations describing the total differential of the slope of the tangent plane to the Gibbs free energy surface of solid solution phases. The variables are the differentials of T, P, chemical potentials of all phase components, and independent compositional terms of solid solution phases. The required input data are entropies, volumes, the compositions of coexisting phases at a reference P and T, and an expression for the curvature of the Gibbs functions for solid solution phases. Results derived are slopes of isopleths (dP/dT, dX/dT or dX/dP) which can be used to contour P-T diagrams with mineral composition.To interpret mineral zoning, T and P can be expressed as functions of n independent composition parameters, where n is the variance of the mineral assemblage. The total differentials of P and T are differential equations that can be solved by finite difference techniques using the derivatives obtained from the analytical formulation of phase equilibria.Results calculated from Zone I and Zone IV garnets of Tracy et al. (1976) indicate that Zone I garnets grew while T increased (T+72° C) and P decreased sharply (P–3 kb). Zone IV garnets zoned in response to decreasing T (T–17° C) and P (P–1 kb). A P-T path calculated for a zoned garnet from the Greinerschiefer series, western Tauern Window, Austria, also indicates growth during decompression (–3kb) and heating (T+15° C). A P-T path calculated for the Wissahickon schist (Crawford and Mark 1982) indicates growth during cooling and compression (T–25 C, P+2.2 kb). The calculated P-T paths differ according to structural environment and can be used to relate mineral growth to tectonic processes.     

Constraining the prograde and retrograde P-T paths of granulites using decomposition of initially zoned garnets: an example from the Central Indian Tectonic Zone

The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (M_o) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M₁) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M₂) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (X_An=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinel_ss {X_Mg (Mg/Mg+Fe²⁺) = 0.23–0.26}, X_Al (Al/Al+Fe³⁺)=0.71–0.81}-plagioclase (X_An=0.91)-cordierite {X_Mg (Mg/Mg+Fe²⁺) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M₃), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M₂ spinel_ss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).Editorial responsibility: W. Schreyer     

Geomorphic investigation of the Late-Quaternary landforms in the southern Zanskar Valley,NW Himalaya

The Suru, Doda and Zanskar river valleys in the semi-arid region of Southern Zanskar Ranges (SZR) preserve a rich repository of the glacial and fluvial landforms, alluvial fans, and lacustrine deposits. Based on detailed field observations, geomorphic mapping and limited optical ages, we suggest four glaciations of decreasing magnitude in the SZR. The oldest Southern Zanskar Glaciation Stage (SZS-4) is inferred from glacially polished bedrock and tillite pinnacles. The SZS-4 is ascribed to the Marine Isotopic Stage (MIS)-4/3. The subsequent SZS-3 is represented by obliterated and dissected moraines, and is assigned to MIS-2/Last Glacial Maximum. The multiple recessional moraines of SZS-2 glaciation are assigned the early to mid Holocene age whereas, the youngest SZS-1 moraines were deposited during the Little Ice Age. We suggest that during the SZS-2 glaciation, the Drang-Drung glacier shifted its course from Suru Valley (west) to the Doda Valley (east). The study area has preserved three generations of outwash gravel terraces, which broadly correlate with the phases of deglaciation associated with SZS-3, 2, and 1. The alluvial fan aggradation, lacustrine sedimentation, and loess deposition occurred during the mid-to-late Holocene. We suggest that glaciation was driven by a combination of the mid-latitude westerlies and the Indian Summer Monsoon during periods of cooler temperature, while phases of deglaciation occurred during enhanced temperature.     

Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya,Zanskar

In this contribution, we highlight the importance of in-situ monazite geochronology linked to P−T modelling for identification of timescales of metamorphic processes. Barrovian-type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis. P−T−t paths are constrained through combined pseudosection modelling and in-situ and in-mount monazite and xenotime laser ablation–split-stream inductively coupled plasma-mass spectrometry. Petrography and garnet zoning combined with pseudosection modelling show that garnet-staurolite schists record burial from ~530 to 560°C and 5.5 kbar to ~630 to 660°C and 7 kbar; staurolite-kyanite schists from ~530 to 560°C and 5 kbar to ~670 to 680°C and 7−9 kbar; and garnet-kyanite migmatites from 540−570°C and 5 kbar to ~680 to 750°C and 7−10 kbar, probably also to >750°C and >9 kbar above the muscovite stability field. The decompression paths of garnet-staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re-equilibration at ~600 to 670°C and 4−6 kbar in some of the staurolite-kyanite schists, and at ~670 to 700°C and 6 kbar in garnet-kyanite migmatites. Some of the staurolite-kyanite schists and garnet-kyanite migmatites also contain andalusite or andalusite-cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts. BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence of c. 44 − 37 Ma ages. The first group of samples—two garnet-staurolite schists—recorded only c. 31 − 27 Ma ages in porphyroblasts and no c. 40 Ma ages. The second group (samples of staurolite-kyanite schist, garnet-kyanite migmatites, augengneiss) have both the older, c. 44 − 37 Ma monazite ages in porphyroblasts and younger ages down to c. 22 Ma. These significantly different ranges of ages from porphyroblasts of 44−37 Ma, and 31−27 Ma, are interpreted as the duration of prograde P−T paths in Eocene and Oligocene, and indicate diachronous two-stage burial of rocks. Early migmatization occurred at 38 Ma. The c. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26−24 Ma. The youngest ages of 23−22 Ma are related to leucogranite emplacement.     

Quaternary geology in Zanskar,NW Indian Himalaya: evidence for restricted glaciation and preglacial topography

Research into the Quaternary geology of the NW Himalaya has concentrated on the elucidation of the glacial sequence. However, whilst the main ranges of the Himalaya have been subjected to numerous glaciations and are now an obvious alpine glaciated terrain, much of the landscape in Zanskar and Ladakh is more equivocal and does not appear to have been glaciated during this time. These landscape facets may therefore have a much older origin and relate to preglacial events.In Zanskar, the main ice source in all glaciations was the strongly glaciated and still glacierized north slope of the main Himalaya. This ice then flowed generally northwards in the valleys of the Zanskar river and its tributaries leaving between them a landscape supporting only a few and scattered minor local glaciers. Evidence of early glaciation has been found on isolated valley-side remnants >200 m above the present rivers. Reconstruction of these preglacial valley cross profiles show them to be generally broad and shallow, with gentle slopes. This is in distinct contrast to the present major valley systems which can usually be divided into two parts—a lower unglaciated fluvially eroded section, such as the Lungnak (Tsarap Lingti Chu) Gorge and an upper broad glacial section, such as the Stod (Doda) valley.Down-valley extent of glaciation is defined by the upper ends of unglaciated fluvial gorges. Laterally, the glaciers were confined progressively to their valleys. Inevitably there is only evidence of successively smaller subsequent glaciations, but the tectonic uplift of the southern ranges may have been a factor in this forming an increasing barrier to the snow-bearing monsoon winds.     

Stratigraphic significance of the Cambrian ichnofauna of the Zanskar region,Ladakh Himalaya,India

The Cambrian succession in the Zanskar Basin of Tethys Himalaya contains an abundant ichnofossils like in the other Tethyan Himalayan successions. The ichnofossils are stratigraphically important as they occur below the trilobite body fossils and are useful to define the basal part of the Cambrian. The ichnofossil assemblage reported from the Zanskar Basin of Ladakh Himalaya is significant to demarcate the Early Cambrian age due to lack of other faunal elements so far. The body fossils of trilobites recorded from the overlying beds indicates the earliest part of the Middle Cambrian age. Sixteen ichnogenera identified include: Bifungites, Cruziana, Diplichnites, Dimorphichnus, Isopodichnus, Lockeia, Merostomichnites; Monomorphichnus, Psammichnites, Palaeophycus, Planolites, Rhizocorallium, Skolithos, Taphrhelminthopsis, Teichichnus, Trepitichnus and trilobite scratch marks etc. The ichnogenera reported so far from this part of the Tethyan Himalayan region belongs mostly to the traces of arthropod origin. The ichnofauna ranges in age from Lower Cambrian to late part of the Middle Cambrian. The ichnofaunal assemblage can be assigned to repichnial, cubichinial, pascichnial, to fodinichnial behaviour. The distribution of ichnofossils in the studied sections shows that the ichnofossils are predominately less in occurrence in the sections were trilobites dominates and higher in the successions the abundance of ichnofossils decreases.     

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.     

Evidence of Late Quaternary seismicity from Yunam Tso,Lahaul and Spiti,NW Himalaya,India

A relict fluvio-lacustrine sediment of an 8 m thick section exposed at Kilang Sarai along Yunam river, near Baralacha La shows presence of cycloids or pseudonodules, ball and pillow structures, flame-like and pocket structures, sand dyke injections, bed dislocation/faulting and flow folds. Within this section four deformed levels of soft sediment structures have been identified which were dated ca. 25 ka BP at level 1 (～0.4 m from the modern river level (mrl), 20.1 ka BP at level 2 (～1.8 m mrl), 17.7 ka BP at level 3 (～2.56 m mrl) and 12.2 ka BP at level 4 (～4.25 m mrl)). Detailed study of these soft sediment structures allow us to demonstrate that deformation level 3 is not related to seismic trigger, but remaining three deformation levels (1, 2 and 4) are ascribed to seismic origin. From compilation of earlier palaeoseismological studies using soft sediment deformational structures (SSDS) in the palaeolake deposits in the adjoining area, suggest that the deformational events identified in the present study are regional in nature and thus tectonic process plays an important role in the evolution of landform in the Spiti region.     

Probabilistic seismic hazard map of NW Himalaya and its adjoining area,India

The seismically active Northwest (NW) Himalaya falls within Seismic Zone IV and V of the hazard zonation map of India. The region has suffered several moderate (~25), large-to-great earthquakes (~4) since Assam earthquake of 1897. In view of the major advancement made in understanding the seismicity and seismotectonics of this region during the last two decades, an updated probabilistic seismic hazard map of NW Himalaya and its adjoining areas covering 28–34°N and 74–82°E is prepared. The northwest Himalaya and its adjoining area is divided into nineteen different seismogenic source zones; and two different region-specific attenuation relationships have been used for seismic hazard assessment. The peak ground acceleration (PGA) estimated for 10% probability of exceedance in 50 and 10 years at locations defined in the grid of 0.25 × 0.25°. The computed seismic hazard map reveals longitudinal variation in hazard level along the NW Himalayan arc. The high hazard potential zones are centred around Kashmir region (0.70 g/0.35 g), Kangra region (0.50 g/0.020 g), Kaurik-Spitti region (0.45 g/0.20 g), Garhwal region (0.50 g/0.20 g) and Darchula region (0.50 g/0.20 g) with intervening low hazard area of the order of 0.25 g/0.02 g for 10% probability in 50 and 10 years in each region respectively.     

Hydrocarbon source potential of the Proterozoic Sirban Limestone Formation,NW Himalaya,Jammu

The Proterozoic Sirban Limestone Formation (SLFm) crops out as detached allochthons in the northwest Himalaya (Jammu region, India) and has its coeval equivalents laterally disposed in the west in Salt Range, in the northwest in Abbotabad (Pakistan) and in southeast in Himachal Pradesh (India). The oil and gas occurrences have been reported from the Proterozoic successions globally and the hydrocarbon potential of the SLFm cannot be ruled out.The interbedded shales and algal laminated dolostones within the SLFm have yielded microflora comparable to those reported in the North African Neoproterozoic sandstones and the Late Proterozoic carbonates of the giant oil and gas fields of the Siberian Platform. The SLFm contains a rich and diverse biota comprising ~ 10% of the rock volume in thin section. The rich organic assemblage justified a hydrocarbon source potential analysis of the SLFm, tested in this study by Rock Eval (RE) pyrolysis.RE pyrolysis yielded a total organic carbon (TOC) content of 0.02 to 1 wt. % with very low Hydrogen Index (HI) values for the shales and TOC content averaging 0.02 wt. % for the dolostones. The organically lean shales and dolostones exhibit T_max values indicative of immature to post mature stage. But, since these values are for the samples with complex thermal and tectonic history the results may be unreliable. The highly altered organic matter and kerogen present in the SLFm had the potential to generate hydrocarbons and presently indicates no significant source potential. This study is important for understanding the hydrocarbon occurrences in the SLFm particularly in light of the recent oil and gas discoveries from the coeval Proterozoic successions.     

Geomorphology,sedimentology and minimum exposure ages of streamlined subglacial landforms in the NW Himalaya,India

Streamlined subglacial landforms that include drumlins in three study areas, the upper Chandra valley around Chandra Tal, the upper Spiti Valley and the middle Yunam Valley of the NW Himalaya of India were mapped and studied using geomorphic, sedimentological and geochronological methods. These streamlined subglacial landforms include a variety of morphological types, including: (i) half egg‐shaped forms; (ii) complex superimposed forms; (iii) dome‐shaped forms; (iv) inverse forms; and (v) flat‐topped symmetrical forms. Sedimentological data indicate that subglacial deformational processes are responsible for the formation of the streamlined subglacial landforms in the Chandra Tal and upper Spiti Valley study areas. In contrast, streamlined landforms in the middle Yunam Valley are the result of melt‐out and subglacial erosional processes. In the Yunam Valley study area, 11 new cosmogenic ¹⁰Be surface exposure ages were obtained for boulders inset into the crests of streamlined subglacial landforms and moraines, and also for a bedrock surface. The streamlined landforms date to 8–7 ka, providing evidence of an early Holocene valley glaciation, and older moraines date to ～17–15 and 79–52 ka, representing other significant valley glacial advances in the middle Yunam Valley. The subglacial landforms in the Chandra Valley provide evidence for a ≥300‐m‐thick Lateglacial glacier that advanced southeast, overtopping the Kunzum Range, and advancing into the upper Spiti Valley. The streamlined subglacial landforms in these study areas of the NW Himalaya highlight the usefulness of such landforms in developing glacial chronostratigraphy and for understanding the dynamics of Himalayan glaciation.     

Metamorphic P-T paths from calcic pelitic schists from the Strafford Dome, Vermont, USA

Metamorphism of the Gile Mountain Formation and Waits River Formation in the Strafford Dome and Townshend-Brownington Syncline in east-central Vermont records two nappe-style events, D1 and D2, followed by doming. D1 formed a muscovite + biotite ± ilmenite schistosity subparallel to compositional layering, SO, and was followed by heating to garnet grade. The temperature and pressure at the end of D1 are estimated to be c . 450 C and 6-8 kbar. D2 variably crenulated and folded S1 during a nearly isothermal pressure increase of 1-2 kbar, calculated from compositions of garnet, which have inclusions trails with progressive crenulation and rotation of the S1 fabric. Similar P-T paths are computed for most of the area, suggesting that the later schistosity developed during emplacement of a regional nappe 3-6 km thick. There is a general lack of D3 (dome-stage) microstructures.
Near the Strafford-Willoughby Arch, staurolite and kyanite overgrew S2 in pelites, and plagioclase with increasing X _An overgrew S2 in calcic pelites, reflecting post-D2 heating to a maximum of 550-600 C. Metamorphic pressures at the end of D2 are fairly constant on the west side of the dome, indicating minor dome-stage uplift. In contrast, pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome. The observed pattern of isotherms and isobars is mainly the result of post-metamorphic, differential uplift and unroofing.
Finally, a minor, retrograde metamorphism produced the assemblage albite + epidote + K-feldspar + muscovite + chlorite, with grade increasing east toward the Connecticut River.     

Radioactivity monitoring in the vicinity of Jawalamukhi thrust NW Himalaya,India for tectonic study

Natural Hazards - This work reports radon-thoron monitoring at two depths (60 and 90 cm) and at 82 sites around Jawalamukhi thrust of NW Himalaya, India using Solid State Nuclear Track...     

Pan-African Magmatism, and Sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.     

Higher Himalaya in the Bhagirathi section (NW Himalaya, India): its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms

The Higher Himalayan Crystalline (HHC) in the Bhagirathi river section (India) on fieldwork reveals two extensional ductile top-to-N/NE shear sub-zones—the ‘South Tibetan Detachment System’ and the ‘Basal Detachment’—besides a preceding top-to-S/SW ductile shear. A top-to-N/NE brittle shear was identified as backthrusts from the HHC (except its northern portion) that occur repeatedly adjacent to numerous top-to-S/SW brittle shears as fore-thrusts. The northern portion of the HHC—the Gangotri Granite—exhibits infrequent total six extensional and compressional brittle shear senses. The backthrusts could be due to a low friction between the lower boundary of the HHC (i.e. the Main Central Thrust-Zone) and the partially molten hot rock materials of the HHC. Subduction of the Eurasian plate towards S/SW below the Indian plate more extensively in the Garhwal sector could be the second possible reason. Presence of two ductile extensional shear sub-zones may indicate channel flow (or several exhumation mechanisms) of the HHC in a shifting mode (similar to Mukherjee et al. in Int J Earth Sci 101:253–272, 2012). The top-to-S/SW extensional brittle shear exclusively within the upper (northern portion) of the HHC and a top-to-S/SW brittle shear within the remainder of it is a possible indicator of critical taper deformation mechanism. Thus, this work provides the field evidences of possibly both channel flow and critical taper conditions from a Higher Himalayan section, besides that by Larson et al. (Geol Soc Am Bull 122:1116–1134, 2010).     

Protracted zircon growth in migmatites and In situ melt of Higher Himalayan Crystallines:U-Pb ages from Bhagirathi valley,NW Himalaya,India

The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.     

Paleobiogeography,paleoecology and paleoenvironmental significance of the Cambrian trilobites from the Zanskar region (Zanskar-Spiti-Kinnaur Basin), Northwest Himalaya

In present study the newly recorded latest Middle Cambrian trilobite fauna from the Cambrian succession of the Zanskar region of Zanskar-Spiti-Kinnaur Basin (Tethyan Himalaya) is analyzed critically to assess relationships with other Cambrian faunal elements of equatorial peri-Gondwanaland. The identification of genus Neoanomocarella, Parablackwelderia, Kunmingaspis, Fuchouia, Damesella and Dorypyge from the Cambrian of the Zanskar region and their comparison with those of South China and Australia is significant. It constitutes the basis for assessing the paleobiogeographic affinities during the Cambrian. The latest Middle Cambrian trilobite fauna from Zanskar shows proximity of Indian margin with that of southwest China “outboard” micro-continent. The recovery of analogous Middle Cambrian species i.e., Dorypyge perconvexlis, Fuchouia bulba, Fuchouia cf. oratolimba, Parablackwelderia sp. and Damesella sp. from the Zanskar region (Tethyan Himalaya) suggests a contiguous close proximity with south China and Australia during the latest Middle Cambrian, which supports the model of Meert and Van der Voo (1997) for assembly of Gondwanaland. The presence of Kunmingaspis in Zanskar and similar reports from northwestern Yunnan-Tibetan region, northern Henan, central and southeastern Hubei, north China, western Xinjiang and Yangtze platform reveal a close affinity between the Indian margin and the Yangtze platform during the Middle Cambrian. The trilobite fauna indicates the deeper shelf-shallow slope environment of deposition under fluctuating conditions of relative sea-level. The faunal elements of the Lejopyge acantha and Proagnostus bulbus zones indicate that the sea inundated the northern margin of Zanskar region during the latest Middle Cambrian time (Teta transgression) which is synchronous with globally recognized eustatic events during Lejopyge laevigata Zone.     

Migmatites and Leucogranites of tertiary age from the high Himalayan Crystallines of Zanskar (NW India): a case history of anatexis of Palaeozoic orthogneisses

Summary In the High Himalayan Crystallines of Zanskar (NW India), migmatites and peraluminous leucogranitic melts were produced by partial melting not only of paragneisses but also of Palaeozoic orthogneisses. Anatexis occurred at T = 650–720°C and P = 4–7 kbar and is related to a decompression path at increasing T of Oligocene-Miocene age. Under vapour-absent conditions anatexis of orthogneisses occurred in response to dehydration melting involving muscovite and produced very low amounts ( 5%) of leucogranitic melt. This melt segregated in situ as homogeneous leucoanatexites. In heterogeneous diatexites, the leucosomes formed by disequilibrium melting probably at T in excess of the H₂O-saturated solidus. Extensive anatexis and melt segregation into dykes or bodies require infiltration of an aqueous fluid. A model is proposed in which large rock-volumes showing low-degrees of vapour-absent melting are associated with ductile shear zones infiltrated by H₂O and showing high-degrees of vapour present melting. With respect to the eastern Himalayas, the relative scarcity of leucogranites in Zanskar depends on: 1- the lack of a high—T, low-P stage; 2- the abundance of dry igneous (i.e. orthogneiss) protoliths relative to more fertile metasedimentary magma sources.

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Tertiäre Migmatite und Leukogranite aus den kristallinen Gesteinen des Himalaya, Zanskar (NW Indien): ein Beispiel von Anatexis paläozoischer Orthogneise

Zusammenfassung In den kristallinen Teilen des Himalayas von Zanskar, Nordwest-Indien, sind Migmatite und aluminiumreiche leukogranitische Schmelzen durch teilweise Aufschmelzung von Paragneisen und von paläozoischen Orthogneisen entstanden. Die Anatexis fand bei T = 650–720° und P = 4–7 kbar statt, und hält sich an einen Dekompressions-Pfad bei zunehmender Temperatur, der oligozänen-miozänen Alters ist. In Abwesenheit einer Dampfphase fand die Anatexis von Orthogneisen in Antwort auf Dehydrations-Aufschmelzung mit Beteiligung von Muskovit statt und führte zur Entstehung von geringen Mengen ( 5%) von leukogranitischer Schmelze. Diese Schmelze segregierte in situ als homogene Leukoanatexite. In heterogenen Diatexiten bildeten sich die Leukosome durch Ungleichgewichts-Aufschmelzung wahrscheinlich bei Temperaturen, die über dem H20-gesättigten Solidus liegen. Extensive Anatexis und Segregation der Schmelzen in Gänge oder unregelmäßige Körper erfordern Infiltration einer wäßrigen Fluid-Phase. Es wird ein Modell vorgestellt, bei dem große Volumina von Gesteinen niedrige Grade von Aufschmelzung in Abwesenheit einer Dampfphase zeigen und mit duktilen Scherzonen assoziiert sind, die mit H₂O infiltriert wurden und hohe Grade von Aufschmelzung in Anwesenheit einer Dampfphase erkennen lassen. Die relative Seltenheit von Leukograniten in Zanskar ist auf zwei Faktoren zurückzuführen; 1. Die Abwesenheit eines Hoch-T, niedrig-P Stadiums; 2. das reichliche Vorhandensein von trockenen magmatischen Protolithen (z.B. Orthogneise) verglichen mit den mehr produktiven metasedimentären Ursprungsgesteinen für Magmen.

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With 8 Figures