High Himalayan Crystalline Series Research Articles

Leucogranites in Lhozag, southern Tibet: Implications for the tectonic evolution of the eastern Himalaya

Petrogenesis of the Himalayan leucogranite is strongly influenced by conditions which are associated with the tectonic evolution of Himalayan orogen. In this article, we present petrological, geochronological and geochemical results of the Lhozag leucogranites that crop out alongside the South Tibetan Detachment System (STDS) in the east of Himalaya. Zircon U-Pb dating revealed three episodes of leucogranitic magmatism in Lhozag at 17.8±0.1Ma, 15.1±0.1Ma, and 12.0±0.1Ma, respectively. The Lhozag leucogranites show relatively low εNd(t), low zircon εHf(t) and high initial 87Sr/86Sr ratios, which are similar to the High Himalayan Crystalline Series (HHCS), indicating that they were derived from the HHCS. The characteristics of relatively high Na2O and Rb contents, high Rb/Sr ratios and low CaO, MgO, TFe2O3, TiO2, and Sr contents indicate that both the ca. 18Ma Lhozag tourmaline leucogranites and the ca. 15Ma Lhozag two-mica granites were derived from fluid-absent muscovite-dehydration melting of metasediments. The opposite geochemistry characteristics of the ca. 12Ma Khula Kangri two-mica granites imply that these granites are derived from fluid-present melting of metasediments. Four Khula Kangri two-mica granite samples with relatively lower TiO2, TFe2O3, MgO, and CaO contents, higher Rb concentrations and Rb/Sr ratios could be evolved from the Khula Kangri two-mica granites with relatively lower Rb/Sr ratios. The melting behaviors of the Lhozag leucogranites varied from fluid-absent melting to fluid-present melting, implying that there were P-T-XH2O variations in the deep crust. The tectonic evolution would give rise to variation of P-T-XH2O variation, and subsequent transformation of melting behavior. Our new results display the transformation of melting behavior of the Lhozag leucogranites, which implies the tectonic evolution from earlier N–S extension to later E–W extension in the eastern Himalaya at ca. 12Ma.

On discrimination between carbonate and silicate inputs to Himalayan rivers

We review new and published analyses of river waters, bedloads and their constituent minerals from the Dhauli Ganga and Alaknanda, headwaters of the Ganges in Garhwal, and the Marsyandi in Nepal and their tributaries. These data are used to discriminate between the inputs of major cations and Sr from silicate and carbonate sources. Methods of estimating the proportion of the carbonate and silicate inputs to river waters using mixing arrays in Sr-Ca-Mg-Na-K-87Sr/86Sr space are shown to suffer from systematic correlations between the magnitude of the precipitation of secondary calcite and the fraction of the silicate component. This results in factor-of-two overestimates of the fractions of silicate-derived Ca, Mg and Sr. To correct for this the magnitude of secondary calcite precipitated and relative fractions of silicate and carbonate-derived cations are instead calculated by modeling the displacement of water compositions from the compositions of the carbonate and silicate components of the bedload in subsets of Sr-Ca-Mg-Na-K-87Sr/86Sr space. The compositions of the carbonate and silicate end-members in the bedload are determined by sequential leaching. The results of this modeling are compared with modeling of the modal mineral inputs to waters where mineral compositions are derived from electron-microprobe analyses of the minerals in the bedload. In the upper Marsyandi catchment, which drains low-grade Tethyan Sedimentary Series formations, a set of mainstem samples collected over a two-year period define tight correlations in Sr-Ca-Mg-Na-K-87Sr/86Sr space. Modeling of the magnitude of secondary carbonate precipitation and fractions of silicate-derived Ca, Mg and Sr in Sr-Ca-Mg-87Sr/86Sr space gives self-consistent results that are compatible with both the calculations of mineral modes and published Mg-isotopic compositions, if the ratio of chlorite to biotite weathering is high or if there is another silicate source of Mg. These calculations imply that between 12 and 31 percent of the Sr and 44 and 72 percent of the Mg is derived from silicate minerals where the range reflects the seasonal change in the ratio of silicate-derived to carbonate-derived cations. Modeling in Sr-Ca-Na and/or K space is inconsistent with the Sr-isotopic and Mg-isotopic constraints and we conclude that in this catchment dissolution of Na and K are incongruent relative to Sr-Ca-Mg. Potassium is preferentially retained in micas whereas the controls on Na are unclear. Modeling of the catchments underlain by High Himalayan Crystalline and Lesser Himalayan Series in Garhwal is complicated by the presence of dolomite as well as calcite in the carbonate and the results imply that dolomite dissolves faster in the acetic acid leaches than in nature. Up to 60 percent of the Sr in the catchment on High Himalayan Crystalline Series and 20 to 30 percent of Sr in the catchments on Lesser Himalayan Series are estimated to be derived from silicates. However it should be noted that the element budgets are not all self-consistent and the use of bedrock-element ratios to model the sources of chemical inputs to river waters remains subject to uncertainties.

Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow

Research Article| July 01, 2014 Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow Melanie Finch; Melanie Finch † 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia †E-mail: melanie.finch@monash.edu Search for other works by this author on: GSW Google Scholar Pavlína Hasalová; Pavlína Hasalová 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia2Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic §Current address: Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, 118 21, Prague 1, Czech Republic. Search for other works by this author on: GSW Google Scholar Roberto F. Weinberg; Roberto F. Weinberg 1School of Geosciences, Monash University, Clayton, Victoria 3800, Australia Search for other works by this author on: GSW Google Scholar C. Mark Fanning C. Mark Fanning 3Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2014) 126 (7-8): 892–924. https://doi.org/10.1130/B30817.1 Article history received: 07 Nov 2012 rev-recd: 09 Aug 2013 accepted: 22 Nov 2013 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Melanie Finch, Pavlína Hasalová, Roberto F. Weinberg, C. Mark Fanning; Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow. GSA Bulletin 2014;; 126 (7-8): 892–924. doi: https://doi.org/10.1130/B30817.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Zanskar shear zone is a ductile, normal-sense shear zone that exploited the contact between the High Himalayan Crystalline series and the Tethyan sedimentary series. The Zanskar shear zone is an extension of the South Tibetan detachment system with similar timing and nature, and, in Zanskar, it accommodated 24 km of normal movement. Early thrusting is preserved in the footwall and hanging wall and is overprinted by normal shearing. Thrusting and normal shearing were coplanar and codirectional, with SW-directed thrusting overprinted by NE-directed normal shearing—a simple inversion of movement sense. The telescoped isograds related to normal shearing define a broad pattern of colder rocks on top of hotter. However, we found preserved thrust-related metamorphic series, with hotter rocks on top of colder, severely telescoped by normal shearing. Some determinations of the amount of displacement and thinning on the Zanskar shear zone prior to the current work have assumed a steady-state crustal profile and have disregarded preexisting perturbations of isograds such as those indicated here. Miocene leucogranitic intrusions accumulated within and below the normal Zanskar shear zone. Intrusions were sheared during thrusting and normal movement, and magmatism outlasted normal shearing. We have dated monazites by U-Pb sensitive high-resolution ion microprobe (SHRIMP) from leucogranite samples that were sheared by the thrusting event, by the normal movement event, and those that postdate all shearing. Results constrain the timing of the switch from thrusting to normal movement to between 26 and 24 Ma and ca. 22 Ma. At ca. 20 Ma, normal shearing in Zanskar shear zone was no longer active, and magmatism was waning, producing late, undeformed leucogranitic dikes. Taking into account the shear zone thickness of 0.83 km, the maximum duration of normal movement of 6 m.y., and the estimated strain of γ = 28.6, we estimate the lower bound of strain rate for the Zanskar shear zone to be 1.5 × 10–13 s–1. Given the short duration of the normal shearing event and magmatism, we find little support for the hypothesis of channel flow in Zanskar. We propose instead that Miocene anatexis weakened the midcrustal levels and caused the switch from thrusting to normal movement, doming, and cooling of the anatectic core of the High Himalayan Crystalline series. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mid-Eocene high Sr/Y granites in the Northern Himalayan Gneiss Domes: Melting thickened lower continental crust

Within the Himalayan collisional belt, granotoids occur along two sub-parallel belts, the Northern Himalayan Gneiss Domes (NHGD) and the High Himalayan Crystalline Series (HHCS). In the Yardoi area of NHGD, two-mica granite, a new type granite occurs in the core of the Yardoi gneiss dome (YGD), Dala and Quedang from north to south, and extends at least 50km long. These granites have similar mineral composition, elemental and radiogenic isotope geochemistry, and age of formation. SHRIMP zircon U/Pb dating indicates that the Yardoi and the Quedang two-mica granites formed at 42.6±1.1Ma and 42.8±0.6Ma, respectively, similar to the Dala pluton. These two-mica granites have (1) high SiO2 (>68wt.%), Al2O3 (>15wt.%), and A/CNK(>1.0); (2) relatively high Sr and LREE, but low Y(<10ppm) and Yb (<1ppm); (3) high Sr/Y (>40 and up to 250) and La/Yb (>30); (4) very weak or no Eu anomalies; and (5) as compared with those in the Himalayan leucogranites, low initial Sr (87Sr/86Sr(i)<0.7120) and similarly unradiogenic Nd (εNd(i)=−8.9–−15.0) isotopic compositions. These granites have initial Sr and Nd isotope compositions similar to those in the amphibolites but significantly different from those in the metapelite and granitic gneiss. Two-mica granites from the Yardoi area are of peraluminous granite with relatively high Na/K and Sr/Y ratios. Such features are distinct from those in the younger leucogranites along the HHCS as well as in the NHGD, and require melting of source consisting dominantly of amphibolite at thickened crustal conditions. This is also supported by the presence of amphibolites with similar Sr and Nd isotope compositions, and similar ages of metamorphism. Two-mica granites of similar age also occur in the other NHGD gneiss domes and along the HHCS belt, implying that Mid-Eocene melting of thickened crustal materials was widespread and might be a primary factor that led to the formation of high density materials (e.g. eclogitic rocks) beneath the Tethyan Himalaya.

Exhumation and incision history of the Lahul Himalaya, northern India, based on (U–Th)/He thermochronometry and terrestrial cosmogenic nuclide methods

Low-temperature apatite (U–Th)/He (AHe) thermochronology on vertical transects of leucogranite stocks and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure dating on strath terraces in the Lahul Himalaya provide a first approximation of long-term (10 4–10 6 years) exhumation rates for the High Himalayan Crystalline Series (HHCS) for northern India. The AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to the surface was ~ 1–2 mm/a and occurred during the past ~ 2.5 Ma. Bedrock exhumation in Lahul fits into a regional pattern in the HHCS of low-temperature thermochronometers yielding Plio-Pleistocene ages. Surface exposure ages of strath terraces along the Chandra River range from ~ 3.5 to 0.2 ka. Two sites along the Chandra River show a correlation between TCN age and height above the river level yielding maximum incision rates of 12 and 5.5 mm/a. Comparison of our AHe and surface exposure ages from Lahul with thermochronometry data from the fastest uplifting region at the western end of the Himalaya, the Nanga Parbat syntaxis, illustrates that there are contrasting regions in the High Himalaya where longer term (10 5–10 7 years) erosion and exhumation of bedrock substantially differ even though Holocene rates of fluvial incision are comparable. These data imply that the orogen's indenting corners are regions where focused denudation has been stable since the mid-Pliocene. However, away from these localized areas where there is a potent coupling of tectonic and surface processes that produce rapid uplift and denudation, Plio-Pleistocene erosion and exhumation can be characterized by disequilibrium, where longer term rates are relatively slower and shorter term fluvial erosion is highly variable over time and distance. The surface exposure age data reflect differential incision along the length of the Chandra River over millennial time frames, illustrate the variances that are possible in Himalayan river incision, and highlight the complexity of Himalayan environments.

Calcium and magnesium isotope systematics in rivers draining the Himalaya-Tibetan-Plateau region: Lithological or fractionation control?

In rivers draining the Himalaya-Tibetan-Plateau region, the 26Mg/ 24Mg ratio has a range of 2‰ and the 44Ca/ 42Ca ratio has a range of 0.6‰. The average δ 26 Mg values of tributaries from each of the main lithotectonic units (Tethyan Sedimentary Series (TSS), High Himalayan Crystalline Series (HHCS) and Lesser Himalayan Series (LHS)) are within 2 standard deviation analytical uncertainty (0.14‰). The consistency of average riverine δ 26 Mg values is in contrast to the main rock types (limestone, dolostone and silicate) which range in their average δ 26 Mg values by more than 2‰. Tributaries draining the dolostones of the LHS differ in their δ 44 42 Ca values compared to tributaries from the TSS and HHCS. The chemistry of these river waters is strongly influenced by dolostone (solute Mg/Ca close to unity) and both δ 26 Mg (−1.31‰) and δ 44 42 Ca (0.64‰) values are within analytical uncertainty of the LHS dolostone. These are the most elevated δ 44 42 Ca values in rivers and rock reported so far demonstrating that both riverine and bedrock δ 44 42 Ca values may show greater variability than previously thought. Although rivers draining TSS limestone have the lowest δ 26 Mg and δ 44 42 Ca values at −1.41 and 0.42‰, respectively, both are offset to higher values compared to bedrock TSS limestone. The average δ 26 Mg value of rivers draining mainly silicate rock of the HHCS is −1.25‰, lower by 0.63‰ than the average silicate rock. These differences are consistent with a fractionation of δ 26 Mg values during silicate weathering. Given that the proportion of Mg exported from the Himalaya as solute Mg is small, the difference in 26Mg/ 24Mg ratios between silicate rock and solute Mg reflects the 26Mg/ 24Mg isotopic fractionation factor ( α silicate – dissolved Mg ) between silicate and dissolved Mg during incongruent silicate weathering. The value of α silicate – dissolved Mg of 0.99937 implies that in the TSS, solute Mg is primarily derived from silicate weathering, whereas the source of Ca is overwhelmingly derived from carbonate weathering. The average δ 44 42 Ca value in HHCS rivers is within uncertainty of silicate rock at 0.39‰. The widespread hot springs of the High Himalaya have an average δ 26 Mg value of −0.46‰ and an average δ 44 42 Ca value of 0.5‰, distinct from riverine values for δ 26 Mg but similar to riverine δ 44 42 Ca values. Although rivers draining each major rock type have δ 44 42 Ca and δ 26 Mg values in part inherited from bedrock, there is no correlation with proxies for carbonate or silicate lithology such as Na/Ca ratios, suggesting that Ca and Mg are in part recycled. However, in spite of the vast contrast in vegetation density between the arid Tibetan Plateau and the tropical Lesser Himalaya, the isotopic fractionation factor for Ca and Mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of Ca and Mg in these catchments. The discrepancy between solute and solid Ca and Mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of Ca and Mg, bound as minerals in rock, and released as cations in rivers. The fractionation of Ca and Mg isotope ratios may prove useful for tracing mechanisms of chemical alteration. Ca isotope ratios of solute riverine Ca show a greater variability than previously acknowledged. The variability of Ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global Ca cycling, if past variations in oceanic Ca isotope ratios are to be of use in constraining the past carbon cycle.

Insights into the patterns and locations of erosion in the Himalaya — A combined fission-track and in situ Sm–Nd isotopic study of detrital apatite

Understanding the role that climate, erosion and tectonics play in determining the mean relief, elevation and general form of a mountain range over time has become the focus of much recent research in the field of tectonic geomorphology. A crucial constraint in these studies is the long-term exhumation rate, which is increasingly being examined using detrital minerals from the sedimentary rock record that represent the eroded remnants of the orogen. A classic field area of such studies is the Himalayan orogen — the Earth's largest mountain range. We describe here a novel approach that uses the in situ analysis of Nd isotopes by laser ablation multicollector inductively coupled plasma mass spectrometry of fission-track dated apatite grains to unambiguously tie them to their source regions. We demonstrate the capability of this new approach on several modern and Holocene river sand samples from the Himalayan orogen and determine, because of the large isotopic differences in the constituent tectonostratigraphic units, both the location and magnitude of long-term erosion. In agreement with previous studies we are able to show that in the Central and Eastern Himalaya long-term denudation is focussed within the High Himalayan Crystalline Series (HHCS) at average rates of ∼ 2 mm/yr. Average long-term rates are similar in the Western Himalaya sampled by the Indus River and largely reflect erosional denudation of the Western Indian Himalaya and the Western Syntaxis (the Nanga Parbat Haramosh Massif and South Karakoram Metamorphic Complex). Notably, the Asian continent supplies significantly more apatites to the mouth of the Indus than they do to the Bengal delta. The ability to reconstruct the patterns of long-term erosion in the Himalaya using detrital apatites from a few samples collected from major transverse drainages provides confidence in the utility of this approach to examine the locations and magnitude of long-term erosion in the past.

Himalayan architecture constrained by isotopic tracers from clastic sediments

We report the first geochemical traverse to integrate U–Pb ages and Hf data on single detrital zircons with bulk-rock Sm–Nd–Rb–Sr isotopic measurements across the breadth of the Himalayan orogen, in northwest India. Our dataset, from key tectonostratigraphic units in the Sutlej valley, provides detailed insights into both the provenance of the sedimentary formations, and their tectonic history. Whole-rock Nd data provide information on the mean age of source areas contributing detritus to the sedimentary units in the mountain belt, regardless of their current metamorphic grade. U–Pb ages of detrital zircons from the same units identify the age of melt formation in the source rocks, while Lu–Hf isotope data from zircons resolve discrete crust formation ages in the multiple eroding terrains. Complex whole-rock Sr systematics hint at the extent of pre-Himalayan thermal events. Our data confirm the geochemical distinction between the older Lesser Himalayan Formations and those of the depositionally younger units (including the High Himalayan Crystalline Series) that has been recorded along the strike of the Himalaya from the western syntaxis to Nepal. Palaeoproterozoic metasediments of the Jutogh Group and Rampur Formation yield ɛ Nd(500) values below − 17, whereas Neoproterozoic and younger metasediments yield ɛ Nd(500) above − 13. The latter group encompasses the Vaikrita Group gneisses of the High Himalayan crystallines and the overlying Haimanta Group, but also includes a number of units in thrust sheets north of the Main Boundary Thrust in the Lesser Himalayan Zone (designated the ‘Outer Lesser Himalaya’ to distinguish them from the Palaeoproterozoic ‘Inner Lesser Himalaya’). Zircon populations from the Inner Lesser Himalaya are characterised by Palaeoproterozoic–Late Archaean ages (2.6–1.8 Ga), whereas the depositionally younger units contain populations both of this age, and a younger period (Meso- to Neo-Proterozoic; 1.1–0.8 Ga). Detrital zircon ages are younger than their respective Hf-isotope derived crustal formation ages by 0.7–2.1 Ga, indicating that the source regions of the detrital zircons consisted of older terranes with considerable amounts of reworking and renewed magmatism. The distinct feature of the High Himalayan crystallines, which is also shared with the Outer Lesser Himalaya of the Sutlej traverse, is the presence of Meso- to Neo-Proterozoic detritus derived from a complex source area. In addition, while 500 Ma granitoids are apparently confined to Neoproterozoic units such as the High Himalayan crystallines and Haimanta Group, 1.8–1.9 Ga granitoid bodies (e.g., the Wangtu gneiss in the Sutlej valley) are exclusively associated with Inner Lesser Himalayan units. Our data suggest that, while the Inner Lesser Himalaya sediments derived their detritus from partially reworked Archaean crust similar to the Aravalli craton, depositionally younger units such as the HHCS represent a mixture of detritus from this ancient crust and a more juvenile source region.

Causes and consequences of protracted melting of the mid-crust exposed in the North Himalayan antiform

Gneiss domes exposed by the North Himalayan antiform in southern Tibet provide evidence for protracted melting of the mid-crust during convergent tectonics. Anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low-viscosity crustal sheet. Sr–Nd isotope systematics establish that the cores of the domes, dominated by Cambrian gneiss, form a window of the High Himalayan Crystalline Series (HHCS; Indian Plate). Combined trace-element and isotopic analyses from 4 two-mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the High Himalayan Crystalline Series by the incongruent breakdown of muscovite and, to a lesser extent, biotite, at temperatures from 680 to 760 °C. U–Pb dating of accessory phases extends the known period of granite magmatism from the Late Oligocene to Mid-Miocene. A Ca-rich, garnet-bearing leucogranite of Oligocene age (27.5 Ma) provides a rare insight into the pre-Miocene history of the underthrust Indian plate, reflecting prograde melting at greater depths during a period of crustal thickening. The origin of the Miocene granites (15–10 Ma) can be modeled from the exhumation of an extruding channel at a rate of ca. 1 mm/year, constrained by Ar-isotope data. Melting over a protracted time scale along the leading edge of the Indian Plate, less than 50 km from the suture, accelerated horizontal shortening within the collision zone and provided a positive-feedback mechanism between magmagenesis and deformation.

The pressure–temperature–time path of migmatites from the Sikkim Himalaya

AbstractA combined metamorphic and isotopic study of lit‐par‐lit migmatites exposed in the hanging wall of the Main Central Thrust (MCT) from Sikkim has provided a unique insight into the pressure–temperature–time path of the High Himalayan Crystalline Series of the eastern Himalaya. The petrology and geochemistry of one such migmatite indicates that the leucosome comprises a crystallized peraluminous granite coexisting with sillimanite and alkali feldspar. Large garnet crystals (2–3 mm across) are strongly zoned and grew initially within the kyanite stability field. The melanosome is a biotite–garnet pelitic gneiss, with fibrolitic sillimanite resulting from polymorphic inversion of kyanite. By combining garnet zoning profiles with the NaCaMnKFMASHTO pseudosection appropriate to the bulk composition of a migmatite retrieved from c. 1 km above the thrust zone, it has been established that early garnet formed at pressures of 10–12 kbar, and that subsequent decompression caused the rock to enter the melt field at c. 8 kbar and c. 750 °C, generating peritectic sillimanite and alkali feldspar by the incongruent melting of muscovite. Continuing exhumation resulted in resorption of garnet. Sm–Nd growth ages of garnet cores and rim, indicate pre‐decompression garnet growth at 23 ± 3 Ma and near‐peak temperatures during melting at 16 ± 2 Ma. This provides a decompression rate of 2 ± 1 mm yr−1 that is consistent with exhumation rates inferred from mineral cooling ages from the eastern Himalaya. Simple 1D thermal modelling confirms that exhumation at this rate would result in a near‐isothermal decompression path, a result that is supported by the phase relations in both the melanosome and leucosome components of the migmatite. Results from this study suggest that anatexis of Miocene granite protoliths from the Himalaya was a consequence of rapid decompression, probably in response to movement on the MCT and on the South Tibetan detachment to the north.

Fluxes of Sr into the headwaters of the Ganges

Himalayan weathering is recognized as an important agent in modifying sea water chemistry, but there are significant uncertainties in our understanding of Himalayan riverine fluxes. This paper examines causes of the variability, including that of the seasons, by analysis of downstream variations in Sr, 87Sr, and major ions in the mainstream, in relation to the composition of tributary streams from subcatchments with differing geologic substrates. Water samples were collected over four periods spanning the premonsoon, monsoon, and postmonsoon seasons. Uncertainties in the relative fluxes have been estimated, using Monte Carlo techniques, from the short-term variability of mainstream chemistry and the scatter of tributary compositions. The results show marked seasonal variations in the relative inputs related to high monsoon rainfall in the High and Lesser Himalaya, contrasting with the major contribution from glacial melt waters from the Tibetan Sedimentary Series (TSS) at times of low rainfall. Much of the spread in previously published estimates of the sources of Sr in Himalayan rivers may result from these seasonal variations in Sr fluxes. The annual fluxes of Sr into the headwaters of the Ganges are derived from the three main tectonic units in the proportions 35 ± 1% from the TSS, 27 ± 3% from the High Himalayan Crystalline Series (HHCS), and 38 ± 8% from the Lesser Himalaya. The particularly elevated 87Sr/ 86Sr ratios characteristic of the HHCS and the Lesser Himalaya enhance their influence on seawater Sr-isotope composition. The TSS contributes 13 ± 1%, the HHCS 30 ± 3%, and the Lesser Himalaya 57 ± 11% of the 87Sr flux in excess of the seawater 87Sr/ 86Sr ratio of 0.709.

Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits

Petrography, Sr–Nd isotope compositions and single-grain laser 40Ar– 39Ar ages of detrital white mica from Early–Middle Miocene molasse of the Dharamsala and Lower Siwalik Formations of Northern India, dated by magnetostratigraphy, determine the sediment sources, their metamorphic grade and exhumation rates in the Himalayan palaeo-hinterland. Deposition of the Lower Dharamsala Member (21–17 Ma) occurred during the period of rapid isothermal decompression and crustal anatexis (24–18 Ma) of the metamorphic core. This episode of decompression is thought to be coeval with thrusting on the Main Central Thrust and normal faulting on the South Tibetan Detachment System. The sediment composition and detrital mica ages indicate erosion from the rapidly exhumed metamorphic slab of the High Himalayan Crystalline Series. Deposition of the Upper Dharamsala Member (17–13 Ma) and basal Siwalik Group (13–12.5 Ma) spanned the period in which thrusting transferred south from the Main Central Thrust. The sediment composition and detrital mica ages contrast strongly with those of the Lower Dharamsala, indicating erosion from sedimentary and low grade rocks. The isotopic composition indicates that these rocks were part of the High Himalayan Series unaffected by Tertiary metamorphism, i.e. from upper structural levels of the High Himalayan Slab. This suggests that a major reorganisation of the orogenic wedge occurred at 17 Ma involving forward propagation of the MCT and cessation of rapid exhumation of the metamorphic slab.

Metamorphism and exhumation of the NW Himalaya constrained by U–Th–Pb analyses of detrital monazite grains from early foreland basin sediments

Single detrital monazite grains from the Dharamsala and Lower Siwalik Formations (early to mid-Miocene continental foreland basin sediments in NW India) have been dated by two techniques; isotope dilution thermal ionization multicollector mass spectrometry (ID-TIMS) and laser ablation plasma ionization multicollector mass spectrometry (LA-PIMMS). The results give U–Th–Pb isotopic ages of c. 400–1300 Ma and 28–37 Ma and suggest that the source of detritus shed from the uplifting Himalayan mountains and captured in the foreland basin included (1) the protolith to the High Himalayan Crystalline Series (HHCS), i.e. rocks unaffected by the Himalayan metamorphism, (2) Cambro-Ordovician granites and (3) HHCS affected by the M 1 phase of Barrovian metamorphism (Eo-Himalayan) related to the Indo-Asian collision. Deposition of the Dharamsala Formation was coeval with M 2 sillimanite grade Himalayan metamorphism and crustal melting. The youngest monazite ( c. 28–37 Ma) ages imply that Indian plate rocks, having experienced the earliest Himalayan metamorphic event which occurred within 10–20 Ma of collision were exhumed, eroded and deposited within c. 10–20 Ma of metamorphism. This indicates a minimum cooling rate of between 60 and 40°C Ma −1 for the period 30–20 Ma. After 20 Ma our study suggests no change in source area and that this same sequence, comprising both metamorphosed and unmetamorphosed rocks, was supplying detritus and being progressively incised by erosion for at least a further 8 million years.

The early Palaeozoic magmatic event in the Northwest Himalaya, India: source, tectonic setting and age of emplacement

In the High Himalayan Crystalline Series of Northwest India, numerous peraluminous granites intruded the metasediments of the late Proterozoic to early late Cambrian Haimanta Group. Nd and Sr isotope systematics confirm that they were derived from heterogeneous crustal sources. New geochronological data from two plutons range in age from late Precambrian to early Ordovician: single zircon U–Pb dating yielded an age of 553 ± 2 (2σ) Ma for the Kaplas granite, whereas mineral Sm–Nd isotope systematics define a crystallization age of 496 ± 14 (2σ) Ma for the tholeiitic mafic rocks in the Mandi pluton, where evidence of magma mingling documents a close association between mafic and granitic melts. The end of this period of magmatic activity coincides with the depositional gap below the Ordovician transgression, caused by surface uplift and erosion, that is an important feature in the stratigraphy of the Northwest Himalaya. In Spiti, the transgression of the Ordovician basal conglomerates on a normal fault indicates pre-Ordovician extensional faulting. Therefore, the early Palaeozoic magmatic activities in the Northwest Himalaya could be correlated with a late extensional stage of the long-lasting Pan-African orogenic cycle which ended with the formation of the Gondwana supercontinent.

Fluid-enhanced melting during prograde metamorphism

Anatexis is a commonly recognized feature of high-grade metamorphism, but segregated melts are generally ascribed to anatexis during peak metamorphic conditions and little is known about melting along the prograde path. A suite of small-volume, deformed, two-mica leucogranites has been recognized within the High Himalayan Crystalline Series of the Garhwal Himalaya. These granites are consistently more siliceous than minimum-melt granite compositions and are characterized by low Rb/Sr ratios, high Ba, low abundances of HFS elements and positive Eu anomalies. Such trace-element characteristics contrast strongly with the geochemistry of the well-studied Early Miocene leucogranites of the High Himalaya, derived from fluid-absent melting. Sm–Nd garnet dating of one deformed granite indicates a crystallization age of 39±3 Ma, c. 15 Ma before the emplacement of the more voluminous High Himalayan leucogranites. Whilst some entrainment of restitic phases cannot be excluded, trace element signatures suggest a low temperature (&lt;650°C) crustal melt formed under conditions of high H 2 O activity. Positive Eu anomalies and unusually low Rb/Sr ratios are indicative of rapid, disequilibrium melting. Fluid-enhanced melting may be a common feature of prograde upper amphibolite-facies metamorphism of orogenic belts, predating peak metamorphism by at least 15 Ma. These melts will only crystallize within this period if they segregate from their protoliths. Subsequent dating of long-lived melts would indicate erroneously young ages for the prograde melting events. However, melts formed in this way may be recognized by their distinctive trace-element chemistry. The persistence of early formed melts within an orogen provides insights into the prograde heating path, and may be critical in controlling the rheology of the middle crust, and hence its deformational history.

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Nd and Sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences. The potential of the technique is illustrated by this isotopic study of the Main Central thrust system of the Himalaya. Nd isotope data from the Garhwal Himalaya indicate that metasedimentary rocks from the Vaikrita Group (Nd = –14 to –19) correlate closely with those from the High Himalayan Crystalline Series, which constitutes the hanging-wall lithologies of the Main Central thrust. In contrast, metasedimentary rocks from the Munsiari Group (Nd = –23 to –28) show marked similarities to the Lesser Himalayan Series in the footwall of the Main Central thrust. Sr isotopes support the correlations in that the Vaikrita Group shows partial reequilibration at 500 Ma, whereas the Munsiari Group has not undergone Sr isotope homogenization since 1800 Ma. Thus, the Vaikrita thrust that juxtaposes these two formations is recognized as the Main Central thrust in Garhwal Himalaya. The thrust coincides, approximately, with the location of the kyanite isograd, confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the Main Central thrust. Along the Tons thrust (known locally as the Srinagar thrust) 50 km south of the Main Central thrust, low-grade quartzarenites with Nd-Sr isotope and trace element characteristics typical of Lesser Himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the High Himalayan Crystalline Series. The field relationships most probably result from out-of-sequence thrusting in which Lesser Himalayan Series rocks to the north were emplaced over low-grade equivalents of the High Himalayan Crystalline Series preserved in the external part of the orogen. This study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts.

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya

Tracing the origins of the western Himalaya: an isotopic comparison of the Nanga Parbat massif and Zanskar Himalaya

AbstractNew Sr and Nd isotope data for basement gneisses and leucogranites are presented from two contrasting areas of the western Himalaya; the Nanga Parbat-Haramosh massif (NPHM) and Zanskar. Sr-isotope systematics of metapelites and anatectic migmatites from the Zanskar Himalaya are characterized by εSr of 515–930, typical of the High Himalayan Crystalline unit as exposed for more than 2000 km along strike. Moreover, Zanskar leucogranites are typical of the belt of Early Miocene granites intruding the High Himalayan Crystallines across the orogen (mean εSr = 834). In contrast, the NPHM leucogranites show an elevated average εSr of 2400, and basement samples show a wide range in εSr from 1850 to 8150. Errorchrons for the metasedimentary gneisses indicate isotopic homogenization of the basement at c. 500 Ma for the Zanskar samples compared with c. 1800 Ma from the NPHM, confirming that the two terrains have experienced contrasting pre-Himalayan histories.Nd isotopic data from the NPHM indicate model ages from 2300 to 2800 Ma, indicating the mean crustal formation ages of the protoliths from which the sediments were derived. A compilation of published Nd data from the Himalaya indicates average protolith formation ages of 2640 ± 220 Ma for the Lesser Himalaya lithologies, compared with 1940 ± 270 Ma for the High Himalaya unit.Gneissic lithologies from Zanskar and the NPHM have previously been correlated with the High Himalayan Crystalline Series, since both display high-grade Himalayan metamorphism and are intruded by syn- to post-tectonic tourmaline-bearing leucogranites. Isotopic systematics in the Zanskar region confirm this correlation. In contrast, the NPHM basement rocks are better correlated with Lesser Himalayan lithologies, exposed south of the Main Central Thrust. We conclude that the NPHM represents either a lower structural level of the Lesser Himalaya Series, or its protolith.

Lithostratigraphic correlations in the western Himalaya—An isotopic approach

We present the results of a whole-rock Nd isotopic study of two contrasting regions of the western Himalaya, using the neodymium model age approach on the scale of a single orogen. High-grade metasedimentary rocks from Zanskar yield model ages ( T DM ) that are similar to those of the High Himalayan Crystalline Series ( T DM = 1.2–2.0 Ga; ϵ Nd = −6 to −16) and distinct from values from the Lesser Himalaya ( T DM = 2.3–3.4 Ga; ϵ Nd = −18 to −27). Hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen. Data for the basement of the Nanga Parbat massif at the western extremity of the Himalaya ( T DM = 2.3–2.8 Ga; ϵ Nd = −18 to −30) suggest that these rocks are not equivalent to the High Himalaya, as previously supposed, but have affinities with the Lesser Himalaya. A thin metasedimentary cover sequence on the margins of the Nanga Parbat massif is isotopically indistinguishable from the High Himalaya ( T DM = 1.6–1.8 Ga; ϵ Nd = −10 to −14). The prior misidentification of the provenance of the massif stems from its high metamorphic grade, characteristic of the High Himalaya, but in this case related to the unique Neogene history of the Nanga Parbat massif, which has exhumed a higher-grade equivalent of the Lesser Himalaya that is not seen elsewhere.

The significance of Himalayan rivers for silicate weathering rates: evidence from the Bhote Kosi tributary

The significance of weathering by Himalayan runoff for both the Sr-isotope marine record and the removal of atmospheric CO 2 through silicate dissolution has been examined by systematic sampling of dissolved loads and bedloads from the Bhote Kosi, a tributary of the Ganges that rises in Tibet from Tethyan sediment bedrock and traverses the major Himalayan lithologies of eastern Nepal before debouching onto the Gangetic plains. Throughout the section, the cation geochemistry of water samples is dominated by Ca and Mg ions, suggesting that carbonates are the predominant lithology undergoing dissolution particularly within the Lesser Himalayas. As the river transects the metasedimentary and granitic lithologies of the High Himalayas the Sr-isotope ratio of the bedload rises rapidly, closely reflecting the isotope geochemistry of the bedrock. In contrast the 87Sr/ 86Sr ratio of the dissolved load remains roughly constant (0.719–0.723). Downstream of the Main Central Thrust, where the river transects the carbonate-bearing lithologies of the Lesser Himalayas, the 87Sr/ 86Sr ratio of the dissolved load rises sharply (>0.768). The relative contributions of silicate and carbonate weathering from each of the main Himalayan units has been estimated from major cation, Sr concentration and Sr isotope mass-balance equations. These calculations suggest that the high Sr and high 87Sr/ 86Sr characteristics of riverine analyses arise initially from a component dissolved from the Tibetan Sedimentary Series which is substantially enhanced by input weathering fluxes, particularly as the river traverses the Lesser Himalayas. Whilst mechanical erosion is maximised within the High Himalayan Crystalline Series, as confirmed by 143Nd/ 144Nd ratios from the bedload, at least 63% of the dissolved load is acquired by chemical weathering of bedrock lithologies and/or of transported particulates within the Lesser Himalayas, enhanced by higher ambient temperatures and slower discharge rates. This may involve continued dissolution of the High Himalayan Crystalline Series particulates in addition to Lesser Himalayan lithologies. Although Himalayan rivers collectively have a major influence on the Sr-isotope marine record, the high 87Sr/ 86Sr ratios of their dissolved load results from the mixing of a small component (<10%) of silicate-derived material with an unusually high 87Sr/ 86Sr ratio (0.75–1.0) and a large component (>90%) of carbonate-derived material some of which is characterised by a high 87Sr/ 86Sr ratio (up to 0.8). Elevated 87Sr/ 86Sr ratios in rivers are therefore not necessarily indicative of anomalously high dissolution rates of silicates.