Talat Ahmad

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.

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.

Controls on the 87Sr/86Sr Ratio of Carbonates in the Garhwal Himalaya, Headwaters of the Ganges

The episodic variation of the seawater 87Sr/86Sr ratio has been attributed to either variations in the Sr flux or the Sr‐isotopic composition of the riverine‐dissolved load derived from weathering of the continental crust. The discovery that Himalayan rivers are characterized by high concentrations of dissolved Sr concentrations with high 87Sr/86Sr ratios has raised the possibility that collisional orogens play a critical role in moderating the variations in seawater 87Sr/86Sr ratios. Here we describe Himalayan carbonates and calc‐silicates from Garhwal, the headwaters of the Ganges, with extreme 87Sr/86Sr ratios (>1.0). Elevated Sr‐isotope ratios result from exchange with Rb‐rich silicate material during both Himalayan and pre‐Himalayan metamorphic episodes, and the carbonates contribute a significant fraction to the Ganges 87Sr flux. Particularly elevated 87Sr/86Sr ratios are found in calc‐silicates from the Deoban Formation of the Lesser Himalaya. A detailed traverse of shales and calc‐silicates from this unit confirms that carbonate horizons have increased 87Sr/86Sr ratios as a result of isotopic exchange over length scales of 10–30 cm. We conclude that metamorphism of carbonates may cause elevation of their 87Sr/86Sr ratios and that uplift of metamorphosed carbonates may be a consequence of collisional orogens, which contributes to the elevation of seawater 87Sr/86Sr ratios.

Geochemistry and petrogenesis of late Archaean Aravalli volcanics, basement enclaves and granitoids, Rajasthan

Abstract Major, trace and rare earth element data are presented for the basal volcanics of the Aravalli supracrustal sequence, for the mafic enclaves within the Banded Gneiss Complex (BGC) adjacent to the supracrustals, and for granitoids within the BGC and their petrogenesis discussed with regard to the problematic relationship between basement and cover. In terms of major elements the Aravalli lavas range from magnesian (22% MgO) komatiites (or picrites) to Fe-rich tholeftes, but differ from a typical Archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements. Moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting. There are two distinct types of mafic enclave within the BGC. The larger lenses correspond quite well with the range of variation seen in the Aravalli sequence, and could be feeders; the other type, close to the BGC/Aravalli junction, has more alkalic characteristics (high Nb Zr ) and could represent rift volcanics, or tectonically accreted fragments of ocean islands. The data suggest that the Aravalli sequence is not an accreted oceanic terrane, but are more consistent with interaction of a deep mantle plume with rather mature sub-continental lithosphere. The early Archaean age of parts of the BGC, the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model: the character of the Aravalli sequence is intermediate between that of an Archaean greenstone belt and a Phanerozoic flood basalt province.

Geochemistry and petrogenesis of Garhwal volcanics: implications for evolution of the north Indian lithosphere

Abstract Middle Proterozoic magmatism in the Garhwal Himalayas consists of an extensive sequence of mafic volcanics plus associated mafic dyke sheets. Geochemical data reveal that the volcanic suite is tholeiitic, with the dyke sheets showing considerably more Fe-enrichment than the lava flows. All the rocks are distinctly enriched in incompatible elements and light REE relative to primordial mantle abundances, but have an equally distinct “continental” signature reflected in marked negative Nb, Sr, P and Ti anomalies in their mantle-normalised spidergrams. Although such geochemical characteristics are commonly thought to indicate contamination of the magmas with a continental crust component, the evidence is strongly in favour of the compositional characteristics being inherited from their mantle source. Other Proterozoic volcanics in northern India have closely similar geochemical features, as have other classic occurrences of younger continental flood basalt suites in adjacent areas of Gondwanaland. These geochemical characters are probably imposed upon the subcontinental lithosphere at the time of continent formation but, in the case of younger CFB suites, this may have been modified to a greater or lesser extent by subsequent additions of “plume” mantle material.

Geochemistry of Precambrian mafic magmatic rocks of the Western Himalaya, India: petrogenetic and tectonic implications

Abstract Precambrian sequences of the Higher Himalayan Crystallines (Vaikrita Group) and the Lesser Himalaya (Chail, Jutogh and Jaunsar Groups), in the Garhwal and Himachal regions of the Western Himalaya, include abundant metamorphosed mafic lavas and dykes (amphibolites). A gabbroic body within the Chail Group has been dated at 1907±91 Ma (initial 87 Sr/ 86 Sr ratio of 0.7022±0.0008) by the whole rock Rb–Sr method. This age is consistent with several age data (1800–2000 Ma: whole rock Rb–Sr method), assigned to the granitoids and gneisses of this region. These amphibolites and gabbros exhibit low-Ti tholeiite characteristics. The Lesser Himalayan samples are enriched in light rare earth elements (LREE) and large ion lithophile element (LILE), with distinct negative Nb, Sr, P and Ti anomalies. Conversely, the Vaikrita Group samples are characterized by less enriched LREE–LILE and absence of the above anomalies but have distinct positive Sr anomalies. The chemical characteristics of the Lesser Himalayan samples are remarkably similar to the basal Aravalli volcanics of the NW Indian shield, inferred to reflect significant components from enriched mantle sources and crustal contamination. Samples of the Vaikrita Group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative Nb, P and Ti anomalies. These distinct geochemical characteristics are used to demarcate the Main Central Thrust along the Vaikrita Thrust. The Chail and Jutogh Groups are affiliated with the Lesser Himalaya and the Chail Group gabbro body may represent portion of one among the numerous magma chambers that fed a large Precambrian magmatic province in the south of the Main Central Thrust. The similar emplacement ages (1800–2000 Ma) for the mafic and felsic magmatic rocks, suggest that this region experienced a major episode of crustal generation and evolution in a rift environment.

Tectonic implications of Palaeoproterozoic anatexis and Late Miocene metamorphism in the Lesser Himalayan Sequence, Sutlej Valley, NW India

Unravelling the kinematic evolution of orogenic belts requires that the defining tectonostratigraphic units, and structural elements that bound them, are properly identified and characterized. In the Sutlej Valley (western Himalaya), the Munsiari and Vaikrita thrusts have both been correlated with the Main Central Thrust. The sequence of amphibolite-grade rocks (the Jutogh Group) bounded by these faults has been variously assigned to the Lesser Himalayan Sequence (based on provenance ages) and to the Greater Himalayan Sequence (from its metamorphic grade). Trace-element and geochronological data from leucogranites in the Jutogh Group (1) indicate crustal melting at c. 1810 Ma, before the deposition of the Greater Himalayan Sequence, thus correlating the Jutogh Group with the Lesser Himalayan Sequence, and (2) record Proterozoic metamorphism overprinted at 10.5 ± 1.1 Ma (established from U–Pb analysis of uraninite) during the Himalayan orogeny. Pressure–temperature–time data indicate that the Jutogh Group and Greater Himalayan Sequence represent distinct tectonic units of the metamorphic core that were decoupled during their extrusion. This precludes extrusion along a single, widening channel, and requires a southward shift of the locus of movement during the Late Miocene, coincident with present-day precipitation patterns.

Geochemistry and petrogenesis of the basal Aravalli volcanics near Nathdwara, Rajasthan, India

The early Proterozoic Aravalli supergroup of Rajasthan near Nathdwara includes a thick basal unit of mafic volcanics with intercalated quartzites. The volcanics have been metamorphosed from greenschist to amphibolite facies which, together with intense deformation, obliterated original mineralogy and textures. Based on geochemical criteria we infer that the volcanics consist predominantly of tholeiitic rocks with minor amounts of komatiitic rocks which are referred to here as picritic because of the absence of textural criteria. Modelling of both major and trace element data suggests that the picritic magmas were generated by different extents of adiabatic melting of mantle sources over a range of P-T conditions, at pressures as high as 50 kbar. Their sources were enriched in light REE elements. The tholeiite series could not be related to the picrites either by fractional crystallization processes or by considering that the former represent lower extents of melting of sources similar to those for the latter under any physical conditions. The tholeiites require shallow lithospheric sources with enrichment in light REE and [Fe]/[Me] ratios which are significantly higher than those of a pyrolite. Addition of melts from deeper levels to the shallow source regions of the tholeite is considered a possible mechanism for the generation of tholeiitic magmas. Source characteristics and physical conditions of magma generation indicate that the basal Aravalli volcanism could be related to deep mantle plumes.

Geochemistry and petrogenesis of the mandidarla volcanics, northwestern Himalayas

Abstract The Proterozoic Mandi-Darla Volcanics (MDV) occur as flows intercalated with the low-grade metasediments of the Sundarnagar Group of the Lesser Himalayas. These volcanics are aphyric and have quenched textures with plagioclase-clinopyroxene skeletal microphenocrysts. They are of tholeiitic composition, and are enriched in FeO t , TiO 2 , incompatible trace and rare earth elements. They have relatively high SiO 2 abundances for their MgO levels, which are attributed to high PH 2 O pertaining during melt generation. Fractional crystallisation does not appear to have played any major role in the evolution of their bulk chemistry. Instead, they appear to have been generated by different extents of isobaric partial melting of a non-pyrolitic and heterogeneous source with enriched and variable Fe/Mg ratios. REE modelling suggests that the REE enriched source consisted of variable proportions of at least two end members differing in their MREE and HREE contents, but with similar LREE abundances. The enrichment of the source is attributed to mantle metasomatism by a melt-dominated phase penecontemporaneous with magma generation.

Bafliaz volcanics, NW Himalaya: origin of a bimodal, tholeiite and alkali basalt suite

The lower Palaeozoic (Ordovician ?) Bafliaz volcanics in the southern part of the NW syntaxial bend of the Himalaya yield information about the tectonic regime and the conditions in the subjacent mantle during the early Palaeozoic. The volcanics occur as a succession of flows; two intercalated sedimentary units (carbonates + clastics) indicate short periods of quiescence. The succession has been sampled systematically, and the chemistry shows a change from dominantly differentiated tholeiites in the basal flows through a mix of less differentiated tholeiites and a few alkali basalts in the middle part, to distinctly alkaline uppermost flows. The alkali basalts in the Bafliaz volcanics are the oldest known alkali basalts in the Himalaya. The tholeiites show chemical characteristics of low-Ti continental flood basalts whereas the alkali basalts show similarities with ocean island alkali basalts. High La/Yb and low Cr, Sc and Yb in alkali basalts indicate variable amounts of residual garnet and clinopyroxene during their source melting. A distinctive feature of the alkali basalts, however, is the negative Zr, Nb and P anomalies in their incompatible-element patterns, most likely reflecting a source feature of these rocks. Bulk chemistry suggests derivation of the two rock types from two different mantle sources by different degrees of melting followed by gabbro fractionation for the tholeiites and olivine + clinopyroxene fractionation for the alkali basalts. A plume tectonic setting for the volcanism is not favoured because of the small volume of the erupted magma. Instead, field relations favour a short-lived rift reactivation phase which induced asthenospheric melting, producing the theoleiites from shallow levels and the alkali basalts from relatively deeper levels.