Crystalline Sequence Research Articles

The Central American Seaway: Do Magdalena Flocs Mimic Deep Water Neodymium?

A common assumption for the interpretation of seawater Neodymium isotopes is that the signal is a mixture of distally advected waters. A review of the local paleogeography and bedrock Nd isotopic composition (n = 712) of the largest drainage basins of the Caribbean Sea shows: (a) unradiogenic sedimentary and crystalline sequences in the cratonic-derived eastern Andes; (b) very radiogenic sequences in the western Andes and Panama Isthmus; and (c) intermediate values in between them (Cauca River drainage basin). These local signatures can be matched to the Miocene-Pliocene record of Ocean Drilling Project sites ODP-998, 999, and 1000, dating the arrival of major inter-Andean drainages to the Caribbean Sea. The birth of the Atrato River at 15 Ma, following closure of the Central American Seaway, delivered very radiogenic Nd isotopes to the Caribbean Sea (εNd: +5.5), while arrival of Magdalena River waters at 11 Ma brought very unradiogenic values (εNd << −8). Later excavation of the Cauca River canyon at ∼7 Ma added a moderately radiogenic source (εNd: +1.9), modulating Magdalena River signatures to the Caribbean Sea. Flocculates forming in tropical deltas and estuarine environments carried these signals in hyperpicnal flows to slowly mixing Caribbean bottom waters, mimicking distally advected water.

Unveiling the geoheritage, cultural geomorphology and geotourism potential of Zanskar region, NW Himalaya, India

The Zanskar region, nestled in the heart of the Tethyan Himalaya, represents a significant geological and cultural repository that has remained relatively unexplored by the global tourism industry. This study investigates Zanskar's geoheritage and cultural geomorphology, highlighting its importance for geotourism and advocating for the implementation of sustainable development to preserve its fragile ecosystem. Flowing northward from the Higher Himalayas, the Zanskar River, one of the largest northeast-flowing tributary of the Upper Indus River Basin (UIRB), traverses the complex, strongly folded, and thrusted Zanskar Range via a steep, narrow 60-kilometer-long gorge. Zanskar has consistently garnered attention from geoscientists and tourists owing to its unique geographic location and remoteness. Many areas and routes in Zanskar are challenging to reach, but it stands out as a prime location where glaciers can be accessed by road, making it an ideal destination for studying geoscience. Geologically, the study area is part of the Zanskar Range, with its boundaries defined by the Ladakh Range to the northeast and the Great Himalayan Range to the southwest. The Indus Tsangpo Suture Zone, the Tethyan Sedimentary Sequence, and the Higher Himalayan Crystalline Sequence are exposed from north to south. Zanskar serves as a pristine field museum for studying young Himalayan orogenic tectonics, featuring prominent exposures such as the Zanskar Shear Zone (ZSZ)/South Tibetan Detachment System (STDS) and the structural outcrop of Zangla, showcasing a variety of folding types. These folds signify different phases of deformation resulting from the Indo-Eurasian collision. Moreover, it serves as a prime sequence of Quaternary Glacial Periods (QGP), providing an ideal natural laboratory and an open, practical classroom for geoscientists and Earth science students. We propose ten geoheritage sites in Zanskar, which encompass cultural, geomorphological, and geological attractions with potential for geotourism in this region.

Tectonic evolution of the Higher Himalayan Crystalline Sequence, Dhauliganga valley, Garhwal Himalaya: Insights from P–T conditions of metamorphism and partial melting

Metamorphism of pelites from the Higher Himalayan Crystalline Sequence (HHCS) in the Dhauliganga valley, Garhwal Himalaya has been studied. Two structurally lower lithounits of the HHCS, namely the Joshimath and Pandukeshwar Formations, contain kyanite-grade mineral assemblages with no evidence of partial melting and yield P–T values in the ranges of 7.8–10.3 kbar and 646–735 °C. The Badrinath Formation, which is the upper lithounit, shows partial melting and increase in P–T conditions from 5.5 to 7.0 kbar and 712–724 °C (sillimanite–K-feldspar grade) in its lower part to 10.5–11.9 kbar and 790–801 °C (kyanite–K-feldspar grade) in the upper part. Corona texture of plagioclase around relict kyanite in the uppermost part of the Badrinath Formation indicates decompression following peak metamorphism. Based on the sharp change in T/depth ratio (geothermal gradient), and diachroneity in peak metamorphism and exhumation from available geochronological data, a metamorphic discontinuity known as the Badrinath Shear Zone (BSZ) is suggested between the lower and upper Badrinath Formation. The upper HHCS, which comprises the upper Badrinath Formation, was tectonically extruded and juxtaposed over the lower HHCS along the BSZ. Decreasing trend of P–T conditions of peak metamorphism and younging of exhumation ages southward from the upper to lower HHCS are compatible with critical taper model for the exhumation of the HHCS.

Zircon U–Pb, Hf, and O isotopic constraints on the tectonic affinity of the basement of the Himalayan orogenic belt: Insights from metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India

Zircon U–Pb geochronology and Hf–O isotope geochemistry of metasedimentary rocks, orthogneisses, and leucogranites in Garhwal, NW India, provide new insights into the tectonic affinity of the basement of the Himalayan orogenic belt. Detrital zircon U–Pb ages from metasedimentary rocks in both the Lesser Himalayan Sequence (LHS) and the overlying Main Central Thrust (MCT) zone yielded dominant Paleoproterozoic ages with the youngest age peak of 1840 Ma. Along with that, the variable ɛHf (t) values (−14.56 to 8.44) indicate that these metasedimentary rocks could have been derived from the juvenile Paleoproterozoic orthogneiss basement in the Himalayas and/or the ancient Indian craton in the south. In contrast, the metasedimentary rocks in the Ediacaran Harsil Formation are dominated by the ca. 1200–800 Ma zircons with the youngest age peak of ca. 630 Ma and variable ɛHf (t) values (−16.57 to 13.32), which belong to the underlying High Himalayan Crystalline Sequence (HHCS), rather than the overlying Tethyan Himalayan Sequence. Orthogneisses in the MCT zone record ca. 1.87–1.85 Ga felsic magmatism associated with the Nuna supercontinent. The Bhagirathi leucogranites intruding the Harsil Formation were formed at ca. 496–490 Ma. These Paleoproterozoic orthogneisses and early Paleozoic leucogranites show only positive ɛHf (t) values (0 to +15) correlatable with a juvenile mantle input. However, the zircon Hf model ages older than the U–Pb ages and the oxygen isotope values (δ18O: 7.74–8.91 ‰), significantly higher than zircons crystallized from mantle-derived magma, indicate large degrees of melting and contamination of crust materials in both the granitoids. In combination with the pre-existing geochronological data, the current study indicates that the Garhwal HHCS is characterized by bimodal Neoproterozoic zircons at ca. 1100–900 Ma and ca. 860–800 Ma with wide εHf (t) values which are similar to those in the outer LHS and the Cathaysia Block (including the Jiangnan Orogen) of the South China Craton. These observations suggest that a close relationship existed between North India and South China during the Neoproterozoic since Tonian. The provenance of the Neoproterozoic HHCS is possibly a mixed source from both southern Gondwana terranes (e.g., east Antarctica) and northern South China.

Modern sedimentation on the eastern continental shelf of India: Assessing the provenance and sediment dispersal pattern

Sediments deposited on the eastern continental shelf of India are studied for texture, mineralogy and their major, trace, and rare earth elemental composition in lithogenic fraction. The objective of this study was to identify the sources, and evaluate the factors affecting the erosion and weathering in the catchment area and dispersal and deposition of sediments over the shelf region. Here, we present an extensive dataset covering the entire eastern continental shelf of India from the Ganges-Brahmaputra (G-B) basin in the north including the shelf off major peninsular Indian river basins of Mahanadi, Godavari, and Krishna up to the Kaveri basin in the southern tip.Textural and geochemical studies have shown that the lithology of the source rocks plays an important role in governing the chemistry and mineralogy of the sediments. The results show that the G-B shelf sediments are primarily derived from the Higher Himalaya Crystalline sequence (HHCS) and the Tethyan sedimentary series (TSS) exposed along the peaks and drainage divide of the Himalayas. Mahanadi shelf sediments are mainly derived from the felsic rocks belonging to Precambrian metamorphic formations of the Eastern Ghats, limestones, sandstones, and shales of the Gondwanas and recent deltaic alluvium deposits, whereas the Krishna-Godavari (K-G) sediments find a part of their source in the Deccan Basalts. The shelf sediments off River Kaveri are derived from a mixture of peninsular granitic gneisses, tonalite-trondjhemite gneisses, and charnockites. The sediment geochemistry has also allowed us in assessing the weathering patterns in the source areas. The sediments have undergone a moderate degree of weathering. Weathering intensities were found to have varied with the source area lithology.The least square regression of the geochemical data allowed us to estimate the relative contribution of each of these sources and their geographic extent. Unlike the massive offshore deposition and formation of a major Fan system (the Bengal fan), the fluvial sediment dispersal of the mighty Himalayan Rivers on the continental shelf is significant only at the G-B mouth and their contribution reduces to <50% at about 18oN. The sediment dispersal from the Godavari River is seen to extend on either side of the mouth of the river which is attributed to the reversing East India Coastal Current (EICC), which plays an important role, in the sediment transport along the central and south-central part of the east coast of India. The influence of the Mahanadi, Krishna, and Kaveri River sediments is however confined to a small area near their mouths and has a limited geographic extent of influence in the shelf region.

Cumulate rocks of the Dumak ophiolitic mélange, Eastern Iranian Ranges: Composition, origin and tectonic implications

The Eastern Iranian Ranges are considered to be either a suture zone produced by closure of a Neo-Tethyan back-arc basin between the Lut and Afghan blocks or an oroclinal buckling of multiple terranes accreted to the active margin of the Neo-Tethyan Ocean. Both models are based on the presence of Cretaceous ophiolite complexes and sequences of Eocene turbiditic sedimentary rocks. The Dumak ophiolitic mélange, a significant ophiolitic assemblage that crops out in the western portion of the orogen adjacent to Lut, contains all the essential elements of a typical ophiolite in a matrix of serpentinite and clay-rich sediments. Ultramafic-mafic cumulates in the mélange are characterized by medium, non-rhythmic bedding in limited outcrops where they are in tectonic contact with other ophiolitic units. The cumulates consist of plagioclase-bearing dunite, troctolite, Cpx-troctolite and gabbro composed chiefly of olivine, plagioclase, and clinopyroxene accompanied by rare orthopyroxene. This assemblage is very similar to the differential crystallization sequence of tholeiitic magma at modern mid-ocean ridges. The clinopyroxene in these rocks is diopside (En = 47–49) and the plagioclase is bytownite (An = 71–77). Whole-rock geochemistry of samples from the crustal sequence of Dumak mélange are characterized by low TiO2 (0.03–0.17 wt%). Investigation of this crystalline sequence and geochemical properties of the rocks suggests that they can be considered as the low-Ti ophiolite originated from mid-ocean ridges. Additionally, the positive Eu anomalies as well as comparison of frequency of LREE and HREE in the mafic and ultramafic samples indicate the cumulates formed by fractional crystallization and differentiation of mantle-derived magmas. According to petrographic, geochemical, and structural evidence, it is possible that the Dumak ophiolite, after being formed or displaced between Lut and Afghan blocks, was firstly accreted to the south of the Lut block and then re-mixed into Eocene sediments emplaced in the current position.

Long-lived magmatic evolution and mineralization resulted in formation of the giant Cuonadong Sn-W-Be polymetallic deposit, southern Tibet

The Cuonadong Sn-W-Be polymetallic deposit is the first Cenozoic leucogranite-related rare-metal deposit with giant metallogenic potential in the Himalayan orogen. However, the controlling factors for the supernormal enrichment of beryllium, tin and tungsten in this deposit remain vague. In this study, we carried out systematic geochronological, whole-rock geochemical, and Sr-Nd isotopic analysis for the Cuonadong leucogranites, as well as detailed ore-forming geochronological study for the rare-metal mineralization. The monazite U-Th-Pb, cassiterite U-Pb and muscovite Ar-Ar dating results, together with previously reported geochronological data, indicate that the major Cuonadong leucogranites (including, from old to young, the weakly-oriented two-mica granite, two-mica granite and the muscovite granite) were formed during ∼ 21–15 Ma, whereas the Sn-W-Be mineralization mainly occurred at ∼ 18–14 Ma. The Cuonadong leucogranites show strong peraluminous (A/CNK = 1.09–1.22) features, and have high SiO2 (71.62–75.97 wt%) and Al2O3 (14.04–16.09 wt%) and low MgO (0.07–0.33 wt%), MnO (0.01–0.15 wt%) and total iron (Fe2O3T = 0.36–1.01 wt%) contents, and are enriched in large ion lithophile elements (e.g., Rb, U, K, and Pb). These geochemical features, together with enriched Sr-Nd isotopes (εNd(t) = -15.7 to −11.7; (87Sr/86Sr)i = 0.71957–0.76313), indicate that the Cuonadong leucogranites belong to S-type granite and were derived from muscovite-induced dehydration melting of metapelites of the Higher Himalayan Crystalline Sequence. Perceptible linear variations of some major elements (e.g., Na2O, K2O, MnO, Fe2O3T, TiO2 and A/CNK) with increasing Rb/Sr ratios suggest that these leucogranites experienced different degrees of evolution. Quantitative simulation calculations based on the whole-rock Rb, Sr, and Ba contents imply that the Cuonadong leucogranites experienced increasingly-strong fractional crystallization of plagioclase, K-feldspar and biotite from the weakly-oriented two-mica granite through the two-mica granite to the muscovite granite. Importantly, intense fractional crystallization leaded to notable enrichment of Sn, W and Be, although these elements are not obviously high in the relatively primitive magma for the Cuonadong leucogranites. Significantly, evident REE tetrad effects and deviation of twin-element pair ratios (e.g. K/Rb, K/Ba, Zr/Hf, Nb/Ta, and Y/Ho) from the chondritic values demonstrate that intense interaction between melts and volatile-rich aqueous fluids occurred during magmatic evolution. This implies that the Cuonadong leucogranites were derived from a volatile-rich magmatic system. The abundant volatiles probably facilitated occurrence of fractional crystallization and extended duration of this process through lowering the solidus and viscosity of the magma. Thus, we propose that the long-lived fractional crystallization (21–15 Ma) and rare-metal mineralization (18–14 Ma) collectively leaded to supernormal enrichment of Sn, W, and Be in the Cuonadong Sn-W-Be polymetallic deposit. In contrast, the enrichment of these rare-metal elements was insignificant during partial melting.

The Fate of Pyroxenes in Mafic Xenoliths from the Kinnaur Kailash Granite, Sutlej Valley, NW Himalaya: Effect of Retrograde Hydration and Insights on the Rare Occurrence of High-Grade Metamorphic Rocks in the Himalayan Orogen

Abstract Pseudosection modelling of post-peak mineral assemblages in metamorphosed mafic xenoliths from the Kinnaur Kailash Granite, Northwest Himalaya, has provided new insights into pre-Himalayan metamorphism and constrained the P–T conditions of hydration. P–T and P–MH2O pseudosections constructed in the NCFMASH system show that retrograde hydration occurred during destabilisation of orthopyroxene and clinopyroxene in mafic granulites after decompression to 0.3 GPa pressure, and as a result, water-containing phases such as cummingtonite and hornblende formed. Results from two of the studied xenoliths indicate that the secondary mineral assemblage is controlled not only by P and T, but also by the amount of water available for hydration. In contrast, another example indicates the development of cummingtonite from hornblende via dehydration reaction after decompression to conditions appropriate for cummingtonite stability. The study shows that the Higher Himalayan Crystalline Sequence (HHCS), from which the xenoliths have been derived, must have been subjected to pre-Himalayan high-grade metamorphism, although it is not evident because of pervasive overprinting by Himalayan metamorphism. The present-day metamorphic pattern of the HHCS is expectedly the result of both pre-Himalayan and Himalayan metamorphism, and careful petrological investigation is required to distinguish their signatures so that a better understanding of the tectonothermal evolution of the Himalayas can come into picture. Isotopic dating of the xenoliths is required to establish the timings of the pre-Himalayan metamorphism and decompression.

Geochemistry and U–Pb zircon geochronology of the Jutogh Thrust sheet, Himachal Pradesh, NW‐Himalaya: Implications to the petrogenesis and regional tectonic setting

We present datasets of the whole‐rock geochemistry and U–Pb zircon geochronology from the Jutogh Thrust (JT) sheet of eastern Himachal Pradesh, NW Himalaya, in order to explain the tectono‐magmatic environment in which they formed. The metapelite and granitoids of the JT sheet have moderate to high SiO2, Al2O3, Na2O, K2O, and rare earth elements (REE). The A/CNK (Al2O3/CaO + Na2O + K2O) ratio with magnitudes ≥1, indicates the peraluminous nature, calc‐alkaline series, and S‐type granitoids. The high values of Rb and Th as compared to Nb and the negative Ba anomaly also suggest the dominance of crustal source material. The distinct enrichment of large‐ion lithophile elements of Rb, Th, K, Ba and Pb and strong depletion in high‐field‐strength elements (HFSE) of Zr, Nb and Ti results in high incompatible elements/HFSE ratios, which suggest that the source is derived from many intra‐crustal tectonic environments. All samples are well‐fractionated REE patterns [(La/Yb)N = 6.06–17.37] and characterized by moderately flat enriched [(La/Sm)N = 2.31–4.71] of light rare earth elements. The negative Eu anomalies (Eu/Eu* = 0.18–0.71), suggest the plagioclase fractionation from the sourced magma and the origin of granitic gneiss through the high temperature (~800°C). The zircon U–Pb ages of granite range between 883.0 ± 15.0 Ma and 1260.0 ± 10.0 Ma, with a consistent peak of ~925.0 Ma. It confirms that the northern edge of the Indian Plate collided with an unnamed microcontinent. Also, in a contemporaneous process, the upper crust underwent partial melting, and the generated magma intruded in an extensional tectonic set‐up of the northern edge (i.e., presently known as the Jutogh and Vaikrita thrust sheets) of the Indian Plate during the Neoproterozoic. The available zircon U–Pb ages of the JT sheet range between 550–2750 Ma. The magmatic nature of zircons, with two peaks of ~825 Ma and ~910 Ma, suggests that episodic magmatic processes have taken place during the Neoproterozoic. This study revealed that the Higher Himalayan Crystalline Sequence was a meta‐sedimentary sequence and was deposited in an active margin set‐up of the northern edge of the Indian Plate.

Eocene to Miocene metamorphic evolution and tectonic implication of the Ilam Nappe in Nepal Himalaya: Constraints from P–T conditions and monazite petrochronology

The Ilam Nappe, consisting of the High Himalayan Crystalline Sequence (HHCS) in far-eastern Nepal, was largely emplaced southward along the Main Central Thrust (MCT). Here we present the P–T–t path of the Ilam Nappe using conventional geothermobarometers, pseudosection modelling, and monazite petrochronology. Amphibole-bearing migmatite just above the MCT experienced prolonged fluid-present melting from prograde metamorphism at 39–31 Ma to peak upper amphibolite-facies metamorphism (694 °C and 10.3 kbar) during 24–19 Ma, followed by cooling. Kyanite-sillimanite migmatites record prograde metamorphism at 35–28 Ma and peak lower granulite-facies metamorphism (738–765 °C and 8.3–9.7 kbar) under kyanite stability via the muscovite dehydration reaction at 25–20 Ma, followed by isothermal decompression under sillimanite stability. The clockwise P–T path and Early-Middle Miocene peak metamorphism of the Ilam Nappe can be correlated to those in the lower-middle HHCS hinterland. The Eocene-Oligocene prograde metamorphism with a low geothermal gradient of 15–25 °C/km in the Ilam Nappe and the entire HHCS hinterland could represent the earlier crustal thickening accompanied by large-scale overthrusting before the normal faulting of the South Thrust Detachment System (STDS), followed by diachronous extrusions of the upper HHCS along the High Himal Thrust, and of the lower-middle HHCS along the MCT with nappe emplacement. The lower granulite-facies Ilam Nappe was exhumed during the Early-Middle Miocene from the deeper or lower structural level of the HHCS, compared to the earlier Oligocene exhumation of the upper amphibolite-facies Karnali and Kathmandu klippes from the shallower or uppermost structural level of the HHCS.

Palaeoproterozoic S‐type granites from Garhwal Himalaya, NW India: Geochemistry, Sm–Nd isotope systematics and tectonic implications

A combined geochemical and Sm–Nd isotopic study on the Palaeoproterozoic (1,845 Ma) granites of the Lesser Himalayan Crystalline Sequence (LHCS) in the Garhwal region of NW India has been done in the present study. These granite samples are characterized by high silica, alumina, and potash and belong to a peraluminous to strongly peraluminous series, having molar A/CNK values of 1.01 to 2.4. The low P2O5 contents and its negative correlation with SiO2 presiding out that the granites have S‐type affinity, also supported by various classification diagrams (ACF; SiO2 vs. P2O5, Na2O + K2O‐CaO, and Th). The concentration of trace elements Ba, Sr, Nb, and Ti are low, and Rb, Th, U, and Pb are found to be high. The granites have low total rare earth elements contents of 56.19–229.16 ppm with enrichment in Light rare earth elements (LREE) ([La/Yb]N = 1.61–15.08) and negative europium anomaly (Eu/Eu* = 0.12–0.31). Sm–Nd isotope studies were also performed for three granite samples. Estimated model ages as 2.5–2.7 Ga, indicates the contribution of the Archean crustal substrate as their protolith source. Therefore, we assume that the melting of metasedimentary rocks with Archean protolith can form these peraluminous granites in an accretional–collisional event, during the Palaeoproterozoic on the western flank of the Columbia supercontinent.

Geochemical and Boron Isotopic Evidence that Tourmaline Records Country Rock Assimilation of Leucogranites in the Himalayan Orogen

AbstractTourmaline geochemical and boron (B) isotopic compositions in two‐mica granites (TMG), tourmaline‐bearing leucogranites (Tou‐LG), tourmalites and metapelites from the Gyirong–Malashan areas of the Himalayan orogen provide evidence for country rock assimilation during the intrusion of Himalayan leucogranite. The schorls in Gyirong leucogranitic plutons show low contents of MgO (0.238%–1.160%) and δ11B values (–12.1‰––11.2‰), while dravites gathered in the contact zone between the leucogranitic veins and metapelites show high contents of MgO (4.815%–6.755%) and δ11B values (–10.7‰––9.3‰). This geochemical and isotopic variation of tourmalines can also be identified in the Malashan gneiss dome. As a result, three types of tourmaline were identified in the Himalayan orogen: (1) Tou‐I in the TMG and Tou‐LG, which is the most common tourmaline type of schorl; (2) Tou‐II (dravite and high‐Mg schorl) in the Tou‐LG and tourmalite at the margins of the leucogranite; and (3) Tou‐III (mainly dravite, with minor high‐Mg schorl) in metapelites of the High Himalayan Crystalline Sequence. The lenses and veins of Tou‐LG may have experienced metasomatism and assimilation as a result of interaction with the High Himalayan Crystalline Sequence metasedimentary country rocks, which can be traced by the geochemical and isotopic characteristics of the tourmaline therein.

Review of the Geology of the Arun-Tamor Region, Eastern Nepal: Present Understndings, Controversies and Research Gaps

Systematic study of the eastern Nepal Himalaya was started after 1950 when Nepal opened up for foreigners. Thereafter, several geological studies have been carried out in the Arun-Tamor region of eastern Nepal Himalaya. The Tibetan-Tethys sedimentary sequence, the Higher Himalayan amphibolite to granulite facies metamorphic crystalline sequence, the Lesser Himalayan sedimentary and greenschist facies metasedimentary sequences, and the Siwalik foreland molassic sedimentary sequence are the four major tectonic units of this area. The individual nomenclature schemes of stratigraphic units, the correlational dispute, the positions and interpretations of regional geological structures are some examples that have created controversies regarding the lithostratigraphy and structural arrangements. The difference in age and genesis of the Main Central Thrust and its effects in the metamorphism of the eastern Nepal Himalaya are the exemplification of the contradiction in the interpretation of the tectonometamorphic history. There is a gap in research in the tectonics and episodic metamorphic evolution of the area owing to the bare approach in the microstructural and geochronological investigation. Future investigations should be focused on solving the above mentioned controversies and narrowing down the research gaps in tectonic and metamorphic evolution.

Tectonic discontinuity, partial melting and exhumation in the Garhwal Himalaya (Northwest India): Constrains from spatial and temporal pressure-temperature conditions along the Bhagirathi valley

The pattern and timing of tectonic discontinuity and partial melting in the Garhwal Himalaya, resulting from the India–Asia collision, are crucial factors for understanding the evolution of the Himalayan orogen, but these parameters remain poorly constrained. Middle- to high-grade metamorphic rocks from the Main Central Thrust (MCT) zone and the High Himalaya Crystalline Sequence (HHCS) along the Bhagirathi valley in northwest India have been investigated in this study. The primary mineral assemblage of the gneisses and migmatites in the HHCS is garnet + biotite + white mica + quartz + kyanite + rutile ± ilmenite ± sillimanite. The metamorphic grade at thermal peak rapidly increases upwards across the MCT from garnet schists (515–550 °C, 8–9 kbar) in the footwall to garnet-bearing migmatite (705–735 °C, 10–13 kbar) in the hanging wall. The P–T condition decreased upwards in kyanite gneiss of the lower HHCS (690 °C, 8.3 kbar), yielding a high field pressure gradient near the MCT. The prominent metamorphic discontinuity could have been caused by the southward extrusion of the HHCS along with the MCT. The base of the HHCS experienced high-grade metamorphism at 38–25 Ma, mainly at 31.9 ± 0.5 Ma. The new data, combined with previous studies, suggest that the prominent metamorphic discontinuity is characterized by the different periods of peak metamorphism and the overprint in the hanging wall (Eocene–Oligocene metamorphism and Early Miocene overprint) and footwall (Early Miocene metamorphism and Late Miocene to Pliocene overprint) of the MCT. The second metamorphic discontinuity occurred near the migmatite zone in the lower-middle HHCS, which experienced the anatexis (690–740 °C, 7–9 kbar) at 37–25 Ma, mainly at 33.9 ± 1.2 Ma. The Bhagirathi HHCS experienced a regional Barrovian-type Eohimalayan metamorphism associated with fluid-present melting of muscovite + plagioclase + quartz + H2O → melt ± kyanite at 34–32 Ma without a strong Miocene overprint. The period of partial melting is significantly older than the intrusion of ca. 22 Ma Early Miocene Gangotri leucogranites at the top of the HHCS. Protracted periods of crystallization (38–22 Ma) indicate a long duration of more than ca. 15 Myr of anatexis, from initiation of migmatization to pluton formation in the Bhagirathi HHCS. The protracted fluid-present melting and subsequent rapid exhumation in the Bhagirathi HHCS could be associated with heterogeneous extrusion forming the tectonic discontinuity in the lower-middle HHCS. The metamorphic grade decreased towards the upper HHCS (620 °C, 6.5 kbar) across the Jhala Normal Fault where occurs in the right-way-up sequence. On the other hand, in the central Himalaya, the thrusting occurred between upper HHCS and lower-middle HHCS in the inverted metamorphic sequence. The Early Miocene migmatites in the footwall of the significant tectonic discontinuity are dominant, and diachronous extrusion accompanied with the fluid-absent melting reactions is the common mechanism to exhume the HHCS, unlike the heterogeneous extrusion in the northwest Himalaya.

Petrogenesis of highly fractionated leucogranite in the Himalayas: The Early Miocene Cuonadong example

There has been considerable debate concerning the petrogenesis of Cenozoic highly felsic, strongly peraluminous, leucogranites within the Himalayan–Tibetan orogen, with two contrasting viewpoints regarding their origin, either as primary melts from parting melting or magmatic differentiation. This debate hampers our understanding of the tectono‐magmatic evolution and associated uplift and exhumation processes during Himalayan orogenesis. This study attempts to resolve this debate by means of an integrated petrographic, geochronological, and geochemical study on the Cenozoic Cuonadong leucogranites (including two‐mica granites, garnet‐bearing granites, and tourmaline‐bearing granites) in the eastern part of the south Himalayan–Tibetan orogen. One SHIRIMP zircon U–Pb dating of two‐mica granites and four LA–ICP–MS monazite U–Th–Pb dating of all three types of granites yield ages of ca. 21.9–19.7 Ma. Combined with previous published continuous U–Th–Pb zircon/monazite ages varying from 16.5 to 21.8 Ma, indicating that Cuonadong leucogranites have experienced continuously multiple episodes of intrusion and long‐term crystallization from ca.16 to 22 Ma, and lasted for ca. 6 Myrs. Major and trace element geochemistry combined with whole‐rock Sr–Nd isotope data suggest that the Cuonadong's three types of leucogranites are highly fractionated and garnet‐ and tourmaline‐bearing granites are more evolved. The Cuonadong leucogranite melts are mainly derived from metapelites within the High Himalayan Crystalline Sequence, and underwent fractional crystallization of biotite, monazite, and plagioclase during melt migration. This study indicates Cuonadong leucogranites are Cenozoic highly fractionated strongly peraluminous leucogranites, which, combined with previous intermittent documentations of fractionated leucogranites, point to an increasing maturation of the continental crust composition during continental orogenesis.

Geochemistry of Granites from Chail Group of Garhwal Region, Lesser Himalaya, NW India

Paleoproterozoic granites are well exposed in the Chail group of Garhwal region, Lesser Himalaya crystalline sequences (LCHS). These granites are less studied in terms of geochemical classification and tectonic settings. In the present work, we carried out the geochemical analysis of granites of the Chail group from the Chirbatiya-Khal and Ghuttu areas. All the samples have high SiO2 (73.24 ̶79.1 wt %), Al2O3 (11.2 ̶12.95 wt %), K2O (3.8 ̶5.9 wt %) and low P2O5 (0.11 ̶0.24 wt %), CaO (0.21 ̶1.02 wt %), and Na2O (2.2-3.03 wt %; exceptionally low in one sample, that is 0.009 wt %) contents. The A/CNK values for the samples are range from 1.19 to 2.91, characteristic of S-type granites. REE patterns for these granites are moderately fractionated with an average (La/Yb)N ̴8.21 and europium anomaly (Eu/Eu*) ̴0.15. The tectonic settings of the studied granite suggest that they are formed in syn-collision tectonic environments.

The genetic landscape of crystallins in congenital cataract

BackgroundThe crystalline lens is mainly composed of a large family of soluble proteins called the crystallins, which are responsible for its development, growth, transparency and refractive index. Disease-causing sequence variants in the crystallins are responsible for nearly 50% of all non-syndromic inherited congenital cataracts, as well as causing cataract associated with other diseases, including myopathies. To date, more than 300 crystallin sequence variants causing cataract have been identified.MethodsHere we aimed to identify the genetic basis of disease in five multi-generation British families and five sporadic cases with autosomal dominant congenital cataract using whole exome sequencing, with identified variants validated using Sanger sequencing. Following bioinformatics analysis, rare or novel variants with a moderate to damaging pathogenicity score, were filtered out and tested for segregation within the families.ResultsWe have identified 10 different heterozygous crystallin variants. Five recurrent variants were found: family-A, with a missense variant (c.145C>T; p.R49C) in CRYAA associated with nuclear cataract; family-B, with a deletion in CRYBA1 (c.272delGAG; p.G91del) associated with nuclear cataract; and family-C, with a truncating variant in CRYGD (c.470G>A; W157*) causing a lamellar phenotype; individuals I and J had variants in CRYGC (c.13A>C; T5P) and in CRYGD (c.418C>T; R140*) causing unspecified congenital cataract and nuclear cataract, respectively. Five novel disease-causing variants were also identified: family D harboured a variant in CRYGC (c.179delG; R60Qfs*) responsible for a nuclear phenotype; family E, harboured a variant in CRYBB1 (c.656G>A; W219*) associated with lamellar cataract; individual F had a variant in CRYGD (c.392G>A; W131*) associated with nuclear cataract; and individuals G and H had variants in CRYAA (c.454delGCC; A152del) and in CRYBB1 (c.618C>A; Y206*) respectively, associated with unspecified congenital cataract. All novel variants were predicted to be pathogenic and to be moderately or highly damaging.ConclusionsWe report five novel variants and five known variants. Some are rare variants that have been reported previously in small ethnic groups but here we extend this to the wider population and record a broader phenotypic spectrum for these variants.

Tuning layered superstructures in precision polymers

An approach to influence and control layered superstructures by varying the methylene sequence length between two consecutive functional groups in linear precision polymers containing 2,6-diaminopyridine (DAP) groups is presented. Layered superstructures with repeating units involving three monomeric units along the chain direction with very high coherence lengths upto 110 nm are observed in case of shorter alkyl segments, (16 and 18 {hbox {CH}}_{2} units), while more conventional layer superstructures incorporating only one monomer are found for related polymers with 20 {hbox {CH}}_{2} units per methylene sequence. A building block model explaining the unusually large periodicity of three monomeric units is proposed wherein layers containing crystalline or amorphous methylene sequences occur in different combinations. Occurrence of different layered structures depending on crystallization conditions, methylene sequence length as well as functional group type is explained by a competition of H-interactions between the DAP groups and the van der Waal forces between the hydrophobic methylene groups.

Spodumene pegmatites from the Pusila pluton in the higher Himalaya, South Tibet: Lithium mineralization in a highly fractionated leucogranite batholith

The Himalayan leucogranite belt is evaluated as a prospect for rare-metal mineralization. The Pusila pluton is located 44 km northwest of Mount Everest, and consists of two-mica granite, muscovite granite, albite granite, and pegmatites. Numerous pegmatite dikes have intruded the Higher Himalayan crystalline sequence and the granites. Oligocene pegmatites in the Chomogu and Cuore areas, which we constrain to have formed at ca. 25–23 Ma, were investigated in this study, because they are strongly enriched in Li (up to 8460 ppm) and contain abundant spodumene±petalite. Lithium was progressively enriched from the two-mica granite to the muscovite granite, and finally Li mineralization occurred in the highly fractionated pegmatites. The Chomogu No. 1 pegmatite is a well-zoned dike that hosts beryl, columbite, cassiterite, and Li minerals, which are concentrated in the inner parts of the dike. Scanning electron microscopy and cathodoluminescence images reveal two stages of Li mineralization. Primary spodumene (Spd-1) crystallized in the magmatic stage, whereas secondary spodumene (Spd-2) and petalite formed later in the hydrothermal stage. The Pusila pluton is thus an important site of Li mineralization in the Himalayan leucogranite belt.

The absence of high-pressure metamorphism in the inverted Barrovian metamorphic sequences of the Arun area, eastern Nepal and its tectonic implication

The metamorphic pressure–temperature (P–T) conditions across the Main central thrust (MCT) in the Arun area have been investigated. The MCT marks the tectono-metamorphic boundary between the overlying high-grade High Himalaya crystalline sequences (HHCS) and the underlying low-grade Lesser Himalaya sequences (LHS). The metamorphic rocks regionally preserve an inverted Barrovian sequence (i.e., intermediate P/T type metamorphism) devoid of previously reported high-pressure metamorphism. The metamorphic grade increases upwards from 670–740 °C and 6.9–9.4 kbar in the MCT zone and lower HHCS to 760–835 °C and 10.0–11.1 kbar in the middle HHCS. Orthoamphibole gneisses in the middle HHCS yield prograde Barrovian-type metamorphism, such as staurolite inclusions in garnets, showing an intermediate P/T gradient. The differences in the tectonic setting and metamorphic evolution imply that the metamorphic units in the Arun area do not correspond to the other high-pressure units in eastern Himalaya. Zircon and monazite U–Pb ages from kyanite gneiss of the lower HHCS reveal the MCT activity, associated with fluid-present anatexis, at ca. 20–14 Ma. Furthermore, similar K–Ar white mica ages (ca. 13–7 Ma) in the hanging wall and footwall of the MCT could represent the timing of later deformation events in shear zones or cooling, possibly associated with exhumation accompanied by activities on younger, structurally lower thrust faults such as the lower MCT. The similar P–T conditions near the MCT in this area could result from recrystallization during syn-metamorphic thrusting, whereas the middle HHCS away from the MCT preserve the original Barrovian metamorphic sequences related to crustal thickening. This and previous studies imply that different P–T profiles near the MCT according to each transect observed in Nepal could be apparent and the cumulative result of polyphase metamorphism.