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Palaeomagnetic studies in the Malani Igneous Suite (ca. 770-750 Ma) of the Marwar Crustal Block, NW India, provide essential constraints on palaeogeographic reconstructions of the Rodinia Supercontinent. The Malani Igneous Suite is intrusive into megacrystic granite and granite-gneissic enclaves of the Marwar Crustal Block. A crustal-scale NE-SW ductile transpressional shear zone with a southeasterly dip known as the Phulad Shear Zone (820-810 Ma) separates this Marwar Crustal Block from the rocks farther east. The pre-shearing tectonic evolution of the Marwar Crustal Block is poorly understood. Three phases of ductile deformations (D1, D2, and D3, with associated fabrics S1, S2, and S3) were identified in the Marwar Crustal Block. The D1 deformation is restricted to enclave gneisses. The megacrystic granite was emplaced syn-tectonically during D2 deformation. The S2 magmatic foliation (NNW-SSE) in the megacrystic granite is oblique to the Phulad Shear Zone. The D3 deformation in the megacrystic granite is synchronous with the Phulad Shear Zone mylonites. Another porphyritic granite (Phulad granite, ca. 820 Ma) emplaced syn-tectonically during D3 deformation along and across the Phulad Shear Zone. U-Pb zircon dates in the Marwar Crustal Block document ca. 890 Ma and ca. 860 Ma magmatic dates. U-Pb-Th monazite dates in the Marwar Crustal Block show a strong peak at ca. 820 Ma. By integrating critical field relations, deformational patterns, and geochronology, we argue that the Marwar Crustal Block shows distinct geological evolution prior to its accretion with the remaining parts of India. We propose that the accretion of the Marwar Crustal Block must be younger than ca. 860 Ma and culminated during ca. 820-810 Ma to form the Greater India landmass along the Phulad Shear Zone. [ABSTRACT FROM AUTHOR]

The South China Craton comprises the Yangtze and Cathaysia blocks and is one of the largest Precambrian continental blocks in East Asia. However, the early geological and geographical evolution of the Cathaysia block is relatively poorly understood, due to the sparse exposure of pre-Neoproterozoic rocks and reworking during Phanerozoic polyphase magmatism and metamorphism. In this contribution, we carried out detrital zircon U-Pb geochronology and sedimentary analyses on five Proterozoic meta-sedimentary rocks collected from the northeastern Cathaysia block, which belong to the previously defined Chencai, Mayuan, and Mamianshan Groups (strata). LA-ICP-MS U-Pb dating results of the detrital zircons show various ~1.85–1.35 Ga maximum depositional ages. They are significantly older than the previously constrained Neoproterozoic formation ages of these Proterozoic strata of northeastern Cathaysia, suggesting that their deposition and formation were probably initiated as early as the late Paleoproterozoic. Provenance analyses reveal that the late Paleoproterozoic to early Mesoproterozoic detrital zircons with igneous-origin were derived from in situ contemporary crystalline basements in eastern Cathaysia. In addition, by implication, the easternmost part of Cathaysia was probably an emerged area (i.e., the "proto-Cathaysia Land") under active erosion. It had a ~NWW orientation and provided detrital sediments to the neighboring marine basin (i.e., the Cathaysia Sea) during the late Paleoproterozoic to early Mesoproterozoic. Finally, the Paleoproterozoic evolution of Cathaysia was involved in the assembly of the Nuna supercontinent. Our results, together with the published data, reveal a distinct late Paleoproterozoic (~1.8 Ga) detrital zircon age peak, which seems to support the view that eastern Cathaysia had close tectonic affinities with terranes such as the Precambrian terranes of current northern India, in the framework of the Nuna supercontinent reconstruction. [ABSTRACT FROM AUTHOR]

• The Sandmata Complex preserves Neoarchaean protolith ages of 2.68–2.42 Ga. • Zircon-monazite ages indicate peak granulite facies metamorphism at 1.90–1.78 Ga. • Sandmata quartzofeldspathic gneisses record an isobaric cooling P–T path. • The complex was tectonically active up to the Neoproterozoic (∼0.90–0.78 Ga). The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Bt matrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny. [ABSTRACT FROM AUTHOR]

• Two-stage clock wise P-T evolution of UHT sapphirine granulites in Madurai block. • U-Pb zircon-monazite-rutile-apatite geochronology record heating/cooling episodes. • Evidence for long-lived Neoproterozoic UHT orogeny and subsequent slow cooling. • Identification of Kambam UHT belt as an important tectonic boundary within SGT. The Precambrian granulite terrane of south India is known for the preservation of regional-scale high to ultrahigh-temperature (HT-UHT) metamorphic rocks. In the Madurai block, the largest crustal block in the terrane, a significant proportion of these HT-UHT rocks occur along a narrow belt referred to as the Kambam ultrahigh-temperature belt. Understanding the P-T-t evolution of these HT-UHT granulites is vital in decoding the spatial and temporal evolution of the Madurai block in relation to other crustal blocks. Here we present the petrology, mineral chemistry, phase equilibrium modelling, and accessory mineral (zircon, monazite, rutile and apatite) geochronological and geochemical data for a hitherto unreported sapphirine granulite from the Kambam ultrahigh-temperature belt. Combined mineral reaction and phase equilibrium modelling indicate extreme P-T conditions up to 1130 °C at 11 kbar. The geometry of the P-T path is clockwise with initial isothermal decompression followed by near isobaric cooling. LA-ICPMS U-Pb geochronological studies on accessory phases report two distinct thermal events: (i) Paleoproterozoic high-T event and associated crustal anatexis at ∼2.5 Ga from zircons and (ii) Neoproterozoic UHT event at ∼550 Ma from monazites. The monazite ages suggest a 50–60 Ma prolonged thermal event, attributed to Ediacaran-Cambrian collisional orogenesis. Additional metamorphic pulses are also identified, demonstrating the polydeformed crustal evolution history of the terrane. U-Pb age dating of rutile and apatite in the sample provide ages of ∼458 Ma and ∼392 Ma and cooling rates of ∼6 °C/Ma and ∼0.3–2 °C/Ma respectively. This suggests a complex history of slow cooling followed by ultraslow cooling. The results bring forth the significance of the Kambam ultrahigh-temperature belt in understanding the tectonothermal evolution of Madurai block and provide additional evidence for long-lived Neoproterozoic UHT orogenesis in south India. [ABSTRACT FROM AUTHOR]

Palaeomagnetic analysis of the Lakhna Dykes (Bastar Craton, India) yields a palaeopole at 36.6°N, 132.8°E, dp=12.4°, dm=15.9°, and the U Pb zircon age obtained from one of the rhyolitic dykes is 1466.4±2.6Ma (MSWD=0.21, concordia age based on two analyses with identical Pb/U ages), similar to previously published U Pb ages. Major and trace element analyses of the Lakhna Dykes show shoshonitic and high-K calc-alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting. This is in keeping with the Palaeo- to Mesoproterozoic tectonic environments in the eastern Indian margin. The new 1460Ma Indian palaeopole was used to test possible palaeopositions of India within the Mesoproterozoic supercontinent Columbia. Of the four palaeomagnetically permissible reconstructions, juxtaposing western India against south-west Baltica is geologically the most reliably constrained and best fitting model. Our preferred reconstruction implies a long Palaeo- to Mesoproterozoic accretionary orogen stretching from south-eastern Laurentia through south-western Baltica to south-eastern India. Breakup of India and Baltica probably occurred in the Late Mesoproterozoic, but additional constraints are needed. [ABSTRACT FROM AUTHOR]

Abstract: The Himalayan orogeny, a result of the collision of India and Eurasia, provides direct evidence of strain accommodation and large-scale rheological behavior of the continental lithosphere. Knowledge of the timing of the India–Eurasia collision is essential to understand the physical processes involved in collisional systems. Here we present a geochronological and multi-isotopic study on rocks from the upper crust of the Kohistan Paleo-Island Arc that formed in the equatorial part of the Neo-Tethys Ocean. In situ U–Pb geochronology and Hf isotopes in zircon, and whole-rock Nd and Sr isotopic data of plutonic rocks from the Kohistan-Ladakh Batholith, are used to construct a continuous record of the isotopic evolution of the source region of these granitoids that are related to both the subduction of the oceanic lithosphere and subsequent arc–continent collisions. We demonstrate that profound changes in the source region of these rocks correspond to collisional events. Our dataset constrains that the Kohistan–Ladakh Island Arc initially collided along the Indus suture zone with India at 50.2±1.5Ma, an age generally attributed to the final India–Eurasia collision for the entire Himalayan belt. In the western Himalaya, the final collision between the assembled India/Arc and Eurasia however, occurred ∼10Ma later at 40.4±1.3Ma along the so-called Shyok suture zone. We present evidence indicating that a similar dual collision scenario can be extended to the east and conclude that a final India/Arc–Eurasia collision at ∼40Ma integrates crucial aspects of the magmatic, tectonic, and sedimentary record of the whole Himalayan mountain belt. [Copyright &y& Elsevier]

In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted SHRIMP U-Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-, Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt (quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light (U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon ages from both plutons exhibit bimodal distributions in U-Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newly precipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and 2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U-Pb analyses document the origin of the leucogranite by partial melting of the tonalite. High alkalis (NaO + KO), Rb, Nb, HREE, FeO/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900-950°C), moderate-pressure (<10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction-zone environment. The leucogranite probably formed in a post-collisional realm immediately after accretion of the tonalitic crust. [ABSTRACT FROM AUTHOR]

Abstract: High-pressure (HP)/low-temperature (LT) metamorphic rocks, such as eclogite and blueschist, are generally regarded as an indicator of subduction-zone metamorphism. Eclogites have recently been discovered in the central Qiangtang Block. Their occurrence is highly significant to the understanding of the closure of the Paleo-Tethys and tectonic evolution of northern Tibet. We report the results of petrological, mineralogical and geochronological investigations of these rocks, and discuss their tectonic implications. The Qiangtang eclogite occurs as blocks and lenses in Grt–Phn schist and marble, and is composed of garnet, omphacite, phengite and rutile. Eclogitic garnet contains numerous inclusions, such as glaucophane and phengite in the core, and omphacite in the mantle or inner rim. In strongly retrograded eclogite, the omphacite is replaced by glaucophane, barroisite and albite. Four stages of metamorphic evolution can be determined: (1) prograde blueschist facies; (2) peak eclogite facies; (3) decompression blueschist facies and (4) retrograde greenschist facies. Using the Grt–Omp–Phn geothermobarometer, a peak eclogite facies metamorphic condition of 410–460°C and 2.0–2.5GPa was determined. Zircon U–Pb dating gave ages of 230±3Ma and 237±4Ma for two eclogite samples. The ages are interpreted as the time of eclogite facies metamorphism. Moreover, 40Ar/39Ar dating of phengite from the eclogite and Grt–Phn schist yielded ages about 220Ma, which are probably indicative of the time of exhumation to the middle crust. We conclude that the HP/LT metamorphic rocks were formed by northward subduction of the Paleo-Tethys Ocean and they marked a Triassic suture zone between the Gondwana-derived block and Laurasia. [Copyright &y& Elsevier]