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The transition from magmatic to solid-state deformation in granitic rocks is observed either in cases of shear-zone-controlled magma emplacement or in cases of magma-assisted shear-zone nucleation. In the latter case, pre-emplaced granitoid magmas that are later subjected to a dynamic regime with decreasing temperature accommodate stress that causes strain localization. Shear zones in this case are generally restricted to the contact zone between wall rock and granitoid magma, as it represents large viscosity discontinuities. In large granitic provinces (LGPs), however, rheological properties are mainly driven by the changes in physical parameters of granitic magmas, and thus, shear-zone nucleation in magmatic or sub-magmatic conditions cannot be attributed to the viscosity discontinuities caused by the presence of wall rock. Rather, in LGPs, it is only dependent on the rheological discontinuities within the granitic magmas, and thus, a detailed knowledge of the transition from magmatic to solid-state deformation can give important insights into the dynamic history of granitoid magmas in LGPs. The ~2.5 Ga old, voluminous granitoids of Bundelkhand Craton have been affected by multiple shear systems. Among these, the NE–SW and E–W shear systems are major systems that affected the granitoid microstructures. The NE–SW shear system transects the craton as multiple small- and large-scale shear zones, while the E–W shear system is mainly present in the central part of the Bundelkhand Craton. A petrographic study of a ~25 km long, NE–SW trending Baldeogarh shear zone and granite microstructures in associated areas show important magmatic and solid-state deformation features. Overprinting of solid-state deformation over sub-magmatic deformation confirms the progression of deformation with decreasing temperature. Contrary to this, E–W trending microstructures are primarily magmatic to sub-magmatic with rare small-scale solid-state deformation. This indicates that the E–W shear system is older in southern Bundelkhand and was followed by the NE–SW shear system in magmatic conditions. Therefore, the presence of a transition from magmatic to solid-state deformation along the NE–SW trending shear zone indicates that bulk cooling of the granitoid took place while the NE–SW shear system was active. Research Highlights: Transition from magmatic to solid-state deformation in Large Granitic Provinces. Magmatic to solid-state deformation along NE-SW shear zone. Overprinting textures of transition from sub-magmatic to solid-state deformation. [ABSTRACT FROM AUTHOR]

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]

Granitic magmatism is considered an essential mechanism for crustal recycling in the rapidly accreted crust. Mode of emplacement and ascent of granitic magma, together with the exhumation of granites, especially along plate margins, hold a vital clue to the Earth's thermomechanical workings. The present study investigates the role of ductile deformation in the exhumation of the granitic rocks of the Karakoram batholith (KB), north of the Shyok Suture Zone (SSZ), NW Trans-Himalaya. Textural and thermobarometric studies suggest that crystallization of the KB initiated at ~ 700–800 °C and ~ 4.2–7.5 kb. Microstructures of feldspar and quartz pertaining to temperatures > 700 °C together with the late crystallization of muscovite evince submagmatic flow. Muscovite-rich domains typically exhibit microstructures indicating temperatures < 650 °C. Aligned mica, along with penetrative grain boundary migration and moderately strong crystallographic preferred orientation in quartz, represents the highest deformation intensity that probably prevailed during the initial stages of collision along the SSZ. On the other hand, randomly oriented muscovites that crystallized following peak deformation intensity occur exclusively in micro-domains with polygonal quartz grains. Phase transformation of micas to chlorite occurred at ~ 280–400 °C, during which deformation progressed by minor GBM at relatively slower rates. This study implies that submagmatic flow followed by subsolidus solid-state ductile deformation was significant in the exhumation of the granites from a depth of ~ 19–28 to ~ 5.5–9.5 km. Temperature estimates, coupled with geochronological data, yield average cooling rates of ~ 11–18 °C/Ma from ~ 110–85 Ma that gradually decreased to ~ 1.8 °C/Ma after ~ 85 Ma. [ABSTRACT FROM AUTHOR]

The soda granite (~Paleoproterozoic) of the Singhbhum Craton (SC) of eastern India occurs along the entire stretch of the Singhbhum Shear Zone (SSZ; or Copper Belt Thrust Zone). The SSZ (marking a thrust contact between the adjacent North Singhbhum mobile belt and SC) serves as the important locale to host apatite‐magnetite, copper sulphide, and uraniferous mineralization. Typical field occurrence of soda granite is either as tongues (or patches) within a shear zone or as lit‐per‐lit style injections into surrounding country rocks which are generally highly sheared and mylonitized. Experimental studies on granite bodies can potentially throw light on their mineralogical and petrogenetic aspects; however, these studies are hitherto unreported in case of soda granites for almost the last 60 years. In this view, during the present research, experimental studies on soda granites have been carried out and these decipher the nature of the stability relationship of K‐feldspar (and transgression to more disordered anorthoclase), amphibole (both Ca‐amphibole and Fe–Mg–Mn amphibole and breaking down of Ca‐amphibole to Fe–Mg–Mn amphibole + non‐quad pyroxene + quartz at an appropriate temperature), biotite, muscovite, and albite (high albite‐ low albite displacive transformation). Whole‐rock chemistry (deduced from homogenized glass compositions) indicates its peraluminous, S‐type nature with an affinity towards Caledonian post‐collisional plutons. Characteristic geochemical plots suggest a pelitic source material for melting (and subsequent internal differentiation of the melt) and this has given rise to both migmatite as well as patchy soda granite exposures in the field. It is suggested that partial melting of mainly metapelitic (and minor mafic magmatic rocks) source (belonging to Singhbhum Series) gave rise to soda granites in this region. [ABSTRACT FROM AUTHOR]

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: Multiple subduction events occurred preceding the collision of Greater India with the Karakoram terrane that consumed the Neotethyan Ocean between Early Cretaceous and Eocene time. Zircon U–Pb dating and Hf isotope analyses of granitic rocks collected from the Karakoram terrane and Ladakh batholith have been conducted to constrain these geological processes. Undeformed Ladakh batholith granodiorite yielded zircon U–Pb ages of 68 and 64 Ma with ε Hf(t) values of +6.4 and +10.3, and three diorites near Leh and Darbuk, gave zircon U–Pb ages of 50.4–51.0 Ma and ε Hf(t) values of +7.4 to +8.9, indicating that the Paleocene–Eocene was the most important period of granitic magmatism that derived from a juvenile source. Three tonalite–granodiorite enclaves at Skyangpoche within the Karakoram batholith have zircon U–Pb ages of 101–103 Ma with ε Hf(t) values of −2.3 to −3.7, indicating generation of granitic rocks by partial melting of the Karakoram basement. Three samples of the Miocene Karakoram batholith, from around Darbuk, yielded ages of 16.6–19.1 Ma with ε Hf(t) values of −8.5 and −10.5, that indicate an input of ancient crust from the Indian plate. Granodiorite within the Karakoram metasedimentary sequence (zircon U–Pb age ∼74 Ma, ε Hf(t) of +5.4 from the Tangtse gorge) and granite around Muglib (zircon U–Pb age of 63 and 56 Ma, ε Hf(t) of +6.8 and +6.0) indicate similar geochronological and isotopic characteristics to those of the Ladakh batholith. Thus the time resolved Hf isotopic compositions of magmatic zircons from the granites in the Karakoram and Ladakh batholith show a large variation that indicate their derivation from both ancient and juvenile continental crusts, in active continental margin since at least the Mid Cretaceous. [Copyright &y& Elsevier]