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We investigate the shape and strength of the magnetic fabrics (anisotropy of magnetic susceptibility (AMS) data) of various massive granitic plutons from different parts of India, using the eigenvalue method. The study aims to analyse eigenvalues and establish their relationship with various deformational attributes. It involves: (1) calculating eigenvectors and their corresponding eigenvalues from magnetic fabric datasets; (2) finding a link between the geometrical appearance of eigenvectors and the mechanistic issues involved with a specific deformation scenario; and (3) determining shape and strength parameters from the magnetic foliation data distribution. The statistical analysis for the unimodal magnetic fabric dataset of orthorhombic symmetry class implies that the plane, consisting of intermediate (V2) and minimum (V3) eigenvectors with pole V1, accurately traces the instantaneous stretching axis (ISAmax) of a particular material flow system under a pure shear regime. Moreover, for the distributions of similar symmetry and modality, we infer that the rotational characteristics of eigenvectors with respect to a fixed coordinate cause a distinct shift of such planes (V2–V3) from the ISAmax of a steady-state flow system under simple shear, where a substantial amount of rotational strain is involved. However, our findings also suggest that variation in symmetry and modality of magnetic fabric data distribution of different studied granitoids can directly influence the relative disposition of V2–V3 with respect to the direction of ISAmax. We conclude that eigenvalue analysis of magnetic fabrics is a powerful approach, which can be utilized while studying the salient deformational aspects of any syntectonic massive granitic body. [ABSTRACT FROM AUTHOR]

The relationships between textural variations and structural trends of the aplite dyke enclosed in the Malanjkhand pluton were investigated in this study. The estimated zircon saturation temperature (747–835°C) and pressure of crystallization (2.5–6.1 kbar) suggested that the aplite dyke was emplaced in the lower-middle level in the continental crust. Water solubility calculations indicated that the aplite dyke originated from the silicic magma under water undersaturated conditions. Primitive mantle normalized spider diagram showed enrichment of large-ion lithophile elements (LILEs) and depletion of high field strength elements (HFSEs). The aplite dyke displayed LREE-enriched and MREE-depleted patterns, with significant positive Eu-anomaly in the REE patterns. This observation alluded the accumulation of plagioclase crystals before the crystallization of felsic magma in the reduced environment. The presence of the positive Eu-anomaly signified that the pre-existing granitic source at the lower-middle level of the crust generated aplitic magma owing to partial melting above the felsic source rock. Trace element discrimination diagrams presented evidence for possible extensional tectonic settings coupled with felsic magmatic episodes and granitic plutonic activity in a continental rift environment, thus favoring the emplacement of the aplite dyke. Th/U ratios in the aplite dyke implied that the melt fractionation in the magma chamber and the post-magmatic hydrothermal processes exerted negligible effect on the crystallization evolution of the aplitic magma. The aplite dyke pointed to a single pulse of silicic magmatism and a continuous process of injection, thus reflecting subtle variations in the physical conditions of the formation of the host Malanjkhand pluton. [ABSTRACT FROM AUTHOR]

Vermiculite, a clay mineral of economic importance, is reported for the first time from the Bundelkhand Craton. The locales of the vermiculite occurrence are mapped within the Bundelkhand granitoids at several places. The identification of the fracture system and the zones of strain localization within Bundelkhand granitoids are critical for targeting the occurrences of vermiculite. A range of studies including meso‐, micro‐, and sub‐microscopic analyses were used to delineate the characteristic features of vermiculite along the brittle/ductile shear zones in the Bundelkhand Craton. SEM‐based EDS, EPMA, and XRD analyses confirm the ubiquitous dominance of vermiculite as fracture infills and/or channel infill materials along the zones of nucleation of ductile shear zones. It is postulated here that the occurrence of vermiculite is a result of supergene alteration of biotite present in the vein material. The parent material of the fracture infills from which the veins crystallized is genetically derived from the host rock (Bundelkhand granitoid), which later healed the fractures and prompted the nucleation of ductile shear zones in the Bundelkhand Craton. [ABSTRACT FROM AUTHOR]

The northward subduction of the Neo-Tethys oceanic lithosphere and subsequent collision between India and Asia continents gave rise to the Tibetan Plateau. However, how and when oceanic subduction started to transform to an intraplate environment are still open questions. The granitoids distributed in Lhasa Terrane of south Tibet offer a unique chance for us to investigate the deep geodynamic processes. Here, we present zircon U–Pb–Hf isotope, whole-rock major and trace element and Sr–Nd isotope data of granitoid intrusions in the Sangsang area of the southern Lhasa Terrane. The Sangsang granodiorites and granites were crystallized at ca. 54 Ma, while the emplacement age of the quartz monzonites is ca. 47 Ma. The granodiorites are characterized by relatively high Mg# values (35.3–41.1) and Fe2O3t (5.16–6.26 wt%) contents, and low Na2O + K2O contents (6.4–6.9 wt%) and A/CNK values (0.91–0.99), which are similar to the geochemical characteristics of I-type, high-K calc-alkaline rocks. They have high 87Sr/86Sri ratios (0.706455–0.706490), and low εNd(t) (− 3.58 to − 2.96) and zircon εHf(t) (− 3.4 to 0.3) values, indicating they were derived from a hybrid source of ancient mafic crust and juvenile lower crust. The coeval granites have lower Mg# values (22.5–27.25) and similar zircon εHf(t) values (− 2.6 to 1.1), suggesting they were probably differentiation productions of the granodiorites. The quartz monzonites have higher Na2O + K2O contents (9.18–9.59 wt%) and A/CNK values (0.98–1.03), higher zircon εHf(t) values (− 2.2 to 2.6) and more depleted Sr and Nd isotopes than the granodiorites and granites. The quartz monzonites were probably produced by melting of mixed juvenile crustal materials and metagreywacke. The new geochronological and geochemical data help constrain the geodynamic processes in the Lhasa Terrane during the Early Cenozoic, as the Sangsang granitoids represent the change from subduction termination to intraplate extension at the southern margin of the Lhasa Terrane. [ABSTRACT FROM AUTHOR]

The Precambrian Singhbhum Craton and the adjoining orogenic belts of eastern India hold a history of continental crust generation by repeated emplacement and reworking of multiphase tonalite-trondjhemite-granodiorite (TTG) and granites throughout the Archean. Temporal classification of this complex Archean TTG and granite sequence have not been straightforward due to repeated magmatism that gave rise to complex intrusive relationships between the magmatic bodies, their complex tectonic juxtaposition and lack of geochronological data. Recent surge of new geochronological and isotope data has brought significantl clarity in the temporal relationships between different crustal components of this craton. In this study, geochemical data (published and new) of the TTG and granites (n = 251) with robust age reference was compiled and classified using a two-fold (viz. age and geochemistry) classification to understand the secular change of crustal composition. Herein, the Archean TTG suites were divided into four age-based groups, viz. Group-I (> ca. 3.5–3.4 Ga), II (ca. 3.4–3.3 Ga), III (ca. 3.3–3.2 Ga) and IV (

High to medium content of radioactive elements (U, Th, K) together heat generation of anorogenic, A-type felsic to intermediate volcano-plutonic rocks (granite, rhyolite and trachyte) of Dhiran Area, Malani Igneous Suite, western India has been reported. These volcano-plutonic suits of rocks are classified as high to medium Heat Production (HP). Leucocratic granite shows higher Heat Production (4.84-8.75 μWm-3) and total Heat Generation Unit (11.52-20.84 HGU) than pink granite (2.09-2.94 μWm-3 HP; 4.98-7.07 HGU) and rhyolites (2.87-6.63μWm-3 HP; 6.83-15.80 HGU). Leucocratic granites, pink granites and rhyolites of the Dhiran area shows higher average value of total Heat Generation Unit (16.51 HGU, 5.79 HGU and 9.33 HGU) than the average value of 3.8 HGU for the continental crust. The high Heat Generation Unit values (4.98-20.84) of the volcano-plutonic rocks of the Dhiran indicate 'hot crust' category and a possible linear relationship between the surface heat flow and crustal heat generation in the Malani Igneous Suite. [ABSTRACT FROM AUTHOR]