Sohini Ganguly

Mesozoic felsic volcanic rocks from the North China craton: Intraplate magmatism associated with craton destruction

The North China craton preserves the history of crustal growth and craton formation during the early Precambrian, followed by extensive lithospheric thinning in the Mesozoic associated with large-scale magmatism and metallogeny. The timing and petrogenesis of the voluminous Mesozoic magmatic rocks are important in understanding the mechanism of craton destruction. Here, we investigate a suite of volcanic rocks including basalt and trachybasalt, basaltic andesite and andesite, dacite and trachydacite, and rhyolite from the Yanshan belt in the northern part of the North China craton and provide evidence for intraplate magmatism along a paleosuture. We present bulk chemistry, zircon U-Pb geochronology and rare earth element data, and Lu-Hf isotopes from the volcanic suite and attempt to constrain the timing of magmatism and source characteristics. The zircon U-Pb data show two age peaks at 175−165 Ma and 155−145 Ma. Geochemically, the rocks are calc-alkaline with arc-related features. The andesites show adakitic affinities with high Sr contents (up to 1140 ppm), high Sr/Y (up to 76.2) and La N /Yb N ratios (up to 21.7), lack of a negative Eu anomaly, extreme depletion in Y and Yb, and relatively low MgO contents (1.3−4.4 wt%), indicating that they were likely derived from the partial melting of thickened lower continental crust. The Zr/Ba ratios indicate interaction between lower crust and primitive magma, possibly through magma underplating at the crust-mantle boundary. However, there was no arc-related tectonic setting in the study area during the Jurassic, which indicates that the elemental and isotopic compositions are possibly inherited from the basement rocks generated during the Paleoproterozoic subduction-accretion-collision process in the North China craton, subsequent to the 2.5 Ga cratonization event. Our zircon Hf isotope data also confirm the incorporation of Paleoproterozoic reworked material. The magmatism and craton destruction along a paleosuture were induced by mantle upwelling through far-field tectonics of the Pacific plate subduction. Our study presents a case where source components have strongly influenced the geochemical imprint of the magmas, with intraplate volcanics preserving a continental arc magmatic signature.

Mineral chemistry of lava flows from Linga area of the Eastern Deccan Volcanic Province, India

Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province (EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG) sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector. A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows. Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ). The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively. The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing. The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range ∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes. Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic (An52.7–An72.9), whereas groundmass plagioclase are relatively sodic (An39.2–An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption.

Mineralogical study of gabbro-anorthosite from Dumka, Chhotanagpur Gneissic Complex, Eastern Indian Shield

The Chhotanagpur Gneissic Complex (CGC), bearing imprints of widespread high grade metamorphic and magmatic history since Palaeoproterozoic, represents an integral crustal segment of Eastern Indian Shield. The gabbroanorthosite intrusives constitute a part of mafic-ultramafic magmatism in the CGC. The study area around Dumka (24°16′ to 24°20′N: 87°13′ to 87°22′E) predominantly comprises of granite gneiss and charnockitic country rocks within which gabbro-anorthosite intrusions occur as lenses. Field relations and structural studies reveal that the country rocks of Dumka have suffered three phases of deformation represented by F1, F2 and F3 folds. The gabbro-anorthosite intrusives maintain a sharp contact with the host rocks, deformed and metamorphosed. Relict igneous layering or primary igneous foliation (Sig) is recorded where metamorphic overprint is minimal. Mineral phases of gabbro-anorthosite rocks suggest that clinopyroxene compositions from gabrro correspond to diopside and clinoferrosilite, while those from anorthosite are clinoferrosilite. Amphiboles from the gabbro-anorthosite rocks are calcic, and range from ferroan pargasite in gabbro to ferroan pargasitic hornblende in anorthosite. Plagioclase from gabbro and anorthosite belong to bytownite and andesine respectively. Chemical composition of garnet in gabbro is almandine. Thermobarometric estimates for Dumka gabbroanorthosites correspond to 511°C to 915°C and 5.0–7.5 kb pressure, comparable to that estimated for Bengal Anorthosite (593–795°C, 4.1–7.3 kb). Fractionation trend of plagioclase substantiates a single parental magma in the evolution of Dumka gabbro-anorthosite intrusives.