Pritam Nasipuri

Crustal geodynamics from the Archaean Bundelkhand Craton, India: constraints from zircon U–Pb–Hf isotope studies

A comprehensive study based on U–Pb and Hf isotope analyses of zircons from gneisses has been conducted along the western part (Babina area) of the E–W-trending Bundelkhand Tectonic Zone in the central part of the Archaean Bundelkhand Craton. 207 Pb– 206 Pb zircon ages and Hf isotopic data indicate the existence of a felsic crust at ~ 3.59 Ga, followed by a second tectonothermal event at ~ 3.44 Ga, leading to calc-alkaline magmatism and subsequent crustal growth. The study hence suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Singhbhum and Bastar cratons of the North Indian Shield.

Semibrittle deformation and partial melting of perthitic K-feldspar: An experimental study

To investigate the relationships between deformation, cracking, and partial melting in the lower continental crust, axial compression and hydrostatic experiments were performed on K-feldspar single crystals at temperatures of 700° and 900°C and confining pressures between 0.75 and 1.5 GPa. Sample deformation was carried out at a constant strain rate of ~ 10−6 s−1. The samples deformed at 700°C show typical brittle behavior with formation of conjugate fractures and peak stresses that increase with confining pressure. Samples deformed at 900°C show formation of shear fractures, peak stresses below the Goetze criterion, and inverse confining pressure dependence of peak stress, indicating that along the fractures deformation was not dominantly friction controlled. Microstructural and chemical analyses reveal the presence of melt (<6 vol %) of inhomogeneous composition along the shear zones and chemical compositional changes of gouge fragments. In a hydrostatic experiment performed at 900°C, no melt and no compositional changes were observed. These observations indicate that deformation of K-feldspars at high pressures and temperatures is controlled by the simultaneous formation of brittle fractures and melt. The formation of melt is strongly accelerated and kinetically favored by cracking, as demonstrated by the absence of melting in the hydrostatic experiments. However, the melt along fractures does not dramatically weaken the samples, as the melt domains remain isolated during deformation. The fine-grained gouge fragments formed along the fracture systems undergo chemical homogenization. The dominant deformation mechanism in the gouge is likely to be melt-enhanced diffusion creep, which may also assist the chemical homogenization process.

Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature (P–T) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

Eastern Ghats Province (India)–Rayner Complex (Antarctica) accretion: Timing the event

There is consensus that, at 1.0–0.9 Ga, the granulites in the Eastern Ghats Province (EGP), Eastern India, and the Rayner Complex, Antarctica, were parts of a coherently evolved crustal block. Paleogeographic reconstructions suggest that in the Neoproterozoic/Early Paleozoic, India and Antarctica were closely positioned at equatorial latitudes in two periods at 1.0–0.9 Ga and 0.6–0.5 Ga. The question is, in which of these periods did the EGP–India vis-à-vis India–Antarctica accretion occur. Top-to-the-west thrusts juxtaposed the EGP with the Bastar Craton, a part of the Greater India landmass. The eastern fringe of the craton underwent anatexis (750–780 °C; 8–9 kbar) and high deformation strain that demonstrably weakened westward. Zircon in the anatectic migmatites at the EGP margin and in the weakly-deformed and non-migmatite granite in the hinterland in the west yields U–Pb upper intercept ages of 2.5–2.4 Ga whereas titanite, hosted in the leucosome of a metatexite and in a granite, has an age of 502 ± 3 Ma coinciding with the lower intercept ages of zircon discordia lines. The lack of 1.0–0.9 Ga dates in the cratonic margin suggests that the EGP accreted with the Bastar Craton and the Greater India landmass at 0.5 Ga during the Gondwanaland assembly, and not in the Early Neoproterozoic. It is within the realms of possibility that the EGP had already separated from the Rayner Complex during the disintegration of Rodinia, and therefore, the 0.5 Ga accretion of the dismembered EGP with Greater India may not be symptomatic of India–Antarctica accretion, in spite of the proximity of the two landmasses inferred from paleogeographic reconstructions. LITHOSPHERE; v. 10; no. 4; p. 523–529; GSA Data Repository Item 2018166 | Published online 18 April 2018 https://doi.org/10.1130/L703.1