Oxidation degree of chromite from Indian ophiolites: a Crystal chemical and 57Fe Mossbauer study

Several samples of Cr-bearing spinel from Indian ophiolites have been studied by X-ray single crystal diffraction and structure refinement, electron probe microanalyses and M\uf6ssbauer spectroscopy. Differences between samples coming from massive chromitite bands and those coming from podiform chromitite deposits have been evidenced: the former have (Mg,Fe)-chromite component 65 75%, the latter from 69 to 74%. In both cases the complementary components are aluminate (mainly spinel sensu stricto MgAl2O4). As magnesiochromite (MgCr2O4) is always dominant with respect to chromite (FeCr2O4), the studied samples are classified as magnesiochromite. The multi-analytical approach has revealed that some of the studied spinels are non-stoichiometric (due to Fe2+ oxidation and vacancy formation), with those coming from massive chromitite being more oxidised than those from the pods. Comparison between our results and recent literature suggested that spinel oxidation is more common than usually believed: it is not restricted to ophiolites but may occur in different geological settings and may help reconstructing the complex thermo-oxidative history of the host rock. Consequently, the proposed multi-analytical approach is crucial for an accurate crystal chemical characterisation of spinels and other Fe-bearing mineral phases, especially when geothermobarometric calculations have to be performed

Crystal chemical and 57Fe M\uf6ssbauer study of chromite from the Nuggihalli schist belt (India)

In the Nuggihalli schist belt (India), chromite bodies were affected by intense serpentinization followed by weathering. In spite of the strong oxidation of the chromite grains, some unaltered cores were preserved, and they were characterised using electron probe microanalysis, single-crystal X-ray diffraction and low temperature 57Fe M\u20acossbauer spectroscopy. Results of such investigations revealed that chromite cores from the Nuggihalli schist belt are effectively not oxidised, and their study revealed that chromite was not affected by greenschist-facies metamorphism. The apparently highly ordered cation distribution of the chromites, which would suggest low equilibration temperature, is just caused by the very high Cr contents, which prevent Mg\u2013Al exchange between T and M sites. Using Fabries (1979) geothermometer a temperature of about 1180 C was retrieved (Mitra and Bidyananda, 2003), which therefore corresponds to the quenched-in igneous equilibrium

Dating blueschist-facies metamorphism within the Naga ophiolite, Northeast India, using sheared carbonate veins

The tectonic significance of blueschist-facies rocks associated with the Indo-Myanmar ophiolite belt is uncertain, given the lack of detailed petrological study and the paucity of reliable age data for different stages in their geological evolution. Here, we present new integrated petrological and geochronological data for samples from the Nagaland complex of the Indo-Myanmar ophiolite belt, northeastern India, which constrains the pressure–temperature conditions and absolute ages of peak and retrograde metamorphism. Several samples of blueschist were collected from the region, which have been variably deformed and subjected to shear recrystallization. Based on microstructural constraints and mineral geochemistry, garnet, omphacite, barroisite, chlorite and muscovite are interpreted to represent a high-pressure prograde-to-peak metamorphic assemblage, and omphacite, actinolite, hornblende and albite represent a lower-pressure retrograde metamorphic assemblage that formed during shear-related exhumation. Petrological modelling and thermobarometry indicates that unsheared samples equilibrated at ~1.9 GPa and ~480–520°C at peak metamorphism, indicating subduction to ~60 km depth, whereas sheared and recrystallised samples re-equilibrated at ~0.6 GPa and ~470°C during retrograde metamorphism associated with obduction of the Naga ophiolite onto the Indian foreland. U–Pb in-situ analysis of carbonate grains (aragonite–calcite) and associated silicate phases (epidote, prehnite, amphibole, etc.) at different microstructural positions, including within dynamically recrystallised shear bands that cross-cut older metamorphic fabrics and cogenetic silicate phases, constrains the age of peak metamorphism to be c. 95 Ma and retrograde metamorphism to be c. 90 Ma. Based on the overall progression of ages in the sheared and unsheared samples, we interpret that the area experienced exhumation at a time-averaged rate of ~1 cm/year in the order of Phanerozoic period plate tectonic rate, which is in the order of rates of plate tectonic processes on the Phanerozoic Earth

Crustal evolution of the amphibolite- to granulite-facies transition zone in the eastern Dharwar Craton, southern India: insight from petrological modelling, zircon U–Pb geochronology and Hf isotopes

Integrated petrological, geochemical, isotopic and thermobarometric study of metasedimentary rocks from the amphibolite- to granulite-facies transition zone of the Eastern Dharwar Craton (EDC), South India, has provided new insight into the evolution of the lower continental crust in this region. Phase equilibrium modelling of metapelites and metagreywackes suggests that they reached peak metamorphism at ~800–850°C and 6–7 kbar (corresponding to a paleodepth of ~20 km), with minor retrograde change occurring at ~700°C and 3–5 kbar during exhumation. U–Pb ages of conventionally separated zircon from metapelite samples range from 2.5 to 3.4 Ga, whereas garnet-hosted zircons yield younger ages of 2.5–2.7 Ga. Zircon Th/U ratios and Hf isotopes reveal several significant pulses of zircon growth at 3.0, 2.95 and 2.7 Ga. Hf isotope data suggest the evolution of juvenile magma at around 3.2 Ga, while Hf model ages show that the crust building process also involved the recycling of pre-existing Mesoarchean crust. Our study confirms the presence of a Paleoarchean component in the EDC lower crust, as well as older metamorphic events in the terrain and the gradational distribution of the metamorphic rocks

Crustal evolution of the amphibolite‐ to granulite‐facies transition zone in the eastern Dharwar Craton, southern India:insight from petrological modelling, zircon U–Pb geochronology and Hf isotopes

Integrated petrological, geochemical, isotopic and thermobarometric study of metasedimentary rocks from the amphibolite‐ to granulite‐facies transition zone of the Eastern Dharwar Craton (EDC), South India, has provided new insight into the evolution of the lower continental crust in this region. Phase equilibrium modelling of metapelites and metagreywackes suggests that they reached peak metamorphism at ~800–850°C and 6–7 kbar (corresponding to a paleodepth of ~20 km), with minor retrograde change occurring at ~700°C and 3–5 kbar during exhumation. U–Pb ages of conventionally separated zircon from metapelite samples range from 2.5 to 3.4 Ga, whereas garnet‐hosted zircons yield younger ages of 2.5–2.7 Ga. Zircon Th/U ratios and Hf isotopes reveal several significant pulses of zircon growth at 3.0, 2.95 and 2.7 Ga. Hf isotope data suggest the evolution of juvenile magma at around 3.2 Ga, while Hf model ages show that the crust building process also involved the recycling of pre‐existing Mesoarchean crust. Our study confirms the presence of a Paleoarchean component in the EDC lower crust, as well as older metamorphic events in the terrain and the gradational distribution of the metamorphic rocks