Mineral parageneses and metamorphic reactions in metasedimentary enclaves from the Archaean Gneiss Complex of north-west India

Three metasedimentary enclaves up to a kilometre in length of contrasting compositions within the polymetamorphic Banded Gneissic Complex (> 2580Ma) have been studied for their mineral parageneses and metamorphic conditions. The largest enclave, consisting of kyanite-chloritoid-muscovite schist with quartz or corundum, and kyanite-fuchsite-corundum ± diaspore, was metamorphosed at most under lower amphibolite conditions, and is thus not isofacial with the surrounding schists and gneisses (of the ‘basement’ complex) which reached sillimanite-grade metamorphism in the last orogenic cycle (Aravalli: 1650–950Ma Orogeny) in Rajasthan.The second enclave is a calc-silicate rock which occurs as a small lens. The presence of two generations of wollastonite which formed during different metamorphic events in the calcite-quartz-grossularite-anorthite-clinopyroxene assemblage indicates polymetamorphism.The third enclave is a metabasic rock which records a complete polymetamorphic history in discontinuous zones in garnet coexisting with hornblende-chlorite-plagioclase-quartz±epidote. To explain the garnet zoning a model involving partial resorption of early garnet during the initial recrystallization stage of superimposed regional metamorphism is preferred to the alternative based on a single prograde metamorphism and retrogression.The mineralogy of the calc-silicate and metabasic enclaves gives a recrystallization temperature of c. 700°C and a pressure in the range of 8–3 kbar during the second metamorphism.

Reinterpretation of the Aravalli basal conglomerate at Morchana, Udaipur district, Rajasthan, western India

SummaryThe supposed Aravalli basal conglomerate near Morchana, considered to mark an erosional unconformity above the older Precambrian Banded Gneissic Complex in central Rajasthan, is a tectonic mélange in a terrain involved in superposed deformations. The supposed pebbles represent tectonic inclusions of various shapes, formed by isoclinal folding, stretching and disruption of concordant, pre- or early-kinematic vein quartz and rare pegmatite sheets in the Aravalli mica schist during the first folding.