The absence of high-pressure metamorphism in the inverted Barrovian metamorphic sequences of the Arun area, eastern Nepal and its tectonic implication

Abstract

The metamorphic pressure–temperature (P–T) conditions across the Main central thrust (MCT) in the Arun area have been investigated. The MCT marks the tectono-metamorphic boundary between the overlying high-grade High Himalaya crystalline sequences (HHCS) and the underlying low-grade Lesser Himalaya sequences (LHS). The metamorphic rocks regionally preserve an inverted Barrovian sequence (i.e., intermediate P/T type metamorphism) devoid of previously reported high-pressure metamorphism. The metamorphic grade increases upwards from 670–740 °C and 6.9–9.4 kbar in the MCT zone and lower HHCS to 760–835 °C and 10.0–11.1 kbar in the middle HHCS. Orthoamphibole gneisses in the middle HHCS yield prograde Barrovian-type metamorphism, such as staurolite inclusions in garnets, showing an intermediate P/T gradient. The differences in the tectonic setting and metamorphic evolution imply that the metamorphic units in the Arun area do not correspond to the other high-pressure units in eastern Himalaya. Zircon and monazite U–Pb ages from kyanite gneiss of the lower HHCS reveal the MCT activity, associated with fluid-present anatexis, at ca. 20–14 Ma. Furthermore, similar K–Ar white mica ages (ca. 13–7 Ma) in the hanging wall and footwall of the MCT could represent the timing of later deformation events in shear zones or cooling, possibly associated with exhumation accompanied by activities on younger, structurally lower thrust faults such as the lower MCT. The similar P–T conditions near the MCT in this area could result from recrystallization during syn-metamorphic thrusting, whereas the middle HHCS away from the MCT preserve the original Barrovian metamorphic sequences related to crustal thickening. This and previous studies imply that different P–T profiles near the MCT according to each transect observed in Nepal could be apparent and the cumulative result of polyphase metamorphism.