Counterclockwise P–T evolution of the Aghil Range: Metamorphic record of an accretionary melange between Kunlun and Karakorum (SW Sinkiang, China)

This paper describes the metamorphic evolution and the tectonic significance of the Aghil Range, a poorly known terrane located between Kunlun and Karakorum north of K2 in the framework of western Tibet. The Aghil Range consists of different units separated by syn- to late-metamorphic thrusts and post-metamorphic faults of similar attitude; among other units, the Surukwat Complex is a composite sequence of thrust sheets trending WNW–ESE and steeply dipping SSW showing a general increase of metamorphic grade from lower to higher structural levels. P–T pseudosections and conventional thermobarometry of metapelites at the top of the Surukwat Complex tightly constrain the P–T path of this highest-grade metamorphic portion of the Aghil Range. The prograde path is characterized by an early increase in both P and T and by a later, nearly isothermal, increase in P, from 500 < T < 530 °C and 0.25 < P < 0.40 GPa to 580 < T < 600 °C and 0.80 < P < 0.90 GPa. The peak metamorphic event is constrained at 550 < T < 590 °C and 0.77 < P < 0.91 GPa. The retrograde path is characterized by decompression associated to a slight cooling to T ~ 500 °C and P < 0.5 GPa. Altogether, the petrology of the studied rocks suggests a P–T path with a narrow counterclockwise shape. The studied sequence could represent the result of the early subduction of an accretionary complex, interpreted further eastward according to a melange underthrusting model. As concerning the tectonic setting of this terrane, a number of geologic and petrologic similarities link the Aghil Range and the central Qiangtang metamorphic belt, suggesting that the Aghil Range is the possible NW extension of the Qiangtang microplate separating Kunlun from Karakorum.