Multi-system geochronological and isotopic constraints on age and evolution of the Gaoligongshan metamorphic belt and shear zone system in western Yunnan, China

Abstract

The Gaoligongshan metamorphic belt, located east of the Eastern Himalayan Syntaxis (EHS) in the Yunnan province, southwestern China, is a compelling region in which to determine temporal constrains on crustal dynamic processes related to the Himalayan orogeny. We applied multi-system geo- and thermochronology (900 °C to <50 °C) to orthogneiss and mylonites from a major shear zone in the southern Gaoligongshan in order to elucidate the magmatic, cooling and exhumation history of this belt. Zircon U/Pb data reveal three magmatic events at ∼486 Ma, ∼136 Ma and ∼76 Ma. Similar ages are found in orthoderivative rocks of the neighboring Tengchong and Baoshan blocks, and the Gangdese batholith, suggesting that the southern Gaoligongshan is composed of an assemblage of Lhasa and Qiangtang terrane derived rocks. Muscovite Rb/Sr ages of 35–21 Ma are coeval with the onset of lateral crustal displacement along major shear zones in Eastern Tibet and Indochina, and with the post-collision volcanic activity in western Yunnan. Biotite Rb/Sr and mica 40Ar/39Ar ages provide evidence that mylonitization along the Gaoligongshan shear zone and crustal rotation of the Tengchong and Baoshan blocks proceeded during the Miocene, between 19 and 12 Ma, when the rocks rapidly cooled through the 350–280 °C temperature range. Almost identical 40Ar/39Ar ages reported for the Karakorum–Jiali–Parlung Fault system in Western Tibet suggest that the Gaoligongshan shear zone is the southeastern continuation of this fault, recording the eastward extrusion of Tibet and crustal movement around the EHS. The final stage of exhumation of the Gaoligongshan occurred between 8 and 5 Ma at an average exhumation rate of ∼3 km/Ma as documented by apatite fission-track and apatite (U–Th–Sm)/He data. This rapid exhumation was triggered by crustal root delamination and opening of the Andaman sea. Our results clearly show that the complex tectonothermal evolution of the Gaoligongshan was influenced by Tibetan extrusion and escape tectonics associated with lower crustal flow around the EHS and the southeastward movement of Indochina and back-arc extension in response to Andaman seafloor spreading.