Deep subduction and exhumation of micro-continents in the Proto-Tethys realm: Evidence from metamorphism of HP-UHT rocks in the North Qinling Orogen, central China

Abstract

The North Qinling Orogen in central China contains a typical continental high- and ultrahigh-pressure (HP-UHP) metamorphic belt, but its metamorphic evolution remains controversial. We report a combined investigation on petrology, geochronology and phase modelling for felsic granulite (samples SN1504 & SN1517) and garnet clinopyroxenite (sample SN1505) from the Songshugou area. Four stages of metamorphic evolution are constrained: (i) A possible prograde from HP amphibolite facies to peak UHP eclogite facies stage (~3–8 GPa/700–1080 °C) recorded by the chemical zoning of garnet core and mantle in sample SN1504 and also the presence of supersilicic titanite in sample SN1505. (ii) A post-peak decompression evolution is recorded by the formation of coronary garnet and plagioclase around kyanite in sample SN1504 and high-Ti amphibole and plagioclase in sample SN1505, including the evolution from eclogite facies to HP–ultrahigh temperature (UHT) conditions of 1.3–2.2 GPa/900–980 °C for sample SN1504 and 1.0–2.1 GPa/960–1010 °C for SN1505, respectively. (iii) Further decompression is revealed from the growth of corundum + spinel in local domains in sample SN1504, suggesting a low-pressure (LP) condition of ~0.8 GPa/950 °C. (iv) A subsequent cooling evolution is recorded by thin films of K-feldspar + quartz + biotite in sample SN1504 and low-Ti amphibole in sample SN1505, indicative of melt crystallization towards the solidus with conditions of 830–770 °C/0.55–0.8 GPa and 880–820 °C/0.8–0.9 GPa, respectively. Zircon and monazite U-Pb dating yields three metamorphic ages of c. 521 Ma, c. 503 Ma and c. 500–480 Ma, which are interpreted to represent the prograde HP amphibolite facies stage, the peak eclogite facies stage and the late cooling stage, respectively. The metamorphic P–T–t path suggests a complete history of deep continental subduction to depths of >90–250 km, followed by rapid exhumation to crustal depths for the Qinling micro-continent during the early Proto-Tethys evolution.