Partial melting of subducted continental crust during the exhumation: Insights from Palaeozoic granitic rocks in South Altyn, western China

The early Palaeozoic continental subduction-collisional belt in South Altyn (SA) western China distributes a large amount of (ultra)-high-pressure ((U)HP) metamorphic and granitic rocks associated with subduction, collision, and exhumation. In this study, four granitic rocks were collected from Bashenwake located in eastern SA. The ages of the garnet-bearing alkali-feldspar granite (GAFG), garnet-bearing granitic gneiss (GGG), alkali -feldspar granite (AFG), and syenogranite (SG) are 458.0 +/- 4.1, 483.0 +/- 4.1, 450.0 +/- 2.4, and 482.0 +/- 4.9 Ma, respectively. Whole-rock major and trace elements and Sr-Nd-Pb isotope, and zircon Hf isotope compositions for all samples exhibit characteristics of a continental crustal source. Based on partial melting modeling, the source temperature of the GGG, GAFG, AFG and SG was concluded as 750-780 degrees C, 730-760 degrees C, 730-770 degrees C and ca. 800 degrees C, respectively. The source pressure was estimated as ca. 0.8 GPa for the GGG, GAFG and AFG, and ca.1.2 GPa for SG. Combined with studies on (U)HP metamorphic rocks in SA, this study reveals that GGG is formed by metamorphism while SG is formed from partial melting of upper crustal rocks heated by upwelling mantle during the initial stage of exhumation (ca. 480 Ma) of subducted continental crust in SA. During further stage of exhumation of the subducted continental crust at ca. 450 Ma, the GAFG and AFG were formed from partial melting of GGG as a result of decreased pressure and further mantle upwelling. Consequently, the exhumation age of subducted continental crust in eastern SA is ca. 30 Myr earlier than that in western SA. Thus, the geochronology and petrogenesis of four granitic samples in this study exhibit one of very few evidence for partial melting of subducted continental crust during the exhumation and also provide magmatism exhibits evidence for the differential exhumation model of eastern SA in Palaeozoic.