The Early Paleozoic Xitieshan syn-collisional granite in the North Qaidam ultrahigh-pressure metamorphic belt, NW China: Petrogenesis and implications for continental crust growth

Syn-collisional magmatism produced by partial melting of subducted oceanic and continental crust during the continental collision plays an important role in understanding the orogenic evolution and crustal growth. This contribution reports zircon LA-ICPMS U–Pb ages and Hf isotopic compositions, whole rock major and trace elements and Sr-Nd isotopic compositions of syn-collisional Xitieshan intrusion within the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt, to study its petrogenesis and contribution to continental crust growth. LA-ICPMS zircon U–Pb dating of the Xitieshan granite yields magmatic crystallization ages of 441±2Ma and 442±2Ma, which are consistent with the peak age of ultrahigh-pressure eclogite-facies metamorphism in the Xiteishan terrane. The temporal correlation between them confirms that the Xitieshan granite was products of syn-collisional magmatism during the continental collision between Qaidam and Qilian Blocks. These granites show high-K calc-alkaline and slightly peraluminous signature, low zirconium saturation temperatures and high contents of K2O with the S-type characteristics. They have rare earth element and trace element patterns resembling those of bulk continental crust, with strong fractionation of light and heavy rare earth elements ((La/Yb)N=19–26), moderately Eu negative anomalies (δEu=0.65–0.71) and the obviously Nb, Ta, P and Ti negative anomalies. The Xitieshan granite also exhibits remarkable Sr-Nd isotopic differences (initial ⁸⁷Sr/⁸⁶Sr=0.70920–0.71080 and εNd(t)=−4.54–4.11) from the contemporaneous granites within North Qaidam UHPM belt. Combined with the positive εHf (t)(0.5–5.3) and ages of inherited zircons (475–518Ma), the magmatism is best explained as resulting from melting of subducted oceanic and continental crust during continental collision. Isotopic mixing calculations suggest that ca. 28–35% ocean crust and ca. 65–72% continental materials contribute to the origin of the Xitieshan granite. Thus the syn-collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the depleted mantle and the hypothesis “continental collision zones are primary sites for net continental crust growth” is applicable in the North Qaidam.