Magmatism and Tectonic Implications of Early Cambrian Granitoid Plutons in Tianshuihai Terrane of the Western Kunlun Orogenic Belt, Northwest China

The Ayilixi and Warengzilafu plutons are located in the Tianshuihai terrane of the Western Kunlun Orogenic Belt (WKOB).Detailed LA-ICP-MS zircon U-Pb dating has yielded the crystallization ages of ca. (530±6) Ma and ca. (515±2) Ma for the Ayilixi pluton and Warengzilafu pluton, respectively, signifying their formation in Cambrian. The Ayilixi rocks are high-K calc-alkaline to shoshonitic granite porphyries, with a limited range of SiO₂ (67.7%~69.4%), high Mg# (44~46), A/CNK ratios (0.81~0.93) and K₂O (2.15%~3.02%). They are also marked by negative ε_Hf(t) values of -3.3 to -0.8, obviously fractionated between light and heavy rare earth element, enrichments in Cs, Rb, K, Th, Zr, Hf, Y, and depletions in Ba, Nb, Ta, Sr, P, T. They are considered to have been most likely derived from magma mixing processes by mantle-derived magma and crustal magmatic melt. The Warengzilafu biotite monzonitic granite gneisses are high-K calc-alkaline and peraluminous. Compared to the rocks of the Ayilixi pluton, they generally have higher SiO₂ (75.0%~76.3%), lower Mg^# (18~26), with higher A/CNK ratios (1.01~1.30) and positive ε_Hf(t) values of +2.5 to +4.3. Elemental and isotopic data suggest that the Warengzilafu rocks are highly fractionated I-type granites, which were mainly generated by partial melting of high-K calc-alkaline andesite and basaltic andesite. The two plutons represent the early Cambrian magmatism under island-arc setting in Tianshuihai terrane, which are related to the subduction of Proto-Tethys.