LA-ICP-MS Zircon U-Pb Chronology and Geological Implications of Sikeshu Stitching Pluton in the North Tianshan Suture Zone, Xinjiang

Sikeshu granitic pluton is located in the North Tianshan suture zone, the biggest granitic body which intrudes the Yilianhabierga island arc belt, and has tectonic significance as one stitching pluton. Sikeshu pluton is composed of various types of rocks generally formed in three intrusive periods. In the first period, the pluton rocks are quartz diorite and granodiorite, while in the second period the major compositions are monzogranite and K-feldspar granite, and in the third period the pluton is made of granite porphyry. Noticeably, rocks occurred in the second intrusive period constitute the main body of the pluton. On the whole, the rocks are featured by high-K calc-alkaline characteristics, with SiO₂ content varying between 57.85 and 74.28 wt.%, K₂O content from 1.95 to 5.25 wt.%, and Na₂O content from 3.00 to 4.50 wt.%. The total REE content is between 113.91×10^-6 ppm and 169.47×10^-6 ppm. In addition, the values of (La/Sm)N, (Gd/Yb)N and (La/Yb)N fluctuate from 1.77 to 3.16, 1.33 to 2.12, and 3.41 to 5.64 respectively, showing a right-inclined distribution pattern characterized by weakly negative Eu anomalies with δEu value from 0.37 to 0.82. Moreover, the rocks are enriched in Th, Zr, Hf and depleted in Nb, Ta and Eu. In terms of deformation environment, quartz diorite and granodiorite of the first period mainly fall into the volcanic arc area of the pre-collision to the post-collision stage; rocks of the second period, namely monzogranite and K-feldspar granite, mainly formed in the post-collision stage; and graniteporphyry in the last period occurred in later orogenic stage. LA-ICP-MS zircon U-Pb dating results of quartz diorite, monzogranite and K-feldspar granite are 324.1±4.3 Ma, 314.9±4.1 Ma and 311.5±3.9 Ma. Considering the age of Bayingou ophiolite, which is 344±3.4 Ma according to previous researches, we can conclude that the suture zone of North Tianshan formed between 344 to 311.5 Ma and the major collision orogenic process took place from 324 to 311 Ma, and then the suture moved into the post-collision stage after 311 Ma. To some extent, the three intrusive rock units of Sikeshu granitic pluton may be representative products of three tectonic-magmatic stages, which are subduction, collision and intracontinental orogenic of North Tianshan orogenic belt.