Petrogenesis and Geodynamic Implications of the Carboniferous Granitoids in the Dananhu Belt, Eastern Tianshan Orogenic Belt

This paper presents new LA-ICP-MS zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Sr-Nd isotopes systematically on porphyritic granitic and K-feldspar granitic intrusions from the Dananhu belt, eastern Tianshan orogenic belt (ETOB). Zircon U-Pb dating indicates that the porphyritic granitic and K-feldspar granitic plutons were formed at 357 +/- 3 and 311 +/- 3 Ma, respectively. The porphyritic granites show geochemical and isotopic characteristics (high SiO2, low MgO and Mg#, depleted Sr-Nd isotopic values (about 0.703 4 and 6.13, respectively), with Nb/Ta (13.3-14.7) and Zr/Hf (31.0-33.9) ratios) similar to those of the crustal-derived magmas. The above characteristics suggest they were probably originated from juvenile lower crustal materials. The K-feldspar granites also have high SiO2, low MgO and Mg-#, depleted Sr-Nd isotopic values (0.703 3-0.704 6 and 4.41-5.67, respectively). But some trace elements contents vary widely, with variable Nb/Ta (12.7-22.7), Zr/Hf (21.3-36.1) and Nb/La (0.38-1.07) ratios, indicating that the K-feldspar granites were formed by partial melting of juvenile lower crustal materials with old crustal materials. Combined with previous data on Carboniferous granitoids in the Dananhu belt, we infer that all the Carboniferous granitic plutons in the Dananhu belt were most likely emplaced in an island arc environment (Dananhu arc). Subsequently, a tectonic transition from oceanic subduction to post-collisional extension probably occurred in the ETOB.