GEOCHEMISTRY AND GEOCHRONOLOGY OF S-TYPE GRANITES AND MP/HT META-SEDIMENTARY ROCKS IN THE SOUTHWEST TIANSHAN, NW CHINA AND THEIR TECTONIC IMPLICATIONS

This paper presents the whole-rock geochemical data, in-situ zircon U-Pb and Hf isotopes, monazite U-Th-Pb age and pseudosection modeling results, in order to determine the genetic relationship between S-type granites and meta-sedimentary rocks in the Central Tianshan and their tectonic implications for the South Tianshan Orogenic Belt. The S-type granites have the LA-ICP-MS zircon U-Pb ages of 275±3 Ma, 266±13 Ma and 278±3 Ma, interpreted as the crystallization ages. They have high contents of SiO₂ (72.41–83.24 wt.%) and K₂O (2.96–8.47 wt.%) but low contents of Fe₂O₃t (0.61–2.19 wt.%), MgO (0–0.49 wt.%) and CaO (0.61–1.51 wt.%), with A/CNK ratios of 1.14 to 1.35. The SHRIMP zircon U-Pb dating for the metapelite yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 393±7 Ma and a significantly younger apparent ²⁰⁶Pb/²³⁸U age of 267±6 Ma, whereas the EMP monazite dating gave three age groups, i.e., 482±19 Ma, 366±10 Ma to 376±13 Ma, and 282±17 Ma. Two metamorphic events are identified, one is the regional anatexis event at ca. 393 Ma based on the recent published result, which is inferred to be related to the arc magmatism, and the other is an anatexis event at ca. 280 Ma in the post-collisional setting. Pseudosection modeling shows that the meta-sedimentary rocks have experienced the anatexis under a P-T condition of 7.21 kbar and 728 °C, consistent with medium-pressure facies series. Thus, the meta-sedimentary rocks are inferred to have been buried to the depths of middle-to-lower crust through the back-arc basin thickening during the continental collision at ca. 320–300 Ma, and then these rocks experienced the anatexis to form the S-type granites in the post-collisional setting with the heat possibly provided by the upwelling of asthenosphere and crustal thinning.