Petrogenesis of the Kekesai composite intrusion, western Tianshan, NW China: Implications for tectonic evolution during late Paleozoic time

详细信息    查看全文

• 作者：Dongyang Zhang ; Zhaochong Zhang ; John Encarnaci¨®n ; Chunji Xue ; Shigang Duan ; Zhidan Zhao ; Junlai Liu
• 关键词：Granitoids ; Geochemistry ; Petrogenesis ; Slab break-off ; Chinese western Tianshan
• 刊名：Lithos
• 出版年：2012
• 出版时间：August, 2012
• 年：2012
• 卷：146-147
• 期：Complete
• 页码：65-79
• 全文大小：2298 K

文摘

The late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB), which is characterized by voluminous igneous rocks, particularly granitoids. The Kekesai composite granitoid porphyry intrusion, situated in the Chinese western Tianshan (southwest part of CAOB) includes two intrusive phases, a monzogranite phase, intruded by a granodiorite phase. LA-ICPMS U-Pb zircon analyses suggest that the monzogranitic rocks were formed at 305.5 ¡À 1.1 Ma, with a wide age range of inherited zircons (358-488 Ma and 1208-1391 Ma), whereas the granodioritic rocks formed at 288.7 ¡À 1.5 Ma. The monzogranitic and granodioritic phases have similar geochemical features and Sr-Nd-Hf isotopic compositions. They exhibit high and variable SiO₂ (66-71 wt. % ) and MgO (0.41-2.14 wt. % ) contents with some arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements and negative anomalies of Nb, Ta and Ti) and relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (I_Sr = 0.7055-0.7059), low positive ¦Å_Nd(t) (+ 0.84 to + 1.03) as well as a large variation in Hf isotopic compositions with ¦Å_Hf(t) between + 3.43 and + 14.8, implying both of them were derived from similar source materials. These geochemical characteristics suggest that they might be mainly derived from the partial melting of arc-derived Mesoproterozoic mafic lower crust with involvement of a mantle-derived component in variable proportions by mantle-derived magma underplating. The presence of late-Ordovician to earliest early Carboniferous inherited zircons and the Hf isotopic compositions in the monzogranitic sample, similar to that of the widespread juvenile arc rocks, indicates some crust contamination during magma emplacement. Our new data, combined with previous studies, imply that extensive post-collisional magmatism due to underplating of mantle-derived magma, could plausibly be explained by slab break-off regime.