Permian–Triassic highly-fractionated I-type granites from the southwestern Qaidam Basin (NW China): Implications for the evolution of the paleo-tethys in the eastern Kunlun orogenic belt

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• 作者：Yinjuan Ju ; Xiaoli Zhang ; Shaocong Lai ; Jiangfeng Qin
• 关键词：highly ; fractionated I ; type granites ; Permain–Triassic ; Qaidam Basin ; eastern Kunlun orogenic belt ; Paleo ; Tethys
• 刊名：Journal of Earth Science
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：28
• 期：1
• 页码：51-62
• 全文大小：
• 刊物主题：Earth Sciences, general; Geotechnical Engineering & Applied Earth Sciences; Biogeosciences; Geochemistry; Geology;
• 出版者：China University of Geosciences
• ISSN：1867-111X
• 卷排序：28

文摘

Late Paleozoic to Early Mesozoic granites are widespread in the southern Qaidam Basin, northern margin of the eastern Kunlun orogenic belt. Their petrogenesis can provide us insights into the tectonic evolution and crustal growth process in the Qaidam Basin. This paper reports Permian–Triassic granites from the Kunbei area, southwestern Qaidam Basin. Detailed zircon LA-ICP MS U-Pb dating reveals that the granites from the four drilling cores (q404, q406, q1612-8, q1613-8) have identical ages of 251±3, 256±4, 247±2, and 251±6 Ma, respectively, these ages are identical with the Permian–Triassic granites from the eastern Qaidam Basin. Detailed geochemical analyses indicate that these granites display typical affinities of highly-fractionated I-type granites: (1) they have high SiO₂ (up to 76.5 wt.%), Na₂O+K₂O (7.91 wt.% to 9.48 wt.%) contents and high FeOT/MgO values of 4.7 to 9.3, suggesting significant fractional crystallization; (2) their low A/CNK values of 0.54 to 1.03, no normative Al-rich minerals, inconsistent with the per-aluminous S-type granites; (3) their low Ga (14.5 ppm to 20.7 ppm) and 10 000×Ga/Al (2.23 to 3.03, most of them <2.6) values are inconsistent with the A-type granites; (4) the high Rb (191 ppm to 406 ppm) contents and Rb/Sr (2.1 to 13.4) ratios, as well as the significant negative Eu anomalies (0.10 to 0.42) also indicate significant fractional crystallization of feldspars; (5) their low P₂O₅ contents (0.02 wt.% to 0.10 wt.%) suggest the limited solubility of phosphorus in primitive metaluminous melts. In combination with the geological background, we propose that the Permian–Triassic highly-fractionated I-type granites resulted from partial melting of intra-crustal mafic rocks, and the primitive I-type granitic melts underwent significant fractional crystallization of feldspars. The occurrence of highly-fractionated I-type granites in the southwestern Qaidam Basin suggests a Permian–Triassic active continental margin in the northern margin of the East Kunlun orogenic belt.