Petrogenesis of an Early Permian bimodal intermediate‐felsic suite in the East Junggar in Central Asian Orogenic Belt and tectonic implications

Abstract

The bimodal intermediate‐felsic suites are rare, and their petrogenesis remains unsolved. Here, we report a bimodal intermediate‐felsic magmatic suite from East Junggar (NW China) composed of rhyolites, basaltic andesites, and andesites. The basaltic andesites and andesites are porphyritic with phenocrysts predominantly of plagioclase. The rhyolites are porphyritic with phenocrysts of alkali‐feldspar, quartz, and minor plagioclase. LA–ICP–MS zircon U–Pb dating yielded an age of 284 ± 2 Ma for rhyolites. The basaltic andesites and andesites show SiO2 ranging from 54.67 to 58.26 wt%, and exhibit moderate TiO2 (1.45–1.77 wt%), and K2O (1.59–2.07 wt%) with low MgO (1.51–2.25 wt%). They display enrichment in HFSE and LILE together with negative Th, Nb, Ta, Ti anomalies, and exhibit low 87Sr/86Sr(t) ratios (0.7040–0.7046), high positive εNd(t) values (+5.82 to +6.16) and young TDM ages (<0.6 Ga). The rhyolites exhibit highSiO2(>75 wt%), negative Nb, Ta anomalies and significant depletion in Ba, Eu, Sr, Ti, and P. They also show high εNd(t) values (+5.51 to +7.08) and young TDM ages (<0.6 Ga), which are close to those of the intermediate rocks. Except for elements dominantly controlled by fractional crystallization, the other trace elements of the bimodal suite display similar patterns. Based on these characteristics, the MELTS simulation further suggests that the felsic rocks were possibly evolved from the intermediate magmas through fractional crystallization of feldspar, clinopyroxene, iron‐titanium oxide, and apatite. In contrast, the magmas parental to basaltic andesites were derived from the remelting of basaltic rocks underplated beneath the lower crust. The petrogenesis of this bimodal suite also provides new insights into Permian crustal reworking in the East Junggar, and suggest that the Junggar Ocean was closed prior to the Early Permian.