Geochronology and geochemistry of late Paleozoic magmatic rocks in the Yinwaxia area, Beishan: Implications for rift magmatism in the southern Central Asian Orogenic Belt

摘要

Mafic-ultramafic rocks are distributed widely in the Beishan rift, which is located in the southern Beishan, central southern Central Asian Orogenic Belt. The Yinwaxia study area is located in eastern Beishan rift, where mafic-ultramafic rocks occur along major faults. The zircon SHRIMP U-Pb age obtained of a gabbro is 281 ±11 Ma, and the age of the basalt is constrained by the youngest xenocrystal with an age of 265 Ma, which substantiate that these mafic rocks formed in Permian. Basalts and gabbros exhibit similar geochemical characteristics including: high SiO_2, total Fe_2O_3 and TiO_2 contents; low MgO contents and Mg# values; and tholeiitic characteristics. Yinwaxia mafic rocks have relatively high total rare earth element contents, enrichment in light rare earth elements, enrichments in the high field strength elements, and obvious negative Nb-Ta-Ti anomalies. Basalts exhibit low (~(87)Sr/~(86)Sr)_i and high ε_(Nd)(t) values, while gabbros exhibit relatively high (~(87)Sr/~(86)Sr)_i and low ε_(Nd)(t) values. Isotopic compositions of these mafic rocks display a mixed trend between depleted and enriched mantles. Meanwhile, differing ε_(Nd)(t) values show that basalts were intensively contaminated by juvenile crustal materials, but gabbros were contaminated by older continental crust. We conclude that Yinwaxia mafic rocks were derived from lithospheric mantle metasomatized by fluids and/or melts from subducted slab; parental magmas underwent AFC processes, then emplaced along faults in a continental rift. We collected geochemical and geo-chronological data in the study area, and collated geochronological data from previous workers in the Beishan orogenic belt to develop a geochronological frequency diagram. From these data and analyses we deduced a model of tectonic evolution for the Beishan orogenic belt. Considering the geochemistry, sedimentological evidence for rifting, and the geochronological frequency diagram, we propose that the Beishan rift had entered a post-collision stage since Early Devonian, and then changed into a continental rift stage around late Carboniferous-early Permian.