Geochronology and petrogenesis of shoshonitic igneous rocks, Luzong volcanic basin, middle and lower Yangtze River reaches, China

Abstract

The Luzong volcanic basin has a unique tectonic background, being located at the northeastern margin of the Yangtze craton and the foreland of the Dabie–Sulu orogen, near the intersection of the renowned Tanlu and Yangtze River faults. The volcanic and subvolcanic rocks in the basin are characterized by high potassium contents and are rich in alkali elements, and belong to a typical shoshonite series. Geochemically, these rocks are depleted in high field strength elements such as Nb and Ta, and enriched in highly incompatible elements such as Rb, Th, U, K, and light rare earth elements. Nd and Sr isotope compositions, combined with elemental geochemical characteristics, indicate that the parental magmas were formed mainly by partial melting of enriched subcontinental lithospheric mantle. The magmas underwent minor or negligible contamination by upper crustal rocks or fractional crystallization at low pressures (i.e., <1.5GPa). Nevertheless, variation trends for major and trace elements indicate that these shoshonitic mantle-derived magmas experienced fractional crystallization at high pressures (i.e., above the stability field of plagioclase, >1.5GPa), dominated by the removal of clinopyroxene and Fe–Ti oxides. The volcanic rocks of the Luzong Basin can be divided into four formations, which, from early to late, are the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan Formations, each separated by clear gaps in eruptive activity. In this study, we obtained zircon U–Pb SHRIMP ages from two subvolcanic samples from the Longmenyuan and Zhuanqiao Formations, which yield emplacement ages of 132±1Ma and 131±1Ma. These data, combined with ages reported by other workers, show that the duration of volcanism in the Luzong Basin (and the whole middle and lower Yangtze River reaches) was very short (⩽8Ma). We consider that in the Early Cretaceous the foreland area of the Dabie orogenic belt was under extension, influenced by the relative motion of the Pacific and Eurasian plates, and by sinistral displacement along the Tanlu Fault. At this time, rapid mechanical delamination of thickened lithosphere caused lithospheric thinning and resultant asthenospheric upwelling in the middle and lower Yangtze River region and the Dabie orogenic belt. These deep geological and tectonic events in turn produced the shoshonite volcanic rocks in the Luzong Basin.