Geochronology and Geochemistry of Mafic Rocks in the Xuhe, Shaanxi, China: Implications for Petrogenesis and Mantle Dynamics

Abstract

AbstractThe Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase‐porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle‐normalized multi‐element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements (LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite‐normalized REE patterns display LREE enrichment (LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SEMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low MgO (basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg# (0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low (5%‐10%) and in garnet‐spinel transition facies. Sr‐Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMU and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back‐arc extension.