Constraints of the Xialan gabbroic intrusion in the Eastern Nanling Range on the early Jurassic intra-continental extension in eastern South China

Abstract

One highly disputed topic about the Jurassic igneous rocks in the eastern South China Block (SCB) is whether they were products of the Pacific plate subduction or the intra-continental extension of the SCB. The early Jurassic mafic rocks, which are regarded as important tracers for the mantle nature, geodynamic processes and tectonic setting, are of great importance to solve this controversy. This paper presents the detailed zircon U-Pb geochronological, elemental geochemical and Sr-Nd-Hf isotopic data for the Xialan gabbroic intrusion from the Eastern Nanling Range. SIMS zircon U-Pb dating results for olivine gabbro and hornblende gabbro yield weighted mean 206Pb/238U ages of 190.6±1.3Ma and 196.0±2.7Ma, respectively. Olivine gabbros are high in MgO, Mg-numbers and CaO and low in SiO2 and depleted in LILEs and LREEs. They have significant Ba, Sr and Eu positive anomalies with εNd (t) of +0.8 - +2.9, suggestive of their origination from an ancient lithospheric source. Hornblende gabbros have a wide compositional range of SiO2, MgO, Cr and Ni and are characterized by enrichment in LILEs and LREEs and insignificantly negative Nb-Ta anomalies, along with positive εNd (t) values ranging from +1.7 to +6.2 and zircon εHf (t) values from +9.7 to +12.7, indicative of their origination from the asthenosphere-lithosphere interaction. The estimated mantle potential temperature (1341–1407°C) of hornblende gabbros is similar to that of the western Basin and Range Province (1350–1450°C). In combination with the E-W-trending early Jurassic rock-association in the Nanling Range, it is herein proposed that the early Jurassic igneous rocks most likely formed in an intra-continental extension setting in response to the asthenospheric upwelling.