U-Pb zircon geochronology, geochemical, and Sr-Nd isotopic constraints on the age and origin of basaltic porphyries from western Liaoning Province, China

摘要

Basaltic porphyries from the northeast North China craton (NCC) provide an excellent opportunity to examine the nature of their mantle source and the secular evolution of the underlying mantle lithosphere. In addition, the study helps to constrain the age and the mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr-Nd isotopic analyses of a suite of mafic lavas. Detailed laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating yielded an age of 223.3 ±1.1 million years, which we regard as representing the crystallization age of the basaltic porphyries. The bulk-rock analysed samples are enriched in both large ion lithophile elements (LILEs) (i.e. Ba, Sr, and Pb) and light rare earth elements (LREEs), but depleted in high field strong elements (HFSEs) (i.e. Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements (HREEs), without significant Eu anomalies (Eu/Eu* = 089-0.98). The basaltic porphyries have undergone low degrees (~5%) of partial melting of a garnet-bearing lherzolite mantle. The rocks display very uniform (~(87)Sr/~(86)Sr); (0.70557-0.70583) and negative ?Nd (t) values (-11.9 to -10.1). These features indicate that the western Liaoning basaltic porphyries were derived from a common enriched lithosphere mantle that had previously been metasomatized by fluids related to subduction of Palaeo-Asian sedimentary units. However, the mafic melts were not affected to a significant degree by crastal contamination. Based on earlier studies, these findings provide new evidence that the northeast margin of the NCC had undergone a phase of post-orogenic extensional tectonics during the Middle Triassic. Furthermore, lithospheric thinning occurring across the northern NCC might have been initiated during Early Triassic times and was likely controlled by the final closure of the Palaeo-Asian Ocean, as well as the collision of Mongolian arc terrenes with the NCC.