U–Pb zircon geochronology, geochemistry and geodynamic significance of basaltic trachyandesites and trachyandesites from the Jianchang area, western Liaoning Province, China

Abstract

Basaltic trachyandesites and trachyandesites from the northern 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, our study of these rocks helps to constrain the age and mechanism of NCC lithospheric destruction. In this paper, we report geochronological, geochemical, and Sr–Nd isotopic analyses of the Niujiaogou (NJG) basaltic trachyandesites and trachyandesites. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb dating yielded an age of 120.7±0.8million years, which we regard as the crystallization age of the rocks. Analysed whole-rock samples are enriched in both light rare earth elements and large ion lithophile elements (i.e. Rb, Sr and Ba), but depleted in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, and Ti), with slightly negative to positive Eu anomalies (Eu/Eu*=0.73–1.09). The NJG basaltic trachyandesites and trachyandesites are characterized by low MgO (2.42–3.69wt.%), Cr (10.3–24.6ppm) and Ni (17.1–25.6ppm), suggesting that they may have originated from an extremely evolved magma. In addition, the rocks display negative εNd (t) values (−12.2 to −8.5), which indicate that they were derived from a common lithosphere mantle that had previously been metasomatized by fluids related to subduction of Paleo-Asian Ocean sedimentary units. This magmatism may have been induced by large-scale, transtensional strike-slip on the Tan-Lu fault zone. In accord with earlier studies, these findings provide evidence that the lithospheric mantle source beneath western Liaoning Province was not changed much by the extensive Mesozoic magmatism during Triassic to early Cretaceous time.