Petrogenesis and tectonic implication of the Triassic syenitic complexes in the eastern Qinling orogen, central China

Abstract

Alkaline igneous rocks usually provide significant information on the mantle-derived components, magmatic processes and crust–mantle interaction due to their deep source. The Songxian syenite, located in the Xiong'er terrane of the Qinling Orogen, is one of the most typical alkaline intrusions in the Early Mesozoic. Geochemical, geochronological and in situ Sr–Nd–Hf isotopic analyses were conducted on the alkali-feldspar syenite and aegirine-augite syenite samples collected from the Songxian syenite, in order to reveal the magma sources, petrogenesis and possible geodynamic implications. All the rock samples are enriched in alkali (Na2O + K2O = 11.74 to 15.14 wt%), light rare earth elements and large ion lithophile elements, and depleted in high field strength elements. Zircon LA–ICP–MS U–Pb dating of medium- and fine-grained alkali-feldspar syenites yield emplacement ages of 235.5 ± 0.9 Ma and 232.7 ± 3.3 Ma, respectively. In addition, the aegirine-augite syenite generally developed as dyke or vein cutting through the two types of alkali-feldspar syenite in the field, indicating multiple pulses of magmatism. In-situ Sr-Nd isotopic analyses on magmatic apatite yielded (87Sr/86Sr)i ratios from 0.707959 to 0.710414 and εNd(t) from −19.3 to −13.9 for alkali-feldspar syenite, while the analyses for aegirine-augite syenite yielded (87Sr/86Sr)i ratios from 0.717948 to 0.721470 and εNd(t) from −25.5 to −24.5. The difference suggests that the Songxian syenite may be generated from multi-stage magmatic episodes. The first magmatic incident mainly originated from the partial melting of the enriched lithospheric mantle, mixed with the lower crust-derived melts. The second mixing magma might involve more proportion of Archean to Paleoproterozoic crust-derived materials. During the Early Mesozoic, the Mian-Lue oceanic slab subducted northward to the asthenosphere mantle and induced asthenosphere upwelling. Under the tectonic setting, the mixture of enriched mantle- and crust-derived melts occurred to form the Songxian syenite.