Geochemical characteristics of Late Triassic lamprophyres from the western Zhen'an, South Qinling and its indicative significance for tectonic environment

The lamprophyres are good geological bodies reflecting deep tectonomagmatism and geochemical properties of source.In this paper, a systematic study has been carried out on the lamprophyre veins in the west of Zhen'an, South Qinling Mountains.Zircon U-Pb isotopes of the lamprophyre have been determined by LA-ICP-MS.The ²⁰⁶Pb/²³⁸U age of the lamprophyre is 222.2±1.2 Ma(MSWD=0.38, n=15), which represents the formation age of the vein body and belongs to the Late Triassic.Geochemical data show that the rocks belong to the calc-alkaline potassium-superpotassic series, enriched in LREE and large ion lithophile elements such as Rb and Ba, and deficient in HREE and high field strength elements such as Nb and Ti, which have the composition characteristics of mantle derived rocks in subduction zones.The isotopic compositions of the rocks are characterized by high initial(⁸⁷Sr/⁸⁶Sr)_i rations(0.7066~0.7076)and negative ε_Nd(t)values(-5.77~ -4.62);Furthermore, the ¹⁷⁶Hf/¹⁷⁷Hf rations of zircons range from 0.28250 to 0.28287, with ε_Hf(t)values from -4.75 to 1.89, The two-stage Hf model ages(T_DM2)and the whole-rock Nd isotopic model ages, are mainly concentrated in mesoproterozoic(1.3~1.5 Ga).Comprehensive analysis shows that the source of lamprophyre is Mesoproterozoic enriched mantle and formed in post-collision extensional tectonic environment.At the Late Triassic(approximately at 220 Ma), the enriched lithospheric mantle experienced metasomatism by subduction fluid, which occurred to partial melting under the conditions of tectonic decompression, heat and fluid from the upwelling deep materials, and produced mafic magma, the magma emplaced to shallow crust along tectonic fracture and formed the lamprophyres.