Origin, petrogenesis and tectonic implications of the Jiasha potassic intrusive complex in the giant Gejiu Sn-Cu polymetallic field, SW China

The potassic intrusive complex is a series of mafic to felsic rock assemblages with high potassium content and is characterized by large amounts of K-feldspar, plagioclase, biotite and, hornblende. The Jiasha potassic intrusive complex (JPIC) comprises monzogabbro, porphyritic monzonite, quartz monzonite, and monzogranite. The zircon U-Pb dating shows that the ages of monzogranite are 86.5-87.0 Ma. The ages of monzogabbro, porphyritic monzonite, and quartz monzonite are 85.7-86.9 Ma, 79.1-83.3 Ma, and 77.2-78.0 Ma. The JPIC belongs to metaluminous to weak peraluminous and show affinities of shoshonite. The JPIC have high (87Sr/86Sr) i values (0.7097-0.7118) and a wide range of epsilon Nd(t) values (-5.03 to -8.24). The zircon 176Hf/177Hf isotopic ratios are homogeneous (0.2825-0.2827), epsilon Hf(t) values vary from -2.1 to -7.1. The ratios of Pb isotope are: (206Pb/204Pb)i = 18.1180-18.5021, (207Pb/204Pb)i = 15.6180-15.7681, (208Pb/204Pb)i = 38.7160-39.3672. The whole-rock geochemical, zircon Hf and whole-rock Sr-Nd-Pb isotopic characteristics, together with mineral composition collectively suggest that the JPIC resulted from fractional crystallization and varying degrees of crustal contamination of enriched lithospheric mantle. The depletion of Nb, Ta and Ti in JPIC is inherited from the source region, which may be caused by the infiltration of slab-derived fluid or melt into the mantle when the Paleo-Pacific subducted to the South China. The late Cretaceous magmatism and mineralization activities in Gejiu is the product of the subduction of the Palaeo-Pacific Plate beneath the Eurasian continent, which probably formed under an Andean-type active continental margin setting.