Neogene porphyries from Gangdese: petrological, geochemical characteristics and geodynamic significances

The distribution of Neogene felsic porphyries located in earlier granitoid batholiths was mainly controlled by NS-striking rifting zones and normal faults. The felsic porphyries were spatially and temporally allocated with the basaltic dike swarms. The main rock types of the porphyries are granodiorite-porphyry, monzonitic granite-porphyry and quartz monzonitic porphyry. The porphyries are characterized by high-SiO2, Al2O3, K2O, Sr and Mg-# and low CaO, Y and HREE (Yb) contents with high Sr/Y and La/Yb ratios, and show shoshonitic or high-K calc-alkaline magmatic affinity. The porphyries have enrichment pattern of LILE and LREE with lack of Eu anomalies, stronger negative HFSE spikes as Nb and Ti. The porphyries show characteristics of adakite. Trace element systems indicate that the source of the porphyries is characterized by the presence of garnet, amphibole, clinopyroxene, and perhaps some titanate phase. On the base of the sequence of observable structural-thermal-magmatic events at Gangdese, Neogene porphyries in Gangdese were formed by partial melting of dead subducted slab during post-collision. K-enrichment in the porphyries is attributed to the interaction of slab-derived melts, i.e., adakites, with the metasomatized mantle during ascent. There might be a delamination of residual phase with eclogite/amphibole-eclogite. The delamination caused latest uplift during 20 similar to 14Ma in Gangdese. The temporal and spatial distribution of the felsic porphyries and basaltic dike swarms indicates that the initial age of EW extension in Gangdese is around 20Ma.