Post‐collisional Adakitic Porphyries in Tibet: Geochemical and Sr‐Nd‐Pb Isotopic Constraints on Partial Melting of Oceanic Lithosphere and Crust‐Mantle Interaction

Abstract

Abstract The distribution of Neogene felsic porphyries intruding in earlier granitic batholiths was mainly controlled by north‐south‐tending rifting zones and normal faults. The main rock types of the felsic porphyries include granodiorite‐porphyry, monzonitic granite‐porphyry and quartz monzonitic porphyry. The porphyries are characterized by high SiO2 (≥64.26%) and Al2O3 (>15% at 70% SiO2), low Y and HREE (Yb) contents, strong enrichment of LILE and LERR, especially K and Sr. Geochemical features of the porphyries show distinct adakitic magma affinity. Nd, Sr and Pb isotopic compositions of the porphyries form a linear alignment from MORB to EM2, suggesting a mixing of the MORB reservoir with the metasomatized mantle reservoir. Considering also the geochemical characteristics of the porphyries and the sequence of observable structural‐thermal‐magmatic events at Gangdise, it is thought that the Neogene porphyries were formed by partial melting of dead subducted oceanic crust in a post‐collision setting. K‐enrichment in the porphyries is attributed to the interaction of slab‐derived melts, i.e., adakites, with the metasomatized mantle during the ascent There might be a delamination of residual eclogites or amphibole eclogites before the eruption of potassic lava on the Tibetan plateau since 13 Ma.