Mesozoic lower crustal xenoliths and their significances in lithospheric evolution beneath the Sino-Korean Craton

Abstract

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+ Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx ±Pl±Rt. P-T estimation indicates that the granoblastic assemblage crystallized at 765-890 °C and 1.25-1.59 GPa, corresponding to crustal depths of ca. 41-52 km with a geotherm of 75-80 mW/m2. Calculated seismic velocities (Vp) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm3. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub- alkaline basaltic compositions, equivalent to olivine- and quartz- tholeiite. REE patterns are flat to variably LREE-enriched (LaN/YbN= 0.98-9.47) without Eu anomaly (Eu/Eu*=0.95-1.11). They possess 48-127 ppm Ni and 2-20 ppm Nb with Nb/U and La/Nb ratios of 13-54 and 0.93-4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton. © 2002 Elsevier Science B.V. All rights reserved.