Xinyang mafic granulitic xenoliths and its significance for the early Mesozoic lower crustal nature on the south margin of the North China Craton

Abstract

Petrography, mineral chemistry, petrochemistry, trace elements, Nd isotope and calculated P-T conditions and petrophyscial nature about the mafic garnet granulite xenoliths collected from a Mesozoic volcaniclastic diatreme in Xinyang, south margin of the North China Craton, have been reported in this paper. The 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. Calculated seismic velocities (Vp) of granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm3. These samples have sub-alkaline basaltic compositions, equivalent to olivine- and quartz-tholeiitic basalts. 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. Present-day 143Nd/144Nd isotope ratios of the granulites are 0.512267 to 0.512700 with ε Nd(T) = -5.72 to + 1.01. Nd depleted mantle model ages (TDM) are 1.8 - 1.9 Ga. The primary materials of these mafic granulites possibly resulted in the magmas underplating in early Proterozoic, and may triggered by the subduction and collision of the Yangtze with the North China Cratons. The newly formed dense lower crust must have been significantly delaminated and modified, as evidenced by the mafic granulites carried up by the Cenozoic basalts on the North China Craton.