Underplating and assimilation-fractional crystallization of Mesozoic intrusions in the Tongling area, Anhui Province, East China: evidence from xenoliths and host plutons

摘要

This paper presents new petrographic observations and geochemical and microp-robe analyses for the Laomiaojishan, Xiaotongguanshan, and Tianebaodanshan intrusions in the Tongguanshan mineral district, East China. The plutons vary in composition from quartz monzonitic diorite to pyroxene monzonitic diorite, and contain gabbroic to dioritic xenoliths. The Xiaotongguanshan intrusion yields a SHRIMP zircon U-Pb age of 139.5 + 2.9 Ma, indicating Late Jurassic to Early Cretaceous magmatism in the Lower Yangtze River Valley. Relative to host rocks, the gabbro and diorite xenoliths are low in SiO_2 (52.03-54.61 wt- percent ), Al_2O_3 (12.87-14.43 wt- percent ), and total alkalis (Na_2O + K_2O; 5.26-6.30 wt- percent ), but high in MgO (5.41-11.66 wt- percent ); the host rocks have high SiO_2 (59.97-64.44 wt- percent ), Al_2O_3(16.43-17.59 wt- percent ), and total alkalis (6.67-8.25 wt- percent ), but are low in MgO (1.52-2.50 wt- percent ). Concentrations of rare earth elements (REEs) in the xenoliths (165.70-190.40 ppm) are similar to those in the host rocks (166.12-185.95 ppm), although the ratio of light REEs to heavy REEs in the xenoliths (3.39—4.27) is lower than that in the host plutons (4.86-5.94). All of the analysed rocks show similar REE patterns, although the xenoliths display marked positive Eu anomalies and the host rocks show slightly negative Eu anomalies. Values of epsilon Nd (t) ranges from —4.9 to -9.9 in the gabbro xenoliths and from — 11.4 to -11.9 in the host intrusives. Initial ~(87)Sr/~(86)Sr ratios are 0.7064-0.7073 in the xenoliths and 0.7072-0.7084 in the quartz monzonitic diorite host rocks. Crystallization temperatures of hornblende and plagioclase in the gabbro xenoliths, diorite xenoliths, and host rocks are 816, 773-790, and 664-725 deg C, respectively, based on an amphibole-plagioclase geotherm-ometer. The pressures recorded by these phases indicate that they formed at depths of 26, 12-15, and 3-4 km, respectively, based on an aluminum-in-hornblende geobarometer. The petrological and geochemical features of the analysed intrusions and xenoliths are consistent with their derivation from basic to intermediate-acidic magmas that possibly formed via a series of complex interactions between underplated, mantle-derived basaltic magma and varying amounts of middle- to lower-crustal material, followed by assimilation-fractional crystallization.