Abstract:

This paper presents petrographic observations and geochemical analyses on the Laomiaojishan, Xiaotongguanshan and Tianebaodanshan intrusions with quartz monzonitic dioritic composition and xenoliths with gabbroic to dioritic compositions exposed in the Tongguanshan ore field, Tongling, Anhui Province, East China, and provides some evidence for the origin of the intrusions and xenoliths. The Xiaotongguanshan intrusion is dated at (139.5±2.9） Ma using the SHRIMP zircon U-Pb method and represents product of the Late Jurassic to Early Cretaceous magmatism in the lower valley of Yangtze River. The gabbro and diorite autoliths are low in SiO₂ (52.03 %-54.61 %), Al₂O₃ (12.87 %-14.43 %) and total alkalis (Na₂O+K₂O) (5.26 % - 6.30 %), but high in MgO (5.41 %-11.66 %), relative to the hosts which have high SiO₂ (59.97 %-64.44 %), Al₂O₃ (16.43 %-17.59 %) and total alkalis (6.67 %- 8.25 %), but low in MgO (1.52 %-2.50 %). The gabbro and diorite autoliths have similar total REE content ranging from 165.70×10^-6 to 190.40×10^-6 to that of the hosts from 166.12×10^-6 to 185.95×10^-6, but have smaller ratios of LREE to HREE ranging from 3.39 to 4.27 than those of the hosts from 4.86 to 5.94. All of the rocks show very similar REE patterns, but the gabbro and diorite autoliths display evident positive Eu anomalies, and the hosts indicate slightly negative Eu anomalies. The values of epsilon Nd (t) vary from -4.9--9.9 in the gabbro autoliths to -11.4--11.9 in the hosts. Initial ⁸⁷Sr/⁸⁶Sr ratios define ranges of 0.706,4-0.707,3 in the gabbro and diorite autoliths and 0.707,2-0.708,4 in the quartz monzonitic diorite hosts, respectively. The petrological and geochemical features of the Late Jurassic to Early Cretaceous intrusions with gabbroic to dioritic autoliths in the Tongguanshan ore field are consistent with an origin from basic to intermediate-acidic magmas that were possibly formed by a series of complex interactions of the underplated basaltic magma with different amount of middle to lower crustal materials through a process of mantle-derived magma underplating followed by assimilationfractionation crystallization.

Key words: Late Jurassic to Early Cretaceous, basaltic magma underplating, assimilation and fractionation crystallization, intermediate-acidic igneous rock, xenolith, lower valley of Yangtze River