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大洪山地区三里岗岩体出露于土门乡、三里岗镇和南必湾之间,整体呈北西—南东向条带状分布,主体为中—粗粒二长花岗岩,北西、南东端分别出露少量石英闪长岩和英云闪长岩。地球化学方面,三里岗岩体具有低钾、富铝的特点,为钙碱性(δ<3.3)I型花岗岩;轻、重稀土元素分异度高,无明显负Eu异常;富集大离子亲石元素Rb、K、Pb,亏损高场强元素Nb、Ta、Ti,应为一套活动大陆边缘岛弧钙碱性花岗岩。LA-ICP-MS锆石U-Pb测年结果显示,15个分析点谐和度都>90%,且分布较为集中,~(206)Pb/~(238)U年龄分布范围在854—904 Ma之间,加权平均年龄为(876.9±9.2)Ma(MSWD=4.1),属于晋宁期,代表三里岗岩体的形成年龄。三里岗岩体与土门—源潭一带的岛弧火山岩一起共同组成大洪山古洋盆的岩浆弧,二者都是大洪山地区晋宁期古洋盆向北俯冲的产物。
The Sanliang granite in the Dahongshan area is exposed between Tumen, Sanliang and Nanbaiwan, with an NW-SW distribution. The main body is medium-coarse-grained granite, northwest and south-east Side reveal a small amount of quartz diorite and diorite. In terms of geochemistry, the Sanliang granitoid is characterized by low potassium and aluminum enrichment, which is characterized by calc-alkaline (δ <3.3) I-type granites. The light and heavy rare earth elements are highly heterogeneous with no obvious negative Eu anomalies. Probite elements Rb, K, Pb, depleted high field strength elements Nb, Ta, Ti, should be a set of active continental marginal island arc calc-alkaline granite. LA-ICP-MS zircon U-Pb dating results show that the 15 analysis points have a harmonic degree> 90% and the distribution is relatively concentrated. The age distribution of 206 Pb / 238 U ranges from 854 to 904 Ma, with a weighted average age of (876.9 ± 9.2) Ma (MSWD = 4.1), belonging to the Jinning Period and representing the age of formation of the Sanliang granite. The Sanliigang granitoid and the island arc volcanic rocks along the Tumen-Yuantan area together form the magmatic arc of the Dahongshan paleo-oceanic crust, both of which are the result of the northward subduction of the ancient oceanic basins in the Jinning Period in the Dahongshan area.