内蒙古贺根山地区早白垩世花岗斑岩时代、成因及其地质意义

Ages, origin and geological implications of Early Cretaceous granite porphyry in Hegen Mountain, Inner Mongolia

贺根山花岗斑岩位于华北板块和西伯利亚板块结合部位,锆石^(206)Pb/^(238)U年龄加权平均值为137.6±1.6Ma,为早白垩世岩浆活动的产物。地球化学特征表现为主量元素总体富硅,且有较高的分异指数(DI)及碱/铝(NK/A)、Fe_2O/FeO值,较低的TFe、Ti、Mg、Ca和P含量;微量元素蛛网图则表现为富集大离子亲石元素Rb、Th、U,弱富集轻稀土元素La,Ce,明显亏损高场强元素Nb、Ba和Sr、Ti。稀土元素配分图表现为相对富集轻稀土元素、亏损重稀土元素,轻、重稀土元素分异较大、重稀土元素曲线较平坦的右倾型特征。综合分析认为,贺根山花岗斑岩属于高分异弱过铝质高钾钙碱性系列的I型花岗岩,其源区物质可能为来自高钾、正常水含量的长英质陆壳岩石物质。花岗斑岩成因可能是晚古生代末—中生代初古亚洲洋闭合引起的一系列板块碰撞作用(包括蒙古-鄂霍次克洋闭合),使造山后期地壳逐渐增厚并发生重力垮塌,导致构造环境由挤压转变为伸展,同时受古太平洋板块西向俯冲影响的结果。

Zircon U-Pb age of granite-porphyry in Hegen Mountain located in the conjunction of the North China plate and the Siberia plate is 137.6±1.6Ma,suggesting that the rockswere formed by magmatic evolution during Early Cretaceous.The geochemi-cal data indicate that major elements are characterized by enrichment of Si,higher DI and NK/A as well as Fe2O/FeO ratios.The trace element spider diagram indicates that graniteporphyry is relatively rich in Rb,Th,U,slightly enriched in light rare earth elements La,Ce,and depleted in high field strength elements such as Nb,Ba,Sr and Ti,while the distribution model of REE is right oblique,exhibiting relative LREE-enrichment and HREE-depletion,and the HREE curve is flat.These results suggest that the granite por-phyry is attributed to high differentiated metaluminous-weakly peraluminous high calc-alkaline series,being typical I-type granite.The source materials might have come from crustal felsic rocks with high K and normal water content.The formation of the granite porphyry may be related to the process from the extrusion to extension resulting from the lithosphere thinning after plates collision(including Mongolia-Okhotsk Ocean)with the closure of Paleo-Asian Ocean during the Late Paleozoic to the Early Mesozoic.It was also affected by the westward subduetion of the Paleo-Pacific Ocean.