胶东早白垩世中-酸性脉岩造岩矿物微区地球化学特征研究

Microregional geochemical characteristics of rock-forming minerals in Early Cretaceous intermediate-felsic dikes in Jiaodong

胶东地区广泛发育早白垩世中-酸性脉岩群,但是其成因演化及成岩地质背景至今仍存在诸多争论。本文利用电子探针(EMPA)与激光剥蚀电感藕合等离子质谱(LA-ICP-MS)技术分析了胶东早白垩世石英闪长脉岩与闪长脉岩中主要造岩矿物(斜长石和黑云母)的主、微量元素组成;并结合岩石地球化学特征,对两者的岩浆源区和岩浆演化进行了研究探讨。石英闪长脉岩与闪长脉岩中黑云母低于检测线的Ca O含量与斜长石主量元素之间良好的线性关系指示两者为未受到后期变质作用影响的原生矿物,进一步说明胶东中生代石英闪长脉岩与闪长脉岩岩浆形成后,在上涌成岩过程中未受到变质作用的影响。石英闪长脉岩中壳源黑云母矿物成分基本一致的,以及斜长石正环带中核边部线性变化的An值与Fe、Mg、Sr、Ba等不相容元素特征指示石英闪长脉岩源于华北克拉通东部古老的加厚下地壳部分熔融作用,并在岩浆演化早期和晚期有一定幔源镁铁质岩浆混入,整个岩浆演化过程并未受到大气、俯冲、变质流体混入或构造作用的影响。闪长脉岩中黑云母矿物较大的Fe2+/(Fe2++Mg)比值范围,Al含量与结晶压力高度正相关以及斜长石中不相容元素特征指示本次研究中胶东闪长脉岩源自俯冲的板片来源的流体或沉积物混入所形成富集岩石圈地幔源区。胶东早白垩世石英闪长脉岩与闪长脉岩形成的大地构造动力学背景为古太平洋板俯冲-回撤引起热-机械侵蚀,进而导致岩石圈地幔减薄。在此情况下软流圈地幔上涌加热导致胶东富集岩石圈地幔部分熔融形成地幔熔体。这些幔源熔体经历分离结晶形成早白垩世闪长脉岩。此外,幔源镁铁质岩浆持续加热导致加厚下地壳部分熔融,形成了石英闪长脉岩。

Early Cretaceous intermediate-felsic dikes swarm widely spread in the Jiaodong Peninsula,however,their genetic relationship,evolution and tectonic setting are still controversial.In this study,the in-situ major-and trace elements of the plagioclase and biotite minerals from the Early Cretaceous quartz diorite and diorite dikes at Jiaodong are analyzed with an electron microprobe(EMPA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS).Combined with the whole rock geochemical components,we further study the origin and magmatic evolution of these dikes.The accurate linear relationship between the major elements of plagioclase and the extremely low CaO content of biotite indicate these minerals didn’t undergo late metamorphism,which illustrate that the quartz diorite and diorite dike magma has not affected by metamorphism during the magmatic evolution.The crustal genesis biotite of quartz diorite dikes shows consistent components,and the plagioclase has continuous trend of decreasing An value from core to rim and show the variation characteristics of Fe,Mg,Sr and Ba elements,both of which indicate that the quartz diorite dikes originated from partial melting of the ancient thickened lower crust of the eastern North China Craton(NCC),and mixed with mantle-derived mafic magma in the early and late stages of magmatic evolution.Quartz diorite was not influenced by the atmosphere,subduction,metamorphism fluids mixing or tectonism during the magmatic evolution.In the diorite dikes,the wide range of Fe^2+/(Fe^2++ Mg) ratio and the accurate linear relationship between Al and crystallization pressure of biotite,and incompatible element characteristics of plagioclase indicate that these diorite dikes derived from partial melting of an enriched lithospheric mantle metasomatized by fluids released by subduction plates and/or sediment.Early Cretaceous magmatism within the Jiaodong Peninsula occurred in an extensional tectonic setting involving lithospheric thinning,which was induced by thermo-mechanical erosion further caused by rollback of Paleo-Pacific slab.The upwelling of asthenosphere mantle resulted in partial melting of the lithospheric mantle enriched in Jiaodong,and these mantle-derived melts underwent crystallization to form the Early Cretaceous diorite dikes.In addition,continuous heating of mantle-derived mafic magma resulted in partial melting of the thickened lower crust and form the quartz diorite dike magma.