摘要
分布于华北陆块南部的熊耳群火山岩由玄武质到流纹质火山岩组成,并以玄武安山岩、安山岩为主,次为英安-流纹岩,SiO2=62％±的岩石较少,显示双峰特点。中基性熔岩的主要造岩矿物是辉石和斜长石,没有角闪石和黑云母,表明熊耳群火山岩形成于相对无水的环境。岩石地球化学上的显著特点是,富K2O、FeO,低Al2O3、MgO、CaO;富含大离子亲石元素(LILE,如K、Rb、Ba)和轻稀土元素(LREE),相对亏损高场强元素(HFSE,Nb、Ta、Ti),表现出岛弧型火山岩的地球化学亲合性。文中详细的岩石地球化学证据表明,熊耳群火山岩具有岛弧型地球化学特征的主要原因是其地幔源区遭受俯冲带组分的改造,而岩浆在上升到地表的过程中混染地壳物质是次要的。基于熊耳群及下伏结晶基底的地质学和地球化学特征,表明熊耳群形成于夭折的三叉裂谷环境,推断俯冲带组分的改造和富集事件发生在华北陆块南部古元古代大陆壳的形成和 拼贴过程中,由于洋壳和地壳物质再循环到上地幔中,使陆下岩石圈地幔源区富集LILE、LREE并亏损HFSE。熊耳期岩浆作用正是源于这样的保留有早期俯冲带组分改造特征的陆下岩石圈富集地幔。
The volcanics of the Xiong'er Group are widely distributed in Henan, Shanxi and Shaanxi provinces, distributing an area of more than 60000 km2. Most of the volcanic sequences vary from 3 km to 7 km in thickness, overlying unconformably the crystalline basement of Archean and palaeo-Proterozoic age, and is covered with the middle-upper Proterozoic terrestrial clastic rocks and carbonate rocks. The volcanic rocks of the Xiong'er Group appear as a bimodal association, and the rocks with SiO2 = 62%± are rare. The rocks are composed predomiantly of basaltic andesites and andesites, and minor dacitic-rhyolitic rocks. The mineral assemblage of mafic-intermediate rocks mainly contains pyroxene and plagioclase, and amphiboles and bitotites have not been observed, suggesting an anhydraous environment of their formation. The volcanic rocks of the Xiong'er Group are rich in K2O and FeO and poor in A12O3, MgO and CaO. Geochemically the rocks are highly enriched in large-ion lithophile elements (LILE, e. g. , K, Rb, Ba) and light REEs (LREE) and are depleted in high field strength elements (HFSE, e.g. , Nb, Ta and Ti), probably reflecting the involvement of the continental materials during their eruption, or a derivation from an enriched lithospheric mantle source. The chemical and isotopic data presented in this paper show that the characteristics of the rocks are mainly controlled by a modified enriched mantle source with ancient subduction belt materials during the formation and consolidation of the Archean to paleo-Proterozoic continental crust. Based on the geological features of the Xiong'er Group and the underlying basement, it is suggested that the Xiong'er Group is originated from a failed trident continental rift environment.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2002年第1期56-69,共14页
Acta Petrologica Sinica
基金
国家自然科学基金项目(编号:49502028
40072061)资助.
关键词
双峰式火山岩
地球化学特征
地幔源区混染
元古宙
熊耳群
华北陆块
Proterozoic, Bimodal volcanic rocks, Geochemical characteristics, Subcontinental mantle source, Ancient subduction belt, North China Craton, Xiong'er Group
