华北陆块南部元古宙熊耳群火山岩的成因与构造环境:事实与争议

Petrogenesis and Tectonic Setting of the Paleoproterozoic Xiong'er Group in the Southern Part of the North China Craton:a Review

位于华北陆块南部的熊耳群形成于古元古代1.80～1.75Ga,以火山熔岩占绝对优势,沉积岩和火山碎屑岩仅占地层总厚度的4.3%。熊耳群火山岩是华北陆块结晶基底形成后规模最大、涉及范围最广的火山活动产物,基岩出露面积约7000km2,地层厚度3000～7000m,以玄武安山岩、安山岩为主,次为英安-流纹岩,SiO2=60%±的岩石较少,显示双峰特点。中基性熔岩的主要造岩矿物是斜长石和辉石,几乎没有角闪石和黑云母。其岩石化学成分的突出特点是高Fe,K,而低Al,Ca和Mg,火山岩显示富集大离子亲石元素和轻稀土元素、相对亏损高场强元素的岛弧型地球化学特征。Nd同位素和微量元素特征表明,这种地球化学特征是地幔源区遭受地壳组分改造及少量岩浆上升过程中的地壳混染造成的。熊耳群中的沉积岩主要分布在大古石组及马家河组。沉积岩的岩石学和地球化学特征表明熊耳群形成于被动大陆边缘的构造环境,结合火山岩的结构、构造特征,表明火山喷发的主体环境是海相,经历了陆相到海相再到陆相的演变过程,即:伴随着裂谷的发育、地面沉降和海水的侵入,以及随着火山喷发的进行最后又高出海面暴露于地表的全过程。熊耳群形成后,海侵范围更广、海水逐渐加深,表明地壳逐步向更大的凹陷发展。熊耳群形成于夭折的三叉裂谷环境,其火山岩所具有的岛弧型火山岩的地球化学特征是继承于受俯冲组分改造的陆下岩石圈富集地幔源区。熊耳期岩浆活动在华北陆块广泛分布,除了陕西宝鸡附近及山西吕梁地区的火山岩,还广泛分布1.75Ga前后的基性岩墙群、A-型花岗岩以及比熊耳群稍晚的斜长岩、奥长环斑花岗岩等(1.75-1.70Ga)和碱性花岗岩类(～1.65Ga)。这些岩浆岩和熊耳群火山岩在源区性质、岩石成因及其形成的构造背景等方面,可能存在必然的联系。上述岩浆岩的发育,说明华北陆块在1.80Ga后处于地壳拉伸、减薄,很可能沿早先拼合的薄弱部位最先发生破裂,是华北陆块裂解的产物。

The Xiong＇er Group formed in 1.80-1.75 Ga of Paleo-Proterozoic and contains predominantly volcanic lavas with minor sedimentary rocks and volcanic clastic rocks （4.3% in thickness）. It is the largest magmatism in scale after the formation of the crystalline basement of the North China Craton, which vary from 3 km to 7 km in thickness, and crops out over an area of 60 000 km^2. The volcanic lavas are composed predomiantly of basaltic andesites and andesites, and minor dacitic-rhyolitic rocks. The volcanic lavas are lack of the rocks with SiO2 = 62% ± and therefore appear as a bimodal association. The mineral assemblage of mafic-intermediate lavas mainly contains pyroxene and plagioclase, and rarely with amphibole and bitotite. The rocks are characteristic of high Fe and K and low Al, Ca and Mg contents. All the volcanic rocks are enriched in large-ion lithophile elements （LILE, eg, Rb, Ba） and light REEs （LREE） and are depleted in high field strength elements （HFSE, e.g., Nb, Ta and Ti）, suggesting an arc-related setting. The geochemical and isotopic compositions of the lavas indicate derivation from a mantle source previously contaminated by subducted material and minor crustal contamination during the upwelling of the magmas. The sedimentary rocks of the Xiong＇er Group formed the Dagushi and Majiahe Formations. The rock assemblage and geochemisty of the sedimentary rocks indicate that the Xiong＇er Group formed in a passive continental margin and it was mainly a marine facies when lava erupted. Therefore the paleogeographic environment of the Xiong＇er Group evolved from continental facies to marine facies then to continental facies again. It is suggested that the Xiong＇er Group was originated from a failed trident continental rift environment. The arc-like geochemisty resulted from an enriched subcontinental lithospheric mantle previously contaminated by subducted slab. The volcanic rocks of the Xiong＇er Group is also widespread in the Baoji and Lvliang area and also appear as 1.75 Ga-old mafic dyke swarms, A-type granites, later anorthosite and rapakivi granites （1.75-1.70 Ga）, and alkaline granitoids （-1.65 Ga）. These intrusive rocks are assumed to have genetic link to the volcanic rocks of the Xiong＇er Group. These intrusive rocks resulted from the breakup of the North China Craton, suggesting that the North China Craton was under an extention environment at -1.80 Ga. However, a detailed work is worthwhile to figure out whether a post-collision magmatism or a mantle plume that triggered the emplacement of the intrusive rocks.