华北克拉通新太古代板块俯冲作用:来自山西云中山地区变质高镁火成岩组合的证据

在华北克拉通中部的山西云中山地区,新太古代花岗闪长质片麻岩中存在一些超镁铁质岩-镁铁质岩块及由斜长角闪岩、角闪变粒岩、石英岩和石榴夕线黑云片岩等岩石类型构成的变质表壳岩残片,其中的超镁铁质-镁铁质岩、斜长角闪岩和角闪变粒岩构成一套高镁火成岩组合。超镁铁质岩已变质为橄榄绿泥阳起片岩等岩石类型,呈变余斑状结构,橄榄石斑晶仍有保存;岩石SiO_2含量为39.22%～44.99%,Al_2O_3为8.82%～13.47%,Mg O为19.24%～22.13%,Na_2O+K_2O=0.71%～1.11%,CaO为5.75%～8.42%;Al_2O_3/TiO_2=14.8～17.4,CaO/Al_2O_3=0.60～0.84;化学成分上与科马提岩有一定的相似性。与之紧密伴生的斜长角闪岩也具有高镁特征,Mg O含量为11.28%～15.09%,铝、硅和碱质均偏低,具正铕异常,显示堆晶辉长岩的特征。非高镁斜长角闪岩有相对高的铝、硅和碱质,其原岩应为钙碱性玄武岩。角闪变粒岩样品的SiO_2含量为54.21%～55.71%,Al_2O_3为14.24%～15.49%,Mg O为6.26%～8.28%,Fe OT/Mg O=1.11～1.58,高钠低钾,Na_2O+K_2O=3.7%～4.78%,Na_2O/K_2O=5.15%～13.13,Mg#=53.0～61.5,属于高镁安山岩。由超镁铁质质岩-斜长角闪岩-角闪变粒岩构成的变质高镁火山岩组合具有钙碱性系列趋势。超镁铁质岩稀土元素含量总量较低,具有轻稀土富集和重稀土亏损的稀土型式;斜长角闪岩与超镁铁质岩比较,除富集大离子亲石元素和Cr、Ni明显较低外,具有相似的微量元素图谱形态。三种岩石类型在微量元素蛛网图上均显示出Ta、Nb、Ti负异常和Pb正异常。野外产状和岩石地球化学特征表明超镁铁质岩和高镁斜长角闪岩属于阿拉斯加型杂岩体,角闪变粒岩属于赞岐岩质高镁安山岩。在Zr/Nb-Nb/Th和Nb/Y-Zr/Y构造环境判别图解上显示出与俯冲相关的演化趋势,在Hf-Th-Ta、Nb/La-(La/Sm)N和Th/Yb-Nb/Yb图解上也落在岛弧钙碱性岩石区域。以上特征表明高镁火成岩组合形成于与板块俯冲相关的岛弧构造背景。野外地质关系和锆石U-Pb年龄限定高镁火成岩组合形成时代在～2.5Ga。云中山地区阿拉斯加型镁铁质-超镁铁质杂岩与赞岐岩质高镁安山岩共生,表明该地区存在新太古代的板块俯冲作用,为太古宙存在板块构造机制提供了新证据。

Neoarchean subduction in North China Craton: New evidence from the metamorphic high-Mg igneous assemblage in Yunzhognshan area, Shanxi Province

Some mafic-ultramafic rock fragments and metamorphosed supracrustal rock fragments consisting of amphibolites, Hb-leptynite, quartzite and garnet-mica-schist were found as residues occur in the Neoarchean augen granodioritic gneiss, in Yunzhongshan area, the central North China Craton (NCC). The mafic-ultramafic rocks, amphibolite and Hb-leptynite constitute a metamorphic high-Mg igneous assemblage. Ultramafic rocks that metamorphosed into olivine-chlorite-actinolite-schists preserve remnant porphyritic texture with olivine-phenocryst, and which are characterized by high contents of MgO (19.24%~22.13%), Cr (1935×10^-6~3345×10^-6) and Ni (706×10^-6~988×10^-6), and low contents of SiO₂ (39.22%~44.99%) and alkalis (Na₂O+K₂O=0.71%~1.11%), with relative low Al₂O₃/TiO₂ (14.8~17.4) as well as CaO/Al₂O₃ (0.60~0.84). In major element compositions ultramafic rocks are somewhat similar to the komatiitic rocks. Amphibolites can be divided into two groups of high-MgO and non high-MgO, the MgO content of the former is 11.28%~15.08%, and of the latter 7.68%~9.07%. The high-MgO amphibolites coexist closely with ultramafic rocks, and are characterized by relative low contents of SiO₂, Al₂O₃ and alkalis, high Mg^# (65.7~72.7), with positive Eu anomalies. These features suggest that the high-MgO amphibolites are metamorphic gabbro from fractional crystallization of mantle-derived magma. The non high-MgO amphibolites have relative high contents of SiO₂, Al₂O₃ and alkalis, with very weak positive Eu anomalies, which infer that their protolith are calc-alkali basalts. The chemical composition of Hb-leptynite (SiO₂ 54.21%~55.71%, Al₂O₃ 14.24%~15.49%, MgO 6.26%~8.28% and FeO^T/MgO=1.11~1.58) reveal the character of high-Mg andesite. The high-Mg igneous assemblage shows a calc-alkaline series trend. REE patterns of light REE enrichment and trace-elements spider diagram with Nb, Ta, Ti negative anomalies, Pb positive anomaly imply crustal material contamination and tectonic setting of arc magmatism. The geological occurrence and the feature of petrology and geochemistry indicate that ultramafic-mafic rocks akin to Alaskan-type complex, and Hb-leptynite is similar to sanukitic high-andesite. High field strength element ratios of high-Mg igneous assemblage indicate effects of subduction on the Zr/Nb vs. Nb/Th and Nb/Y vs. Zr/Y diagrams, and drop on the field of island-arc calc-alkaline rocks on the Hf-Th-Ta, Nb/La vs. (La/Sm)_N and Th/Yb vs. Nb/Yb diagram. Above characteristics show that the igneous assemblage formed in island-arc tectonic setting related to subduction. Field geological relations and zircon U-Pb age constraint that high-Mg igneous assemblage and other metamorphic supercrustal rocks form in~2.5Ga, and mean that Neoarchean subduction occur in the central NCC.