摘要
胶北高压麻粒岩地体位于华北克拉通东部,是目前为止华北克拉范围内唯一一处已知的古元古代高压基性麻粒岩、高温泥质麻粒岩和变质超镁铁质岩共存的地区。变质超镁铁质岩通常呈透镜状产出在TTG片麻岩之中,并在空间上与基性麻粒岩伴生。由于高级变质和蚀变作用的叠加,胶北变质超镁铁质岩的原岩时代和成因仍是尚未解决的难题。本文对莱西-莱阳一带的变质超镁铁质岩进行了详细的岩相学、矿物化学和全岩地球化学研究,并对其原岩结晶条件进行了估算,厘定其原岩主要为辉石角闪橄榄岩和橄榄辉石角闪石岩,主要由橄榄石、斜方辉石和角闪石组成,发育近等粒粒状变晶结构,普遍的三联点结构指示这三个矿物为平衡共生关系。同时,还可以看到橄榄石、斜方辉石和角闪石之间还存在着相互包裹的关系。橄榄石属贵橄榄石(Fo=79.87~89.23),辉石大多为古铜辉石(En=80.73~88.55),角闪石属于镁角闪石且在不同样品中的含量变化较大(20%~50%)。橄榄石(Mg^(#)=80~89)和斜方辉石(Mg^(#)=81~89)的Mg^(#)值变化范围较大,且均低于地幔成因的范围。角闪石的Mg^(#)值(82~91)、Ni(~1100×10^(-6))和Cr(平均~3600×10^(-6))含量较高,但仍远低于地幔捕虏体中角闪石的Cr含量。同时在主量元素协变图解中,变质超镁铁质岩的MgO与TiO_(2)、Al_(2)O_(3)、CaO和Cr_(2)O_(3)等主量元素显示出与堆晶岩一致的变化特征,而低的MgO(25.94%~34.91%)、Cr(平均2310×10^(-6))和高的FeO^(T)(9.34%~11.41%)含量,以及平坦的REE配分模式则能很好的与地幔岩区别开。因此研究区变质超镁铁质岩的原岩不是地幔残片,而是幔源岩浆底垫到下地壳后在~1200℃开始结晶分异形成的堆晶岩,随后与就位在下地壳的变基性岩和变泥质岩共生在一起并一同经历了麻粒岩相变质作用和抬升至地表的过程。
The Jiaobei high-pressure granulite terrane,located in the eastern part of the North China Craton,is currently the only known area within the North China Craton where Paleoproterozoic high-pressure mafic granulites,high-temperature pelitic granulites,and metamorphosed ultramafic rocks coexist.The meta-ultramafic rocks typically occur in lens-shaped bodies within TTG(tonalite-trondhjemite-granodiorite)gneisses,spatially associated with mafic granulites.Due to the superimposed effects of high-grade metamorphism and alteration,the protolith age and genesis of the Jiaobei meta-ultramafic rocks remain unresolved.In this study,we conducted detailed petrographic,mineral-chemical,and whole-rock geochemical investigations of the meta-ultramafic rocks from the Laixi-Laiyang region and estimated their crystallization conditions.The protolith of meta-ultramafic rocks are primarily composed of pyroxene hornblende peridotite and olivine pyroxene hornblendite,with the main minerals being olivine,orthopyroxene,and hornblende.They exhibit a nearly homogranular granoblastic texture,and the common triple-point structure suggests a balanced co-existence of these three minerals.Additionally,the olivine,orthopyroxene,and hornblende display mutual inclusions.The olivine belongs to chrysolite(Fo=79.87~89.23),the pyroxene is predominantly enstatite(En=80.73~88.55),and the hornblende is magnesiohornblende,with a varying content(20%~50%)across different samples.The Mg^(#)values of olivine(Mg^(#)=80~89)and orthopyroxene(Mg^(#)=81~89)show significant variation and are generally lower than those typical of mantle-derived materials.The hornblende has high Mg^(#)values(82~91),and elevated concentrations of Ni(~1100×10^(-6))and Cr(average~3600×10^(-6)),though still far below those of hornblendes in mantle xenoliths.In the major element covariation diagrams,the meta-ultramafic rocks show variations in MgO that are consistent with those of cumulate rocks,with low MgO(25.94%~34.91%),high FeO^(T)(9.34%~11.41%),average Cr(2310×10^(-6)),and a flat REE(rare earth element)distribution pattern,which clearly distinguish these rocks from mantle peridotites.Therefore,the protolith of the meta-ultramafic rocks in this study area is not a mantle residue but rather a cumulate formed through fractional crystallization of mantle-derived magma emplaced at the lower crust,initiating at approximately 1200℃.Subsequently,these cumulates coexisted with metamorphosed mafic and metapelitic rocks within the lower crust,undergoing granulite-facies metamorphism and subsequent exhumation to the surface.
作者
张华桥
高亢亢
商光锐
申梦梦
周李岗
周艳艳
翟明国
ZHANG HuaQiao;GAO KangKang;SHANG GuangRui;SHEN MengMeng;ZHOU LiGang;ZHOU YanYan;ZHAI MingGuo(State Key Laboratory of Lithospheric and Environmental Coevolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China)
出处
《岩石学报》
北大核心
2025年第7期2335-2362,共28页
Acta Petrologica Sinica
基金
国家自然科学基金项目(42220104008)资助.
关键词
胶北地体
古元古代
变质超镁铁质岩
原岩成因
Jiaobei terrane
Paleoproterozoic
Meta-ultramafic rocks
Protolith origin
