Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen: Accretion of a Cambrian Oceanic Plateau?
Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen: Accretion of a Cambrian Oceanic Plateau?
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祁连造山带南祁连增生带羽状蛇绿岩中的玄武岩和苦陨石：寒武纪大洋高原的增生？
DOI：
10.1016/j.lithos.2017.01.027
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发表时间：
2017
期刊：
影响因子：
3.5
通讯作者：
Xu Xin
中科院分区：
文献类型：
--
作者：
Zhang Yuqi;Song Shuguang;Yang Liming;Su Li;Niu Yaoling;Allen Mark B.;Xu Xin
现今洋壳中含高镁岩的海洋高原因其地幔-羽成因和异常高的地幔位温（Tp）而备受关注。然而，在古代海洋环境中，由于变形和变质作用，等效岩石通常保存较差。本文报道了中国北部祁连造山带南段拉集山和永靖地区寒武系蛇绿岩枕状熔岩的岩石学、年代学和地球化学资料。地球化学上可识别出3组岩石：(1)富洋中脊玄武岩（E-MORB）亲和的亚碱性玄武岩；(2)具有洋岛玄武岩（OIB）特征的碱性玄武岩，可能来源于富集地幔源的部分熔融；(3)含氧化镁的苦味酸（18-22 wt%）。苦橄岩尖晶石的Cr-数[Cr#= Cr/(Cr + Al)]表明，在1489 ~ 1600℃的地幔位温（Tp）估算值（相当于新生代夏威夷苦橄岩的1500 ~ 1600℃）下，其部分熔融程度为18 ~ 21%。其中一个辉长岩样品的锆石U-Pb Concordia年龄为525±3 Ma，表明其洋壳形成于寒武纪。现有证据表明，南祁连增生带保留了寒武纪地幔柱活动，并影响了区域构造：厚洋壳对海沟的“堵塞”解释了海洋高原的侵位和保存，并通过该地区俯冲带的重组产生了伴生的奥陶系内洋岛弧玄武岩。
Oceanic plateaus with high-Mg rocks in the present-day oceanic crust have attracted much attention for their proposed mantle-plume origins and abnormally high mantle potential temperatures (Tp). However, equivalent rocks in ancient oceanic environments are usually poorly preserved because of deformation and metamorphism. Here we present petrological, geochronological and geochemical data for pillow lavas from Cambrian ophiolites in the Lajishan and Yongjing regions of the South Qilian Accretionary Belt (SQAB), from the southern part of the Qilian Orogen, northern China. Three rock groups can be identified geochemically: (1) sub-alkaline basalts with enriched mid- ocean ridge basalt (E-MORB) affinity; (2) alkaline basalts with oceanic island basalt (OIB) features, probably derived from partial melting of an enriched mantle source; and (3) picrites with MgO (18–22 wt%). Cr-numbers [Cr#= Cr/(Cr + Al)] of spinels from the picrites suggest 18–21% degree of partial melting at the estimated mantle potential temperature (Tp) of 1489–1600 °C, equivalent to values of Cenozoic Hawaiian picrites (1500–1600 °C). Zircons from one gabbro sample yielded a U–Pb Concordia age of 525 ± 3 Ma, suggesting the oceanic crust formed in the Cambrian. Available evidence suggests that Cambrian mantle plume activity is preserved in the South Qilian Accretionary Belt, and influenced the regional tectonics: “jamming” of the trench by thick oceanic crust explains the emplacement and preservation of the oceanic plateau, and gave rise to the generation of concomitant Ordovician inner-oceanic island arc basalts via re-organisation of the subduction zones in the region.