The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology
The subducted oceanic crust within continental-type UHP metamorphic belt in the North Qaidam, NW China: Evidence from petrology, geochemistry and geochronology
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DOI：
10.1016/j.lithos.2007.12.001
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发表时间：
2008-08
期刊：
影响因子：
3.5
通讯作者：
Guibin Zhang;Shuguang Song;Lifei Zhang;Y. Niu
中科院分区：
文献类型：
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作者：
Guibin Zhang;Shuguang Song;Lifei Zhang;Y. Niu
柴北缘超高压大陆型变质带东段沙流河剖面沿着花岗质和泥质片麻岩中，三种类型的榴辉岩与一条蛇纹化方辉橄榄岩块体共存。橄榄岩（Ol 1）和斜方辉石在harzburtite的成分类似于今天的深海橄榄岩。蓝晶石-榴辉岩来源于橄榄质原岩，绿帘石-榴辉岩来源于辉长质原岩，二者均具有明显的Eu正异常，低TiO 2，高Al 2 O 3和MgO。蓝晶石-榴辉岩具继承性堆晶分层。多硅白云母-榴辉岩具有高TiO 2、低Al 2 O3和MgO，微量元素不相容，类似于富集型MORB。Sr-Nd同位素数据表明蓝晶石榴辉岩和绿帘石榴辉岩的原岩（[87 Sr/86 Sr]i~0.703-0.704; εNd（T）~5.9-8.0）均为地幔成因（例如，海洋地壳特征）。另一方面，多硅白云母-榴辉岩的εNd（T）值较低（1.4-4.1），与MORB原岩的富集程度基本一致，而其高[87 Sr/86 Sr]比值（0.705-0.716）则表明其在俯冲带环境中经历了额外的富集。野外对比和地球化学分析表明，蛇纹化方辉橄榄岩和三种类型的榴辉岩构成了从地幔橄榄岩到堆晶岩再到上部玄武质岩石的蛇绿岩序列的大洋岩性剖面。绿辉石加温压计算中柯石英假像和石英出溶体的存在表明榴辉岩经历了超高压变质作用（即，峰值P≥ 2.7GPa）。方辉橄榄岩可能也经历了同样的变质作用，但缺乏石榴石，表明压力条件≤3.0 GPa。锆石U-Pb SHRIMP测年表明，榴辉岩的原岩年龄为516±8 Ma，变质年龄为445±7 Ma。这些资料表明早奥陶世之前在柴达木地块和祁连地块之间存在古祁连洋。蛇绿岩组合可能是奥陶纪-志留纪大陆物质俯冲及最终大陆碰撞前洋壳俯冲的遗迹。这些岩石记录了从洋壳俯冲到大陆俯冲再到大陆碰撞的完整历史。
Three types of eclogite, together with a serpentinized harzburgite, coexist as blocks within granitic and pelitic gneisses along the Shaliuhe cross section, the eastern part of the North Qaidam continental-type ultrahigh-pressure (UHP) metamorphic belt, NW China. The olivine (Ol1) and orthopyroxene in the harzburgite are compositionally similar to present-day abyssal peridotites. The kyanite–eclogite is derived from a troctolitic protolith, whereas the epidote–eclogite from a gabbroic protolith, both having distinct positive Eu anomalies, low TiO2, and high Al2O3and MgO. The kyanite–eclogite shows inherited cumulate layering. The phengite–eclogite has high TiO2, low Al2O3and MgO with incompatible trace elements resembling enriched-type MORB. Sr–Nd isotope data indicate that the protoliths of both kyanite–eclogite and epidote–eclogite ([87Sr/86Sr]i~0.703–0.704; εNd(T)~5.9–8.0) are of mantle origin (e.g., ocean crust signatures). On the other hand, while the lower εNd(T) value (1.4–4.1) of phengite–eclogite is more or less consistent with an enriched MORB protolith, their high [87Sr/86Sr]iratio (0.705–0.716) points to an additional enrichment in their history, probably in an subduction-zone environment. Field relations and geochemical analyses suggest that the serpentinized harzburgite and the three types of eclogite constitute the oceanic lithological section of an ophiolitic sequence from mantle peridotite, to cumulate, and to upper basaltic rocks. The presence of coesite pseudomorphs and quartz exsolution in omphacite plus thermobarometric calculations suggests that the eclogites have undergone ultrahigh pressure metamorphism (i.e., peak P≥2.7 GPa). The harzburgite may also have experienced the same metamorphism, but the lack of garnet suggests that the pressure conditions of ≤3.0 GPa. Zircon U–Pb SHRIMP dating shows that the eclogites have a protolith age of 516±8 Ma and a metamorphic age of 445±7 Ma. These data indicate the presence of a Paleo-Qilian Ocean between Qaidam and Qilian blocks before the early Ordovician. The ophiolitic assemblage may be the relics of subducted oceanic crust prior to the subduction of continental materials during Ordovician–Silurian times and ultimate continent collision. These rocks, altogether, record a complete history of ocean crust subduction, to continental subduction, and to continental collision.