Why do we need to propose the Early Cretaceous Comei large igneous province in southeastern Tibet?
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摘要:
近年在西藏东南部特提斯喜马拉雅带东段大规模白垩纪火成岩受到了很多学者的关注。这里的火成岩岩石类型包括玄武岩、镁铁质岩墙/岩床、辉长岩侵入体以及少量层状超镁铁质岩和酸性火山岩。锆石U-Pb定年结果指示现今覆盖面积约50000km2的岩浆活动发生在130~136Ma(峰期约132Ma)之间。镁铁质岩显示OIB型(高Ti)、N-MORB型(低Ti)和过渡型(介于二者之间)三种地球化学类型，其中未受地壳混染的镁铁质岩的Sr-Nd同位素和锆石Hf同位素成分类似于Kerguelen地幔柱产物。在扣除堆晶橄榄石之后，通过橄榄石-熔体平衡计算，苦橄玢岩母岩浆的MgO含量约20%，对应的地幔潜温>1560℃。西藏东南部白垩纪火成岩浆活动这种覆盖范围大、持续时间短和地幔潜温高等特征，非常类似于世界上其它地幔柱成因的大火成岩省或热点，因而将其描述和命名为措美(Comei)大火成岩省是合理的。年代学、地球化学和古地理重建资料显示藏南措美大火成岩省和南西澳大利亚同期的Bunbury玄武岩可能代表了同一个大火成岩省(即Comei-Bunbury大火成岩省)。Comei-Bunbury大火成岩省很可能记录了Kerguelen地幔柱在132Ma左右的早期岩浆作用，拉开了大印度从澳大利亚分离出来的序幕，影响了同期Weissert大洋缺氧事件的形成。
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关键词:
- 锆石U-Pb年龄 /
- 早白垩世措美大火成岩省 /
- Kerguelen地幔柱 /
- 地幔潜温 /
- 东冈瓦纳大陆裂解 /
- Weissert大洋缺氧事件 /
- 特提斯喜马拉雅带 /
- 藏东南
Abstract:Growing attention has been focused on the extensive igneous rocks in eastern Tethyan Himalaya, southeastern Tibet. The rock types here include basalts, mafic dikes/sills, gabbroic intrusions, and minor layered ultramafic and silicic volcanic rocks. Zircon U-Pb dating indicates that these rocks were emplaced between 130~136Ma (peaked at ca. 132Ma), covering an areal of ca. 50000km2 in the present-day southeastern Tibet. The mafic rocks show OIB-type (high-Ti), N-MORB-type (low-Ti), and transitional (between the first two types) geochemical types. Whole-rock Sr-Nd and zircon Hf isotopic compositions of the uncontaminated mafic rocks resemble the products of the Kerguelen mantle plume. After removing the effect of cumulated olivine, about 20% MgO content is obtained for the parental magmas of the picritic porphyrites through olivine-liquid equilibrium calculation, corresponding to mantle potential temperature at about 1560℃. The large areal extent, short duration of magmatism, and high mantle potential temperature of the extensive igneous rocks in southeastern Tibet are well comparable with mantle plume-related large igneous province (LIP) worldwide and thus can be best described as a large igneous province, i.e. the Comei LIP. Geochronological, geochemical, and paleogeographical data suggest that the Comei LIP in SE Tibet and the coeval Bunbury basalts in SW Australia may represent a single LIP (i.e. the Comei-Bunbury LIP). The Comei-Bunbury LIP documents the early stage magmatism of the Kerguelen mantle plume at about 132Ma, resulting in the separation of the Great India from Australia and the development of the Weissert Oceanic anoxic events.
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图 1
西藏东南部特提斯喜马拉雅带中东部地质简图(据Zhu et al., 2008, 2009; 裘碧波等, 2010修改)
Figure 1.
Simplified geological map of the east-central Tethyan Himalaya, southeastern Tibet (after Zhu et al., 2008, 2009; Qiu et al., 2010)
图 2
西藏东南部措美大火成岩省的野外出露情况
Figure 2.
Field occurrences of the Comei LIP in southeastern Tibet
图 3
西藏东南部措美大火成岩省的锆石U-Pb年龄柱状图
Figure 3.
Histograms of zircon U-Pb age dates for the Comei LIP
图 4
西藏东南部措美大火成岩省镁铁质岩的地球化学类型图
Figure 4.
Geochemical types of the mafic rocks from the Comei LIP in southeastern Tibet
图 5
西藏东南部措美大火成岩省的原始岩浆成分和地幔潜温
Figure 5.
Primitive magma compositions and mantle potential temperature calculated for the Comei LIP
图 6
东冈瓦纳大陆约132Ma构造重建图(据Heine and Müller, 2005; Zhu et al., 2008修改)
Figure 6.
Reconstructions of Eastern Gondwana at ca. 132Ma (after Heine and Müller, 2005; Zhu et al., 2008)
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