Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates
Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates
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DOI：
10.1016/j.chemgeo.2007.10.016
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发表时间：
2008-01
期刊：
影响因子：
3.9
通讯作者：
Yongsheng Liu;K. Zong;P. Kelemen;Shan Gao
中科院分区：
文献类型：
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作者：
Yongsheng Liu;K. Zong;P. Kelemen;Shan Gao
对中国大陆科学钻探深部100~680米深度的三组榴辉岩(低镁钛榴辉岩、高钛榴辉岩和富镁榴辉岩)和超镁铁质岩石进行了研究。超镁铁质岩石的低镁#S(=100ppm)(摩尔镁/(镁+铁))(81-84%)、低镍(115-1220ppm)和高的Fe2O3总含量(13-15wt.%)暗示了堆积型成因。富镁榴辉岩具有中、重度稀土元素的富集性，这不是石榴石变质生长所能产生的。相反，如果这些岩石是由富含轻稀土的岩浆形成的，那么它们的原岩中可能存在一些石榴石的火成岩前体。或者，它们可能是由没有石榴石的轻稀土贫化岩浆形成的。高钛榴辉岩具有异常高的Fe2O总含量(高达24.5wt.%)和高TiO2与低Nb、Ta解偶的特征。这些特征不能通过高钛玄武岩的超高压变质过程中金红石的聚集(即使是含Ti02；∼；4wt.%的样品)而产生，但可归因于钛磁铁矿(对于含TiO2%；∼；4%的样品)或钛磁铁矿+钛铁矿的晶体分馏(对于含TiO2%和4%的样品)。因此，我们认为高钛榴辉岩的原岩是钛磁铁矿/富钛铁矿的辉长岩堆积体。总体来说，低镁钛榴辉岩与高钛榴辉岩、富镁榴辉岩和超镁铁质岩在地球化学上具有互补性，可能是辉长岩/闪长岩堆积岩高度结晶分馏的变质产物。这些观察表明，超镁铁质岩石-榴辉岩组合的母质可能代表了拉斑玄武岩或苦橄岩岩浆在中高压下分离结晶的完整序列，这些岩浆后来在大陆碰撞事件中被带到超高压条件下。
Three distinct groups of eclogites (low-Mg–Ti eclogites, high-Ti eclogites and Mg-rich eclogites) and ultramafic rocks from the depth interval of 100–680 m of the Chinese Continental Scientific Drill Hole were studied. The low Mg#s (=100⁎molar Mg/(Mg+Fe)) (81–84%) and low Ni (1150–1220 ppm) and high Fe2O3total(13–15 wt.%) contents of ultramafic rocks suggest a cumulate origin. Mg-rich eclogites show middle and heavy REE enrichments, which could not be produced by metamorphic growth of garnet. Instead, if the rocks formed from a light REE enriched magma, there may be an igneous precursor for some garnets in their protolith. Alternatively, perhaps they formed from a light REE depleted magma without garnet. The high-Ti eclogites are characterized by unusually high Fe2O3totalcontents (up to 24.5 wt.%) and decoupling of high TiO2from low Nb and Ta contents. These features cannot be produced by concentration of rutile during UHP metamorphism (even for samples with TiO2>4 wt.%) of high-Ti basalts, but could be attributed to crystal fractionation of titanomagnetite (for those with TiO2<∼4 wt.%) or titanomagnetite+ilmenite (for those with TiO2>∼4 wt.%). Thus, we suggest that protoliths of the high-Ti eclogites were titanomagnetite/ilmenite-rich gabbroic cumulates. As a whole, the low-Mg–Ti eclogites are geochemically complementary to the high-Ti eclogites, Mg-rich eclogites and ultramafic rocks, and could be metamorphic products of gabbroic/dioritic cumulates formed by high degree crystal fractionation. All these observations suggest that parental materials of the ultramafic rock-eclogite assemblage could represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas at intermediate to high pressure, which were later carried to ultrahigh-pressure conditions during a continental collision event.