Origin of carbonatites in the South Qinling orogen: Implications for crustal recycling and timing of collision between the South and North China Blocks
Origin of carbonatites in the South Qinling orogen: Implications for crustal recycling and timing of collision between the South and North China Blocks
复制标题
南秦岭造山带碳酸岩成因：地壳循环及华北地块碰撞时间的意义
DOI：
10.1016/j.gca.2014.03.041
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发表时间：
2014-10
影响因子：
5
通讯作者：
许成
中科院分区：
文献类型：
--
作者：
许成;许成
地幔成分不均匀性的研究大多来自大洋玄武岩，与地壳沉积物的再循环或大陆岩石圈的拆离有关。在这项工作中，我们提出了直接的地质年代学和地球化学证据，再循环的地壳物质在南秦岭造山带（SQ）的碰撞相关的碳酸盐岩，合并与小秦岭造山带（LQ）分离的南，华北地块。SQ碳酸岩主要以与正长岩共生的岩株形式产出。正长岩中锆石的年龄为766 ± 25 Ma，与碳酸岩中原生独居石的年龄（233.6 ± 1.7 Ma）相差很大。正长岩具有较低的初始87 Sr/86 Sr和较高的ε Nd值。这表明碳酸岩与硅酸盐岩没有成因联系，而是直接来源于地幔中生成的原始碳酸盐岩浆。碳酸岩的Sr-Nd同位素特征与碳酸岩均匀储集层（CHUR）相似，Sm-Nd模式年龄（TCHUR）为190-300 Ma。然而，岩石有非常可变的铅同位素值之间的HIMU和EM 1地幔端员跨越。SQ碳酸岩的大多数碳、氧同位素组成都在原生火成碳酸盐岩之外。它们的δ 13 C值比“正常”地幔高，这意味着再循环无机碳的掺入。碳酸岩侵位于勉略缝合带附近，并伴随着勉略洋的闭合和三叠纪南北陆块的碰撞。然而，以高ε Nd和低（类EM 1）206 Pb/204 Pb值为特征的勉略洋壳俯冲直接熔融不能解释SQ碳酸岩中类CHUR Nd特征和Pb同位素向HIMU方向的趋势。我们的结论是，他们的母岩浆来自一个源合并勉略洋壳混合软流圈（或更深）的材料，其特点是高铅和低Nd同位素值。这种物质代表了通过地幔对流或局部上涌再循环的深源元古宙碳酸盐成分。值得注意的是，该模型不能解释晚三叠世（209-221 Ma）LQ碳酸岩的同位素组成，其特征是幔源δ 13 C，但EM 1的Sr-Nd-Pb同位素组成。这一特征可以用南、华北陆块碰撞过程中增厚的下陆壳的拆沉作用以及部分拆沉物质进入LQ地幔源区来解释。对测得的Sr-Nd-Pb同位素变化的模拟表明，LQ碳酸岩的源区可能是由80-85%的地幔物质和15-20%的拆离下陆壳混合而成。SQ和LQ碳酸岩的侵位标志着三叠纪造山带由南北陆块碰撞形成的挤压构造体制逐渐过渡到造山带内伸展。
Most studies of compositional heterogeneities in the mantle, related to recycling of crustal sediments or delaminated subcontinental lithosphere, come from oceanic setting basalts. In this work, we present direct geochronological and geochemical evidence for the incorporation of recycled crustal materials in collision-related carbonatites of the South Qinling orogenic belt (SQ), which merges with the Lesser Qinling orogen (LQ) to separate the South and North China Blocks. The SQ carbonatites occur mainly as stock associated with syenites. The data presented here show that zircon from the syenites yields an age of 766 ± 25 Ma, which differs significantly from the age of primary monazite from the carbonatites (233.6 ± 1.7 Ma). The syenites contain lower initial87Sr/86Sr and higherεNdvalues. This indicates that the carbonatites do not have genetically related with the silicate rocks, and were directly derived from a primary carbonate magma generated in the mantle. The carbonatites show a Sr–Nd isotopic signature similar to that of the chondritic uniform reservoir (CHUR), and parallel Sm–Nd model ages (TCHUR) of 190–300 Ma. However, the rocks have extremely variable Pb isotopic values straddling between the HIMU and EM1 mantle end-members. Most carbon and oxygen isotopic compositions of the SQ carbonatites plot outside the field for primary igneous carbonates. Their δ13C shows higher value than a ‘normal’ mantle, which implies an incorporation of recycled inorganic carbon. The carbonatites were emplaced close to the Mianlue suture, and followed the closure of the Mianlue ocean and Triassic collision of the South and North China Blocks. However, direct melting of the subducted Mianlue oceanic crust characterized by highεNdand low (EM1-like)206Pb/204Pb values cannot explain the CHUR-like Nd signature and the Pb isotopic trend toward HIMU in the SQ carbonatites. We conclude that their parental magma was derived from a source incorporating the Mianlue oceanic crust mixed with an asthenospheric (or deeper) material characterized by high Pb and low Nd isotopic values. This material represents a deep-seated Proterozoic carbonate component recycled via mantle convection or localized upwelling. Notably, this model cannot explain the isotopic compositions of the Late Triassic (209–221 Ma) carbonatites in the LQ, characterized by a mantle-derived δ13C, but EM1-like Sr–Nd–Pb isotopic compositions. This signature is best explained in terms of delamination of the lower continental crust thickened during the collision of the South and North China Blocks, and partial incorporation of the delaminated material into the LQ mantle source. Modeling of the measured Sr–Nd–Pb isotopic variations suggests that the source of the LQ carbonatites could be produced by mixing of 80–85% of mantle material and 15–20% of delaminated lower continental crust. The emplacement of the SQ and LQ carbonatites marked a gradual transition from a compressional tectonic regime, brought about by the collision of the South and North China Blocks to intra-orogenic extension in the waning stages of the Triassic orogeny.
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影响因子：
12.1
作者：
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通讯作者：
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影响因子：
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通讯作者：
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影响因子：
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作者：
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通讯作者：
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影响因子：
3.5
作者：
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影响因子：
2.3
作者：
通讯作者：
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