Melting of carbonate wall rocks and formation of the heterogeneous aureole of the Panzhihua intrusion, China
Melting of carbonate wall rocks and formation of the heterogeneous aureole of the Panzhihua intrusion, China
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DOI：
10.1016/j.gsf.2013.01.012
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发表时间：
2013-09
影响因子：
8.9
通讯作者：
C. Ganino;N. Arndt;C. Chauvel;Alexandre Jean;Charlotte Athurion
中科院分区：
文献类型：
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作者：
C. Ganino;N. Arndt;C. Chauvel;Alexandre Jean;Charlotte Athurion
攀枝花辉长岩侵入体是峨眉山大型火成岩省管道系统的一部分，大约距今 263 Ma 侵入晚元古代白云岩和泥灰岩。接触光环中的白云石转化为水镁石大理石和多种镁橄榄石、透辉石和石榴石矽卡岩。矿物学的变化部分是由于原岩成分的差异，特别是二氧化硅矿物和粘土的比例，部分是由于侵入岩浆中元素的转移。大多数大理石和夕卡岩的微量元素组成与未变质白云岩和泥灰岩非常相似，但有些含有较高的 Si、Ti 和 Fe 含量，被解释为来自岩浆源。从侵入体接触面约 10 m 处取样的三块水镁石大理岩被命名为“富集水镁石大理岩”，其微量元素组成与其白云质原岩非常不同：它们的稀土元素高度富集，而 Nb-Ta、Zr-Hf 和 Ti 的含量非常低。这些特征类似于碳酸盐液体与硅酸盐液体的平衡，或者更可能与攀枝花的硅酸盐矿物的平衡，我们认为这种相似性表明样品经历了部分熔融。从接触点最远 300 m 处采集的样本中含有水镁石，表明高温持续深入到围岩中。然而，在距接触点仅数十米处采集的其他样品仅轻微重结晶，表明光环中的条件变化很大。我们认为，光环内的温度是通过主要侵入物的热传导和光环内丰富的小堤坝提供的额外热量来控制的。来自光环更深层次的流体循环冲洗了白云石中的二氧化碳，将部分熔化碳酸盐所需的温度降低到侵入附近达到的温度。不规则但广泛的加热破坏了光环碳酸盐的稳定性，与碳酸盐分解和熔化相关的脱碳反应排放了大量二氧化碳，对全球气候产生潜在影响。
The Panzhihua gabbroic intrusion, part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about ∼263 Ma ago. The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite, diopside and garnet skarns. The variation in mineralogy is explained in part by differences in the composition of the protolith, particularly the proportion of silica minerals and clay, and in part by transfer of elements from intruding magmas. The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls, but some contain high Si, Ti, and Fe contents that are interpreted to have come from a magmatic source. Three brucite marbles sampled ∼10 m from the contact of the intrusion and named “enriched brucite marble” have trace element compositions very different from their dolomitic protolith: their rare earth elements are strongly enriched whereas levels of Nb-Ta, Zr-Hf and Ti are very low. These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua, a similarity we take to indicate that the sample underwent partial melting. Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks. However, other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable. We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole. Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite, lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion. Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO2, with potential impact on global climate.