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首页> 外文期刊>Contributions to Mineralogy and Petrology >Differentiation, crustal contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China
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Differentiation, crustal contamination and emplacement of magmas in the formation of the Nantianwan mafic intrusion of the ~260 Ma Emeishan large igneous province, SW China

机译：中国西南部〜260 Ma峨眉山火成岩大省南天湾黑铁质侵入体形成过程中的分化，地壳污染和岩浆侵位

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The Nantianwan mafic intrusion in the Panxi region, SW China, part of the ~260 Ma Emeishan large igneous province, consists of the olivine gabbro and gabbronorite units, separated by a transitional zone. Olivine gabbros contain olivine with Fo values ranging from 83 to 87, indicating crystallization from a moderately evolved magma. They have 0.2 to 0.9 wt % sulfide with highly variable PGE (17–151 ppb) and variable Cu/Pd ratios (1,500–32,500). Modeling results indicate that they were derived from picritic magmas with high initial PGE concentrations. Olivine gabbros have negative εNd(t) values (−1.3 to −0.1) and positive γOs(t) values (5–15), consistent with low degrees of crustal contamination. Gabbronorites include sulfide-bearing and sulfide-poor varieties, and both have olivine with Fo values ranging from 74 to 79, indicating crystallization from a more evolved magma than that for olivine gabbros. Sulfide-bearing gabbronorites contain 1.9–4.1 wt % sulfide and 37–160 ppb PGE and high Cu/Pd ratios (54,000–624,000). Sulfide-poor gabbronorites have 0.1–0.6 wt % sulfide and 0.2–15 ppb PGE and very high Cu/Pd ratios (16,900–2,370,000). Both sulfide-bearing and sulfide-poor gabbronorites have εNd(t) values (−0.9 to −2.1) similar to those for olivine gabbros, but their γOs(t) values (17–262) are much higher and more variable than those of the olivine gabbros. Selective assimilation of crustal sulfides from the country rocks is thus considered to have resulted in more radiogenic 187Os of the gabbronorites. Processes such as magma differentiation, crustal contamination and sulfide saturation at different stages in magma chambers may have intervened during formation of the intrusion. Parental magmas were derived from picritic magmas that had fractionated olivine under S-undersaturated conditions before entering a deep-seated staging magma chamber, where the parental magmas crystallized olivine, assimilated minor crustal rocks and reached sulfide saturation, forming an olivine- and sulfide-laden crystal mush in the lower part and evolved magmas in the upper part of the chamber. The evolved magmas were forced out of the staging chamber and became S-undersaturated due to a pressure drop during ascent to a shallow magma chamber. The magmas re-attained sulfide saturation by assimilating external S from S-rich country rocks. They may have entered the shallow magma chamber as several pulses so that several gabbronorite layers each with sulfide segregated to the base and a sulfide-poor upper part. The olivine gabbro unit formed from a new and more primitive magma that entrained olivine crystals and sulfide droplets from the lower part of the staging chamber. A transitional zone formed along the boundary with the gabbronorite unit due to chemical interaction between the two rock units.
机译：中国西南地区盘溪地区的南天湾黑铁质侵入岩是约260个Ma Emeishan火成岩大省的一部分，由橄榄石辉长岩和辉长辉石单元组成，由过渡带隔开。橄榄石长辉岩包含的Fo值在83至87之间的橄榄石，表明从中等程度的岩浆中结晶出来。它们的硫化物含量为0.2至0.9 wt％，PGE高度可变（17-151 ppb），Cu / Pd比率可变（1,500-32,500）。建模结果表明，它们来自具有较高初始PGE浓度的山性岩浆。橄榄石辉长岩的εNd（t）值为负值（-1.3至-0.1），而γOs（t）值为正值（5-15），这与地壳污染程度较低相符。辉长岩包括含硫化物和贫硫化物的变种，并且都具有橄榄石，Fo值在74至79之间，这表明其演化出的岩浆比橄榄石长辉石更容易形成岩浆。含硫辉长岩含有1.9–4.1 wt％的硫化物和37–160 ppb PGE，以及高的Cu / Pd比（54,000–624,000）。硫化物贫乏的辉长岩具有0.1–0.6 wt％的硫化物和0.2–15 ppb PGE，并且具有很高的Cu / Pd比（16,900–2,370,000）。含硫化物和贫硫化物辉长岩的εNd（t）值（-0.9至-2.1）与橄榄石辉长岩相似，但它们的γOs（t）值（17-262）远高于和大于橄榄石辉长岩。橄榄石辉长岩。因此，从乡村岩石中选择性吸收地壳硫化物导致了辉长岩中更多的放射源 187 Os。岩浆室内不同阶段的岩浆分化，地壳污染和硫化物饱和等过程可能在侵入形成过程中介入。母岩浆来自于在S欠饱和条件下分馏的橄榄岩岩浆，然后进入深层分期岩浆室，在那里，母岩浆结晶出橄榄石，吸收了较小的地壳岩石并达到硫化物饱和，形成了富含橄榄石和硫化物的岩浆。下部有水晶糊状物，上部有岩浆。演化的岩浆被迫离开分级室，并由于上升到浅岩浆室期间的压力下降而变得S不饱和。岩浆通过吸收富含S的乡村岩石中的外部S重新达到硫化物饱和度。它们可能以几个脉冲的形式进入了浅岩浆室，因此几块辉长岩层的每个硫化物都被隔离在基部，而硫化物贫乏的上部则被隔离。橄榄石辉长岩单元是由一个新的，更原始的岩浆形成的，该岩浆从阶跃室的下部夹带了橄榄石晶体和硫化物液滴。由于两个岩石单元之间的化学相互作用，沿着与辉长岩单元边界形成的过渡带。

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