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首页> 外文期刊>Contributions to Mineralogy and Petrology >Granite, gabbro and mafic microgranular enclaves in the Gejiu area, Yunnan Province, China: a case of two-stage mixing of crust- and mantle-derived magmas
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Granite, gabbro and mafic microgranular enclaves in the Gejiu area, Yunnan Province, China: a case of two-stage mixing of crust- and mantle-derived magmas

机译：中国云南省个旧地区的花岗岩，辉长岩和镁铁质微颗粒飞地：以地壳和地幔衍生的岩浆两阶段混合为例

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摘要

Geochronological, geochemical, whole-rock Sr-Nd, and zircon Hf isotopic analyses were carried out on the Jiasha Gabbro, mafic microgranular enclaves (MME) and host Longchahe Granite samples from the Gejiu area in the southeast Yunnan province, SW China, with the aim of characterizing their petrogenesis. Compositional zoning is evident in the gabbro body as the cumulate textures and mineral proportions in the gabbro interior are distinct from the gabbro margin. The Longchahe Granite largely comprises metaluminous quartz monzonite with distinctive K-feldspar megacrysts, but also contains a minor component of peraluminous leucogranite. The MME have spheroidal to elongated/lenticular shapes with sharp, crenulated and occasionally diffuse contacts with the host granite, which we attribute to the undercooling and disaggregation of mafic magma globules within the cooler host felsic magma. Field observations, geochronology, geochemistry, Sr-Nd and zircon Hf isotopic compositions point to a complex petrogenesis for this granite-MME-gabbro association. Zircon ~(206)Pb/~(208)U ages determined by LA-ICP-MS for a mafic enclave, its host granite and the gabbro body are 83.1 ± 0.9 Ma, 83.1 ± 0.4 Ma and 83.2 ± 0.4 Ma, respectively, indicating coeval crystallization of these igneous rock units. Crystal fractionation processes can explain much of the compositional diversity of the Jiasha Gabbro. The geochemical features of the gabbro, such as high Mg# (up to 70) and Cr (up to 327 ppm), enrichment in LILEs (e.g., Rb, Ba, K_2O) and LREEs, and depletion in HFSE (e.g., Nb, Ta, Ti), together with initial ~(87)Sr/~(86)Sr ratios of 0.708-0.709 and negative εNd(l) values (-5.23 to —6.45), indicate they were derived from a mantle source that had undergone previous enrichment, possibly by sub-duction components. The Longchahe Granite has a large range of SiO_2 (59.87-74.94 wt%), is distinctly alkaline in composition, and has Sr-Nd-Hf isotopic compositions ((~(87)Sr/~(86)Sr)_i> 0.712, εNd(t) =-6.93 to -7.62 and sHf(t) = —5.8 to —9.9) that are indicative of derivation from a crustal source. However, the most primitive rocks of Longchahe Granite are compositionally distinct from any feasible crustal melt. We interpret the spectrum of rock types of the Longchahe Granite to have formed via mixing between crustally derived peraluminous leucogranite magma and mantle-derived magma of similar heritage to the Jiasha Gabbro. We speculate that this mixing event occurred early in the magmatic history of these rocks at relatively high temperature and/or deep in the crust to allow efficient physical mixing of magmas. Saturation and accumulation of K-feldspar and zircon in the mixed magma is invoked to explain the megacrystic K-feldspar and elevated K_2O and Zr content of some of the granitic rocks. A later episode of magma mixing/mingling is preserved as the MME that have geochemical and isotopic compositions that, for the most part, are intermediate between the granite and the gabbro. The MME are interpreted to be fractionated melts of mafic magma related to gabbro that were subsequently injected into the cooler, partly crystalline granitic magma. Mingling and mixing processes within the convectively dynamic upper crustal magma chamber resulting in a hybrid (MME) magma. During this second mixing episode, element interdiffusion, rather than bulk physical mixing, is interpreted to be the dominant mixing process.
机译：地质，地球化学，全岩石Sr-Nd和锆石Hf同位素分析分别在中国西南部云南省个旧地区的贾沙嘎布布鲁，镁铁质微颗粒飞地（MME）和寄主龙岔河花岗岩样品上进行，目的是表征其成岩作用。由于辉长岩内部的累积纹理和矿物质比例与辉长岩边缘不同，因此辉长岩体中的成分分区明显。龙岔河花岗岩主要包含具有独特的钾长石巨型晶体的金属石英蒙脱石，但也含有少量的钙铝辉石花岗岩。 MME具有球状到细长形/透镜状，与主体花岗岩有尖锐的，锯齿状的和偶发性的扩散接触，这归因于较冷的主体长浆岩浆中的镁铁质岩浆小球过冷和分解。现场观察，地质年代学，地球化学，Sr-Nd和锆石Hf同位素组成表明了该花岗岩-MME-ga钴矿的复杂成岩作用。通过LA-ICP-MS确定的镁铁质飞地，其主体花岗岩和辉长岩体的锆石〜（206）Pb /〜（208）U年龄分别为83.1±0.9 Ma，83.1±0.4 Ma和83.2±0.4 Ma，表明这些火成岩单元的近代结晶。晶体分级分离过程可以解释贾沙嘎布布鲁的大部分成分多样性。辉长岩的地球化学特征，例如高Mg＃（高达70）和Cr（高达327 ppm），LILE（例如Rb，Ba，K_2O）和LREE中的富集以及HFSE（例如Nb， Ta，Ti）以及初始〜（87）Sr /〜（86）Sr比率0.708-0.709和εNd（l）负值（-5.23至-6.45）表示它们是从经历过地幔源以前的浓缩，可能是通过俯冲成分。龙岔河花岗岩具有较大的SiO_2范围（59.87-74.94 wt％），成分明显为碱性，且具有Sr-Nd-Hf同位素组成（（〜（87）Sr /〜（86）Sr）_i> 0.712， εNd（t）= -6.93至-7.62和sHf（t）= -5.8至-9.9），表示来自地壳来源。但是，龙叉河花岗岩最原始的岩石在成分上与任何可行的地壳熔体都不相同。我们解释了龙壳河花岗岩的岩石类型光谱，这些岩石是通过地壳衍生的钙铝辉石花岗岩岩浆和地幔衍生岩浆混合而成的，这些岩浆具有与佳沙格布布鲁相似的遗产。我们推测，这种混合事件发生在这些岩石的岩浆历史的早期，处于相对较高的温度和/或地壳深处，以实现岩浆的有效物理混合。并利用混合岩浆中钾长石和锆石的饱和和堆积来解释某些花岗岩中的巨晶钾长石以及升高的K_2O和Zr含量。岩浆混合/混合的后期事件被保留为MME，其具有地球化学和同位素组成，这些组成大部分位于花岗岩和辉长岩之间。 MME被解释为与辉长岩有关的铁镁质岩浆的分馏熔体，随后被注入到较冷的部分结晶的花岗岩岩浆中。对流动力上地壳岩浆室内的混合和混合过程导致了混合（MME）岩浆。在第二次混合过程中，元素相互扩散而不是整体物理混合被认为是主要的混合过程。

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