Origin of K-feldspar megacrysts in rhyolites from the Emeishan large igneous province, southwest China
Origin of K-feldspar megacrysts in rhyolites from the Emeishan large igneous province, southwest China
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DOI：
10.1016/j.lithos.2017.10.018
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发表时间：
2017-12
期刊：
影响因子：
3.5
通讯作者：
Lilu Cheng;Yu Wang;J. S. Herrin;Zhong‐Yuan Ren;Zongfeng Yang
中科院分区：
文献类型：
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作者：
Lilu Cheng;Yu Wang;J. S. Herrin;Zhong‐Yuan Ren;Zongfeng Yang
硅质岩产于中国西南部二叠纪峨眉山溢流玄武岩省宾川地区最长火山序列（约 5000 米厚）的最上部单元。它们主要是流纹岩，其次是粗面岩，两者都含有钾长石巨晶作为主要斑晶相，尺寸可达约 20 毫米。这些巨型晶体包含排列成脉状网络的钠长石区域，可能是由岩浆后蚀变形成的。晶体尺寸分布（CSD）表明这些巨晶体生长在稳定的岩浆系统中，与相对均匀的核心到边缘成分（K2O：~14-16wt%）和同位素分布208Pb/206Pb比率（~2.06-2.08±0.005）一致。这些硅质岩石的全岩微量元素和铅同位素比率与峨眉山玄武岩相似，表明镁铁质和长英质单元具有共同来源，并且地壳熔融在长英质单元的成因中的作用有限。主量和微量元素模型进一步表明，这些岩石不可能完全由旧地壳或凝固玄武岩的重新熔化形成，而必须通过溢流玄武岩的晶体分异或玄武岩可能部分熔化然后分异结晶而形成。在其他大型火成岩省的最后阶段也观察到含钾长石的流纹岩。我们认为它们代表了最终的熔体分数，并且它们在岩浆系统中的出现与大量岩浆活动的减弱相一致。
Silicic rocks occur in the uppermost units of the longest volcanic succession (~ 5000 m thick) in the Binchuan area of the Permian Emeishan flood basalt province of SW China. They are predominantly rhyolites and to a lesser extent trachytes, both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size. These megacrysts contain domains of albite arranged in vein-like networks, likely formed by post-magmatic alteration. Crystal size distributions (CSD) suggest that these megacrysts grew in a stable magmatic system, consistent with relatively uniform core-to-rim compositional (K2O: ~ 14–16 wt%) and isotopic profiles208Pb/206Pb ratios (~ 2.06–2.08 ± 0.005). Both whole-rock trace elements and Pb isotope ratios of these silicic rocks are similar to the Emeishan basalts, suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units. Major and trace element models further indicate that these rocks could not have formed exclusively by re-melting of old crust or solidified basaltic rock, but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization. K-feldspar-bearing rhyolites are also observed in the last stages of other large igneous provinces. We suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities.