Complementary crystal accumulation and rhyolite melt segregation in a late Miocene Andean pluton
Complementary crystal accumulation and rhyolite melt segregation in a late Miocene Andean pluton
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DOI：
10.1130/g39167.1
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发表时间：
2017-09
期刊：
影响因子：
5.8
通讯作者：
A. Schaen;J. Cottle;B. Singer;C. B. Keller;N. Garibaldi;B. Schoene
中科院分区：
文献类型：
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作者：
A. Schaen;J. Cottle;B. Singer;C. B. Keller;N. Garibaldi;B. Schoene
高硅花岗岩被假设为在地壳浅部通过分馏形成，但预测的残留物很少被识别，并且很难在岩体中区分，否则其岩石会沿着沿着液体下降线分布。在中新世晚期Risco Bayo-Huemul深成杂岩（智利）的大块岩石成分质量平衡表明，Huemul深成杂岩内部的岩性差异反映了熔体提取过程中残留的晶体浓度。从70至75重量%的SiO2和强烈的亏损Ba和Eu的组成差距表明，Huemul碱长石（AFS）花岗岩是从糊状物中提取的高度演化的流纹质熔体的冷冻残留物。石英二长岩富含Zr和Ba与Eu/ Eu* 近统一的解释，代表互补的剩余的分馏过程的累积。成分变化Afs花岗岩锆石（Eu/Eu*，Dy/ Yb）进一步支持提取这种熔体从锆石饱和糊状物。锆石U-Pb年龄表明，Huemul岩的形成时间约为800 k. y。在更多的基性岩Risco Bayo岩石的初始结晶之后，可能排除了它们通过从基性岩先驱分离而演化的可能性。这个岩体记录了在地壳上部产生流纹岩的一种方式，这种方式在安第斯俯冲带的第四纪期间推动了大规模的火山喷发。
High-silica granites are hypothesized to form via fractionation in the shallow crust, yet the predicted residues are rarely identified and can be difficult to distinguish within plutons whose rocks otherwise plot along liquid lines of descent. Bulk-rock compositional mass balance in the late Miocene Risco Bayo–Huemul plutonic complex (Chile) suggests that lithological differences within the Huemul pluton reflect residual crystal concentration in response to melt extraction. A compositional gap from 70 to 75 wt% SiO2 and strong depletion in Ba and Eu suggest that Huemul alkali feldspar (Afs) granites are frozen remnants of highly evolved rhyolitic melt extracted from a mush. Quartz monzonites enriched in Zr and Ba with Eu/ Eu* near unity are interpreted to represent the complementary residual silicic cumulates of this fractionation process. Compositional variations in Afs granite zircon (Eu/Eu*, Dy/ Yb) further support extraction of this melt from a zircon-saturated mush. U-Pb zircon dates indicate that Huemul rocks evolved ~800 k.y. after initial crystallization of more mafic Risco Bayo rocks, likely precluding their evolution via fractionation from mafic forerunners. This pluton records a means to produce rhyolite in the upper crust, which has propelled large silicic eruptions during the Quaternary within the Andean subduction zone.