Interpreting Granitic Fabrics in Terms of Rhyolitic Melt Segregation, Accumulation, and Escape Via Tectonic Filter Pressing in the Huemul Pluton, Chile

The physical process of rhyolite segregation from crystal mushes remains elusive as microstructural evidence of conventional segregation mechanisms is not available. This study provides direct fabric evidence for deformation‐assisted segregation of eruptible rhyolite in the Chilean Andean arc. The shallow (<7 km), 6.4–6.2 Ma Huemul pluton comprises domains of quartz monzonite, granite, and high‐silica granite. Compositional modeling shows that rhyolitic melt (high‐silica granite) was extracted from a granitic parent, leaving behind silicic cumulates (quartz monzonite). To understand mechanisms of rhyolite segregation, we investigate magmatic fabrics in the pluton. Anisotropy of Magnetic Susceptibility analyses reveal oblate magnetic fabrics and NNW‐striking, subvertical magnetic foliations throughout Huemul. Within the high‐silica granite, magnetic lineations are subvertical and parallel to elongate miarolitic cavities. Magnetic lineations in the quartz monzonite plunge moderately to the NNW, away from the high‐silica granite. In the quartz monzonite, the Shape‐Preferred Orientation of early feldspars is parallel to the magnetic lineation and developed while suspended in melt. Estimations of early feldspar clustering and crystallinity yield ~38% of interstitial volume loss in the quartz monzonite and no volume loss in the granite. These fabric data suggest ENE tectonic shortening coeval with rhyolite extraction. We explain these observations with a model of tectonic filter pressing in which shortening is accommodated by interstitial melt flow at slow (10−5 km3/yr) rates, segregating moderate volumes of rhyolite in Myr time scales. These interactions link plutonism, tectonic deformation, and upward mobility of eruptible rhyolite in tectonically active margins.

流纹岩从晶粥中分离的物理过程仍然难以捉摸，因为常规分离机制的微观结构证据并不存在。这项研究为智利安第斯弧中可喷发流纹岩的变形辅助分离提供了直接的组构证据。浅部（<7千米）、6.4 - 6.2百万年的休穆尔深成岩体包含石英二长岩、花岗岩和高硅花岗岩区域。成分模拟显示，流纹质熔体（高硅花岗岩）是从花岗岩母体中提取出来的，留下了硅质堆晶（石英二长岩）。为了理解流纹岩分离的机制，我们研究了该深成岩体中的岩浆组构。磁化率各向异性分析揭示了整个休穆尔地区的扁球形磁性组构以及北北西走向、近垂直的磁性叶理。在高硅花岗岩内，磁线理近垂直且与拉长的晶洞平行。石英二长岩中的磁线理向北北西适度倾斜，远离高硅花岗岩。在石英二长岩中，早期长石的形状择优取向与磁线理平行，并且是在悬浮于熔体中时形成的。对早期长石聚集和结晶度的估算得出，石英二长岩中约38%的间隙体积损失，而花岗岩中没有体积损失。这些组构数据表明，ENE向构造缩短与流纹岩提取同时发生。我们用构造压滤模型来解释这些观测结果，在该模型中，缩短是通过缓慢（10⁻⁵立方千米/年）的间隙熔体流动来调节的，在百万年时间尺度上分离出适量的流纹岩。这些相互作用将深成作用、构造变形以及可喷发流纹岩在构造活动边缘的向上迁移联系起来。