CRUSTAL SHORTENING INDUCED FILTER PRESSING AND RHYOLITE MELT EXTRACTION IN A SHALLOW PLUTONIC COMPLEX, SOUTHERN ANDES

In the Chilean southern Andes (Lat. 36° S), we assess the role of crustal shortening as a mechanism of rhyolite melt extraction within the ~7-6 Ma Risco Bayo – Huemul plutonic complex. The shallow (<5 km depth) Huemul pluton comprises adjacent quartz monzonites, granites, and alkali feldspar granites that form a NNW elongate body (16 x 2 km). Bulk rock and zircon trace element compositions show that the alkali feldspar granites are a rhyolitic melt extracted from a crystal mush, leaving behind a silicic cumulate of porphyritic quartz monzonite (Schaen et al., 2017). Microstructural analyses indicate the presence of solely magmatic fabrics that we further characterized via Anisotropy of Magnetic Susceptibility. In the three compositional domains, magnetic fabrics are consistently oriented, dominantly oblate (T > 0), and exhibit low anisotropy (Pj < 1.1). Magnetic foliations strike NNW, dip steeply (> 60°), and cross cut magmatic contacts between domains. Magnetic lineations generally plunge shallowly to moderately (< 60°) to the NNW and S. The one area of steeply (> 60°) plunging lineations is located on the northern edge of the alkali feldspar granite and exhibits a spatial association with vertically elongate, cm-scale miarolitic cavities.

We hypothesize that magnetic fabrics in Huemul pluton record magmatic flow in response to tectonic deformation and melt extraction/eruption. The NNW-striking magnetic foliations throughout the complex are consistent with ENE-WSW regional shortening. In contrast, we interpret the steep magnetic lineations associated with the miarolitic cavities as a channel of vertical melt extraction that occurred during the regional contraction. The quartz monzonites and granites show evidence for melt extraction via filter pressing. First, the fabrics are oblate, indicating that flattening was likely caused by melt extraction. Second, the rocks exhibit cumulate microstructures - slightly bent plagioclase twins and undeformed, optically continuous interstitial quartz – but with vertical foliation. To quantify the volume of extracted melt, we model crystal packing by calculating the cumulus phase clustering index and apparent crystallinity. The large volumes (>40%) of extracted melt estimated suggest that crustal shortening is an effective driver of filter pressing in arc plutons.