Pre-eruptive rejuvenations of crystalline mush by reservoir heating: the case of trachy-dacitic lavas of Quetrupillán Volcanic Complex, Chile (39º30’ lat. S)

Abstract:

The Southern Volcanic Zone of the Andes is dominated by mainly basaltic to andesitic products, as is the case of the Villarrica 2015 eruption. An exception is the case of the nearby Quetrupillán Volcanic Complex (QVC). The QVC forms part of the NW-SE Villarrica-Quetrupillán-Lanín active volcanic chain and contains products of mainly trachy-dacitic composition that differs from the basaltic and basaltic-andesite composition of the neighboring stratovolcanoes.

The studied Holocene QVC trachy-dacites exhibit similar geochemical, mineralogical features. They are crystal-poor (<12% phenocrysts) with phenocrysts occurring as isolated crystals or forming crystal clots within a glass-rich groundmass. Two groups of plagioclase phenocrysts were found: Group 1 plagioclases are commonly associated with pyroxene in crystal clots and with oscillatory zoning of An_40-50, and Group 2 plagioclases occur as isolated crystals of variable composition (An_70-85), with rims of similar composition to Group 1. Some Group 2 crystals have cores with compositions similar to the rims with resorption features. Clinopyroxenes within clots have compositions of En_40-46Fs_11-21Wo_39-43 similar to the isolated clinopyroxenes. Orthopyroxene phenocrysts have composition of En_36-68Fs_15-34Wo_03-04. Microphenocrysts of magnetite-ilmenite pairs (Mt_14-36-Il_90-94) were identified.

Pyroxene phenocrysts are in disequilibrium with the whole-rock composition, suggesting that they are antecrysts. Two-pyroxene thermometry gives temperatures of 911-980ºC, while the Mt-Il equilibrium gives higher temperatures of 963-1114ºC, for the later stages of lava crystallization.

The presence of a crystalline mush is suggested by the abundance of crystal clots in disequilibrium with the host rock composition whose calculated formation conditions of 0.5-1.5 kbar, 950-990ºC and 2.1-3.1 wt% of H₂O, were obtained from a Rhyolite-MELTS simulation. Rejuvenation of a crystal mush by reservoir heating events is suggested due to resorption, regrowth and complex zonation of plagioclases and the higher temperatures obtained from the late stage of Fe-Ti oxide equilibrium. These events would generate the opening of the mush at the bottom of the magma chamber, allowing the interstitial melt extraction prior to eruption.