Contrasting conditions between reservoirs of two nearby volcanic complexes of the SVZ, Chile: Caburgua-Huelemolle Small Eruptive Centers and Villarrica Volcano

Abstract:

Small eruptive centers of the Chilean Southern Andes are built over the Liquiñe-Ofqui Zone, a major structure, and close to large stratovolcanoes, however the relationships between these two styles of volcanism are poorly known. This contribution presents thermobarometric results obtained from lavas of the Caburgua-Huelemolle Small Eruptive Centers (CHSEC), composed of pyroclastic cones and associated lava flows of basaltic composition, belonging to the Southern Volcanic Zone. These results are compared with those available data in Holocene volcanic products of the Villarrica Volcano located 10 km south of the CHSEC.

Phenocrysts consist of isolated crystals and clots of plagioclase, clinopyroxene, and olivine. Plagioclase phenocrysts are commonly unzoned (An88-92), although a very small (> 20 μm) rim of more albitic (An55-70) composition occurs. Olivines mainly have composition Fo85 with a 50-80 μm rim of Fo79. Clinopyroxene phenocrysts exhibit a narrow compositional range (Wo44-46, En45-47, Fs7-9). Chromian spinels (#Mg= 43-94, #Cr= 16-33) are commonly found as inclusions in olivine phenocrysts. Microlites include plagioclase (An59-77), clinopyroxene (Wo8-40, En45-63, Fs13-31), olivine (Fo70-76) magnetite and titanomagnetites (Mgt = 77, Usp = 23).

Pre-eruptive temperatures of the CHSEC between 1,221 and 1,227 ± 6 °C were obtained from olivine-augite phenocrysts. Olivine and clinopyroxene phenocrysts crystallized at pressures between 8.6 and 12.8 kbar consistent with a reservoir located at the base of the crust. The maximum syn-eruptive temperatures of 1,170 ± 6 °C were obtained in olivine-augite groundmass microcryst pairs, the loss of temperature is consistent with an adiabatic ascent from the base of the crust without an intermediate reservoir.

The CHSEC pre-eruptive conditions are different from those obtained from Holocene lavas and bombs of the Villarrica Volcano. Pre-eruptive temperatures between 1,041 and 1,168 °C and pressures between 0.2 and 0.6 kbar were obtained in the Villarrica Volcano. In addition, chromian spinel inclusions in olivine of Villarrica Volcano bombs (#Al= 2-19) have compositions that differ from those of the CHSEC (#Al= 29-33) , suggesting independent reservoirs.