Contrasting records from mantle to surface of Holocene lavas of two nearby arc volcanic complexes: Caburgua-Huelemolle Small Eruptive Centers and Villarrica Volcano, Southern Chile

Abstract

Most of the small eruptive centers of the Andean Southern Volcanic Zone are built over the Liquine-Ofqui Fault Zone (LOFZ), a NS strike-slip (>1000 km length) major structure, and close to large stratovolcanoes. This contribution compares textural features, compositional parameters, and pre- and syn-eruptive P,T conditions, between basaltic lavas of the Caburgua-Huelemolle Small Eruptive Centers (CHSEC) and the 1971 basaltic andesite lava of the Villarrica Volcano located 10 km south of the CHSEC. Olivines and clinopyroxenes occur as phenocrysts and forming crystal clots of the studied lavas. They do not markedly show compositional differences, except for the more scattered composition of the CHSEC clinopyroxenes. Plagioclase in CHSEC lavas mainly occur as phenoaysts or as microlites in a glass-free matrix. Two groups of plagioclase phenoaysts were identified in the 1971 Villarrica lava based on crystal size, disequilibrium features and zonation patterns. Most of the CHSEC samples exhibit higher La-N/Yb-N and more scattered Sr-Nd values than 1971 Villarrica lava samples, which are clustered at higher Nd-143/Nd-144 values. Pre-eruptive temperatures of the CHSEC-type reservoir between 1162 and 1165 +/- 6 degrees C and pressures between 10.8 and 11.4 +/- 1.7 kb consistent with a deep-seated reservoir were obtained from olivine-augite phenocrysts. Conversely, olivine-augite phenocrysts of 1971 Villarrica lava samples record pre-eruptive conditions of two stages or pauses in the magma ascent to the surface: 1208 +/- 6 degrees C and 6.3-8.1 kb +/- 1.7 kb (deep-seated reservoir) and 1164-1175 +/- 6 degrees C and <= 1.4 kb (shallow reservoir). At shallow reservoir conditions a magma heating prior to the 1971 Villarrica eruption is recorded in plagioclase phenocrysts. Syn-eruptive temperatures of 1081-1133 +/- 6 degrees C and 1123-1148 +/- 6 degrees C were obtained in CHSEC and 1971 Villarrica lava, respectively using equilibrium olivine-augite microlite pairs. The LOFZ could facilitate a direct transport to the surface of the CHSEC magmas and explain the observed differences with the pre-eruptive conditions of the 1971 Villarrica lava.