Tortonian trachyte volcano in the Alta Sierra de Somún Curá Volcanic Complex. Northern Extrandean Patagonia. Argentina.

REMESAL MARCELA; SALANI FLAVIA MARÍA; CERREDO MARÍA ELENA; PARICA CLAUDIO; ALBITE JUAN MANUEL

Hamburgo

Congreso; LatinAmerica Coloquium; 2019

Universidad de Hamburgo, Alemania

The Miocene Alta Sierra de Somún Curá Volcanic Complex (ASSCVC) is the northernmost post-plateau complexof the Somún Curá Magmatic Province (SCMP), at the northern extrandean Patagonia, Argentina (Fig. 1a, b).ASSCVC is made of several volcanic buildings which span the basaltic to trachyte realm. Monogenetic basalticvolcanoes (i.e. Cuatro Cerros), trachyte apparatus (i.e. Cerro Pancho) and polygenetic basaltic to trachyteconstructions (i.e. Cerro Corona) compose the Complex. Four features distinguish ASSCVC from other post-plateau complexes of the SCMP: its age, it is the youngest of the post-plateau assemblages; the lack of acompositional gap between basaltic and trachyte terms; the absence of rhyolite differentiates and the presenceof several emission centers associated with prominent volcanic constructions.This contribution deals with one of the chiefly trachyte volcanoes of ASSCVC. The polygenetic stratovolcano ofCerro Mimbre (CMTV), located in the southern complex (Fig. 1c), is the northernmost of three volcanoesarranged along a NW-SE trend. CMTV is set within a local low relief area dominated by volcanic landforms withgeneral SE gentle slope. Two semicircular escarpments in the uppermost CMTV are compatible with anexplosion crater. Associated loose deposits of white color and fine texture, located among coulées, are indicativeof explosive eruptive facies. At least four lobular trachytic coulées emerge from the domes distributed in theCMTV; coulées locally preserve marginal ridges.Trachyte coulées include autobrecciated facies of porphyric texture with anorthoclase phenocrysts and minorferromagnesian minerals. Groundmass is trachytic composed of sanidine microliths and clinopyroxene.Trachyte lava flows are porphyric with feldspar phenocrysts (of oligoclase to anorthoclase composition) set in atrachyte groundmass with some bostonitic patches. Microliths of anorthoclase to sanidine composition, olivine,clinopyroxene (Mg-rich augite), F-apatite and magnetite complete the groundmass assemblage.Two long basaltic flows extend for several kilometers enveloping the CMTV. Three scoria cones surround theCMTV; within the main crater, there are two oxidized deposits of scoria that would correspond to a very erodedcone, younger than the trachyte coulées. The lowermost basaltic flows surrounding the main volcanic buildingare vesicle rich, either aphyric or porphyritic with plagioclase phenocrysts and olivine microphenocrysts set in anintergranular groundmass with ophitic patches made of clinopyroxene and plagioclase.Trachyte flows of the CMTV yielded a new whole rock K/Ar age (Fig. 1d) of 9.8 ± 0.3 Ma, which fits well (withinerrors) with already published chronological data for other trachyte lavas of ASSCVC ( 9-13 Ma).