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

M. REMESAL; CERREDO M. E.; ALBITE, J.M; FLAVIA M. SALANI; PARICA C.A.

Hamburgo

Congreso; 25Latin-American Colloquium og Geosciences; 2019

Universitát Hamburg

The Miocene Alta Sierra de Somún Curá Volcanic Complex(ASSCVC) is the northernmost post-platea u complexof the Somún Curá Magmatic Province (SCMP), at thenorthern extrandean Patagonia, Argentina (Fig. la, b).ASSCVC is made of severa! volcanic buildings whichspan 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 postplateaucomplexes of the SCMP: its age, it is the youngest of thepost-plateau assemblages; the lack of a compositionalgap between basaltic and trachyte terms; the absence ofrhyolite differentiates and the presence of severa! emissioncenters associated with prominent volcanic constructions.This contribution deals with one of the chiefly trachytevolcanoes of ASSCVC. The polygenetic stratovolcano ofCerro Mimbre (CMTV), located in the southern complex(Fig. le), is the northernmost of three volcanoes arrangeda long a NW-SE trend. CMTV is set within a local low reliefarea dominated by volcanic landforms with general SEgentle slope. Two semicircular escarpments in the uppermostCMTV are compatible with an explosion crater.Associated loose deposits of white color and fine texture,located among coulées, are indicative of explosive eruptivefacies. At least tour lobular trachytic coulées emergefrom the domes distributed in the CMTV; coulées locallypreserve marginal ridges. Trachyte coulées include autobrecciatedfacies of porphyric texture with anorthoclasephenocrysts and minorferromagnesian minerals. Groundmass is trachyticcomposed of sanidine microliths and clinopyroxene. Trachytelava flows are porphyric with feldspar phenocrysts(of oligoclase to anorthoclase composition) set in a trachytegroundmass with sorne bostonitic patches. Microlithsof anorthoclase to sanidine composition, olivine,clinopyroxene (Mg-rich augite), F-apatite and magnetitecomplete the groundmass assemblage. Two long basalticflows extend for severa! kilometers enveloping theCMTV. Three scoria eones surround theCMTV; within the main crater, there are two oxidizeddeposits of scoria that would correspond to a very erodedcone, younger than the trachyte coulées. The lowermostbasaltic flows surrounding the main volcanic buildingare vesicle rich, either aphyric or porphyritic withplagioclase phenocrysts and olivine microphenocrystsset in an intergranular groundmass with ophitic patchesmade of clinopyroxene and plagioclase. Trachyte flows ofthe CMTV yielded a new whole rock K/Ar age (Fig. ld) of9.8 ± 0.3 Ma, which fits well (within errors) with alreadypublished chronological data for other trachyte lavas ofASSCVC (121 9-13 Ma).