Geología de los complejos eruptivos de La Esperanza, provincia de Río Negro

LLAMBÍAS, E.J. ; RAPELA, C.W.

Revista Asociación Geológica Argentina

Asociación Geológica Argentina

Lugar: Buenos Aires; Año: 1984 vol. 39 p. 220 - 243

0004-4822

A sector of approximately 700 km2 in the northwestern part of the Nordpatagonian Massif (La Esperanza area, Ro Negro province, Long. 68°28’ and Lat. 40028P S) was in detail. The rocks that crop out are mostly granodiorites, granites and related volcanic rocks of Late Paleozón age. Exposures of the wall rocks of the eruptive sequences are scarce and they belong to the metamorphic rocks of the Cushamen formation (upper Precambrian to lower Paleozoic) and to a flysch sequence of a probable Silurian to Devonian age (Llanquil formation). The outcrops of both formations are considered as small pendants included in the plutonic rocks. Two igneous complexes have been recognized. 1) The oldest one, La Esperanza plutonic complex, is characterized by distinct petrographic units that in order of decreasing age consists of hornblende-biotite granodiorites; hornblende biotite granites with K-feldspar-megacrysts; biotite granites; and biotite granites with K-feldspar megacrysts. The contact between some units are sharp, although it is also rather common gradational contacts between some transitional facies. Several microdioritic to spessartitic dikes intruded these rocks, but its stratigraphical position is not clear for they could belong to the culmination of this complex or to the begining of the 2) Dos Lomas plutonic-volcanic complex, which is composed by dacitic ignimbrite sheets; a rhyolitic dome; rhyolite-aplite dikes; rhyolite ignimbrite sheets; and leucogranites. The leucogranites intruded the lower units of the volcanic sequence. The field relationships and chemical similarities between the volcanic rocks and the leucogranitic plutons conform the idea they are evolving from the same magmatic source. The ignimbrites are part of an extense ignimbritic plateau whose outcrops extend mainly to the east southeast of La Esperanza, covering the Nordpatagonian Massif, Fifty five chemical analysis, which will be published proximately, show that both complexes are similar in their chemistry and evolutive trends, Between the both complexes there is an erosion surface elaborated on the granitic rocks of La Esperanza complex. The dacitic ignimbrites of the Dos Lomas complex flowed over this almost horizontal surface. However, by the chemical and evolutive bends similarities of the two complexes, it is probably that they belong to one major eruptive cycle, which may be divided in two subcycles. Preliminary Rb-Sr data suggest a lower and middle Carboniferous ages for the La Esperanza complex and Dos Lomas complex respectively.