Geologia, petrologia e geoquímica dos granitóides cálcico-alcalinos da região de Portovelo-Zaruma, El Oro-Equador

Abstract

In the Portovelo-Zaruma region, southwest Equator, occidental side of Cordillera Occidental of the Equatorian Andes, were mapped, at 1: 60.000 scale, five granite bodies: El Prado, Guayabo-Porotillo, Ambocas, Rios Luis-Ambocas anda Amarillo-Pindo. The other geological units are the Paleozoic Tahuim Group, composed by the San Roque Formation (quartzites, schists, gneisses, migmatites and amphibolites) and the Capiro Formation (quartzites, schists and phyllites); the Cretaceous Celica Formation (basalts and andesites), folded and discordantly overlay by Tertiary pyroclastics and andesitic flows. The granite bodies outcrop in very close areas, and probably are part of a unique batholith, only discontinuously exposed by the erosive processes. The granites are not dated, and are supposed to be of Triassic to Cretaceous age. The El Prado Granite intrude the Capiro Formation. It shows a elongated shape in the NW-SE trend (10 x 3 km). The Guayabo-Potillo Granite cuts the Tahuim Group, also showing an elongated shape in the NW-SE trend (7 x 3 km). The Ambocas Granite shows a near circular shape small bodies, and intrude the San Roque and Capiro formations, respectively. The relationships between all these granites and the Celica Formation are not conclusive due to the inaccessibility of the contact zones or by the fact that they are covered by colluvial deposits. The modal compositions of the granites shows span a expanded range, including gabbros, quartz gabbros, quartz diorites, tonalites, granodiorites and monzogranites. In the QAP diagram the rock follow preferentially the calc-alkaline tonalitic trend, but some granodiorites and monzogranites display a calc-alkaline granodioritic trend. Two rock groups were distinguished: (a) mafic to intermediate mafic-rich rocks - gabbros, quartz gabbros and quartz diorites; (b) felsic granitoids rocks - tonalites, granodiorites, and monzogranites. The dominant mafic minerals of the (a) group are pyroxenes, amphiboles and biotite, and biotite in the (b) group. The felsic rocks of the Guayabo-Porotillo body commonly show primary muscovite, are quartz-rich, and were affected by a strong hydrothermal alteration. Muscovite is also relatively abundant in the granitoids of the Ambocas, Luis-Ambocas rivers and Pindo-Amarillo rivers bodies. Amphibole is an important phase only in some granitoids of the El Prado body. The textures of the mafic and intermdiate rocks are porphyritic, pilotaxitic or seriated. Locally the plagioclase shows sieve texture. The felsic rocks are not deformed, and display a medium grained hypidiomorphic texture. According to the geochemical data about these granites there is a compositional gap between the mafic/intermediate and felsic rocks. Nevertheless, the studied rocks, fit the calc-alkaline trend displayed by the continental arc plutonic magmatism of the Andean chain. The mafic rocks are metaluminous and the amphibole-bearing felsic rocks are weakly metaluminous. The muscovite-bearing rocks are strongly peraluminous. A deep compositional gap between the two sets is clearly seen. In the diagram Ca+alkalis vs. SiO2 the rocks plot in the normal arc-related calc-alkaline granites field. However, some samples do not follow the general trend, probably due to crustal contamination or hydrothermal alteration. The El Prado rocks are less disturbed, following a trend very similar to that dis played by New Guinea Series or by the late granodiorites of Panama. The trace elements contents are similar to that of the volcanic arc granites (VAG). The metaluminous granites are petrographic and geochemically very similar to the I-type granites, but their tectonic enviroment is close to that of the Cordilleran I-type granites. The peraluminous granites, at a first sight, are similar to the S-type granites Australian granites, but are not cordierite-bearing, an index mineral of the Australian S-type granites. To explain the constrasting aspects between the metaluminous and the strongly peraluminous granitoids, it is proposed the hypothesis that the latter are derived from an I-type magma, contaminated by metasedimentary crustal rocks, by an assimilation process, similar to that described in the SW of USA. Other hyphotesis, considered more specifically for the extremely peraluminous granitoids, are: (a) their derivation from sedimentary sources; (b) a leaching of alkalis related to strong subsolidus hydrothermal processes, modifying the primary igneous compositions.