Evidence of mingling between contrasting magmas in a deep plutonic environment: the example of Várzea Alegre, in the Ribeira Mobile Belt, Espírito Santo, Brazil

Silvia R Medeiros,Cristina M Wiedemann-Leonardos,Simon Vriend

doi:10.1590/s0001-37652001000100009

Silvia R Medeiros, Cristina M Wiedemann-Leonardos + Show 1 more

Open Access

https://doi.org/10.1590/s0001-37652001000100009

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Abstract

At the end of the geotectonic cycle that shaped the northern segment of the Ribeira Mobile Belt (Upper Proterozoic to Paleozoic age), a late to post-collisional set of plutonic complexes, consisting of a wide range of lithotypes, intruded all metamorphic units. The Várzea Alegre Intrusive Complex is a post-collisional complex. The younger intrusion consists of an inversely zoned multistage structure envolved by a large early emplaced ring of megaporphyritic charnoenderbitic rocks. The combination of field, petrographic and geochemical data reveals the presence of at least two different series of igneous rocks. The first originated from the partial melting of the mantle. This was previously enriched in incompatible elements, low and intermediate REE and some HFS-elements. A second enrichment in LREE and incompatible elements in this series was due to the mingling with a crustal granitic magma. This mingling process changed the composition of the original tholeiitic magma towards a medium-K calc-alkalic magma to produce a suite of basic to intermediate rock types. The granitic magma from the second high-K, calc-alkalic suite originated from the partial melting of the continental crust, but with strong influence of mantle-derived melts.

Highlights

Most of the history of the geological evolution of the coastal region in the states of Rio de Janeiro and Espírito Santo, in Brazil, is comprised of the development of a Mobile Belt of Upper Proterozoic to Paleozoic age (Cordani et al 1973, Söllner et al 1991, Machado et al 1996)
Younger ages obtained for charnoenderbitic rocks from the Coastal Complex in this region have been related to the thermal evolution of the metamorphism, which produced medium to low presssures granulites from previous high-amphibolite facies and migmatitic gneisses. (Sluitner & Weber-Diefenbach 1989, Söllner et al 1989, Fritzer 1991, Seidensticker & Wiedemann 1992)
Best and Mercy (1967) in a classic work on the composition of mafic minerals in the calc-alkalic igneous complex of Guadalupe pointed out the uniformity of coexisting Ca-rich CPX and OPX, indicating relatively high fH2O and low crystallization temperatures

Summary

Introduction

Most of the history of the geological evolution of the coastal region in the states of Rio de Janeiro and Espírito Santo, in Brazil, is comprised of the development of a Mobile Belt of Upper Proterozoic to Paleozoic age (Cordani et al 1973, Söllner et al 1991, Machado et al 1996). High Sr-contents in the more basic rocks contrast with the normally low values obtained for gabbros from the tholeiitic series, studied in this region (Wiedemann et al 1995, Ludka et al 1998). The progressive increase of the REE-contents from the gabbroic to the dioritic rocks (Fig. 5), without Eu-anomalies suggests the predominance of a weakly process of fractional crystallization of a restricted magmatic series.

Results

Conclusion