Caracterização litoquímica e geocronologia Rb-Sr das rochas granitóides e ortognaisses da região de Santa Quitéria-Sobral, NW do Ceará

Abstract

In the SSW of the Sobral-Pedro II Lineament between the cities of Sobral, Forquilha and Santa Quitéria, in the northwest of the State of Ceará, there is a variety of isotropic and foliated granitic bodies, with a predominance of the former. The isotropic granites constitute batholithic masses and bodies of smaller dimensions, intrusive in the rocks of the gneissic-migmatitic complexes, in the set of supracrustal rocks, including those of the Ubajara and Jaibaras groups. The Forquilha orthogneisses, of tonalitic composition with subordinate trondhjemitic and granodiorite terms, constitute one of the individual units within the gneiss-migmatitic complexes. These rocks provided a total rock Rb-Sr age of 1981 ± 45 Ma, which is interpreted as a minimum age for gneissification processes. The similarity of this result with the ages obtained in the region of Granja evidences a significant performance of the Transamazonian Event in the NW of Ceará. The initial isotopic ratio (0.70138 ± 13) of these rocks suggests an evolution from a mantle source or from short-residence crustal protoliths. The Brasiliano granites have a predominantly monzogranitic modal composition, ranging from microclimate granites to granodiorites. They are bodies of alkaline composition, per to metaluminous, with geochemical characteristics that reveal a strong similarity with type A granites, which may represent alkaline granites of intraplate or post-tectonic tectonic environment. These characteristics, including the behavior of REEs, are also similar to those of the Mucambo and Meruoca granites. The Rb-Sr geochronological studies carried out on these granites provided an age of 524 ± 12 Ma and 482 ± 8 Ma for the Pajé and Serra da Barriga granites, respectively. As for the Morrinho Granite, conventional ages between 480 and 510 Ma were obtained. Adding these results to the existing data for the Meruoca and Mucambo granites, an epoch of intense granite genesis is evident in the entire NW region of Ceará, from the late Proterozoic to the Eo-Paleozoic. Foliated granites occur as elongated bodies in the gneissic terrains of the High Grade Belt of Cariré, in agreement with the regional structure. The geochemical characteristics obtained in these bodies show a lot of similarity with those of the isotropic granites set. However, the Rb-Sr age of 475 ± 15 Ma obtained in these granites is smaller than that of the isotropic granites and certainly corresponds to a rejuvenated age. This rejuvenation is probably related to the effects of Graben Jaibaras reactivations. It was not possible to determine the true age of the crystallization of these bodies, as well as the deformational processes that affect them, in a situation very similar to that found for the Chaval Granitoid. The arrangement of the initial ratios of the foliated granites (0.70521 ± 56) and the Pajé Granite (0.70488 ± 71) in relation to the evolution of the Rb/Sr ratios of the Forquilha orthogneisses is compatible with a derivation of the granites from these gneisses. This is not valid for Serra da Barriga Granite, which has a high initial ratio (0.70963 ± 160).