Coexistence and mixing of magmas in the late of cambrian Itaporanga Batholith, State of Paraíba, Northeastern Brazil

Abstract

The Precambrian Cachoeirinha-Salgueiro Fold Belt (CSF) located in the western portion of the states of Pernambuco and Paraiba is intruded, in its northern portion, by several coarsely porphyritic potassic calc-alkalic .batholiths. These batholiths were syntectonicaly unplaced in relation to the Brasiliano cycle (=Pan-African) and are commonly associated with potassium diorites suggesting coexistence and mixing between felsic and mafic magmas. In the Itaporanga batholith three petrographic domains-were mapped. A hybrid zone characterized by intense mechanical mixing of granite to granodiorite and potassium diorite magmas is located towards the border of the batholith. A commingling zone where felsic porphyritic granite to granodiorite and potassium diorite rocks are individualized at outcrop scale is located towards the center of the batholith. Finally a felsic porphyritic fades occur in the hybrid zone. The intense mechanical mixing observed in the hybrid zone developed migmatite-like structures (stromatic and less frequently agmatic), pillov -like structures, mafic enclaves with irregular shape and cuspate contacts between mafic and felsic rocks, suggesting diffusion of chemical species across contact. Major and trace element plotted against SiO2 agree with a mixing model to explain the hybrid samples plotted at intermediate position between felsic and mafic rocks. Similarity among chemical analyses of amphiboles from potassium dioritic enclaves of the Itaporanga batholith and from the potassium diorite stock east of it suggest a common source for both magmas. This hypothesis is corroborated by similar REE patterns for potassium dioritic enclaves of the Itaporanga batholith and for the potassium diorite stock. The batholiths shows a well developed foliation which dips towards its core suggesting that the present level of exposure represents the root zone of a diapir, where intense interaction between felsic and mafic magmas took place.