The Potassic Calc-Alkalic Itaporanga Batholith, Northeastern Brazil: Mineral Chemistry and Oxygen-Isotope Data

Abstract

The Itaporanga batholith (state of Paraíba, northeastern Brazil) is composed of an intimate association of potassic, calc-alkalic, porphyritic quartz monzonitic to granitic rocks and biotite diorites. Net-veined structures demonstrate that the main petrogenetic process during evolution of this batholith involved coexistence and local mixing of magmas. In this work, mineral chemistry and oxygen-isotope data are discussed as an attempt to establish the origin of these magmas. Temperatures obtained through mineral chemistry vary from 690° to 730° C for felsic and mafic rocks, suggesting control by the processes of mingling and mixing of magmas. Chemistries of amphibole and biotite are controlled by the bulk chemistry of K-dioritic to quartz monzodioritic and porphyritic quartz monzonitic rocks, which were influenced by coexistence and local mixing of magmas. A biotite diorite enclave that did not equilibrate with the host quartz monzonite has amphibole and biotite compositions that differ from those diorites and quartz diorites that more intimately interacted with the porphyritic quartz monzonites. Oxygen-isotope data for quartz show that the K-diorite is enriched in 18O relative to the porphyritic quartz monzonite, suggesting that these rocks were not related through crystal fractionation processes and that their magmas had different isotopic signatures. This information, combined with mineral chemistry data, strongly suggests that K-dioritic and porphyritic quartz monzonitic magmas were not consanguineous.