O BATOLITO CALCIO-ALCALINO DE ALTO- K DA SERRA DA LAGOINHA, ESTADOS DO CEARÁ E PARAÍBA: COEXISTÊNCIA E MISTURA DE MAGMAS DIORÍTICOS COM MAGMAS QUARTZO MONZONÍTICOS A GRANÍTICOS

Abstract

The Serra da Lagoinha batholith (SLB) hás an outcrop área of 200 km 2 is located in the Borborema Province, northeastern Brazil, and intrudes schists and phylites at the border of the Cachoeirinha-Salgueiro fold belt. Its southern border is defmed by a major curved NE-SW sinistral transcurrent shear zone (Boqueirão dos Cochos) connecting the E-W dextral shear zones of Patos to the north and Pernambuco to the south. The SLB hás three lithologic domains: the felsic porphyritic domain composed of K-feldspar megacrystic monzonitic, quartz monzonitic, quartz monzodioritic, granitic to granodioritic rocks; the K-dioritic domain composed of biotite (quartz) dioritic to tonalitic rocks and the hybrid domain composed of monzonitic and biotite (quartz) dioritic rocks. Field evidences show that these lithologic domains were produced by the coexistence and mixing of K-diorites with potassic mozonitic to granodioritc magmas. The mafïc rocks are enriched in Nb, Ba, and Zr and LREE in relation to the felsic porphyritic rocks. The REE patterns for mafïc and felsic rocks are similar and enriched in the LREE in relation to the HREE, with (La/Yb)N varying from 22.43 to 36.10 for the felsic and from 27.21 to 58.87 for the mafïc rocks. AlT in amphibole geobarometer (Hollister et al. 1987) shows that amphibole crystallization in the SLB occurred at approximaatelly 4.5 kbar. Geothermometry based on the coexistence of amphibole and plagioclase, (Blundy & Holland 1990) gives a temperature of 742 ° C. Whole rock Oxygen isotope data showed average values for mafïc of δ 18 O = + 7.9 permilSMOW and for the felsic rocks of δ 18 O = + 9. l permilsMOW, suggesting either isotopic equilibrium of coexistent magmas of distinct sources or crystal fractionation process controlling the evolution from mafïc to felsic rocks. Quartz corrected values for the felsic pophyritic rocks averaged δ 18 O = + 9.2 permilSMOW, suggesting that the whole rock data represent magmatic values. Sm-Nd sistematics show T(DM) of 1.78 and 1.70 Ga and εNd(600) of -11.5 and -10.2 for two felsic porphyritic samples and T(DM) of l .78 Ga and εNd(600) of -11.15 for a K-diorite. This data may reflect a mixed source rocks formed essentially by Transamazonia (ca 2.0 Ga) with reduced participation of Caririris Velhos (ca l .0 Ga) lithospheric mantle. Major, trace and REE associated with the field evidence support the coexistence and local mixing of K-dioritic with Quartz monzonitic to granitic magmas of distinct sources as the major petrogenetic process for the SLB.