Amphibole-rich clots in calc-alkalic granitoids in the Borborema province, northeastern Brazil

Abstract

Metaluminous granodioritic–tonalitic plutons intruded low-grade metaturbidites of the Cachoeirinha–Salgueiro terrane and low- to intermediate-grade metasediments of the Macururéterrane, northeastern Brazil, around 630 Ma ago. Four types of amphibole-rich polycrystalline clots, up to 20 cm long, are found within mafic microgranular enclaves or in their granodioritic–tonalitic hosts. Type-I clots are usually angular, with granoblastic textures, composed of amphibole with patchy actinolite cores and magnesiohornblende margins, in polygonal packing where grains commonly display near-120° triple junctions. Interstitial biotite and calcic clinopyroxene are the other component phases. Magnesiohornblende+biotite cumulates form the type-II clots that sometimes constitute an external layer around type-I clots. Actinolite pseudomorphs after clinopyroxene-rich restites form the type-III. A fourth type, texturally similar to type-I clot, contains pyrite as an additional accessory phase and is found only in plutons in the Macururéterrane. Amphibole cores in clots of types I and IV are actinolite with SiO2 around 55%, MgO (6.5%, FeO 10.5% and Al2O3 around 2%, while margins are magnesiohornblende with SiO2 around 50%, FeO≈12%, MgO≈14.5%, and Al2O3≈5%). Amphibole aggregates in type-II clots display compositions identical to those in the granodiorite hosts. Biotite in types I and IV clots show SiO2, TiO2, K2O and CaO equivalent to those in the granodiorite host, but are about 4% lower in FeO and about 3% higher in MgO. Although all studied plutons are oxidized I-type granites, those from the Cachoeirinha–Salgueiro terrane display quartz-corrected w.r. δ18O (+11 to +13‰SMOW) in the range for S-type granites, while in the Macururéterrane values from +9 to +10‰ are observed. Amphibole-rich clots usually have δ18O values 1.5‰ lower than those observed in their corresponding hosts. Initial Sr ratios for plutons and their mafic microgranular enclaves are 0.70598 in the Cachoeirinha–Salgueiro terrane. Host plutons show εNd (0.6 Ga) values from −1.0 to −3.0 and −4.6 to −6.7, respectively in the Cachoeirinha–Salgueiro and Macururéterranes, and δ34S from +1 to +9‰CDT in the Cachoeirinha–Salgueiro terrane. Whole-rock δD values for amphibole clot-bearing granodiorites, mafic microgranular enclaves and clots vary from −69 to −114‰SMOW, a variation much broader than for corresponding δ18O values. Partial fusion of low-T altered basaltic source (amphibolites) generated the calc-alkalic granodioritic–tonalitic magmas in the Cachoeirinha–Salgueiro terrane. Type-I clots are probably fragments or restites of melting of this source. High δ18O in amphibole-rich clots and granodiorite hosts support this hypothesis. Actinolite pseudomorphs after calcic clinopyroxene (type-III clots) attest to the incorporation of extraneous H into host magmas, causing a wide δD isotopic range. In the Macururéterrane, type-IV amphibole-rich clots are restite remnants of dehydration melting of an amphibolite source.