Mistura de magmas e a formação do Batólito Rio Jacaré: evidências de manto litosférico enriquecido no sistema orogênico sergipano, NE-Brasil

Abstract

The Rio Jacaré Batholith (RJB; 617 ± 4 Ma) is an intrusion of the Poço Redondo Domain, Sergipe Orogenic System. The batholith is formed of quartz monzonites, monzogranites, and granodiorites, which are arranged in inequigranular (IF) and porphyritic (PF) facies. Microgranular enclaves (ME) occur abundantly in the BRJ and preserve mixing and mingling features. MEs range in color from light gray to dark gray and have globular to elongated shapes. Their contacts are clear-cut, crenulated and cuspate, or, more rarely, diffuse. The ME are diorites, monzodiorites, quartz monzodiorites and monzonites. MEs represent the breakdown and cooling of initially mafic magma that has been injected into a cooler felsic magma chamber. The emplacement of mafic magma occurred at different stages of the crystallization of the BRJ magma chamber. The maximum degree of hybridization of these MEs is 57%. ME mafic magma has an affinity with shoshonitic series and was generated by the partial melting of approximately 3% of a lithospheric mantle source enriched in incompatible elements. The plagioclase crystals of the RJB rocks exhibit compositional zoning, patchy zoning, boxy cellular texture, embayed crystal cores, cores with homogeneous composition, inclusions zones of mafic minerals, and synneusis. The composition of plagioclase in FI varies from albite to andesine (An7-33), in PF it is albite and oligoclase (An5-23) and in ME it varies from albite to labradorite (An6-51). The albite compositions in these crystals are limited to their periphery and suggest the action of hydrothermal fluids, that is, they did not result from magma crystallization. The textures identified in the plagioclase crystals allow to infer a period of stable magmatic conditions followed by a period with instabilities marked by mafic magma injections, which modified the conditions (temperature, pressure, and H2O activity) of the magmatic system and stimulated mixing between magmas during the evolution of the RJB. The mafic magma injections probably generated convection currents in the RJB magma chamber. Crystals with embayed cores suggest at least five episodes of resorption in these crystals. Primary and reequilibrated Mg-biotite crystals are found in the RJB rocks. Primary crystals mainly occur as inclusions in plagioclase crystals, while reequilibrated crystals do not occur as inclusions and contain anhedral titanite crystals in their cleavages. The crystallization temperature of primary Mg-biotite (IF: 683 to 713 °C; FP: 678 to 704 °C; ME: 685 to 745 °C) is consistent with the crystallization temperature of biotite in granitic systems. The crystallization pressure of primary Mg-biotite was 1.8 to 2.7 kbar in IF, 1.2 to 2.2 kbar in PF, and 1.2 to 2.9 kbar in ME. The compositions of the primary crystals of IF and PF indicate that they were formed by magmas with H2O content between 5 and 7%. The fO2 (ΔNNO) during the formation of primary crystals ranged from -16.3 to -15.0 in IF, from -15.9 to -15.4 in PF, and from -15.6 to -13.9 in ME. The reequilibrating of the studied crystals probably resulted from the exsolution of Ti, which together with hydrothermal fluids containing Ca2+, formed anhedral titanite crystals in their cleavage planes and edges.