Efeito térmico provocado pelo pluton ediacarano Umarizal em rochas do Grupo Seridó (RN): implicações estruturais, mineralógicas e petrofísicas

Abstract

The northeastern portion of the Borborema Province (PB), in northeastern Brazil, is marked by a voluminous Ediacaran magmatism that intrudes the paleoproterozoic gneiss-migmatitic basement (the Caicó Complex) and neoproterozoic metasupracrustal rocks (the Seridó Group). In this context, the Umarizal pluton, the focus of this work, is classified among the magmatic Ediacaran types as the Charnockite Alkaline suite (ChAlk). The main objective of this research is the reevaluation of previous interpretations through new field and geochemical data, emplacement mechanism reevaluation, dating of plutonic rocks and neosome of thermally affected hosting units, and thermal modeling of magma cooling. The obtained results showed that the magmatic suite is composed of three distinct petrographic facies, where the Umarizal and Ação facies are restricted to the Umarizal pluton and the Lagoa facies is found as minor bodies intruding the Tourão pluton and satellites bodies. Usually, the ChAlk Suite is composed of quartz-syenite and quartz-monzonite and their charnockitic equivalents. Structurally, the main pluton is emplaced between the NE-SW Portalegre (PaSZ) and NE-SW Frutuoso Gomes (FGSZ) shear zones. Field criteria suggest that these lineaments had progressive movements, beginning with synistral strike-slip kinematic for the PaSZ and extension for the FGSZ that favoured the emplacement of the Umarizal magma, and finished with dextral transcurrence for the PaSZ and thrusting for the FGSZ. Synchronously with magma emplacement, a thermal aureole of up to 2 km thickness was formed in Formação Jucurutu country rocks, with Buchan-type metamorphic associations registered in andaluzite + sillimanite in paragnaisse, scapolite in calciosilicatic gneiss and flogopite in marble. Zircon U-Pb age of 583 ± 1.8 Ma of a neosome is similar, within error, to the zircon U-Pb ages of the Tourão (589.3 ± 4.4 Ma) and Umarizal (587,2 ± 2,3 and 563,7 ± 6,2 Ma) plutons. Modal and lithogeochemical compositions configure two suites for the AlkCh association, one of probable mantle-derived (gabbro-norites, anorthosites) and another crustal-derived (granites and charnoquites), thus characterizing an anorthosite - mangerite - rapakivi-granite – granite magmatic association. Mineralogical variation from the least differentiated to the most evolved facies suggests the dominance of anhydrous conditions (fayalite, hyperstene, diopside-hedenbergite) at the beginning of crystallization to water subsaturated at the end of evolution and then favouring the formation of hornblende and biotite. Initial crystallization conditions of the charnockite suite indicate control by the fayalite-magnetite-quartz buffer, with pressures of ~9 kbar and temperatures in the order of 900-1100 ° C. On the other hand, geothermobarometric data using Al-in hornblende allowed to calculate approximate conditions of 4.5 ± 0.6 kbar and 755 ± 32 ° C for the final crystallization stage and emplacement. The integration of mineralogical and petrophysical data of the plutonic units and the thermal aureole permitted the thermal modeling of the cooling magma, being coherent with conditions of high geothermal gradient (40 °C/km), thermal metamorphic peak about 15,000 - 80,000 years after emplacement, and thermal conditions extending to 2 km away from the contact. Considering the existence of countless other batholithic bodies of the same age in this part of the PB, it is proposed that the Ediacaran was a period of extensive crustal heating under conditions of high geothermal gradient, possibly due to the heating driven by underplating of mantelic (gabbro-norite) magmas.