Petrogênese do Gabro de Caracaraí - RR

Abstract

The Caracaraí gabbro outcrops as a stock that intrudes the hypersthene monzogranite, monzogranite and K-feldspar granite association of the Serra da Prata Intrusive Suite. This pluton occurs in the central region of the Guiana shield (north of the Amazonian Craton). The understanding of the petrogenesis of Caracaraí gabbro is supported by petrographic evidence, lithogeochemical and mineral chemistry analyzes to define two facies: i) Olivine gabbro with medium and mesocumulatic texture, consisting of hyalossiderite, enstatite, diopside, labradorite, spinel associated with ilmenite- hematite, hornblende, phlogopite. Locally it shows feldspar and quartz xenocrystals. This facies is restricted to Hornblenda Gabronorite porphyritic coarse composed of olivine crystals with millimeter pyroxene reaction rims followed by millimeter hornblende edges that range in total from 5 to 20 mm wide. ii) Gabronorite with fine to medium texture and adcumulatic is constituted by enstatite, diopside, pigeonite and labradorite and by the association spinel, magnetite, hematite, hornblende, and biotite. Cumulative textures show elliptical-shaped mafic crystal aggregates, these cumulative textures occur in a magmatic flow corridor approximately 200 m long and 60 m, In this band, gabbronorite and mafic mineral aggregates as well as tabular plagioclase crystals occur aligned, marking magmatic foliation (Smg). This Smg foliation is arranged according to two attitudes: i) alignment of mafic mineral aggregates (most representative) 120°-146°/30°-65° (strike/dip), ii) alignment of tabular plagioclase crystals (subordinate) 028°- 064°/58°-72° (strike/dip). The orientation of the magnetic fabric (magnetite and hematite as markers) is 180°-240° with high dips greater than 59°. These are rocks of basic composition with tholeiitic character, which show variation in composition to intermediate of calcium-alkaline character due to partial assimilation effects of the host rocks. The variation in the SiO2 content observed in Hacker Diagrams is compatible with the development of reaction rims in olivine and proves the fractionation process as dominant in the crystallization of gabbro. In gabbro olivine MgO levels (11 to 8.87% wt), and in gabbronite MgO (4.37 to 7.16% wt) are compatible with the increase in SiO2, Al2O3 and CaO levels, and were interpreted as evidence of assimilation. Geochemical discriminants suggest a subduction-related tholeiitic continental arc environment as a probable tectonic context for gabbro emplacement. The analysis of mineral chemistry data allowed us to interpret that plutonic accommodation would have occurred at temperatures ranging from 1160°C to 680°C and pressures from 4 to 10 Kbar. A SHRIMP U-Pb Dating on inherited zircon provided a crystallization age of 1946.7 ± 4.9 Ma, which is compatible with the age of the host rocks of the Serra da Prata intrusive suite. The magnetic susceptibility values > 1.05 Km in 92%, the proof of isothermal magnetic remanence with magnetic saturation induced by the application of low fields (< 0.3 mT), the record of the Verwey transition around -160 °C and the temperature close to 573 °C indicate the presence of magnetite in the studied gabbro, and its prevalence as a marker of the pluton magnetic structure.