Petrologia do magmatismo Aricamã na região da Vila do Tepequém (RR), Domínio Urariquera – Cráton Amazônico

Abstract

In the northwest portion of the Urariquera Domain, the north central portion of Roraima, Vila do Tepequém region, occur several granitic bodies of alkaline affinity, associated with alkaline volcanic rocks of the Cachoeira da Ilha Formation and calc-alkaline rocks of the Pedra Pintada Intrusive Suite and Surumu Group. This study were carried out into alkaline plutonic rocks, based on remote sensors, were delimited in six plutons, with varying sizes, called Areia Branca, Ametista, Serra Aricamã, Santa Luzia, Cantinho and Flecha. The petrographic data, whole rock chemistry, mineral chemistry and isotopic data discriminate then in two major groups: (i) the alkali-feldspar granite (Serra Aricamã, Santa Luzia, Cantinho, Flecha and the western portion of Areia Branca) and (ii) the biotite alkali-feldspar granite (Areia Branca and Ametista). Fine to coarse-grained inequigranular, the alkali-feldspar granite consists of alkali feldspar, quartz, plagioclase, amphibole and subordinate epidote, titanite, Fe oxides, fluorite, garnet, allanite, apatite and zircon. Feldspar with perthitic texture and plagioclase-rich in albite is an indicative of a subsolvus nature. The natural chemistry allows us to classifies the amphiboles in Actinolite, Mghornblende, Fe-edenite and Fe-hornblende and estimate the pressure in 3.2 kbar (geobarometer of AlT hornblende), the liquidus temperature of, at least, 700 to 900 ºC (geothermometer plagioclaseamphibole), and high ƒO2. The contents of epidote displays pistacita that differentiate into primary (Ps = 28.3%) and formed by alteration or late subsolidus (Ps = 33.4 to 35.5%), Fe oxides form a solid solution in range of magnetite-hematite and feldspars are classified as perthitic (Or = 61.5 to 68.2%), albitic (Ab = 85.4 to 98.7%) and microcline (Or = 89.1 to 97%). The set of lithogeochemistry data shows a character meta to peraluminous, alkaline affinity, probably generated by fractional crystallization process, with probable fractionation of amphibole, titanite, epidote, apatite, feldspars and iron oxides. The REE distribution show moderate fractionation of LREE relative to HREE and moderate negative anomaly of Eu. The isotopic data indicate that magma has mantle origin (ƐNd(t) + 0.11 to 2.26), TDM ages from 1960 to 2124 Ma and possible age of 1859 ± 7 Ma Rb-Sr isochron. These rocks are correlated with rocks of the Saracura Intrusive Suite. The fine to medium-grained inequigranular biotite alkali-feldspar granite, with alkali feldspar, quartz, biotite, as the main mineral phases, and subordinate Fe oxides, garnet, apatite, zircon and fluorite. The mesoperthitics and perthitics texture, in feldspars indicates a hypersolvus nature. The mineral chemistry analyze identify the biotite is more enriched iron molecule and is considered primary with substitutions like anita-phlogopite and siderophyllite. The iron oxides form a solid solution in the range of magnetite-hematite and feldspars are classified as mesoperthitics, perthitics (Or = 31 to 73%), albitic (Ab = 98.5 to 99.7%) and microcline (Or = 90.6 to 98%). According to geothermometer of Zr saturation, in whole rock, these rocks exhibit temperatures between 820 and 918 °C. It shows peraluminous to metaluminous nature, alkaline affinity, and was formed mainly by partial melting. Process the REE pattern is characterized by gull wing type envelopes with sharp negative anomaly of Eu. The isotopic data indicate mantle sources (ƐNd(t) 2.96 and 3.25) and TDM ages of 2003 Ma and 2036 Ma. Because these rocks show unique lithogeochemistry characteristics compared to the alkaline rocks in the region, we propose to name then as Areia Branca Magmatism.