FLUID ORIGIN AND EVOLUTION DURING THE FORMATION OF RARE-ELEMENT PEGMATITES FROM THE BORBOREMA PROVINCE, NORTHEAST BRAZIL.

HARTMUT BEURLEN, MARCELO REIS RODRIGUES DA SILVA, CLÁUDIO DE CASTRO

Resumo


Microthermometric studies of fluid inclusions in the Borborema Pegmatitic Province, Northeast Brazil, allowed to recognize the following fluid types: A) primary aqueous carbonic (17-21 mol% CO2) inclusions in early pegmatite minerals, such as garnets and tourmaline (wall zone I) and beryl, quartz, tourmaline and manganotantalite (zone II and limit zone II/III); B) primary aqueous fluid inclusions with low CO2 contents (£ 7 mol%), hosted in zoned quartz crystals, apatite and euclase from zone III, core and replacement bodies of the pegmatites; C) late aqueous inclusions barren in CO2 in the same minerals than type B; D) secondary inclusions in all minerals throughout the whole pegmatite, being either aqueous carbonic, showing wide ranges of CO2 /H2O ratios (type D1) or aqueous, with no CO2 (D2). The salinity of these fluids range from low (A and B) to moderate (C) and very low (D). Trapping temperatures decrease from type A to type D inclusions. Laser Raman Microspectrometry analyses of the carbonic phase of the type A inclusions confirmed the absence of methane, but detected significant N2 contents. The observed N2/CO2 ratios, ranging between 1/5 and 1/70 are higher than those reported in magmatic fluids associated with pegmatites or granitic magmas (£ 1/140). The high N2/CO2 ratios could be the result of wall rock fluid contamination. High N2 contents in metamorphic fluids are frequently explained by N2 liberation during the mica break down.

Palavras-chave


Aqueous-carbonic fluid inclusions; Raman data; Pegmatites.

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