The Troia–Pedra Branca mafic–ultramafic complex, Borborema Province, Brazil: Records of 2.04 Ga post–collisional Alaskan–type magmatism and PGE mineralization
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Abstract
The Troia–Pedra Branca complex is the most extensive exposure of mafic–ultramafic rocks in the Borborema Province, northeastern Brazil. These rocks have been known for a long time, particularly because of their platinum group element (PGE) mineralization associated with chromitites. The Troia–Pedra Branca complex consists of a succession of serpentinites (after dunite), metachromitites, metaperidotites, hornblendites and metagabbros. PGE–bearing metachromitites are hosted by the serpentinite–peridotite unit, occurring as dispersed blocks, well preserved from weathering. However, host metadunites are poorly preserved, and are only accessed by drill core samples. Scanning Electron Microscopy (SEM) analysis in thin sections of selected metachromitite samples revealed that most of the platinum group minerals (PGM) occur in the chlorite–serpentine matrix, generally in contact with chromite grains. The main PGM are sperrylite (PtAs2), cooperite (PtS), irarsite ((Ir,Pt,Rh)AsS) and hollingworthite ((Rh,Pd,Pt,Ru)AsS). Within chromite grains, very few PGM were found, and sulfide inclusions are mainly chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4). Whole–rock geochemical data reveal that metagabbros are LILE–enriched and show subduction–related signature similar to that of Alaskan–type intrusions. Mineral chemistry of chromite and olivine is also compatible with arc–related Alaskan–type complexes. The U–Pb SHRIMP zircon age for a metachromitite sample yielded an upper intercept age of 2036 ± 27 Ma, which we interpret as the crystallization age. However, dispersion in the data implies that zircons lost variable amounts of radiogenic Pb at around 749 ± 54 Ma (lower intercept), which may be related to Neoproterozoic metamorphism. The age obtained for the Troia–Pedra Branca metachromitites (2036 Ma) is younger than the 2190–2130 Ma arc–related plutons of the area, and it is closely related in age to the 2.10–2.04 Ga syn– to late–collisional plutonism and high–temperature metamorphism. Therefore, this Alaskan–type mafic–ultramafic magmatism may be related to the post–collisional setting of the 2.2–2.0 Ga Eburnean/Transamazonian orogeny.
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