| Popis: |
The Troia–Pedra Branca complex is the most extensive exposure of mafic–ultramafic rocks in the Borborema Province, northeastern Brazil. Chromitite occurrences in this unit are known for a long time, particularly because of its platinum group element (PGE) mineralization. The Troia–Pedra Branca complex consists of a succession of serpentinites (after dunite), metachromitites, metaperidotites, hornblendites and metagabbros. The PGE–bearing metachromitites are hosted by the serpentinite–peridotite unit. In the field, metachromitites generally occur as dispersed blocks well preserved from weathering. However, the host metadunites are poorly preserved, being only accessed by drill core samples. Scanning Electron Microscopy (SEM) analysis in thin sections of selected metachromitite samples reveled that most of the PGE minerals occurs in the chlorite–serpentine matrix, generally in contact with chromite grains. The main PGE minerals are sperrylite (PtAs2), cooperite (PtS), irarsite ((Ir,Pt,Rh)AsS) and hollingworthite ((Rh,Pd,Pt,Ru)AsS). Within the chromite grains, very few PGE minerals were found, and the main metallic inclusions are mainly chalcopyrite (CuFeS2), pentlandite ((Fe,Ni)9S8) and bornite (Cu5FeS4). Whole–rock geochemical data reveal that the metagabbros are LILE–enriched, and show subduction–related signature similar to Alaskan–type intrusions. Chromite and olivine mineral chemistry 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. Dispersion of the data, however, implies that the zircons have lost variable amounts of radiogenic Pb at about 749 ± 54 Ma (lower intercept), which may be related to Neoproterozoic metamorphism. The obtained age for the Troia–Pedra Branca metachromitites (2036 Ma) is quite younger than the 2190–2130 Ma tonalitic to granodioritic arc–related plutons of the area, and is closely related in age with 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. |
