Pegmatito várzea torta: caracterização geoquímica e relações com os granitos de anatexia adjacentes

Abstract

Várzea Torta is one of five pegmatites already partially exploited and included in the field 3 of Solonópole-CE district, and it is located about 18 km to NW from the headquarter of that community. It is a complex heterogeneous evolved pegmatite with five distinctive zones: 1) the border zone, a fine-grained one rich in feldspar, quartz, muscovite and schorl; 2) wall zone, a coarse-grained one with similar mineralogy to the border zone, with less schorl crystals, and rich in large plates of muscovite; 3) intermediate zone, rich in feldspars; 4) core of quartz and 5) replacement zone, with essential albite and muscovite. Várzea Torta had already been produced, in commercial quantities, columbite-tantalite, beryl, feldspars, spodumene, lepidolite and some varieties of black and green tourmalines. The geological mapping in an area of 100 km2 and 1:50.000 scale around the Várzea Torta pegmatite shows the presence of isotropic to deformed muscovite-biotite granites to muscovite ones, the late with or without garnet and less abundant than the former. The granites occupy about 2/3 of the mapping area and show graded-contacts, also observed in that outcrops that present irregular strip of both. They are albite granites and the petrographic analyses reveals that part of the muscovite of the extreme terms result from the deuteric alteration of the biotite. Thus, the muscovite granites come from the metassomatic alteration of the granites riches in biotite. Inside the granites above quoted are found centimetric to hectometric enclaves of metamorphic rocks of igneous and sedimentary origin, built up mainly of biotite schist, gneisses, meta-arkose, and migmatitos. Locally are found also restites of partial fusion of biotite schists riches in garnet. Late intrusions (in relation to Dn) of weakly deformed to no deformed ultramafic and gabbroic rocks, in spread rounded blocks towards Sn, are found mainly in the SW portion of the mapping area. Sn is the prominent foliation that is imprinted on deformed granites and Supracrustal enclaves, with direction ranging from N-S to NNE-SSW and strong dip (>65°) for W to NW. Supposedly generated in a phase of deformation of the Brasilian Cycle. The pegmatites are intruded into mega-enclaves of sedimentary origin (schists and gneisses), deformed granites, and into contact among both. Várzea Torta are hosted by a biotite schist mega-enclave while the Bom Jesus pegmatite are found intruded into a normal fault that cut the muscovite-biotite granites. Those information, together with some lithogeochemical data, suggest that the pegmatites are late differentiated from fractional crystallization of the same magma that initially give rise to granites.