Geologia e petrografia do picrito do boqueirão e sua correlação com outras rochas máficas-ultramáficas no SW do escudo Sul-Riograndense

Abstract

Picrites are ultramafic, volcanic or hypabyssal rockas, which contain abundant olivine and have high MgO (> 12wt. %) and low alkalis (< 3 wt. %) contents. These rocks represent the crystallization of mantle-derived primitive magmas produced by a high degree of partial melting and/or by decompression of deeper sources. They commonly occur in oceanic and continental basaltic plateaus, ocean islands, middle ocean ridges and in komatiitic sequences. This monography is focused on the geologic and petrographic features of the Boqueirão picrite, in order to investigate its relationships with the main tectonic lineaments, the enclosing rocks and the other mafic-ultramafic bodies occurring along the Ibaré Lineament, sothwest of the Sulriograndense Shield. The Boqueirão picrite, placed in the Bagé municipal district,is about 20 km to the south of the Lavras do Sul town. This picrite forms a small tabular body (40 m x 200 m), aligned in the direction N88ºE that is concordant with the foliation of the enclosing metassedimentary rocks of the Arroio Marmeleiro metamorphic complex. It is emplaced near the intersection of two important fault zones, Porteira (ENE-WSW) and Graciano (NNE-SSW), which marks the limits between these metassedimentary rocks and the volcanogenic ones of the Hilário Formation. The analysis of tectonic alignments and lineaments shows that the picrite is placed along the E-W older tectonics structure, near its intersection with the Porteira fault zone (ENE-WSW). The latter, which had affected intensely the metassediments, was later reactived and acted as a conduct for trachyandesitic magmas of the Hilário Formation. The Boqueirão picrite is a medium to fine-grained rock, consisting of partially serpentinised, cumulate olivine (80-63 vol. %), subhedral or poikilitic grains of orthopyroxene and diopside (23-12 vol. %) and intercumulus material including diopside, kaersutite (< 2.5 vol. %), phogopite (< 1 vol. %), partially altered interstitial glass (9-2 vol. %) and accessory apatite. The opaque minerals ( 10 vol. %) include common Mg-chromite and very rare magnetite. Mafic minerals have magnesian composition, as shown by olivine (Fo84-83), orthopyroxene (En84Wo2 to En80Wo7) and diopside (en51Wo41 to En48Wo43). Textural and compositional variations in the picrite are consistent with a model involving a tholeiitic primitive magma flowing in a tabular body, with a mildly alkalic residual melt. Comparing this rock with the other regional ones, it is possible to suggest that all (picrite, olivine gabbro, noritic gabbro and diabase) belong to a same rock association formed during the initial events of the mesozoic basaltic magmatism of the Paraná Province.