Abstract
The Piquiri Syenite Massif, southernmost Brazil, is part of the post-collisional magmatism related to the Neoproterozoic Brasiliano-Pan-African Orogenic Cycle. The massif is about 12 km in diameter and is composed of syenites, granites, monzonitic rocks and lamprophyres. Diopside-phlogopite, diopside-biotite-augite-calcic-amphibole, are the main ferro-magnesian paragenesis in the syenitic rocks. Syenitic and granitic rocks are co-magmatic and related to an ultrapotassic, silica-saturated magmatism. Their trace element patterns indicate a probable mantle source modified by previous, subduction-related metasomatism. The ultrapotassic granites of this massif were produced by fractional crystallization of syenitic magmas, and may be considered as a particular group of hypersolvus and subsolvus A-type granites. Based upon textural, structural and geochemical data most of the syenitic rocks, particularly the fine-grained types, are considered as crystallized liquids, in spite of the abundance of cumulatic layers, schlieren, and compositional banding. Most of the studied samples are metaluminous, with K2O/Na2O ratios higher than 2. The ultrapotassic syenitic and lamprophyric rocks in the Piquiri massif are interpreted to have been produced from enriched mantle sources, OIB-type, like most of the post-collisional shoshonitic, sodic alkaline and high-K tholeiitic magmatism in southernmost Brazil. The source of the ultrapotassic and lamprophyric magmas is probably the same veined mantle, with abundant phlogopite + apatite + amphibole that reflects a previous subduction-related metasomatism.
Highlights
Syenitic or trachytic magmas can occur in the silicaundersaturated or saturated alkaline igneous series (Lameyre and Bowden 1982)
The post-collisional stage in the eastern portion of this region is marked by voluminous magmatism along the transcurrent lithospheric discontinuities of the Southern Brazilian Shear Belt (Bitencourt and Nardi 2000) which has led to the construction of the Pelotas Batholith (Philipp et al 2002)
Plutonic and volcanic acid to basic rocks of shoshonitic affinity are widespread in the 610590 Ma age interval (Lima and Nardi 1998a), and are followed by (i) voluminous sodic, silica-saturated alkaline magmatism, the Saibro Intrusive Suite (Nardi and Bonin 1991), mostly composed of metaluminous granites, with minor peralkaline components, and (ii) large volcanic plateaus where acid lavas and pyroclastics are dominant, with minor intermediate and basic components (Sommer et al 1999, Wildner et al 1999)
Summary
Syenitic or trachytic magmas can occur in the silicaundersaturated or saturated alkaline igneous series (Lameyre and Bowden 1982). Potassic (or shoshonitic) and ultrapotassic magmatism are frequently associated, either in volcanic sequences like the Roman Province (Civetta et al 1981) or the Aeolian arc (De Astis et al 2000), or in plutonic associations, such as those referred by Silva et al (1987, 1993) and Thompson and Fowler (1986) In many cases, they are related to minettes (Leat et al 1989, Conceição et al 1997, Stabel et al 2001, Paim et al 2002, Plá Cid et al 2005, among other authors). The derivation of potassic and ultrapotassic trachytes and syenites from magmas of minette compositions are suggested by several authors, such as Thompson and Fowler (1986), Rock (1987), Leat et al (1989), Janasi et al (1993), Conceição et al (2000a), and Plá Cid et al (2005). A similar source is here proposed for the primary intermediate melts that generated the syenitic and granitic rocks of Piquiri Massif
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