Geochemistry and petrogenesis of post-collisional ultrapotassic syenites and granites from southernmost Brazil: the Piquiri Syenite Massif

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 K 2 O/Na 2 O 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.

Crystallization conditions and controls on trace element residence in the main minerals from the Pedra Branca Syenite, Brazil: An electron microprobe and LA-ICPMS study

“…In this respect, we concur with the current tendency in literature that gives no support to former suggestions that similar K-rich syenites would correspond to feldspar-rich cumulates (e.g. Bitencourt and Nardi, 2004;Nardi et al, 2008).…”

Linking enclave formation to magma rheology

“…The wide variety of sizes and shapes of preserved magmatic enclaves highlights the inherent complexity of the interactions between the input and resident magmas (Figure 1). Where they are observed, enclaves range in size from a few millimeters to several meters in diameter and their shapes, although often axisymmetric, can also be ovoid, elongate, disk‐like, blade‐like, or elliptical [ Kumar , 2010; Alves et al , 2009; Nardi et al , 2008; Barbey et al , 2008; Feeley et al , 2008; Wiebe et al , 2007; Donaire et al , 2005; Barbarin , 2005; Paterson et al , 2004; Kim et al , 2002; Silva et al , 2000; Akal and Helvaci , 1999; Clynne , 1999; Wiebe and Adams , 1997; Wiebe et al , 1997; Thomas and Tait , 1997; Barbarin , 2005; Pin et al , 1990; Didier and Barbarin , 1991; Bacon , 1986; Vernon , 1984; Bacon and Metz , 1984]. …”

Zircon U-Pb geochronology, Sm-Nd and Pb-Pb isotope systematics of Ediacaran post-collisional high-silica Acampamento Velho volcanism at the Tupanci area, NW of the Sul-Rio-Grandense Shield, Brazil

“…1). Magmatism in the W and NW portion of the SRGS developed mainly between 650-530 Ma and shares a similar geochemical evolution as observed in the eastern part of the SRGS, with the presence of high-K, calc-alkaline granitoids, shoshonitic and tholeitic plutono-volcanic sequences (Nardi et al 2008). The sedimentary sequences in the Camaquã Basin evolve from shallow marine in the Maricá Formation to lacustrine and alluvial in the Bom Jardim and Camaquã groups (Paim et al 2000).…”

“…The Sul-Rio-Grandense Shield (SRGS) in southernmost Brazil is constituted by rocks related to the Brasiliano/PanAfrican orogenic cycle emplaced in a Paleoproterozoic metamorphic basement (Hartmann et al 1999, Soliani et al 2000, Nardi et al 2008. The magmatism in this era was developed in two main phases: arc-related environment, with ages from 850-700 Ma; and post-collisional environment (Liégeois 1998), with ages from 650-535 Ma, marked by magmatism along transcurrent shear zones (Fernandes et al 1995, Babinski et al 1997, Bitencourt & Nardi 2000, Chemale Jr. 2000, Hartmann et al 2000, Nardi & Bitencourt 2009).…”