Contribution to the understanding of the Pinheiro Machado Complex (Dom Feliciano Belt, Brazil): A study of textures, mineral chemistry, and crystallization conditions

Abstract

This article presents textures, mineral chemistry, and geothermobarometric data for the southeastern Pinheiro Machado Complex rocks (PMC), eastern domain of the Dom Feliciano Belt, Rio Grande do Sul, Brazil. The PMC is composed by comagmatic intrusions of quartz diorite, tonalite, and granodiorite with medium‐ to fine‐grained equigranular and medium‐grained inequigranular textures with subordinate alkali‐feldspar macrocrystals. Partial melting features are observed in the comagmatic as melt pools and melt films. Amphibolite xenoliths also occur, presenting sharp‐angled to rounded contacts with the intrusions and evidences of partial assimilation, marked by mafic schlierens. Electron probe microanalyzer punctual analyses were used to classify amphibole, plagioclase, biotite, epidote, and titanite, as well as for the calculations of the crystallization P–T conditions. Amphiboles consist of pargasite for the comagmatic intrusions, and Mg‐hornblende for the amphibolite. Plagioclase ranges from oligoclase to andesine (An15‐40) in both units. Biotite corresponds to annite, with Fe/(Fe + Mg) > 0.3. The epidote has mostly magmatic characteristics in the comagmatic intrusions, with pistacite content 25–30%. Titanite has two distinct textures, titanite I is well formed, subhedral to euhedral, with up to 3 mm; while titanite II is anhedral with size >1 mm and rounded shapes, being both chemically homogeneous. The estimated P–T conditions for the amphibolite crystallization, using plagioclase‐hornblende geothermometer are 650–850°C and 3.62–4.95 kbar, whereas the conditions from the comagmatic intrusions are 714–871°C and 4.44–6.10 kbar. In the mineral phases there is a prevalence in the recrystallization field between subgrain rotation (SGR) and grain boundary migration (GBM) in submagmatic flow. These new data indicate that possibly at the end of PMC crystallization, late magmatic fluids caused re‐equilibrium of mineral phases, mainly in biotite and feldspars. Finally, we suggest that the PMC is the result of continuous and repeated comagmatic intrusions in a continental magmatic arc setting at a depth of 20–30 km in an amphibolitic continental crust.

Slab failure‐related magmatism in the Pinheiro Machado Complex, southern Dom Feliciano Belt, Brazil

“…The PMC comprises plutonic rocks with a wide compositional range along a northeast‐southwest trending belt in the southeastern region of the Pelotas Terrane. Bastos et al (2021) investigated the crystallization conditions of the PMC rocks and demonstrated multiple pulse magmatism besides other important mechanisms, which was confirmed by Loureiro et al (2021), who described the studied area as a result of multi‐intrusive pulses subdivided into distinct magmatic phases.…”

“…Slab rupture and failure are essential components of plate tectonics and a natural consequence of the subduction process (Davies & Von Blanckenburg, 1995;Sacks & Secor Jr, 1990). These processes include The processes of mixing, assimilation and hybridization are common evidence discussed by authors studying the PMC (Bastos et al, 2021;Cruz et al, 2023;Loureiro et al, 2021). Approaching a more regional context, Cruz et al (2023) describe these mixing processes and assimilation of country rocks, suggesting multi-stage magmatism related to episodes of intense magmatic peaks, potentially triggered by changes in the tectonic setting.…”

“…The magmatic rocks petrology of the Monte Bonito region (black stars in Figure 1b,c), referred to as 'comagmatic' by Bastos et al (2021), is the focus of the present study. The Pinheiro Machado Complex is the predominant lithology in the Monte Bonito region.…”

“…Syenogranitic melt (Sn). Syenogranitic melt is associated with modified early magmatism due to partial melting (Bastos et al, 2021;Loureiro et al, 2021). These rocks exhibit inequigranular to porphyritic textures with macrocrystals of hornblende and titanite, generally larger than 1 cm, immersed in a medium equigranular leucocratic matrix.…”

Possíveis fontes e processos magmáticos do Granito Chasqueiro, SE do Cinturão Dom Feliciano, Brasil

“…1B). Este embasamento é representado pelos ortognaisses Arroio Pedrado (idade de cristalização magmática datada em 680 Ma), interpretados como remanescentes do Arco Piratini (Vieira et al, 2019b); as rochas meta-máficas-ultramáficas-sedimentares do Complexo Arroio Grande (800-600 Ma - Ramos et al, 2020); e os granitoides mais antigos do Complexo Pinheiro Machado (630-609 Ma -Philipp et al, 2002;Loureiro et al, 2015;Bastos et al, 2020). O magmatismo sin-cinemático principal relacionado às referidas zonas de cisalhamento da região de Arroio Grande resulta no alojamento do granito peraluminoso Três Figueiras (Klein et al, 2018;Vieira et al, 2021) e no Granito Chasqueiro (Vieira et al, 2016), foco do presente estudo.…”