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.

No abstract

No abstract

This study experimentally investigates the Kalsilite-Nepheline-Diopside-Silica system at high pressure and temperature, with emphasis on silica-undersaturated volume (leucite-nepheline-diopside -Lct-Nph-Di; and kalsilite-nepheline-diopside -Kls + Nph + Di -planes), at 4.0 GPa (~120 km deep), temperatures up to 1,400ºC and dry conditions, to better understand the influence of K 2 O, Na 2 O, and CaO in alkali-rich silica-undersaturated magma genesis. In the Lct-Nph-Di plane, we determined the stability fields for kalsilite (Kls ss ), nepheline (Nph ss ) and clinopyroxene (Cpx ss ) solid solutions, wollastonite (Wo) and sanidine (Sa); and three piercing points: (i) pseudo-eutectic Kls + Nph + Di + liquid (Lct 62 Nph 29 Di 9 ) at 1,000ºC; (ii) Kls + Sa + (Di + Wo) + liquid (Lct 75 Nph 22 Di 2 ) at 1,200ºC; and (iii) pseudo-eutectic Kls + Di + Wo + liquid (Lct 74 Nph 17 Di 9 ) at 1,000ºC. Kalsilite stability field represents a thermal barrier between ultrapotassic/potassic vs. sodic compositions. In the plane Kls-Nph-Di, we determined the stability fields for Kls ss , Nph ss and Cpx ss and two aluminous phases in smaller proportions: spinel (Spl) and corundum (Crn). This plane has a piercing point in Kls + Nph + Di(± Spl) + liquid (Kls 47 Nph 43 Di 10 ) at 1,100ºC. Our data showed that pressure extends K dissolution in Nph (up to 39 mol%) and Na in Kls (up to 27 mol%), and that these solid solutions, if present, determinate how much enriched in K and Na an alkaline magma will be in an alkaline-enriched metasomatic mantle. Additionally, we noted positive correlation between K 2 O and SiO 2 concentration in experimental melts, negative correlation between CaO and SiO 2 , and no evident correlation between Na 2 O and SiO 2 .Keywords: Experimental petrology; Alkaline rocks; Potassium; Mantle. ResumoEste trabalho trata do estudo experimental do sistema kalsilita-nefelina-diopsídio-sílica em condições mantélicas, com ên-fase em sua parte subsaturada em sílica (planos leucita-nefelina-diopsídio -Lct-Nph-Di; e kalsilita-nefelina-diopsídio -Kls + Nph + Di), a 4 GPa (profundidade de aproximadamente 120 km), até 1.400ºC em condições anidras. O objetivo é melhor compreender como K 2 O, Na 2 O e CaO influenciam a gênese de magmas ricos em álcalis e subsaturados em sílica. Para o plano Lct-Nph-Di, foram determinados campos de estabilidade para as soluções sólidas kalsilita (Kls ss ), nefelina (Nph ss ) e clinopiroxênios (Cpx ss ), além de wollastonita (Wo) e sanidina (Sa). Foram também definidos os pontos invariantes: (i) pseudoeutético Kls+Nph+Di+líquido (Lct 62 Ne 29 Di 9 ) a 1.000ºC; (ii) Kls + Sa + (Di + Wo) + líquido (Lct 75 Nph 22 Di 2 ) a 1.200ºC; (iii) pseudoeutético Kls + Di + Wo + líquido (Lct 74 Nph 17 Di 9 ) a 1.000ºC. O campo da kalsilita, nesse diagrama, representa um alto termal entre os extremos composicionais potássico/ultrapotássico vs. sódico. No plano Kls-Nph-Di, encontramos campos de estabilidade para Kls ss , Nph ss e Cpx ss , além de pequenas quantidades de espinélio (Spl) e coríndon (Crn) e um ponto...