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It is widely acknowledged that thermal models clearly demonstrate that crustal magma bodies should solidify rapidly upon emplacement. Small plutons can cool below the solidus in thousands of years, while even large plutons require hundreds of thousands of years, but not more than a million years. However, recent U-Pb zircon geochronological data contradict these results, strongly suggesting that magmatic systems are often characterized by protracted events. Therefore, a conceptual framework that reconciles thermal models and the geochronology data is necessary. The Devonian foreland magmatism of the Sierra de San Luis is made up of two distinctive suites, the Monzonite suite (<65 wt.% SiO2) and the Granite suite (>65 wt.% SiO2), both emplaced at ca. 3.7 kbar. Classification of the studied Devonian granitoids is debatable because they have a hybrid I- to A-type granite signature. Based on a robust geochronological U-Pb zircon dataset we corroborate the development of a protracted magmatic activity with three major crystallization events for this Devonian magmatism: 391 ± 1, 384 ± 1, and 379 ± 2 Ma. Considering these geochronological data, we postulate the presence of a deep mush reservoir, where a prolonged magmatic activity, permitted the prolonged crystallization of antecrysts (ca. 395–384 Ma). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber located at shallow levels, where zircon autocrysts crystallized (ca. 379 Ma). Age spectra reported within individual samples support the idea of a massive migration of magma when conditions were favourable (e.g. thermally matured crust). Individual crystallization ages recorded by monazites hosted in two-mica granites are comparable to those obtained from zircons, supporting the presence of a long-lasting hot source. Additional geochronological data indicate that a later thermal event (ca. 353 Ma) could have partially affected some areas of the Devonian magmatic zircons promoting Pb loss, with subsequent partial resetting of the isotopic clock. [ABSTRACT FROM AUTHOR]

Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical 'spider-diagrams' are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin. [ABSTRACT FROM AUTHOR]

The Cerro Chato region is located in southernmost Brazil and is characterized by associations of acid volcanic and subvolcanic rocks. The region is affected by NW and NE‐ trending faults and is grouped into 2 geomorphologically distinct features: Cerro Chato and Cerro Partido. Cerro Chato is represented by ignimbrites that occur in 2 main facies: lithic‐rich ignimbrites and crystal‐rich ignimbrites. They are poorly sorted and consist of lapilli‐sized pyroclasts in a tuffaceous matrix with eutaxitic texture. Hemi‐crystalline rhyolitic flows represent effusive events, with porphyritic texture, flow structures, and spherulites. Cerro Partido is characterized by a subvolcanic rhyolitic body, with porphyritic texture, elongated in the NE–SW direction. Through geochemical data, the lithologies were characterized as high‐silica type rhyolites, correlated to the alkaline series, close to the subalkaline series limit; display metaluminous to peraluminous character and high contents of alkalis, FeOt/FeOt + MgO and agpaitic index. The Cerro Partido rhyolites were classified as high‐Ti with higher CaO, P2O5, FeOt, MgO, and K2O contents than the Cerro Chato low‐Ti rhyolites. The REE pattern is slightly enriched in LREE in relation to the HREE and has a strong negative Eu anomaly. U–Pb zircon dating indicates an age of 561 ± 2 Ma for Cerro Partido, suggesting contemporaneity with the granitoids of Dom Feliciano Suite. Additionally, Cerro Chato rhyolites indicate an age of 630.4 ± 2.8 Ma. These ages are in agreement with those obtained in the syn‐sedimentary volcanism at the base of the Maricá Formation, Camaquã Basin. [ABSTRACT FROM AUTHOR]

A potential zircon reference material ( BB zircon) for laser ablation-inductively coupled plasma-mass spectrometry ( LA- ICP- MS) U-Pb geochronology and Hf isotope geochemistry is described. A batch of twenty zircon megacrysts (0.5-1.5 cm3) from Sri Lanka was studied. Within-grain rare earth element ( REE) compositions are largely homogeneous, albeit with some variation seen between fractured and homogeneous domains. Excluding fractured cathodoluminescence bright domains, the variation in U content for all analysed crystals ranged from 227 to 368 μg g−1 and the average Th/U ratios were between 0.20 and 0.47. The Hf isotope composition (0.56-0.84 g/100 g Hf) is homogeneous within and between the grains - mean 176Hf/177Hf of 0.281674 ± 0.000018 (2 s). The calculated alpha dose of 0.59 × 1018 g−1 for a number of BB grains falls within the trend of previously studied, untreated zircon samples from Sri Lanka. Aliquots of the same crystal (analysed by ID-TIMS in four different laboratories) gave consistent U-Pb ages with excellent measurement reproducibility (0.1-0.4% RSD). Interlaboratory assessment (by LA- ICP- MS) from individual crystals returned results that are within uncertainty equivalent to the TIMS ages. Finally, we report on within- and between-grain homogeneity of the oxygen isotope systematic of four BB crystals (13.16‰ VSMOW). [ABSTRACT FROM AUTHOR]