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1. Evolution of Mafic Tungnárhraun Lavas: Transcrustal Magma Storage and Ascent Beneath the Bárðarbunga Volcanic System

   https://doi.org/10.3390/min15070687  

   Furman, Tanya; Kincaid, Denali; Brady, Collin Oborn (July 2025, Minerals)

   Costa, Simone; Caracciolo, Alberto (Ed.)

   The Tungnárhraun basalts in southern Iceland record a transcrustal magma system formed during Holocene deglaciation. These large-volume (>1 km3) Early through Mid-Holocene lavas contain ubiquitous plagioclase feldspar macrocrysts that are too primitive to have grown from the host lavas. Thermobarometry based on plagioclase melt and clinopyroxene melt equilibrium reveals a transcrustal structure with at least three distinct storage regions. A lower-crustal mush zone at ~14–30 km is fed by primitive, low 87Sr/86Sr magmas with diverse Ti/K and Al/Ti signatures. Plagioclase feldspar growth is controlled by an experimentally determined pseudoazeotrope where crystals develop inversely correlated An and Mg contents. The rapid ascent of magmas to mid-crustal levels (~8–9 km) allows the feldspar system to revert to conventional thermodynamic phase constraints. Continued plagioclase growth releases heat, causing olivine and pyroxene to be resorbed and giving the magmas their characteristic high CaO/Al2O3 values (~0.8–1.0) and Sc contents (~52 ppm in matrix material). Mid-Holocene MgO-rich lavas with abundant plagioclase feldspar macrocrysts erupted directly from this depth, but both older and younger magmas ascended to a shallow-crustal storage chamber (~5 km) where they crystallized olivine, clinopyroxene, and plagioclase feldspar and evolved to lower MgO contents. The Sr isotope differences between the plagioclase macrocrysts and their carrier melts suggest that the fractionation involves the minor assimilation of country rock. This model does not require the physical disruption of an established and long-lived gabbroic cumulate mush. The transcrustal structures documented here existed in south Iceland at least throughout the Holocene and likely influenced much of Icelandic magmatism. 

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2. Formation of Chromitite Seams and Associated Anorthosites in Layered Intrusion by Reactive Volatile-rich Fluid Infiltration

   https://doi.org/10.1093/petrology/egaa109  

   Marsh, Jennifer S; Pasecznyk, Michael J; Boudreau, Alan E (February 2021, Journal of Petrology)

   null (Ed.)

   Abstract Drilling related to development of the platinum-group element deposit of the J-M Reef of the Stillwater Complex returned samples of a rare chromitite seam between anorthosite and norite in a discordant anorthositic body. Plagioclase core An concentrations are marginally higher and modestly reversely zoned on the norite side (average Ancore = 83·8; average Ancore – Anrim = –1·1) as compared with the anorthosite side (average Ancore 82·5; average Ancore – Anrim = +1·0). The anorthosites are also characterized by a slightly smaller average plagioclase grain size than plagioclase in the norite (1·41 mm and 1·54 mm, respectively). The chromite can contain single and polyphase inclusions of orthopyroxene, plagioclase, amphibole, biotite and Cl-rich apatite. These and other compositional and textural features, as well as inference from other discordant anorthositic bodies in the Banded series, are all consistent with a chromatographic model of chromite precipitation at a reaction front as a norite protolith reacts with a Cl-rich aqueous fluid saturated in plagioclase alone. Chromitite seam formation is modeled using an infiltration metasomatic model, in which a fluid becomes progressively undersaturated in pyroxene as it rises into the hotter part of the crystal pile. As this pyroxene-undersaturated fluid moves through a noritic protolith, it dissolves the Cr-bearing orthopyroxene to produce an anorthosite. Chromite precipitates at the reaction front between the anorthosite and the norite owing to liberation of Mg and Cr from pyroxene. Continuous redissolution and reprecipitation of chromite occurs as the pyroxene dissolution front moves in the direction of fluid flow, collecting the Cr lost from the anorthosite. Owing to Cr dissolved mainly as a neutral divalent cation complex, CrCl(OH)0, in the solution, but incorporated as a trivalent cation in chromite, the required redox reaction can involve concurrent precipitation of sulfide with chromite. This mechanism differs from some recent models in that the anorthosites are themselves replacement bodies and are not original precipitates from a magma nor formed by loss of mafic material by partial melting. The results show the need for experimental mineral solubility data at T and P conditions appropriate to upper crustal mafic–ultramafic intrusions. 

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3. Petrological, geochemical and geodynamic evolution of the Wadi Al-Baroud granitoids, north Arabian-Nubian shield, Egypt

   https://doi.org/10.1016/j.jafrearsci.2023.105044  

   Abuamarah, Bassam A; Alzahrani, Hassan; Matta, Marian J; Azer, Mokhles K; Asimow, Paul D; Darwish, Mahmoud H (November 2023, Journal of African Earth Sciences)

   The Wadi Al-Baroud area, in Egypt’s Eastern Desert, exposes Neoproterozoic rocks of the Arabian-Nubian Shield (ANS), including both syntectonic granitoids (granodiorite and tonalite) and post-collisional granites. We present field work, petrographic study, mineral compositions, and whole-rock geochemistry results from these granitoids and discuss their petrogenesis, magmatic sources, evolution, and tectonic significance. The syntectonic granitoids show subduction affinity and an anomalous steep trend of K-enrichment that suggests assimilation of a granitic component during their evolution. The post-collisional granites form two plutons, on opposite sides of Wadi Al- Baroud, named here the Ras Baroud pluton (RBP) and the Abu Hawis pluton (AHP). They intruded the syn- tectonic granitoids with sharp intrusive contacts. The post-collisional plutons are devoid of mafic enclaves and are cut by very few dikes. They dominantly consist of biotite monzogranite that grades into muscovite mon- zogranite. The latter lithology hosts Nb-Ta oxide minerals (columbite, tantalite, and wodginite) displaying a variety of textural and compositional features. The cores are primary columbite-(Mn), whereas rims are over- grown or partly replaced by tantalite-(Fe) and wodginite due to late interactions with highly fractionated re- sidual melt. The highly-evolved AHP and RBP granites are typical of the post-collisional granitoids of the ANS, including high concentrations of rare earth elements (REE), Ta, Hf, Nb, Zr, Y, and Rb; elevated ratios of Ga/Al; and low contents of Sr, CaO, and MgO. Their geochemistry suggests that the parental magma of both plutons formed from an I-type tonalitic source rock that underwent partial melting during the thermal disturbance that followed a lithospheric delamination event during the post-collisional stage of the East African Orogeny. The variations in major oxide and trace element contents among individual samples of the AHP and the RBP cannot be explained as a liquid line of descent due to fractional crystallization; rather we interpret them as sampling variable proportions of an evolved liquid and the solid crystals in equilibrium with that liquid. 

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4. Distribution of REE between amphibole and pyroxenes in the lithospheric mantle: An assessment from the lattice strain model

   https://doi.org/10.2138/am-2022-8831  

   Wang, Chunguang; Liang, Yan; Xu, Wenliang; Sun, Chenguang; Shimizu, Kei (November 2024, American Mineralogist)

   Abstract Amphibole and pyroxenes are the main reservoirs of rare earth elements (REEs) in the lithospheric mantle that has been affected by hydrous metasomatism. In this study, we developed semi-empirical models for REE partitioning between orthopyroxene and amphibole and between clinopyroxene and amphibole. These models were formulated on the basis of parameterized lattice strain models of mineral-melt REE partitioning for orthopyroxene, clinopyroxene, and amphibole, and they were calibrated using major element and REE data of amphibole and pyroxenes in natural mantle samples from intraplate settings. The mineral-melt REE partitioning models suggest that amphibole is not in equilibrium with coexisting pyroxenes in the mantle samples and that the amphibole crystallized at a lower temperature than that of the pyroxenes. We estimated the apparent amphibole crystallization temperature using major element compositions of the amphibole and established temperature- and composition-dependent models that can be used to predict apparent pyroxene-amphibole REE partition coefficients for amphibole-bearing peridotite and pyroxenite from intraplate lithospheric mantle. Apparent pyroxene-amphibole REE partition coefficients predicted by the models can be used to infer REE contents of amphibole from REE contents of coexisting pyroxenes. This is especially useful when the grain size of amphibole is too small for trace element analysis. 

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5. WHEN DOES A GRANITE BECOME GRANITIC? A RECORD OF MAJOR AND ACCESSORY MINERAL RE-EQUILIBRATION FROM SUPER- TO SUB-SOLIDUS WITHIN THE HALF DOME GRANODIORITE, SIERRA NEVADA, CA

   https://doi.org/10.1130/abs/2023AM-395025  

   Saunders, Emmaline M; Stearns, Michael A; Ward, Ami (January 2023, Geological Society of America Abstracts with Programs)

   The Tuolumne Intrusive Suite (TIS), Sierra Nevada, California, accumulated magmatic rock from 95 to 85 Ma. Ar-Ar biotite dates require that temperatures within the TIS remained above ~300°C until ~79 million years ago. The protracted thermal history resulted in five texturally and chemically distinct units that young towards the center and was recorded by chemical and isotopic re-equilibration of the minerals. Challener and Glazner (2017) demonstrated that amphibole phenocrysts from the Half Dome Granodiorite (Khd) experienced greenschist-facies metamorphism. Amphibole phenocrysts host abundant inclusions of biotite, chlorite, feldspar, titanite, epidote, and apatite, which are interpreted to have crystallized via breakdown of magnesiohornblende. Additionally, Al zoning suggests fracturing and subsequent healing of the amphibole crystals occurred at near- or subsolidus temperatures. New EPMA and LASS-ICP-MS analyses of texturally related amphibole, titanite, feldspar, and biotite from the equigranular Khd place limits on the timing of amphibole breakdown and contextualize the low-temperature re-equilibration of many of the major minerals in the rock. Most of the amphiboles analyzed contain 0.5–6 wt. % Al2O3 corresponding to actinolite compositions, while feldspar pairs record ~475 ºC apparent temperatures. Titanite grains (re)crystallized between 91–80 Ma and contain 25–825 ppm Zr, which correspond to apparent temperatures between 550–710 ºC (150 ± 50 MPa, aTiO2 = 0.5 ± 0.1). The distribution of Zr in titanites is bimodal with the majority having <200 ppm Zr. Titanites younger than 87 Ma have decreasing Zr content and titanites included within actinolite amphibole contain the lowest Zr content (25–50 ppm) and youngest dates (85–80 Ma). Melt-present crystallization of titanite began at ~91–90 Ma, followed by both near and subsolidus (re)crystallization from ~88–86, concluding with titanite growth via hornblende breakdown from 82–80 Ma. These data taken together with previous investigations provide a continuous record of the rock’s chemical evolution driven by incremental emplacement and subsequent episodic autometamorphism of the equigranular Khd, and critically, any inferences regarding magmatic processes in the TIS must first account for the metamorphic re-equilibration of the rock. 

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