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Abstract Understanding the behavior of chalcophile elements during the evolution of arc magmas is critical to refining models for the formation and distribution of porphyry copper deposits used in mineral exploration. Because magmas in continental arcs undergo copper depletion during their early differentiation, a widely held hypothesis posits that the removed copper is locked at the base of the crust in copper-rich cumulates that form due to early sulfide saturation. Testing this hypothesis requires direct evidence for such copper-rich reservoirs and a comprehensive understanding of the mechanisms driving sulfide saturation. Interaction between oxidized magmas and reducing crustal material in island arcs has been shown to be an efficient process causing sulfide saturation. However, the extent to which crustal assimilation impacts the flux of chalcophile elements during magmatism in thick continental arcs remains to be established. Here, we provide a deep perspective into these problems by studying a suite of subarc cumulate rocks from the Acadian orogen, New England (USA). These cumulates record the imprint of subduction zone magmatism and represent the residues left behind during the genesis of intermediate to evolved Acadian magmas (ca. 410 Ma). We find that the most primitive Acadian cumulates are enriched in copper (up to ~730 µg g–1) hosted by sulfide phases, providing direct evidence for the formation of lower crustal copper-rich reservoirs. The Acadian cumulates reveal a wide range of δ34S values, from –4.9‰ in the ultramafic rocks to 8‰ in the most evolved mafic rocks. The negative δ34S values observed in the most primitive and copper-rich cumulates (avg –3‰) reflect the assimilation of isotopically light sulfur from surrounding sulfidic and graphite-bearing metasedimentary rocks (δ34S of –19 to –12‰), whereas the more evolved cumulates with positive δ34S signatures may have formed from different magma batches that experienced less sediment assimilation. The assimilation of these reducing metasedimentary rocks caused a critical drop in oxygen fugacity (~DFMQ –2.5 to –1.9; FMQ = fayalite-quartz-magnetite buffer) in the evolving magmas, ultimately leading to extensive sulfide saturation and the consequent formation of copper-rich subarc cumulates. Assimilation-driven sulfide saturation may be a common process at the root of thickened arc crusts that triggers the formation of lower crustal copper-rich reservoirs, which play a pivotal role in the fate of copper during arc magmatism. Thus, deeply buried reducing metasedimentary crustal material at the base of continental arcs can act as a barrier to the magmatic flux of chalcophile elements and may play a crucial role in the genesis and distribution of porphyry copper deposits. 

Abstract Inclusions of relic high-pressure melts provide crucial information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000 °C. However, discoveries of high-pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, U.S.A.). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism and melting for these felsic granulites. The trapped melt has a hydrous, granitic, and peraluminous character typical of crustal melts from metapelites. This melt is higher in mafic components (FeO and MgO) than most of the nanogranitoids investigated previously, likely the result of the extreme melting temperatures—well above 1000 °C. This is the first natural evidence of the positive correlation between temperature and mafic character of the melt; a trend previously supported only by experimental evidence. Moreover, it poses a severe caveat against the common assumption that partial melts from metasediments at depth are always leucogranitic in composition. NanoSIMS measurement on re-homogenized inclusions show significant amounts of CO2, Cl, and F. Halogen abundance in the melt is considered to be a proxy for the presence of brines (strongly saline fluids) at depth. Brines are known to shift the melting temperatures of the system toward higher values and may have been responsible for delaying melt production via biotite dehydration melting until these rocks reached extreme temperatures of more than 1000 °C, rather than 800–850 °C as commonly observed for these reactions. 

Abstract The dearth of cumulate magmatic roots in accretionary orogens is a cornerstone of models that postulate redistribution of mass and energy within the crust for the genesis of intermediate to silicic magmatism. Likewise, the origin of the evolved Acadian (Devonian) plutonism in the New England Appalachians (northeastern USA) has long been explained by closed-system crustal melting due to the absence of associated coeval deep mafic counterparts. Here, we report the discovery of Acadian hydrous ultramafic cumulate rocks that formed by deep-seated (∼1.1 GPa) fractional crystallization processes from a mantle-derived parental melt (Connecticut, southern New England, USA). These rocks are the first of their kind identified in the Appalachian orogen, and one of only a handful of preserved deep subarc hydrous cumulates worldwide. We propose a genetic link between the studied rocks and the evolved coeval plutonism in central-southern New England, where the former represent the missing deep cumulate roots of the same magmatic arc. Our findings support the hypothesis that differentiation of mantle-derived hydrous magmas by fractional crystallization and assimilation processes in the deep crust is a fundamental process in the production of intermediate to silicic magmatism and the geochemical evolution of the continental crust. 

Diamond and coesite are classic indicators of ultrahigh-pressure (UHP; ≥100-kilometer depth) metamorphism, but they readily recrystallize during exhumation. Crystallographically oriented pyroxene and amphibole exsolution lamellae in garnet document decomposed supersilicic UHP majoritic garnet originally stable at diamond-grade conditions, but majoritic precursors have only been quantitatively demonstrated in mafic and ultramafic rocks. Moreover, controversy persists regarding which silicates majoritic garnet breakdown produces. We present a method for reconstructing precursor majoritic garnet chemistry in metasedimentary Appalachian gneisses containing garnets preserving concentric zones of crystallographically oriented lamellae including quartz, amphibole, and sodium phlogopite. We link this to novel quartz-garnet crystallographic orientation data. The results reveal majoritic precursors stable at ≥175-kilometer depth and that quartz and mica may exsolve from garnet. Large UHP terranes in the European Caledonides formed during collision of the paleocontinents Baltica and Laurentia; we demonstrate UHP metamorphism from the microcontinent-continent convergence characterizing the contiguous and coeval Appalachian orogen. 

Abstract Garnet is a common metamorphic and igneous mineral with extensive solid solution that can be stable to mantle depths ≥400 km. High-T and/or high-P garnet may contain oriented lamellae of other minerals, most commonly simple oxides (e.g., rutile, ilmenite), apatite, and, in ultrahigh-P cases, silicates including pyroxene and amphibole. Lamellae have classically been considered to be precipitation features preserving a record of former garnet chemistry richer in the lamellae nutrients (e.g., Ti4+). Such microtextural origins in precipitation systems (e.g., alloys) have long been studied via the crystallographic orientation relationships (COR) that form between a host and a separating phase, and by the shape-preferred orientation (SPO) of the lamellae. Recently, however, alternative hypotheses to precipitation have been suggested that require emplacement of lamellae in garnet by fluids, or co-growth, overgrowth, or inheritance mechanisms. These hypotheses posit that lamellae cannot be used to study former garnet chemistry. Moreover, they predict that lamellae phases, SPO, and COR should differ widely between localities, as lamellae formation will be controlled by various local rock-specific factors such as fluid presence, fluid chemistry, or mineral growth sequence. On the other hand, if lamellae characteristics are largely consistent between localities, it likely reflects control by precipitation energetics, rather than external factors. There have been few comparative COR studies in geologic systems, but the integrative assessment of COR, SPO, and lamellae assemblages should fingerprint lamellae growth process. To test the precipitation and alternative hypotheses, we collected large electron backscatter diffraction (EBSD) data sets for rutile, ilmenite, and apatite lamellae in garnet from the Brimfield Schist, Connecticut (≥1000 °C metamorphism; Central Maine Terrane, U.S.A.). We analyzed these data alongside published EBSD data for rutile, ilmenite, and corundum from metapegmatites metamorphosed in the eclogite facies from the Austrian Alps (Griffiths et al. 2016). The apatite data set is the first of its kind, and reveals that apatite preferentially aligns its close-packed direction parallel to that of garnet (c-axisapatite//<111>garnet). We also recognize a rutile-garnet COR related to those in meteorites with Widmanstätten patterns that are unequivocal products of exsolution. This is the first identification of direct similarities between silicate-oxide and metal-metal COR of which we are aware. Significantly, this rutile-garnet COR is found in diverse geologic settings including Connecticut and Idaho (U.S.A.), Austria, Germany, Greece, and China over a broad range of bulk-rock compositions. Results for all lamellae minerals show that COR are largely consistent between localities and, furthermore, are shared between apatite, ilmenite, and corundum. Moreover, between 70% and 95% of lamellae have COR and there is a dominant COR for each lamellae phase. Calculations show that d-spacing ratios of host-lamellae pairs can successfully predict the most commonly observed specific COR (those COR with two or more axial alignments with the host). These results, especially similarity of COR from markedly different geologic settings and a low diversity of lamellae minerals, are fully consistent with lamellae formation by precipitation (likely via exsolution). In contrast, the alternative hypotheses remain unsupported by COR results as well as by mineralogical and petrological evidence. Lamellae with similar traits as those in this work should thus be considered precipitates formed during unmixing of garnet compositions originally stable at elevated or extreme pressures and temperatures. 

We quantify the metamorphic pressure-temperature (P-T) conditions for a newly discovered silica-undersaturated high-pressure granulite (HPG) from the Central Maine Terrane (CMT) in northeastern Connecticut, U.S.A. The rocks lie within the Acadian-Neoacadian orogenic belt (Devonian) and form part of the Brimfield Schist. The Brimfield and the adjacent Bigelow Brook Formation contain silica-saturated rocks that have previously been shown to have undergone ~1000 °C metamorphism. The pressure was less well constrained at ≥ ~1 GPa. Silica-undersaturated rocks hold underutilized potential for pinpointing peak metamorphic conditions, particularly pressure, because of their resilience to melting and the variety of refractory minerals they contain. The typical silica-undersaturated mineral assemblage is garnet + spinel + corundum + plagioclase + K-feldspar + biotite + ilmenite. Leucosomes are syenites consisting of two feldspars ± biotite. Plagioclase is commonly antiperthitic, particularly in feldspathic domains surrounding peritectic garnet; such garnet crystals reach ~10 cm in diameter. Alkali feldspars are perthitic. The rocks contain remarkable ellipsoidal spinels as much as 5.5 cm long comprising discrete crystallographic domains hosting crystallographically oriented lamellae of a Fe-Ti phase, most likely ilmenite. Corundum is usually colorless, but can also be found as sapphire in shades of pink, purple, and blue, particularly in antiperthite-rich domains surrounding large garnets. Some sapphires are concentrically color zoned. We carried out a P-T estimation using ternary feldspar reintegration thermometry of metamorphic antiperthites together with pseudosection modeling. Samples texturally and chemically record near-eclogite facies equilibration at minimum conditions of ~1040 °C and ~1.8 GPa, establishing the CMT in northeastern CT as the first known HPG locality in the U.S. These results are consistent with high P2O5 levels found in garnet (0.18 wt%), Ti-in-biotite thermometry, regional sillimanite pseudomorphs after kyanite, and preliminary experimental work on melt inclusions in garnet (Ferrero et al. 2017). The leucosomes provide strong evidence that partial melting of silica-undersaturated rocks at HPG conditions can produce syenitic magmata. Strongly melt-depleted silica-undersaturated rocks may also be protoliths for garnet + spinel + corundum xenoliths reported from kimberlites. The presence of HPG gneisses demonstrates that the large-scale thrusts of the CMT sample the deepest roots of the orogenic belt (60–70 km), and perhaps even deeper subduction zone lithologies as well.