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Igneous rocks with high Ta concentrations share a number of similarities such as high Ta/Nb, low Ti, LREE and Zr concentrations and granitic compositions. These features can be traced through fractionated granitic series. Formation of Ta-rich melts begins with anatexis in the presence of residual biotite, followed by magmatic crystallization of biotite and muscovite. Crystallization of biotite and muscovite increases Ta/Nb and reduces the Ti content of the melt. Titanium-bearing oxides such as rutile and titanite are enriched in Ta and have the potential to deplete Ta at early stages of fractionation. However, mica crystallization suppresses their saturation and allows Ta to increase in the melt. Saturation with respect to Ta and Nb minerals occurs at the latest stages of magmatic crystallization, and columbite can originate from recrystallization of mica. We propose a model for prediction of intrusion fertility for Ta.

The Dabie-Sulu orogenic belt was formed by the Triassic continental collision between the South China Block and the North China Block. There is a large area of Mesozoic magmatic rocks along this orogenic belt, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. The Late Triassic alkaline rocks and the Late Jurassic granitoids only crop out in the eastern part of the Sulu orogen, whereas the Early Cretaceous magmatic rocks occur as massive granitoids, sporadic intermedi- ate-mafic intrusive and volcanic rocks throughout the Dabie-Sulu orogenic belt. Despite the different ages for their emplacement, the Mesozoic magmatic rocks are all characterized not only by enrichment of LREE and LILE but depletion of HFSE, but also by high initial Sr isotope ratios, low εNd(t) values and low radiogeneic Pb isotope compositions. Some zircons from the Jurassic and Cretaceous granitoids contain inherited magmatic cores with Neoprotozoic and Triassic U-Pb ages. Most of the Cretaceous mafic rocks have zircon δ18O values and whole-rock δ13C values lower than those for the normal mantle. A systematic comparison with adjacent UHP metaigneous rocks shows that the Mesozoic granitoids and mafic rocks have elemental and isotopic features similar to the UHP metagranite and metabasite, respectively. This indicates that these magmatic and metamorphic rocks share the diagnostic features of lithospheric source that has tectonic affinity to the northern edge of the South China Block. Their precursors underwent the UHP metamorphism and the post-collisional anatexis, respectively at different times and depths. Therefore, the Mesozoic magmatic rocks were derived from anatexis of the subducted continental lithosphere itself beneath the collision-thickened orogen; the geodynamic mechanism of the post-collisional magmatisms is tectonic collapse of orogenic roots in response to lithospheric extension.

Mechanisms relating to growth and/or compositional modification of zircon occur at the atomic scale. For felsic igneous systems, processes responsible for growth patterns in zircon have previously remained elusive as the volume of material needed to analyze these compositional features using traditional in-situ methods is considerably larger than the typical sub-micron scale distribution of trace elements. To illuminate some of these driving forces, we characterize and quantify minor and trace element concentrations in igneous zircon grains by combining methods of cathodoluminescence (CL) imaging, electron microprobe microanalysis (EMPA) elemental maps for Hf, Y, Yb and U or Th, and atom probe tomography (APT). We focus on igneous zircon from the Chon Aike Silicic Large Igneous Province (Patagonia) that provide novel insights into (1) dissolution and re-crystallization during crustal anatexis, (2) crystallization to produce oscillatory zonation patterns that are typical of igneous zircons, and (3) the incorporation of trace element impurities (e.g., P, Be, and Al) at the nanoscale. Significantly, these APT volumes provide nanoscale sampling of boundaries between oscillatory growth zones in an igneous zircon to reveal compositional zoning of Y and, to a lesser extent P, which appear as high-angle, planar features. These concentration boundaries measured on the order of 10 to 12 nm are difficult to reconcile with proposed mechanisms for generating fine-scaled oscillations. Lastly, we fit diffusional profiles to measured Y concentrations to provide an estimate on the maximum timescales of zircon growth prior to eruption, as a function of the temperature at which diffusion occurred. When combined with known pressure-temperature-time paths for the magmatic system considered, these extremely short diffusion profiles that are resolvable by APT provide a powerful method to constrain timescales of crystal growth.

This review presents a compositional database of primary anatectic granitoid magmas, entirely based on melt inclusions (MI) in high-grade metamorphic rocks. Although MI are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks, MI in crustal rocks that have undergone anatexis (migmatites and granulites) are a novel subject of research. They are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions. Primary MI in high-grade metamorphic rocks are small, commonly 5–10 μm in diameter, and their most common mineral host is peritectic garnet. In most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage (nanogranitoid inclusions) with quartz, K-feldspar, plagioclase, and one or two mica depending on the particular circumstances. After their experimental remelting under high-confining pressure, nanogranitoid MI can be analyzed combining several techniques (EMP, LA-ICP-MS, NanoSIMS, Raman). The trapped melt is granitic and metaluminous to peraluminous, and sometimes granodioritic, tonalitic, and trondhjemitic in composition, in agreement with the different conditions of melting and protolith composition, and overlap the composition of experimental glasses produced at similar conditions. Being trapped along the up-temperature trajectory—as opposed to classic MI in igneous rocks formed during down-temperature magma crystallization—fundamental information provided by nanogranitoid MI is the pristine composition of the natural primary anatectic melt for the specific rock under investigation. So far ~600 nanogranitoid MI, coming from several occurrences from different geologic and geodynamic settings and ages, have been characterized. Although the compiled MI database should be expanded to other potential sources of crustal magmas, MI data collected so far can be already used as natural “starting-point” compositions to track the processes involved in formation and evolution of granitoid magmas.

Constraining the source, genesis, and evolution of Archaean felsic crust is key to understanding the growth and stabilization of cratons. The Akia Terrane, part of the North Atlantic Craton, West Greenland, is comprised of Meso-to-Neoarchaean orthogneiss, with associated supracrustal rocks. We report zircon U–Pb and Lu–Hf isotope data, and whole-rock geochemistry, from samples of gneiss and supracrustals from the northern Akia Terrane, including from the Finnefjeld Orthogneiss Complex, which has recently been interpreted as an impact structure. Isotope data record two major episodes of continental crust production at ca. 3.2 and 3.0 Ga. Minor ca. 2.7 and 2.5 Ga magmatic events have more evolved εHf, interpreted as reworking of existing crust perhaps linked to terrane assembly. Felsic rocks from the Finnefjeld Orthogneiss Complex were derived from the same source at the same time as the surrounding tonalites, but from shallower melting, requiring any bolide-driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust. A simpler alternative has the Finnefjeld Complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth. Hafnium isotope data from the two major Mesoarchaean crust-forming episodes record a contribution from older mafic Eoarchaean crust. Invoking the involvement of an Eoarchaean root in the growth of younger Mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form Earth’s cratons.

Recently reported major element, trace element, and volatile compositions of dacites erupted from Hekla Volcano, Iceland are inconsistent with an anatectic origin. In contrast, all of the data are consistent with dacitic melts being derived by modest amounts of crystallization of an anhydrous assemblage from an andesitic melt. Models that apply straightforward Gd, Yb, Th, and U concentrations, instead of ratios, to the problem show that the crystallization hypothesis remains valid, especially when uncertainties are propagated.

The Wolfsberg lithium deposit in Austria is one of the largest Li-Cs-Ta pegmatite resources in Europe. The deposit is part of the Austroalpine Unit Pegmatite Province in the Eastern Alps that formed during the high-temperature, low-pressure Permian extensional event and was subsequently overprinted by Cretaceous eclogite-facies metamorphism during the Alpine orogeny. The two pegmatite types distinguished at the deposit, amphibolite hosted-(AHP) and mica schist hosted-(MHP) pegmatite, consist of quartz, albite, K-feldspar, muscovite and spodumene with accessory apatite, beryl and columbite. Both pegmatite types have similar peraluminous granitic compositions and element distribution patterns. However, the AHP contains higher Li and Cs concentrations. Both pegmatite types display LREE-enriched/HREE-depleted chondrite-normalized REY patterns that suggest derivation from partial melting of basement mica schist during the Permian HT/LP extensional event. The Alpine metamorphism more strongly affected the MHP relative to the AHP, resulting in recrystallization of primary assemblages by metamorphic assemblages with lower rare-metal concentrations, and development of a strong foliation, during which (re)mobilized elements (e.g., Li, Cs) were concentrated along localized shear zones. Recognition of element remobilization in MHP associated with metamorphic overprinting may bear important implications towards mineral exploration for Li-Cs-Ta pegmatite in other strongly metamorphosed terranes.

While basaltic volcanism is dominant during rifting and continental breakup, felsic magmatism may be a significant component of some rift margins. During International Ocean Discovery Program (IODP) Expedition 396 on the continental margin of Norway, a graphite‐garnet‐cordierite bearing dacitic unit (the Mimir dacite) was recovered in two holes within early Eocene sediments on Mimir High (Site U1570), a marginal high on the Vøring Transform Margin. Here, we present a comprehensive textural, petrological, and geochemical study of the Mimir dacite in order to assess its origin and discuss the geodynamic implications. The major mineral phases (garnet, cordierite, quartz, plagioclase, alkali feldspar) are hosted in a fresh rhyolitic, vesicular, glassy matrix that is locally mingled with sediments. The major element chemistry of garnet and cordierite, the presence of zircon inclusions with inherited cores, and thermobarometric calculations all support an upper crustal metapelitic origin. While most magma‐rich margin models favor crustal anatexis in the lower crust, thermobarometric calculations performed here show that the Mimir dacite was produced at upper‐crustal depths (<5 kbar, 18 km depth) and high temperature (750–800°C) with up to 3 wt% water content. In situ U‐Pb analyses on zircon inclusions give a magmatic crystallization age of 54.6 ± 1.1 Ma, consistent with emplacement that post‐dates the Paleocene‐Eocene Thermal Maximum. Our results suggest that the opening of the Northeast Atlantic was associated with a phase of low‐pressure, high‐temperature crustal anatexis preceding the main phase of magmatism. Plain Language Summary Fifty‐six million years ago, the continents were beginning the final phase of their journey to their modern‐day locations. This included the rifting and formation of the Northeast Atlantic Ocean, known in particular for producing considerable magmatism during continental break‐up. In summer 2021, Expedition 396 of the International Ocean Discovery Program drilled the oceanic floor off the coast of present‐day Norway to collect volcanic and sedimentary rocks deposited at this time. Their main goal was to investigate the cause of the excess magmatism and its potential implications for the global climate. While sampling sediments on the expedition, an unexpected volcanic unit, a glassy garnet‐cordierite dacite, was recovered. To determine its origin, we combined multiple methods (petrography, stratigraphy, thermodynamic calculations, petrochronology, in situ compositional analyses) and showed that the unit is a product of melting of in the continental crust at shallow depth during the rifting process and likely later emplaced in shallow water. Our results demonstrate that the rifting process in the Northeast Atlantic included a long and intense period of continental crustal thinning. This research provides evidence needed to reconstruct the evolution of the Northeast Atlantic Ocean. Key Points A dacitic unit was recovered in early Eocene sediments on the Vøring margin during International Ocean Discovery Program Expedition 396 The Mimir dacite was formed by upper crustal anatexis at 54.6 ± 1.1 Ma, shortly after the Paleocene‐Eocene Thermal Maximum The dacite is evidence for a break‐up phase associated with significant continental lithospheric extension

The genetic mechanism of rare-metal pegmatites – whether they form primarily by extreme fractionation of granitic magmas or direct crustal anatexis – remains highly controversial. The Azubai Be deposit in the Xinjiang Altay provides an ideal case to address this controversy, as its Be-mineralized pegmatites show a significant age gap (∼170 Ma) with the spatially associated Halong S-type granite, ruling out a simple fractional crystallization origin. This study presents an integrated Hf–Li isotopic and geochemical investigation of the Azubai Be-mineralized pegmatites, the Halong granite, and the potential metasedimentary source (Kulumuti Group). The Azubai Be-mineralized pegmatites exhibit homogeneous, positive ) values (−1.2 to +3.4) and Li values (+2.5 to +3.8 ‰), which contrast sharply with the broad and predominantly negative isotopic signatures of the Kulumuti Group metasedimentary rocks ( ): −4.3 to +4.6; Li: −8.6 to −0.4 ‰), precluding a direct anatectic origin. In contrast, the Azubai Be-mineralized pegmatites isotopic compositions show significant overlap with those of the Halong granite ( ) = −2.8 to +3.4; Li = +0.3 to +3.7 ‰). These Hf–Li isotopic results provide robust support for the recently proposed “two-stage model,” which entails: (1) Early Devonian formation of the Halong granite via crustal anatexis, and (2) Triassic post-orogenic extensional setting remelting of this pre-existing, Be-enriched granite, leading to the generation of the Azubai Be-mineralized pegmatites. This model reconciles the apparent paradox of spatial association without a direct genetic-temporal link and underscores the role of crustal recycling through reactivation of pre-existing granites in rare-metal metallogeny.

Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database. While Zr isotopes in magmatic rocks are often used to trace the evolution of magmas through fractional crystallization, it is intriguing how highly heterogeneous Zr isotopes were produced by the growth of zircon during crustal anatexis. We address this issue by in-situ zircon Zr isotope analyses of migmatites from two high-temperature metamorphic terranes in the South Lhasa zone and the North Dabie zone, respectively, in China. The results show highly variable δ 94 Zr values from −0.30‰ to +0.81‰ and from −0.58‰ to +0.49‰, respectively. In addition to the relict zircon of magmatic origin, two types of newly-grown zircons were identified in terms of their occurrences, trace elements and δ 94 Zr values. The peritectic zircon, mainly occurring in the in-situ leucosomes, exhibits the highest Nb-Ta-Hf-U contents and variably higher δ 94 Zr values than those of the relict zircon. The anatectic zircon, mainly occurring in the leucocratic veins, shows higher Nb-Ta-Hf-U contents than and similar δ 94 Zr values to those of the relict zircon. Model calculations demonstrate that the variable Zr isotope compositions of newly-grown zircons would result from decoupled release of Zr from zircon and non-zircon phases. The Zr supply of the peritectic zircon is mainly derived from the decomposition of Zr-bearing minerals in the in-situ anatectic melt (the non-zircon effect), whereas the Zr supply of the anatectic zircon is mainly from the dissolution of pre-existing zircons in the evolved melt (the zircon effect). The significant Zr isotope variations in the migmatites well illustrate the generation, migration and accumulation of the anatectic melts during the partial melting. Therefore, Zr isotopes can be used as a powerful means for distinguishing between the peritectic and anatectic zircons during crustal anatexis.