Fractionation Of Magma Research Articles

Zircon fertility indicators compromised by mineral inclusion contamination: A case study from the Taoxikeng W deposit, South China

Zircon trace element compositions have been widely used as a tool to assess the ore-forming potential of magmatic-hydrothermal systems. However, many zircons contain accessory mineral inclusions which, if accidentally analyzed, could compromise reported zircon trace element signatures, especially when the mineral inclusions are sub-micrometer and thus hard to monitor using conventional microscopes. However, how mineral inclusion contamination influences the robustness of zircon fertility indicators (e.g., Ce and Eu anomalies) is still not well illustrated. In this study, we report U-Pb ages and trace element compositions of zircons from ore-forming muscovite granite plutons of the Taoxikeng W deposit, South China. The studied zircons are characterized by abundant small apatite inclusions. Although these had little influence on zircon Eu/Eu* (and thus probably the interpreted level of magmatic fractionation and melt water concentration), they could have resulted in a misestimation of the magmatic oxidation state either via the zircon Ce/Ce* or Loucks et al. (2020) methods. This is especially important for granite-related W-Sn systems since their magmatic zircons generally contain abundant mineral inclusions, which may be accidentally sampled during zircon trace element analysis. In practice, we can use La as a monitor of REE-enriched mineral (i.e., apatite, titanite) inclusions, although the criterion for identifying pure zircon trace element data is still controversial and awaits further work.

Do Pegmatites Crystallise Fast? A Perspective from Petrologically-Constrained Isotopic Dating

Most recent studies consider the formation of individual pegmatite bodies to be a fast process with estimated crystal growth rates reaching a walloping 10 m/day. This opinion is presumably underpinned by the traditional way of thinking of them as the end products of magmatic fractionation. Indeed, modelling has shown that if a pegmatite-forming substance with a temperature near granitic solidus intrudes into a much colder host rock, as recorded in some outcrops, it must cool rapidly. From here, a conclusion is made that the crystallisation must likewise be rapid. However, this view is challenged by several studies that published isotopic dates supported by petrological characterisation of the analysed materials, which suggested or can be used to suggest that some minerals in pegmatites grew over millions of years. Surprisingly, such in-depth work on the geochronology of individual pegmatite bodies is relatively uncommon, so it is early to make generalisations. Here, I highlight some of the existing evidence with the aim to stimulate further research into the timescales of pegmatite crystallisation, including the use of petrologically constrained isotopic dating.

Relating steep REE patterns in modern volcanism and the development of an amphibole-rich middle to lower crust at the Colombian magmatic arc: A geochemical and receiver functions approach

In this study, I integrated geochemical modeling and receiver functions to test for magmatic fractionation above a garnet pyroxenite arc root structure beneath the Colombian magmatic arc (Northern Andes), as a feasible mechanism to explain the record of steep rare earth element (REE) patterns in modern volcanism and the development of an amphibole-rich middle to lower crust. Magmatic fractionation was evaluated through the geochemical modeling of volcanic rocks that show steep REE patterns with high Sr/Y and La/Yb ratios. A well-defined REE geochemical trend was accomplished for all samples at temperature and pressure estimates that are consistent with middle to lower crustal conditions (∼825-935 °C, ∼10.0–14.7 kbar). From the integration of receiver functions, after analyzing the middle to lower crustal signals at multiple stations on top of active volcanoes, the deep crust of the arc is interpreted as a seismically fast (Vs: ≥4.5 km/s) and dense arc root (≥3.3 g/cm3), above which, lies a low-velocity zone with a horizontal to tilted anisotropy axis. This anisotropic low-velocity region is proposed as a partially molten domain related to the evolution of the arc system. The proposed magmatic fractionating model explains satisfactorily the genesis of rocks with steep REE patterns and high Sr/Y (>40) and La/Yb (>20) ratios as a counterpart for a clinopyroxene amphibolite residuum composing deep levels of the arc crust. I suggest that these fractionating processes are playing an important role in the construction of the Colombian magmatic arc, enabling the formation of a dense garnet pyroxenite arc root, a genetically related amphibole-rich middle to lower crust, and the derivation of steep REE patterns with high Sr/Y and La/Yb ratios in the modern Colombian volcanism.

Geochemical and mechanical interactions within sheet-like granite intrusion during stress-field changeover (Moldanubian Batholith; Central European Variscides)

Late-orogenic granitoids carry unique geochemical and structural evidence of post-collisional exhumation, commonly associated with decompression melting, mixing and mingling of individual magma pulses and differentiation. The Weinsberg Type Granitoids (WTG) forming the 313.6 Ma Kaplice Sheet and 322 Ma Hojná Voda Sheet (northwestern Weinsberg Composite Pluton) have geochemical signature that demonstrates the substantial role played by the partial melting of metasedimentary sequences and the magmatic fractionation of K-feldspar, plagioclase and biotite during magma ascent and emplacement. The composition of highly peraluminous WTG were partly modified by the assimilation of host metasedimentary xenoliths. The textures of the WTG are characterised by straight crystal size distribution (CSD) consistent with in-situ crystallization and crystal/melt separation, forming abundant K-feldspar phenocryst-rich domains (KPD). The KPD were formed by redistribution of K-feldspar phenocrysts (crystal accumulation) in a suspension during magma movement followed by interstitial melt extraction and escape from the highly crystallised mush. The multiple fabric pattern within the Kaplice Sheet is the result of changeover of local stress field by varying increment of regional stress conditions and buoyancy forces at the time of magma ascent and emplacement. The steeply dipping foliation (M1) reflects the magma flow with the increment of regional stress-field (ENE–WSW oriented extension). The superimposition of flat-lying foliation (M2) with abundant particle accumulations (i.e. K-feldspar phenocrysts, enclaves and host-rock xenoliths) were associated with (a) increasing buoyancy force in ascending magma above the rate of the regional stress field or (b) collapse of ascending magma after the decreasing the buoyancy force resulting in the mush moving down.

Geologic scenario from granitic sheet to Li-rich pegmatite uncovered by Scientific Drilling at the Jiajika lithium deposit in eastern Tibetan Plateau

The Jiajika pegmatite-type lithium deposit emplaced in Upper Triassic turbidites is located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau and is the largest hard-rock lithium deposit in China. The Jiajika Scientific Drilling Project (JSD) consists of the JSD-1 borehole, which is 3211 m deep, and two 1000 m boreholes, JSD-2 and JSD-3. The JSD-1 borehole is composed of the upper pelitic metasediment unit (0–900 m) and the lower are calc-silicate unit (900–3211 m). Pelitic metasediments at the depth of 0–400 m experienced Barrovian-Buchan metamorphism, while metasediments at the depth of 0–900 m were associated with domal deformation (D2). The calc-silicate unit below 900 m only experienced earlier orogenic deformation (D1). The mineralization of Li-Be and Nb-Ta occurs at a depth of 0–1200 m and 0–1800 m, respectively, while the Rb enrichment occurs throughout the whole borehole. Analyses of K-Fe-Ba-B-Li-Rb isotopes suggest that the Jiajika lithium deposit resulted from high-degree fractionation of the parent granitic magma and strong fluid immiscibility. The pegmatites may have crystallized from volatile-rich melts during two-stage magmatic-hydrothermal events at 205–210 Ma and 192–193 Ma. The JSD-1, JSD-2 and JSD-3 indicate that S-type granite related to the Jiajika lithium deposit was emplaced as multi-layer domal sheets, rather than as a deeply rooted pluton. The domal granitic sheets are surrounded by pegmatites and meta-sedimentary rocks. Therefore, we propose a “domal granitic sheet” model to interpret the geologic scenario from granite sheet to Li-rich pegmatite in the Jiajika ore field. The Upper Triassic turbidites in the Songpan-Ganze terrane underwent significant orogenic decollement at ∼ 230 Ma, followed by deep anatexis, basement doming, magma rising and emplacement of granitic sheets. Finally, the rare-metal-rich pegmatites intruded under domal decompression condition, forming a multi-layer architecture intercrossed by the granitic sheets and the meta-sediments.

Geochronology and geochemistry of early Paleozoic granitic and coeval mafic rocks from the Tannuola terrane (Tuva, Russia): Implications for transition from a subduction to post-collisional setting in the northern part of the Central Asian Orogenic Belt

Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks.

The Geological and Tectonic Evolution of Feni, Papua New Guinea

Feni is located at the southeastern end of the NW-trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, in northeastern Papua New Guinea. This island chain is renowned for hosting alkaline volcanics, geothermal activity, copper–gold mineralization, and mining. There is no agreed consensus on the tectonic and petrogenetic evolution of Feni. Thus, the purpose of our paper is to present the geology of Feni within the context of the regional tectonic evolution of the TLTF chain and offer a succinct and generic geodynamic model that sets the stage for our next paper. The methodologies used in this study include a critical review of published and unpublished literature in conjunction with our geological observations on Feni. The Pliocene-to-Holocene TLTF chain is a younger arc situated within the greater Eocene-to-Oligocene Melanesian Arc bounded by New Ireland to the west, the Kilinailau Trench and Ontong Java Plateau in the east, and the New Britain Trench to the south. The geological units mapped on Feni include a large volume of basaltic lava flow and trachyandesite stocks intruding a limestone and siltstone basement. Younger units include the trachyte domes, pyroclastic flow, and ash fall deposits. The major structures on Feni are normal or extensional faults such as the Niffin Graben. Feni magmatism is attributed to the petrogenetic processes of polybaric or decompression melting and crystal fractionation of magmas previously influenced by sediment assimilation, mantle wedge metasomatism, slab tears, slab melts, and subduction. Deep lithospheric normal faults provide the fluid pathways for the Feni alkaline magmas.

Magmatic Evolution and Rare Metal Mineralization in Mount El-Sibai Peralkaline Granites, Central Eastern Desert, Egypt: Insights from Whole-Rock Geochemistry and Mineral Chemistry Data

The Ediacaran peralkaline granites, which were emplaced during the post-collisional tectonic extensional stage, have a limited occurrence in the northern tip of the Nubian Shield. In this contribution, we present new mineralogical and geochemical data of Mount El-Sibai granites from the Central Eastern Desert of Egypt. The aim is to discuss their crystallization condition, tectonic setting, and petrogenesis as well as the magmatic evolution of their associated mineralization. Mount El-Sibai consists of alkali-feldspar granites (AFGs) as a main rock unit with scattered and small occurrences of alkali-amphibole granites (AAGs) at the periphery. The AAG contain columbite, nioboaeschynite, zircon and thorite as important rare metal-bearing minerals. Geochemically, both of AFG and AAG exhibit a highly evolved nature with a typical peralkaline composition (A/CNK = 0.82–0.97) and formed in within-plate anorogenic setting associated with crustal extension and/or rifting. They are enriched in some LILEs (Rb, K, and Th) and HFSEs (Ta, Pb, Zr, and Y), but strongly depleted in Ba, Sr, P and Ti with pronounced negative Eu anomalies (Eu/Eu* = 0.07–0.34), consistent with an A-type granite geochemical signature. The calculated TZrn (774–878 °C) temperatures indicate that the magma was significantly hot, promoting the saturation of zircon. The texture and chemistry of minerals suggest that they were crystallized directly from a granitic magma and were later subject to late- to post-magmatic fluids. Both granitic types were most likely generated through partial melting of a juvenile crustal source followed by magmatic fractionation. The lithospheric delamination is the main mechanism which causes uplifting of the asthenospheric melts and hence provides enough heat for crustal melting. The produced parent magma was subjected to prolonged fractional crystallization to produce the different types of Mount El-Sibai granites at different shallow crustal levels. During magma fractionation, the post-magmatic fluids (especially fluorine) contribute significantly to the formation of rare metal mineralization within Mount El-Sibai granites.

Apatite CO2 and H2O as Indicators of Differentiation and Degassing in Alkaline Magmas

Abstract Apatite can incorporate significant amounts of H2O and CO2, potentially recording volatile abundance and behavior during magma evolution. Here we conducted in situ measurements of CO2 and H2O concentrations in apatite, along with elemental compositions, from two contemporaneous alkaline volcanic suites (Tianbao and Tudiling) in the South Qinling belt in central China to better understand the CO2 and H2O contents and behavior in evolving alkaline melts. Apatite from alkali basalts in Tianbao contains variable CO2 contents ranging from <100 ppm to 2.7 wt.% and H2O contents ranging from 0.1 to 0.6 wt.%. Apatite from REE-enriched trachytes, which evolved from alkali basalt, shows significantly lower CO2 contents and a continuous decrease in H2O during magma fractionation. The observations suggest that CO2 loss commenced at the early stages of magma evolution, whereas significant H2O loss occurred during subsequent magma fractionation in tandem with REE-enrichment. In comparison, apatite grains from the Tudiling trachyte, which is genetically linked with carbonatite, contain higher CO2 contents (0.6 to 1.5 wt.%) but lower REE concentrations than the Tianbao trachytes. Apatite in the Tudiling trachyte is inferred to have crystallized from a carbonated alkaline magma prior to significant CO2 loss and the separation of Tudiling carbonatitic melts, where subsequent liquid immiscibility led to REE enrichment into the carbonatitic melts. The volatile characteristics of apatite from the two volcanic suites provide valuable insights into two different evolutionary processes of alkaline/carbonatitic magmas, the behavior of CO2 and H2O, and the enrichment of REE in alkaline systems.

Unraveling 800 Ma of pegmatite formation in the western Grenville Front tectonic Zone near Sudbury, Canada

The origin of pegmatites has been at the core of petrological research for decades, but despite much effort their origin remains controversial – protracted fractionation of a causative felsic magma versus anatexis of an appropriate protolith. To address this controversy, we present the results of U-Pb analyses (zircon (n = 774), monazite (n = 230), xenotime (n = 295)) for 32 well characterized pegmatites from an extensive swarm of felsic dike rocks outcropping, along a 12 km transect ca. 12 km south of Sudbury (Ontario, Canada) in the westernmost part of the Grenville Front Tectonic Zone (GFTZ). Collectively these rocks exhibit a remarkably large morphological, textural and mineralogical variation related to the crystallization of three distinct pegmatite-forming events that span 800 Ma. The derived concordant U-Pb ages of the dated phases coincide with the late Paleoproterozoic (ca. 1740 Ma) Yavapai orogeny, the Mesoproterozoic (ca. 1450–1430 Ma) Chieflakian event, and the early Neoproterozoic (ca. 1000–970 Ma) Grenville orogeny. Field observations and related geochemical data suggest the two older events are genetically related to regional magmatism (i.e., fractionation of a progenitor), whereas the Neoproterozoic pegmatites were generated via partial melting of an appropriate protolith at depth. The data therefore support a duality of models for pegmatite genesis. We emphasize the important role of imaging (CL and BSE) as an integral part of our protocol, as this revealed widespread coupled dissolution-precipitation (CDP) replacement textures and metamictization in both zircon and xenotime, the latter due to high U contents (i.e., >1000 pm). Interestingly, the Neoproterozoic pegmatites show similar U(±Th), Nb(±Ta) and REE enrichments like their contemporaneous Sveconorwegian counterparts. Many zircons record U-Pb system resetting at ca. 220–200 Ma which is possibly related to the far-field effects of the onset of the Central Atlantic Magmatic Province (CAMP) related to the opening of the Atlantic Ocean. Finally, this study shows that anatexis is a viable process of rare-element pegmatite formation, even in areas rich in potential parental granites, which suggests that anatectic rare-element pegmatites might be globally under-recognized.

Constraining mixing and mingling processes from zoned magmatic enclaves: An example from the Taejongdae granite in Busan, Korea

Zoned magmatic enclaves (MEs) within granitic intrusions provide valuable insights into the pre-intrusive processes involved in the formation of granitic magmas. This study focuses on the Taejongdae granite in Busan, Korea, which features uniquely zoned MEs, and aims to constrain the processes involved in their formation. Through a whole-rock geochemistry analysis, SHRIMP U-Pb zircon dating, and Lu-Hf isotope data, we investigated the geochemical characteristics and origin of the two magmas responsible for the zoned MEs (that is, dioritic and host granite magmas). Our findings have shown that the host granite aligns with the Bulguksa granites in the Gyeongsang Basin, which represent the I-Type magma resulting from the fractionation of magma generated through partial melting of the continental crust. In contrast, the dioritic magma exhibits a low K2O composition, high Sr content, and a positive-to-no Eu anomaly, indicating its formation via amphibole-dominated fractional crystallization under different pressure–temperature conditions. Both magmas share the same Cretaceous age and Hf ratios. The zoned MEs in the study area provide compelling evidence of magma mixing processes that occur between the outer dioritic zone and mafic core of the enclaves. However, the relationship between the outer dioritic zone and the host granite suggests limited interaction between the enclaves and host granite. This implies that mixing within the zoned enclaves occurred prior to enclosure in the host granite. By highlighting the significance of zoned magmatic enclaves, this case study advances our understanding of the pre-intrusive processes involved in the formation of granitic magmas, including magmatic mixing, mingling, and fractionation. These results offer important insights into the origin and geochemical characteristics of the two magmas and their interactions, thus contributing to a broader understanding of crustal evolution in subduction-related settings.

Wet calc-alkaline magmatic fractionation in the middle−upper crustal sections of the continental arc: Insights from the Neoproterozoic Nanba intrusive complex, western Yangtze Block, South China

Elucidating the petrological and geochemical characteristics of continental arc crustal fragments can provide unique insights into the magmatic evolutional path and physicochemical conditions (e.g., P−T−H2O) of continental magmatic arcs. This contribution provides a comprehensive set of petrological, mineralogical, geochronological, and geochemical data for the Neoproterozoic Nanba intrusive complex of the western Yangtze Block, South China, which features well-exposed middle−upper crustal sections of the Neoproterozoic continental magmatic arc, to evaluate their magmatic source and fractionation process. The Neoproterozoic Nanba intrusive complex, composed of cumulate gabbros, hornblende gabbros, gabbro-diorites, diorites, granodiorites, K-feldspar granites, and monzogranites, was emplaced at ca. 796−790 Ma. The different lithologies of the Nanba intrusive complex display relatively homogeneous Sr-Nd-Hf isotopes, which indicates a cogenetic evolutional process. The predominantly depleted isotopic characteristics, together with high values of Ba, Rb/Y, and Ba/La, demonstrate that these rocks mainly originated from a subduction fluid−metasomatized mantle source. Petrological and geochemical characteristics, as well as hornblende thermobarometric data, collectively support that the ca. 796−790 Ma Nanba intrusive complex, following the calc-alkaline trend, was dominated by the wet cogenetic fractionation of hornblende accompanied by plagioclase and accessory minerals beneath the middle−upper crustal levels (2.4−4.7 kbar). The cumulate gabbros represent the residue of accumulated hornblende + plagioclase + Fe-Ti oxides from fractionated basaltic melts. The high-SiO2 K-feldspar granites and monzogranites display complementary trace elements, which represent residual silicic cumulates and extracted interstitial liquids in the shallower crystal mush reservoir, respectively. In combination with previous studies, we propose that the middle−upper crustal sections of the Neoproterozoic continental arc of the western Yangtze Block underwent hornblende-dominated fractionation that was followed by significant crystal-liquid separation, which led to the genesis of high-SiO2 melts at the shallower crustal level. The cogenetic evolution played a significant role in molding the petrological and geochemical diversity of Neoproterozoic igneous rocks and the differentiation process of continental arc crust in the western Yangtze Block.

A post-collisional batholith from Southern Iberia rooted in the Earth's mantle: Los Pedroches batholith

The Los Pedroches batholith (SW Spain) is one of the most salient igneous alignments of the Iberian massif. The batholith is considered one of the best examples of post-collisional batholiths of Iberia and is dominated by a biotite granodiorite that was emplaced in the shallow crust postdating the main deformation phases of the Variscan orogeny. In the Quintana de la Serena area, to the northwest of the batholith, granodiorites were partially emplaced into dacites characterized by the presence of abundant orthopyroxene. The quality of outcrops and abundant quarries in the area, together whit the near absence of local crustal contamination, make this zone an ideal location for petrogenetic studies about the origin of granodiorite magmas and their related microgranular enclaves. We present in this paper new SRHRIMPII-based zircon geochronology data that indicate orthopyroxene-bearing subvolcanic rocks emplaced and cooled at the age of 310 ± 2 Ma, slightly earlier than the intrusion and cooling of the massive granodiorites (307 ± 1 Ma and 306.6 ± 0.7 Ma) and their related mafic microgranular enclaves (307.8 ± 0.8 Ma). Whole rock geochemistry demonstrates that the three groups of rocks, dacites, granodiorites and mafic microgranular enclaves, define a coherent geochemical trend indicating that they are cogenetic and related by fractionation of a common parental magma. This study proves the role played by intermediate magmas of sanukitoid affinity in the generation of the most abundant and massive granodiorite rocks that form large post-collisional batholiths in Iberia.

Indium mineralization and genesis of the Bainiuchang Ag–Sn–polymetallic deposit in southeast Yunnan, China: Evidence from mineral chemistry and U–Pb geochronology

The Bainiuchang Ag–Sn–polymetallic deposit is located in the world-calss Southeast Yunnan Indium Metallogenic Belt (SYIMB). The deposit contains approximately 1000 tons of In, but the mechanisms of mineralization and hosts of In in this deposit remain unclear. To clarify these ambiguities, we carried out fieldwork, major- and trace-element analyses of In-bearing minerals, and cassiterite and zircon U–Pb geochronology. Five ore zones were identified, which, from contact of the granitic intrusion upwards, are the In-poor magnetite–cassiterite ore zone, the In-poor Cu ore zone, the In-rich Zn–Sn ore zone, the In-rich Sn–Pb–Zn ore zone, and the In-poor Pb–Zn ore zone. Cassiterite and zircon U–Pb geochronology demonstrate that mineralization at Bainiuchang occurred at ca. 85.80±0.45 Ma (MSWD = 1.5), which is similar to the 85.34±0.42 Ma (MSWD = 1.7) age of the granitic intrusion. This temporal relationship suggests that there is a genetic link between In mineralization and granitic magmatism in this deposit. Ore petrography and mineral chemical analyses indicate that sphalerite, chalcopyrite, and stannite are main In-bearing minerals. From a mass balance perspective, sphalerite is the most important In-bearing mineral, accounting for ∼85% of the In budget at Bainiuchang. Indium is incorporated into sphalerite via the coupled substitution 2Zn2+ ↔ Cu+ + In3+ and/or 3Zn2+ ↔ Ag+ + Sn2+(+In3+). Two generations of sphalerite occur in the deposit, SpI and SpII, which formed during the cassiterite–sulfide and carbonate–sulfide stages, respectively. Based on textural relationships, SpI is further subdivided into SpIa, SpIb, and SpIc. These temporally and texturally distinct sphalerite have variable In contents, with SpIa having the highest In content (0.08–0.44 wt%), followed by SpIb (0.02–0.39 wt%), SpIc (187–236 ppm), and SpII (0.64–7.14 ppm). It is suggested here that the variations in In contents of the sphalerite reflect the evolution of the magmatic–hydrothermal fluids from which the sphalerite precipitated under different fS2 and temperature. By comparing the Bainiuchang deposit to other In-bearing deposits in the southwestern Yangtze Block, it is evident that that the nature and source of the associated granites, and the degree of fractionation of the granitic magmas are likely important controls on In mineralization in the SYIMB and other regions.

Making Andesite Through Shallow Hybridization of Magmas Derived From Variably Enriched Lithospheric Mantle

AbstractWe integrate textural and in situ compositional information from plagioclase and clinopyroxene (Cpx) phenocrysts together with groundmass compositions in early Cretaceous andesite dykes within the Sulu belt of China to propose a new petrogenetic model for andesite. Plagioclase phenocrysts are mostly andesine; they are depleted in high field strength elements (HFSE). However, Cpx phenocrysts are either reversely zoned (type I) or homogeneous (type II), with the zoned Cpx divided into subtypes IA and IB. All Cpx has high Mg#, low Na2O and generally low Al2O3, with depletions in HFSE and variably high 87Sr/86Sr ratios, suggesting crystallization above the Moho from magmas derived from enriched lithospheric mantle. The cores of type IA/IB and type II Cpx have normal major‐ and trace‐element compositional variations and similar 87Sr/86Sr ratios to each other and to plagioclase, consistent with fractional crystallization from a common magma (magma 1). The rims of type IA and IB Cpx also have normal major‐ and trace‐element compositional variations, but these are not as evolved as the cores, and the rims have lower 87Sr/86Sr ratios, demonstrating crystallization from an isotopically distinct magma (magma 2). Based on modeled major and rare earth element compositions of magmas inferred to have been in equilibrium with different Cpx (±plagioclase) domains, the measured groundmass compositions can be reproduced by variable mixing between the two magmas. Our study demonstrates for the first time that andesite magma can be made through fractionation and shallow hybridization of magmas derived from variably enriched lithospheric mantle.

Genesis and interaction of magmas at Nishinoshima volcano in the Ogasawara arc, western Pacific: new insights from submarine deposits of the 2020 explosive eruptions

Sudden changes of eruption styles and magma compositions at arc volcanoes are enigmatic processes. Nishinoshima volcano, western Pacific, has had historical eruptions in 1973–1974 and from 2013 on and off to the present day. These eruptions were characterized by effusive Strombolian eruptions of andesite magmas until mid-June 2020, when they suddenly transitioned to violent explosive Strombolian eruptions that produced tephra fallout over a wide area. To understand this transition, we conducted marine surveys and sampling of the extensive submarine deposits of the tephra fallout. Our new data demonstrate that the full compositional range of the 2020 eruptions spans from basalt to dacite. We present evidence for magma mixing of newly injected basalt with andesite magmas. Nishinoshima consists of an andesitic main edifice surrounded by basaltic knolls: previous studies have shown that Nishinoshima andesite compositions can be generated by olivine fractionation of primary andesitic magmas that result from partial melting of hydrous mantle at relatively low pressures under the thin crust of the Ogasawara arc; knoll basalt compositions can be generated by partial melting of mantle at greater depths and were interpreted as older events of the volcano. We show that basalt magmas could have been generated throughout the entire history of Nishinoshima. In addition, we show that andesites from Nishinoshima and nearby Nishinoshima-Minami Knoll, which are only ∼8 km apart, have distinct subduction components. Together, these data improve our understanding of the diverse primary magmas responsible for the construction and continuing eruptive activity of an active island arc volcano.

Petrogenesis of Shihuiyao Rare-Metal Granites in the Southern Great Xing’an Range, NE China

Shihuiyao Rb–Nb–Ta-rich granites from the Late Jurassic period are newly discovered rare-metal-bearing granites found in the southern Great Xing’an Range, NE China. Further research of these granites may contribute to better understanding the petrogenesis of rare-metal granites and their associated mineralization mechanisms. The granites are high-silica (SiO2 = 73.66–77.08 wt%), alkali-rich (K2O + Na2O = 8.18–9.49 wt%) and weakly to mildly peraluminous with A/CNK values (molar ratios of Al2O3/(CaO + Na2O + K2O)) ranging from 1.06 to 1.16. High differentiation indexes (DI = 95–97) and low P2O5 contents demonstrate that Shihuiyao rocks are low-P and peraluminous rare-metal granites. Mineral chemistry and whole-rock geochemistry can be used to obtain the following lithological sequence: zinnwaldite granite, muscovite–zinnwaldite granite, amazonite-bearing granite and amazonite pegmatite. The effect of the rare-earth element tetrad; low K/Rb (18.98–32.82), Nb/Ta (2.41–4.64) and Zr/Hf (5.99–8.80) ratios; and the occurrence of snowball-textured quartz suggest that extreme magmatic fractionation might be the key factor that causes Rb–Nb–Ta enrichment.

Granitoids indicating moderate to high heat production associated with oceanic subduction system in the NE Tibetan Plateau

Regional heat flow provides a direct surface indication of the thermal state and energy balance of the lithosphere. Heat flux in the Tibetan Plateau remains poorly studied due to the lack of adequate information on heat flow. In this study, we investigated granitoids from the Gonghe-Guide district located in the NE Tibetan Plateau, which have important significance as hot and dry rocks with potential for geothermal resources. Samples from granitoid rocks as well as felsic dikes were collected from this area, and their geochemical data were combined with regional geophysical and drilling core data to evaluate a high-heat-flow anomaly. Our data show that granitoid rocks, including granodiorite, monzogranite, and syenogranite, crystallized at ca. 253−240 Ma, while felsic dikes formed ca. 230 Ma. The former were emplaced during the oceanic subduction process, whereas the latter formed in the syncollisional stage associated with the closure of the paleo-Zongwulong Ocean. Geochemically, these granitoid rocks are metaluminous to slightly peraluminous, with an average differentiation index (DI) value of 80, and they are classified as weakly to moderately fractionated I-type granites. The felsic dikes are peraluminous with high DI values (average of 95), typical of highly fractionated I-type granites. In terms of their high K and especially U and Th abundances, the calculated radioactive heat generation produced by the granodiorite, monzogranite, syenogranite, and felsic dikes is 1.85, 2.91, 6.85, and 3.02 μW/m3, respectively. Their moderate to high heat production generates a local heat-flow anomaly of 14.8−22.2 mW/m2 above the regional value of subduction zones, accounting for 11%−18% of the total regional heat flow. In combination with regional magnetotelluric data, the high-heat-flow anomaly may be attributed to an additional heat-flow contribution from a partial melt layer beneath this region. Furthermore, there is a significant positive correlation between the radioactive heat generation and magmatic fractionation, indicating that the enrichments of heat-producing elements are controlled by fractional crystallization and subsequently source composition. The granitoid rocks with high heat generation correspond to the large-scale intermediate-felsic magmatism during the subduction stage. We propose a model wherein a continuous subduction process during the closure of the paleo-Zongwulong Ocean and protracted cooling and crystallization processes resulted in the enrichment in heat-producing elements.

Carbonatitic Magma Fractionation and Contamination Generate Rare Earth Element Enrichment and Mineralization in the Maoniuping Giant REE Deposit, SW China

Abstract Carbonatite intrusions host the world’s most important light rare earth element (LREE) deposits, and their formation generally requires extraordinary fertile sources, magmatic evolution, and hydrothermal events. However, carbonatitic magma evolution, particularly the role of fractional crystallization and contamination from silicate rocks in REE enrichment, remains enigmatic. The Maoniuping world-class REE deposit in southwestern China, is an ideal target to decipher magmatic evolution and related REE enrichment as it shows continuous textual evolution from medium- to coarse-grained calcite carbonatite (carbonatite I) at depth, to progressively pegmatoidal calcite carbonatite (carbonatite II) at shallow levels. In both types of calcite carbonatites, four generations of calcite can be classified according to petrographic and geochemical characteristics. Early-crystalizing calcite (Cal-I and Cal-II) are found in carbonatite I and exhibit equigranular and a polygonal mosaic textures, while late calcites (Cal-III and Cal-IV) in carbonatite II are large-size oikocrysts (>0.5 mm in length) with strain-induced undulatory extinction and bent twinning lamellae. All these generations of calcite yield similar, near-chondritic, Y/Ho ratios (26.6–28.1) and are inferred to be of magmatic origin. Remarkably, gradual enrichment of MgO, FeO and MnO from Cal-I to Cal-IV is coupled with a significant increase in REE contents (~800 to 2000 ppm), with LREE-rich and gentle-to-steep chondrite-normalized REE patterns ((La/Yb)N = 3.1–26.8 and (La/Sm)N = 0.9–3.9, respectively). Such significant REE enrichment is ascribed to protracted magma fractional crystallization with initial low degree of fractional crystallization (fraction of melt remining (F) = ~0.95) evolving to late stage (F = 0.5–0.6) by formation of abundant calcite cumulates. Differential LREE and HREE behavior during magma evolution largely depend on separation of phlogopite, amphibole, and clinopyroxene from the carbonatitic melt, which is indicated by progressively elevated (La/Yb)N ratios ranging from 3.1 to 26.8. The four generations of calcite have significantly different C and Sr isotopic compositions with δ13CV-PDB decreasing from −3.28 to −9.97‰ and 87Sr/86Sr increasing from 0.70613 to 0.70670. According to spatial relations and petrographic observations, the relative enrichment of δ13C and depletion in 87Sr/86Sr ratios of Cal-I and Cal-II show primary isotopic characteristics inherited from initial carbonatitic magma. By contrast, the variable Sr and C isotopic compositions of Cal-III and Cal-IV are interpreted as the results of contamination by components derived from silicate wall rocks and loss of CO2 by decarbonation reactions. To model such contamination processes, Raleigh volatilization and Monte Carlo simulation have been invoked and the model results reveal that carbonatitic melt-wall rock interaction requires 40% radiogenic Sr contamination from silicate rocks and 35% CO2 degassing from carbonatitic melt. Moreover, positive correlations between decreasing δ13C values and increasing REE contents, together with bastnäsite-(Ce) precipitation, indicate further REE accumulation during the contamination processes. In summary, alongside REE-rich magma sources, the extent of fractional crystallization and contamination during carbonatitic magma evolution are inferred to be important mechanisms in terms of REE enrichment and mineralization in carbonatite-related REE deposits worldwide.

A Global Assessment of the Controls on the Fractionation of Arc Magmas

AbstractDuring the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.