Extensive Fractional Crystallization Research Articles

The role of protolith composition in the formation of tin-enriched granitic melts: A modeling study using the example of the southwest China tin province

Important features of Sn mineralization are the heterogeneous geographic distribution and frequent regional separation from W mineralization in spite some similarities of Sn and W behavior during magmatic processes. Major Sn and W mineralization is often spatially associated with peraluminous granites, which are derived from partial melting of metasediments. Several concepts have been suggested to explain those features, such as a weathering-related Sn-enriched source, Sn redistribution between melts and restite during protolith melting, and extensive fractional crystallization. We demonstrate the importance of protolith composition for the formation of Sn (and W) granites by using a comprehensive bulk-rock composition dataset from Precambrian metasediments of the South China Sn-W province and employing a thermodynamic modeling approach. We used four compositional proxies for phase equilibria calculations, which are the metasediments of the Mengdong, Sibao, Pingbian, and Shuangqiaoshan Groups. It is well documented that those Precambrian metasediments are important protoliths of Sn granites in South China. We present quantitative evaluation of the control of protolith composition in the generation of Sn-enriched granitic melts using South China as example, but our conclusions may also be applicable to worldwide Sn–enriched granites. Our results indicate that the protolith major-element geochemistry controls the anatectic reactions and melt productivity at specific melting conditions, and consequently the partitioning behavior of Sn. Further, pre-enrichment of Sn is crucial to the fertility of granitic melt and may be a prerequisite, particularly for the formation of giant Sn deposits. We propose that the heterogeneous distribution of favorable source rocks is one of the important factors that control the spatial distribution of major Sn (and W) districts in South China and other regions worldwide.

Zinc isotopes of high silica granites in the Qilian Orogenic belt and implications for mineralization potential

Many high silica granites (>70 wt% SiO2) are closely linked to ore deposits. There has been an ongoing debate about whether they are formed through extensive fractional crystallization or partial melting, as traditional major-trace element and radiogenic isotopes make it challenging to distinguish among these mechanisms. Recent studies suggest that zinc isotopes have the potential to trace crustal differentiation and monitor fluid evolution during the late stage of magma evolution. In this study, we investigated the Lenglongling leucogranites in the North Qilian Orogenic belt, characterized by high SiO2 contents ranging from 72 % to 78 wt%, extremely low Zr/Hf ratios (15.8–21), low Nb/Ta ratios (3.8–9.3), and strongly negative Eu/Eu* anomalies (0.04–0.32). The invariant and basalt-like zinc isotopic compositions of the Lenglongling high silica leucogranites (0.27 ‰ ±0.03 ‰ to 0.38 ‰ ± 0.03 ‰) reveal that they did not undergo extensive fractional crystallization or fluid exsolution. Instead, their homogeneous zinc isotope compositions and distinct geochemical characteristics were inherited from their protoliths during partial melting.

Petrogenesis and Tectonic Setting of the Baluogenguole Mafic Dykes, Zongwulong Belt: Implications for Evolution of the Northern East Paleo-Tethys Ocean

The late Paleozoic tectonic setting of the Zongwulong Belt (ZWLB), a significant unit located in the northern Qaidam margin, Qinghai province, remains uncertain. Diabase dykes in the western part of the Zongwulong Belt offer insights into this issue. Field investigations reveal that the dips of the dykes are almost vertical, and they have sharp boundaries with the host rocks. These dykes consist of plagioclase, clinopyroxene, and opaque minerals exhibiting a characteristic porphyritic texture and massive structure. Zircon U-Pb dating of the dykes yields a weighted 206Pb/238U age of 289 ± 1 Ma. The dykes exhibit relatively high concentrations of TFeO, K2O + Na2O, and TiO2, while the SiO2 and MgO concentrations are relatively low. They display relative light-over-heavy REE enrichment, and lack negative Nb-Ta and Eu anomalies. The dykes underwent negligible crustal contamination, and experienced extensive fractional crystallization of olivine, clinopyroxene, and Fe-Ti oxides. Originating from the spinel–garnet transition zone at depths of approximately 75 km, the dykes result from garnet facies low-degree melting (5%–10%) in a continental rift setting. Combining these findings with regional geological data, we propose that the ZWLB likely experienced a continental rift in the west and exhibited a narrow oceanic environment in the east in the late Paleozoic period, potentially representing the most distant north branch of the East Paleo-Tethys Ocean.

Comparison of Sn-related granitoids in subduction and collision settings by accessory mineral geochemistry: A case study in the Tengchong-Lianghe tin belt, SW China

Magmatic-hydrothermal Sn deposits can develop in both subduction and collision settings. However, a systematic comparison of the geochemical fingerprints and petrogenetic evolution of Sn-related granitoids from both settings is still lacking. The Tengchong-Lianghe tin belt hosts numerous Sn deposits but of different metallogenic ages, such as the Xiaolonghe and Lailishan Sn deposits, which provide an ideal study area for addressing this issue. Zircon U–Pb dating suggests that the Xiaolonghe and Lailishan Sn-related granitoids were emplaced at 74.2–75.5 and 50.0–51.1 Ma, respectively. Given the initial India-Asia collision commenced at ca. 65–60 Ma, these ages, consistent with available Sn metallogenic ages, suggest the formation of the Xiaolonghe and Lailishan Sn deposits in subduction and collisional settings, respectively. New zircon and apatite compositions, and apatite Sr isotopic data, combined with previously reported whole-rock geochemical results, all indicate that the Xiaolonghe and Lailishan Sn-related granitoids share similar magma sources, evolution processes, redox states, and volatile contents. The presence of hornblende, high apatite NdN/NdN* (mostly > 1), and whole-rock evolutionary trends, together indicate that they represent fractionated I-type granites. Distinctly elevated apatite 87Sr/86Sr ratios (0.70988–0.71430) suggest that they originated from an ancient lower crust. Subsequently, they underwent an extensive fractional crystallization dominated by feldspar, as revealed by whole-rock, apatite, and zircon elemental variation trends. The low zircon CeN/CeN* (mainly < 200) and ΔFMQ (–1.9 to 1.7), together with elevated apatite F content (2.30–3.69 wt%), reveal that they crystallized from reduced and F-rich magmas. The relatively high whole-rock zircon saturation temperatures (mainly > 800 ℃) and Ba/Pb ratios imply that they were produced at a higher temperature by biotite-dehydration melting, which requires additional heat from the mantle. The specific mechanism that triggers mantle upwelling (oceanic slab subduction or break-off) could be the most significant difference between Sn-related granitoids formed during subduction and collision processes.

Petrogenesis and rare-metal mineralization of the Ririwai alkaline granitoids, north-central Nigeria: mineralogical and geochemical constraints

ABSTRACT The Ririwai anorogenic alkaline complex harbours extensive rare metal (Nb-Zr-Sn-Zn) mineralization. However, uncertainty still surrounds their origin and magmatic-hydrothermal processes responsible for their rare metal mineralization. The complex is composed of peralkaline arfvedsonite ± albite-bearing granites, mildly peralkaline fayalite-bearing granite porphyries, and weakly peraluminous biotite-bearing granites, all of which show diagnostic A1-type characteristics. Zircon U-Pb dating reveals emplacement ages between 181.4 ± 0.81 and 188.6 ± 7 Ma for the three rock units. Sm-Nd and Pb isotopes (εNd (t) = –0.6 to −5.4, 206Pb/204Pb = 17.68–17.93) constrained the Ririwai A-type granites to represent highly fractionated products of enriched mantle-derived magmas. Mass-balance modelling suggests that magma evolution was fuelled by extensive fractional crystallization of 50 ~ 60 vol.% K-feldspar, 30 ~ 35 vol% fayalite, 5 ~ 10 vol.% amphibole, and ~ 5 vol.% apatite under reduced and water-undersaturated (ΔFMQ = –1.55; logfo2 = –18.63; H2O = <2.0 wt.%) conditions and assimilation of ~ 50 vol.% crust. Whole rock and amphibole compositions revealed an increasing degree of fractionation from the fayalite-bearing granite porphyries through biotite-bearing granites to the arfvedsonite ± albite-bearing granites. This extended fractionation at the late stage of magma differentiation, facilitated by fluorine dissolution and concentration of excess HFSE and REE as alkali-fluorocomplexes in the fluorine-rich silicate melt, allowed the accumulation of economic rare metal pyrochlore and columbite mineralizations in the highly fractionated arfvedsonite± albite- and biotite-bearing granites, respectively. Nonetheless, the metasomatic reaction rim textures around the magmatic pyrochlore, coupled with the M-type lanthanide tetrad effect (TE1,3 >1.10) indicate the subsequent effect of hydrothermal processes. Tectonically, reactivated shear fractures and transcurrent faults following lithospheric stresses in a transtensional regime allowed magma uprising and emplacement of A-type granites in the Ririwai complex.

Origins and evolution of two types of Late Triassic granitic magmas in the Caolong-Xiangkariwa area of central-eastern Songpan-Ganze terrane, northern Tibet: Implications for pegmatite lithium mineralization

Pegmatite-type lithium (Li) deposits are an important source of the low-carbon energy metal, which is generally considered to be formed by the high degrees of differentiation of strongly peraluminous granitic magma. However, the question of whether the parental rocks of these peraluminous magmas are igneous or sedimentary has been widely debated. Recent studies have identified a world-class pegmatite-type Li deposit belt (up to 2800 km long) closely associated with the Late Triassic granitoids and (meta)sedimentary rocks along the West Kunlun and Hoh Xil-Songpan-Ganze terranes in northern Tibet. This study presents a comprehensive petrological, geochronological, and geochemical study on the two types of Late Triassic granitoids in the Caolong-Xiangkariwa area of the Yushu region of central-eastern Hoh Xil-Songpan-Ganze terrane in northern Tibet: type A includes the Tongtianhe-Zhenqin-Zhaduo-Zhaya-Xiangkariwa (TZX) diorites and granodiorites (218−212 Ma), and type B includes the Caolong two-mica and muscovite granites (213−205 Ma), as well as ore-barren, beryl-bearing, and spodumene-bearing pegmatite dikes (209−196 Ma). The TZX diorites and granodiorites contain variable amounts of amphibolite and biotite. They are metaluminous to slightly peraluminous with variable SiO2 (54.4−71.3 wt%), MgO (0.72−7.26 wt%) contents, and Mg# values (40−70). Some samples with low SiO2 (54.4−58.5 wt%) contents have geochemical features similar to high-Mg andesites: e.g., high MgO contents (5.24−7.26 wt%) and Mg# values (61−70). The TZX diorites and granodiorites exhibit less enriched (86Sr/87Sr)i (0.7080−0.7118) and εNd(t) (−4.8 to −8.0). By contrast, the Caolong two-mica and muscovite granites are characterized by the occurrence of abundant muscovite (plus tourmaline and garnet) and are strongly peraluminous. The Caolong granites and pegmatites are geochemically characterized by high SiO2 contents (68.8−79.8 wt%) and low MgO contents (0.01−0.10 wt%) and Mg# values (4−23). They have εNd(t) values (−8.8 to −11.4; granites: −8.8 to −10.8; pegmatites: −9.2 to −11.4) that are more enriched than the TZX diorites and granodiorites but similar to those of Songpan-Ganze (meta)sedimentary rocks (−7.4 to −12.9), and the Caolong two-mica granites have high zircon δ18O values (11.6−12.1). In addition, decreasing K/Rb ratios correspond to increasing Cs contents in K-feldspar and muscovite from the Caolong granites and pegmatites. Taking into account the Late Triassic granitoids and (meta)sedimentary rocks in the Hoh Xil-Songpan-Ganze terrane, we suggest that the TZX diorites and granodiorites were most probably formed by the partial melting of metasomatized lithospheric mantle that subsequently underwent extensive fractional crystallization. Conversely, the Caolong two-mica and muscovite granites were likely generated purely by the partial melting of (meta)sedimentary rocks rather than via the evolution of dioritic-granodioritic magmas. In summary, there were two kinds of granitic magmas with different sources in the Caolong-Xiangkariwa area. Finally, the Caolong pegmatites were most likely formed by the extreme differentiation of two-mica granitic magmas rather than the dioritic-granodioritic magmas. Therefore, the evolution of magmas derived from (meta)sedimentary rocks were most likely to have formed the Li-rich pegmatites. Moreover, these (meta)sedimentary rocks representing the ultimate Li source must have undergone strong chemical weathering, resulting in significant Li enrichment.

The formation of lithium-rich pegmatites through multi-stage melting

Abstract Lithium-cesium-tantalum–type pegmatites (the primary source of lithium) crystallize from highly evolved, volatile felsic melts that incorporated crustal material in their source. Pegmatites are classically thought to form either from extreme fractionation of a parental granite body or via low-degree partial melting of a metamorphic rock (anatectic origin). However, the processes that lead to the formation of economic lithium pegmatite deposits remain enigmatic, because precipitation of lithium ore minerals requires melt lithium concentrations in excess of 5000 ppm—~500 times upper crustal abundances. We use petrological modeling to quantify lithium enrichment in an anatectic-origin scenario and show that it is primarily driven by the relative stability of residual biotite and muscovite at medium to high pressures (~8 kbar), and biotite and cordierite at low pressures (~3 kbar). We show anatexis of an average lithium-enriched metasedimentary source cannot sufficiently elevate the lithium content of the ensuing melt to form economic deposits; however, if this first-generation melt—now crystallized as granitic crust—is re-melted, the second-generation melt will be sufficiently concentrated in lithium to crystallize lithium ore minerals. We propose a petrogenetic model for anatectic-origin lithium pegmatites, in which a region experiences at least two stages of partial melting, ultimately generating lithium-rich melts without invoking extensive fractional crystallization. This mechanism can both account for the occurrence of unzoned lithium pegmatites and explain why economic pegmatites in many terranes are younger than their inferred source granites.

The youngest lunar zircon reveals an extremely fractionated nature of Chang’e-5 basalt

The U(Pb)-Pb age of zircon is commonly used to represent the crystallization age for igneous rocks due to its high closure temperature and robust resistance to impact disturbance. However, no zircon crystallization age has yet been reported for Chang'e-5 (CE-5) basalt due to their limited occurrence in mare basalt. In this study, rare zircon grains from CE-5 lunar samples were investigated by the in-situ Pb isotopic analysis, and a precise zircon crystallization age of 2036 ± 19 Ma was determined from Pb-Pb isochron. This is hitherto the youngest reported crystallization age of lunar zircon, similar to the ages of CE-5 lunar basalts obtained by zirconium (Zr)-bearing minerals such as baddeleyite, tranquillityite, and zirconolite. Petrographic evidence and rare-earth element geochemistry indicate that zircon in the CE-5 lunar basalts were formed by the reaction of early-formed baddeleyite with SiO2 melt within the latest residue of extreme fractionation of a non-KREEP (an acronym for potassium, REE, and phosphorus) basaltic magma. In contrast to the prevailing view that lunar zircon have formed in late-stage enriched melts resulting from extensive fractional crystallization of the Lunar Magma Ocean, this study shows that zircon could be derived from extreme fractionation of non-KREEP basaltic magma unrelated to Lunar Magma Ocean.

Sources and petrogenesis of Late Triassic granitoids in the Yajiang gneiss dome group (eastern Songpan-Ganze orogenic belt) with tectonic implications

Numerous Mesozoic granitoid plutons with a substantial variety of petrologic and geochemical signatures intrude the Triassic metasedimentary rocks of the Songpan-Ganze accretionary-orogenic wedge in the eastern Tibetan Plateau. These granitoid rocks generally exhibit sharp contact with the Triassic sedimentary rocks. To better understand the genesis and emplacement of magmas associated with tectonic accretion along convergent plate boundaries, we present new LA-ICP-MS zircon U-Pb geochronology, major and trace elements, and Sr–Nd–Pb–Hf isotopic compositions for intermediate-to-felsic plutons in the Yajiang gneiss dome group (YGDG), including the Changzheng pluton, the Jiajika (Majingzi) pluton, the Rongxuka pluton, the Xiya pluton, the Xinduqiao pluton, and the Waduo diorite dikes in the eastern margin of the Songpan-Ganze orogenic belt (SGOB). By their typical petrological and geochemical characteristics, these plutons are classified as I-type granodiorite-diorite (Rongxuka, Xiya, Xinduqiao plutons, Waduo dikes) or S-type granite (Changzheng and Jiajika plutons). I-type granodiorite-diorites exhibit a high Mg# (average 49), medium-K cala-alkiline signature and are enriched in large-ion lithophile elements (e.g., Rb, Th, and U) but depleted in high field strength elements (e.g., Nb, Ta, and Ti), and with lower initial 87Sr/86Sr values (0.70622 to 0.70863) but a relatively high and concentrated εHf(t) (−27 to −1.9) and εNd(t) (−4.02 to −7.51); while S-type granites exhibit a high silica, high-K cala-alkiline signature with strong negative Eu anomalies (Eu/Eu* = 0.17–0.64), and possess high initial 87Sr/86Sr values but low εHf(t) (−40.1 to −0.9) and εNd(t) (−8.72 to −12.14) signatures, forming through partial melting of metasedimentary rocks from an ancient crust source. LA–ICP–MS zircon U-Pb dating indicates that they were emplacement between 207.2 ± 1.4 and 223.6 ± 2.2 Ma. Both types of granitoid rock were formed by the crustal partial melting followed by extensive fractional crystallization under high-temperature (700–850 °C), low-pressure (<15 kb) conditions. In combination with previous studies, our data provide evidence for an affinity between the eastern Songpan-Ganze andthe Yangtze craton basements, and suggest that the granitoid plutons formed in a syn-collisional to post-collisional setting at ca. 207–223 Ma.

Rare-metal Be–Nb–Ta mineralisation by pegmatite remelting: Insights from Dakalasu deposit in the Chinese Altai orogenic belt

The rare metals Li, Be, Cs, Nb and Ta are mainly extracted from pegmatite deposits that may have formed through extensive fractional crystallisation or local partial melting of crustal rocks. There are a large number of rare-metal pegmatites that formed through crustal melting in the Chinese Altai, but what controls rare-metal enrichment remains enigmatic. Here, a new columbite UPb age of pegmatite mineralisation (247 ± 1 Ma) that is distinct from the ages of surrounding biotite granites (290–270 Ma) in the Dakalasu Be–Nb–Ta deposit in the Altai is reported. Zircon from the border zone with a medium to coarse-grained graphic texture of pegmatite yields two age groups: 290–270 Ma and 249 ± 7 Ma, corresponding to the age of biotite granite and the mineralisation age of pegmatite, respectively. The consistent age of the older group and biotite granite (290–270 Ma) and the elevated εHf(t) ratios and lower Nb/Ta ratios of pegmatite suggest that pegmatite is the end member of biotite granite after fractional crystallisation. The similar age of the younger group and mineralisation, the myrmekitic texture and albite trace elements of pegmatite imply pegmatite remelting and mineralisation occurred at ∼250 Ma. Our modelling agrees with geochronology, petrology and geochemistry that Be–Nb–Ta pegmatite mineralisation requires two melting events. The first step with anatexis of metasedimentary rocks generated biotite granitic melts whose extensive fractional crystallisation generated pegmatite with preliminary Be–Nb–Ta enrichment. The second step with pegmatite remelting (<5% degree) generated the significant enrichments of Be, Nb and Ta, which are sufficient for beryl and columbite crystallisation. The Be–Nb–Ta mineralisation mechanism through pegmatite remelting is likely common in orogenic belts with high-grade metamorphism and, therefore, should be considered in future explorations.

Comparing the biotite and amphibole single-mineral thermobarometric models: A case study from the Cordillera de San Buenaventura volcanic rocks, Puna plateau of Central Andes (Argentina)

We applied a new thermobarometric model based solely on biotite composition to the trachyandesitic to rhyolitic volcanic rocks of the Cordillera de San Buenaventura, a long-lived magma plumbing system (spanning from the Late Miocene to the Holocene) in the Southern Puna plateau (Central Volcanic Zone of the Andean Cordillera). Within these rocks, biotite is a widespread mineral phase found in association with amphibole, as either glomerocrysts, mutual inclusions, or free phenocrysts. This allowed us to compare the biotite-only model with other thermobarometric estimates that rely on amphibole, mineral-melt, and mineral-mineral composition. Our goal was to determine the conditions of formation for biotite and its petrogenetic significance in the evolution of the Cordillera de San Buenaventura magma system. The results indicate that biotite crystallization occurred mainly at middle-to upper-crustal depths (2.1–5.1 kbar) with temperatures below 1000 °C (763–919 °C). The biotite-only T-P estimates for individual lithologies are consistent with those obtained using the amphibole-only model, and this is particularly true for the thermometer. Furthermore, our findings align with the existing petrogenetic models, which suggest extensive fractional crystallization, and with the analyses of mineral textures, that indicates events of magma replenishment. Therefore, our results support the validity of the biotite-only model and its ability to elucidate magmatic processes, including magma differentiation via fractional crystallization, magma ascent and cooling, and magma replenishment and/or mixing events.

Crystal entrainment from cool, low-silica rocks into hot, high-silica melts: diverse primary melt compositions at Taranaki volcano, New Zealand

The prevalence of antecrysts in arc volcanic rocks is widely accepted, yet the origin of their carrier melts remains debated. Crystal cargo in lava flows from Taranaki volcano, New Zealand, is dominated by plagioclase, clinopyroxene and amphibole. Except for some crystal rims, mineral phases are in disequilibrium with the melt they are entrained in. Major element chemistry reveals an almost complete compositional overlap between the crystals in the lava and those in xenoliths. The large volume fraction of crystals (35–55 vol%) exerts a strong control on whole-rock compositions, reducing silica by 5–11 wt% compared with the carrier melt. Yet there is no clear relationship between mineral proportion and bulk-rock compositions. Our data are inconsistent with extensive fractional crystallization, commonly invoked as a driver of magma evolution towards silica-rich compositions. Instead, high-temperature, aphyric carrier melts with varied compositions (55–68 wt% SiO 2 ) entrain crystal cargo while ascending through colder, low-silica rocks. Thus, some parental melts at Taranaki volcano are significantly more silica-rich than arc basalts commonly invoked as primary magmas. Further, thermometric and hygrometric constraints preclude a deep crustal hot zone for the source of these melts, which we argue are of subcrustal origin. Supplementary material: Supplementary figures and databases are available at https://doi.org/10.6084/m9.figshare.c.6406813

Geochemistry and origin of felsites and associated A-type granites at Wadi Atalla area, Central Eastern Desert, Egypt: Implication for genesis of Neoproterozoic post-collisional highly silicic melts

Alkaline volcanics (felsites) and A-type granites are spatially and temporally associated in the Wadi Atalla area, Central Eastern Desert, Egypt. These rocks represent final Neoproterozoic magmatic activity in the Northern Nubian Shield. Wadi Atalla felsites form NW-trending elongate mass consisting of silicic volcanics, which is intruded by A-type granites of Um Had and Um Effein plutons. Field relations indicate that both felsites and A-type granites were emplaced in a series of magmatic pulses. Felsites include tuffaceous, porphyritic and hypabyssal varieties, while A-type granites comprise syenogranite and alkali feldspar granite. Geochemically, they are highly silicic rocks and display a narrow range of SiO2. They have high-K alkaline nature with metaluminous to peraluminous character and were erupted in a post-collisional tectonic setting. Mineral chemistry of biotite supports the alkaline and A-type characters of the studied rocks. Atalla felsites and associated A-type granites show light REE enrichment relative to heavy REE [(La/Lu)n = 2.67–5.72] with pronounced negative Eu anomalies (Eu/Eu* = 0.15–0.52). Compositional similarities between Atalla felsites and A-type granites in the studied area suggest that the intrusive and extrusive events are broadly related. The systematic variation of major- and trace-element contents of the felsites and A-type granites indicates derivation from similar sources through partial melting of juvenile crustal rocks, followed by extensive fractional crystallization. The main fractionated phases are feldspars, with minor role of fractionation of amphibole, biotite, apatite and FeTi oxides. Multiple saturation calculations indicate water-saturated storage and final equilibration conditions for the felsites ranging from 788 °C at 90 MPa to 740 °C at 255 MPa. The parental magmas of the felsites and A-type granites were linked to lithospheric delamination and upwelling of asthenospheric mantle material. This process led to generation of mantle melts that supplied heat to melt the juvenile crust of the Arabian-Nubian Shield (ANS), along with crustal uplift recorded by intersecting strike-slip faults and shear zones. Atalla felsites represent a distinctive post-collisional alkaline volcanic phase in the Nubian Shield that has been emplaced in an extensional tectonic regime, during a phase of fracturing and crustal uplift, which followed the end of the Pan-African orogeny. This volcanic phase is younger than the Dokhan volcanics in the Eastern Desert and older than the typical Katherina volcanics in southern Sinai.

Characterizing a new type of nelsonite recognized in the Damiao anorthosite complex, North China Craton, with implications for the genesis of giant magmatic Fe-Ti oxide deposits

Abstract Nelsonite (Fe-Ti oxide-apatite rock) devoid of silicates offers a rare opportunity to investigate the magma processes for the formation of magmatic Fe-Ti oxide deposits. Both fractional crystallization and silicate liquid immiscibility have been put forward, but the lack of robust evidence has hindered unambiguously distinguishing the role of these two processes in Fe-Ti mineralization. The nelsonite and associated Fe-Ti-P-rich rocks hosted in the Proterozoic Damiao anorthosite complex represent a typical example for studying Fe-Ti ore-forming processes. We recognized a new type of nelsonite (type-I) in the Damiao complex, which is distinct from the two known types of nelsonite (type-II and type-III) from the same complex. The type-I nelsonite is characterized by its coexistence with oxide-apatite gabbronorite and granite in the same dike, and all these rocks have identical emplacement ages (1740 ± 7 Ma), subparallel REE patterns, and major-element compositions lacking intermediate compositions, suggesting derivation from conjugate Fe- and Si-rich melts generated by silicate liquid immiscibility. The large type-II nelsonite bodies form irregular dikes along fractures in anorthosite and constitute the major ore type. The type-III nelsonite occurs as conformable layers or pods within oxide-apatite gabbronorite and pyroxenite, and occupies the end part of the type-II dike. The latter two types of nelsonites formed by extensive fractional crystallization of residual magma with crystal accumulation and subsequent hydrothermal replacement. During residual magma evolution, silicate liquid immiscibility was crucial for Fe-Ti-P enrichment, fractional crystallization was responsible for enhancing oxide-apatite concentrations, and hydrothermal replacement was effective for mobilizing oxide-apatite concentrations. Our newly recognized nelsonite provides an unambiguous, outcrop-scale, field evidence for the operation of silicate liquid immiscibility process. We show that giant magmatic Fe-Ti oxide orebodies can form by a combination of processes involving silicate liquid immiscibility, fractional crystallization and hydrothermal mobilization.

Geochemical and thermodynamic constraints on Archean comagmatic volcanic and cumulate rocks from southern West Greenland

The extensive exposure of the Archean continental crust in southern West Greenland makes it an important window into the tectonic evolution of early Earth. Here, we provide a comprehensive geochemical data set for tholeiitic amphibolites (meta-basalts), calc-alkaline leucoamphibolites (meta-andesites), and ultramafic rocks (meta-cumulates) for the Mesoarchean Bjørneøen Supracrustal Belt, Nuuk region, SW Greenland. This data helps constrain the geodynamic setting in which these rocks formed. The volcanic rocks display two distinct geochemical trends in terms of their La/Sm ratios. Nonetheless, both tholeiitic amphibolites and leucoamphibolites have negative Nb-Ta-Ti anomalies and thus geochemical features associated with apparent island arc or crust contamination processes. Uranium-lead zircon dating of a leucoamphibolite yields an age of 3077 ± 6 Ma, which is older than regional orthogneisses. A series of models for both major element variation (thermodynamics-based) and trace element variation (partition coefficient-based) implies that fractional crystallization of tholeiitic basalt can effectively produce the observed ultramafic rocks. Such ultramafic cumulates had low degrees of crystallinity, reflecting open system magmatic process at shallow depths likely representing magma conduits in a volcanic pile. The geochemical features of the andesites are distinct from the basalts and our modeling excludes a connection via fractional crystallization or crustal assimilation of the two suites. Instead, the andesites formed via high degrees of mixing between basaltic and felsic endmember magmas, requiring elevated temperatures in the mid- to lower-crust. The introduction of felsic components could be derived from partial melting of mafic lower crust, for example by mafic underplating or via some other process that achieves such anatexis, or alternatively by the addition of rhyolitic melt from extensive fractional crystallization. Mixing and homogenization of basaltic and felsic endmembers to produce andesites may occur in modern-style subduction environments, although this could also be feasible in other geodynamic settings in a hotter early Earth.

Origin of silicic rocks of the Deccan Traps continental flood basalt province: Inferences from field observations, petrography, and geochemistry

The origin of silicic rocks (SiO2 > 65 wt%) in Continental Flood Basalt (CFB) provinces could be attributed to complex petrogenetic processes. The 65.5–66 Ma old Deccan Traps CFB contains eight sporadic but significant silicic rock exposures that are studied here in a comprehensive framework using field observations, petrography, major oxides (n = 56), and trace element chemistry. Rhyolite and granophyre, as well as subordinate felsite, ignimbrite, trachyte, pitchstone, and microgranite coexist with volcanic and plutonic mafic rocks such as basalt, basaltic andesite, and gabbro. Multiple isolated and circular/semi-circular hills and linear dykes of silicic rocks are present in the form of lavas with prominent flow folding, rheomorphic ignimbrite, and tuffs. The ‘Rheological Agpaitic Index’ (RAI) indicates that most of the silicic rocks in the Deccan Traps are effusive in nature, except for Rajpipla, Alech, Bombay, and Osham silicic rocks, which are marked by explosive volcanism. Thermodynamic-based Rhyolite-MELTS modelling suggests that the major oxide composition of Pavagadh and Barda basalt is a likely candidate for the parental melt composition of the silicic rocks of the Deccan Traps. Ba, Sr, P, Zr, and Ti anomalies are consistent with the fractionation of K-feldspar, plagioclase, apatite, zircon, and Fe-Ti oxides, respectively. Two broad REE patterns are noticed in the Deccan Traps silicic rocks: a flat pattern for Barda, Alech, and Chogat-Chamardi silicic rocks, and a steep REE pattern for Osham, Rajula, Pavagadh, Rajpipla, and Bombay silicic rocks. Trace element modelling reveals that 5–10 % partial melting of a spinel peridotite source could produce an REE pattern and abundances similar to the associated basalts. Further extensive fractional crystallization (60–90 %) of the parental mafic melt at a deeper depth (where spinel is stable) could produce the REE composition and pattern observed in most silicic rocks except for those of Barda, Alech, and Chogat-Chamardi, which require fractional crystallization of the same parental melt at a shallower depth (where spinel is not stable). The geochemical variability of Deccan Traps silicic rocks reveals an origin from a mantle-derived parental mafic melt that evolved via the assimilation and fractional crystallization (AFC) process to form the silicic exposures, which is typical of silicic volcanism in other global CFBs.

Systematic and Temporal Geochemical Changes in the Upper Deccan Lavas: Implications for the Magma Plumbing System of Flood Basalt Provinces

AbstractLarge Igneous Provinces (LIPs) are unusual volcanic events in which massive amounts of melt (∼106 km3) erupt in relatively short time periods (&lt;106 years). Most LIP magmas have undergone extensive fractional crystallization and crustal contamination, but the crustal magmatic plumbing systems and the processes triggering eruptions are poorly understood. We present new major and trace element and radiogenic isotope data for 43 individual lava flows from a continuous 1,200 m thick stratigraphic profile through the upper, most voluminous part of the Deccan LIP (Bushe to Mahabaleshwar Formations). Eruption rates for this section are constrained by published paleomagnetic directions and absolute U‐Pb ages for zircons from weathered flow tops exposed in the profile. We find four magmatic sequences each lasting ∼104–∼105 years during which major and trace element compositions change systematically, followed by an abrupt change in geochemistry at the start of a new sequence. Within each sequence, the MgO content and proportion of crustal contamination decrease progressively, indicating a continuous replenishment of the associated magma reservoirs with less contaminated but more evolved melts. These geochemical signatures are best explained by repeated episodes of melt recharge, mixing, and eruption of variably evolved magmas originating from relatively small magma reservoir located in different crustal levels.

Rutile Ages and Thermometry Along a Grenville Anorthosite Pathway

AbstractThe Grenville Orogen, in North America, contains a belt of Proterozoic anorthosite massifs with an episodic emplacement history. These distinctive magmas reflect extensive fractional crystallization of plagioclase from a mafic magma and contain layers dominated by Ti‐bearing minerals including ilmenite and rutile. U‐Pb geochronology and Zr‐in‐rutile analyses from the Lac Malbaie complex in the Saint Urbain area of Quebec help to constrain the tectonothermal history of repeated anorthosite emplacement into the crust. Geologically significant dates were obtained from low‐U (&lt;2 ppm) rutile in anorthosite drill cores and a boulder. Rutile ages of c. 1158, 1069–1056, and 936–910 Ma are interpreted to reflect variable modification of cooling ages during later intrusion and fluid mobilization events. Zr‐in‐rutile temperatures track higher apparent temperatures of &gt;800°C in earlier events. Apparently younger rutile grains, and those influenced by fast pathway diffusion, record lower apparent temperatures. The Grenville Orogen comprises major trans‐lithospheric structures that facilitate the repeated bottom‐up transit of mantle‐derived magmas. At depth this led to the emplacement of anorthositic magmas whose thermal pathway is tracked by rutile passing through their Pb blocking temperature during crustal unroofing. This same broad crustal architecture, in the comparatively shallow crust, provided a favorable pathway for top‐down hydrothermal fluids to interact with younger rutile as successive anorthosites were emplaced. Rutile demonstrates that the current crustal section represents the juxtaposition of minerals preserving information from different thermal regimes, retaining vestiges of a protracted history due to incomplete re‐equilibration with subsequent chemical or thermal conditions.

The Andaluca plutonic unit, Vinquis Intrusive Complex, Argentina: An assessment of mantle role in the genesis of Early Carboniferous weakly peraluminous A-type granites in the pre-Andean SW Gondwana margin

During the Early Carboniferous, significant intracontinental magmatism was developed in the retro-arc region of the SW margin of Gondwana between ca. 27° and 31°S. This magmatism consisted of metaluminous to weakly peraluminous A-type granites, strongly peraluminous A-type granites (A- to S- hybrid granitoids) and, in a lesser extent, (sub-)volcanic felsic (rhyolites and trachytes) and mafic (alkaline basalts and lamprophyres) rocks. The Vinquis Intrusive Complex (VIC) registers these two compositional varieties of A-type granites: the Vinquis Unit (VU) represents the strongly peraluminous granites and the Andaluca Unit (AU) the weakly peraluminous granites. The adjacent Zapata Intrusive Complex (ZIC) also contains metaluminous to weakly peraluminous A-type granites. The AU is a semi-elliptical plutonic body of approximately 60 km 2 located in the southwestern part of the VIC, intruding the Vinquis Unit. The AU comprises three dominant rock types: i) quartz monzonite, ii) syenogranite and iii) alkali feldspar granite that can be classified as F-rich (calculated F content: 1950–5700 ppm), ferroan (FeO t /(FeO t + MgO) = 0.86–0.95) and weakly peraluminous (ASI = 1.03–1.10) A-type (Zr + Nb + Ce + Y = 364–570 ppm; Ga = 20–23 ppm) granitoids (SiO 2 wt% = 67.8–75.4% wt%). This unit was emplaced in early Carboniferous time (U Pb zircon crystallization ages of 346 ± 3 Ma and 342 ± 3 Ma). The AU has 87 Sr/ 86 Sr t , εNd t and εHf t values ranging from 0.7092 to 0.7140, −2.6 to −3.6 and − 16.5 to −3.3, respectively. These isotopic suggest that the parental magma of AU dominantly involved variable mixtures of mantle-derived components (magmas/fluids) and peraluminous continental crust. The extremely felsic granites of the AU (SiO 2 > ~72%) with a strongly fractionated composition (Rb > 336 and Sr < 70 ppm and Eu/Eu* < 0.22) supports extensive fractional crystallization. The high contents of fluorine and HFSE in AU could be explained as derived from an F- HFSE- and alkali-rich parental mafic magma plus contributions from F-bearing minerals assimilated from the country rock. Likewise, the magmatic differentiation processes from these reduced mafic magmas could explain the ferroan character of the AU granitoids. We envision three stages in the origin of these rocks: (1) fluids and magmas from the asthenospheric mantle metasomatize and melt the subcontinental lithospheric mantle (SCLM), the alkali-rich mafic melts produced are emplaced at the base of the crust, transfer heat to the crust and melt it, generating strongly peraluminous A-type magmas; (2) continued introduction of alkali-rich mafic magmas into the deep crust produce extensive assimilation of the preheated crust in equilibrium with mafic cumulates (generated by reaction and/or fractionation); (3) the hot magmas so produced migrate into the middle-upper crust to produce ferroan weakly peraluminous A-type granitoids. • The Andaluca plutonic unit of the Vinquis Intrusive Complex crystallized between 346 ± 3 Ma and 342 ± 3 Ma. • Andaluca Unit shows a F-rich weakly peraluminous ferroan A 2 -type affinity. • The high contents of fluorine and HFSE in Andaluca Unit can be explained from an F-rich parental alkaline mafic magma. • The Andaluca Unit granitoids can be generated through combined crustal assimilation and fractional crystallization of magmas derived from the metasomatized subcontinental lithospheric mantle.

Petrogenesis and geodynamic implications of Oligocene A-type granite in the Guadalcazar area, San Luis Potosi, central Mexico

The Guadalcazar is located in the Mesa Central (MC) province, which is mainly composed of granitic rocks and is known for its metallogenetic. The granitic rocks contain complex Sn-Hg-Ag-F mineralization and were emplaced during Eocene to Oligocene. However, the source, origin, and evolution of magma and the tectonic setting of this magmatic area have never been explained. In this study, we have conducted petrology, whole-rock geochemistry, and U–Pb zircon geochronology on granitic rocks from the Guadalcazar to constrain the petrogenesis and tectonic environment. LA-ICP-SF-MS zircon U–Pb dating shows that the Guadalcazar granite was emplaced ca. 31 Ma. These rocks are characterized by high (SiO2) contents (64–75 wt%), low CaO (0.28–1.78 wt%), with relatively high (FeOt)adj/(FeOt + MgO) values ranging from 0.90 to 0.98. The geochemical diagrams of SiO2 vs [(FeOt)/(FeOt + MgO)] and SiO2 vs [(Na2O + K2O) − CaO] show the ferroan and mostly alkali-calcic nature of these rocks. The granite shows an A2-type affinity and is strongly peraluminous, with ASI (molar Al2O3/[CaO + Na2O + K2O]) values of 1.13 to 2.60. These granitic rocks are characterized by enrichments in rare earth elements (REE) and high field strength elements (HFSE), and depletion in Ba, Nb, Sr, Ti, and Eu. These features suggest that these A-type granites were derived from the metasedimentary rocks and evolved through extensive fractional crystallization. The multidimensional discrimination diagrams showed a continental rift or within-plate setting. By combining previous and new data, we proposed a new magmatic evolution model that supports an extension during ca. 34–28 Ma in the Guadalcazar, central Mexico.