Fractional Crystallization Research Articles

Zircon U-Pb Ages of the Granitoids in Shanxi and Its Significance for Tectonic Evolution of North China Craton in Mesozoic

Mesozoic granitoid formations offer crucial insights into the tectonic history of the North China Craton. New zircon U-Pb ages of two Mesozoic granitoids in the Huai’an terrane yield ages of 226.4 ± 1.1 Ma for the Yihe (YH) granite and 156.3 ± 2.9 Ma for the Zhujiagou (ZJG) granodiorite. The negative Nb, Ta, and Ti anomalies; high Nb/Ta ratios (20.4 to 24.1); high (La/Yb)N (30–84); low initial 87Sr/86Sr ratios (0.707725–0.708188); and negative ƐNd(t) values of the Yihe complex suggest that it originated from the partial melting of the lower crust and lithospheric mantle. However, the geochemical and Sr-Nd isotopic results of the ZJG granodiorite are characterized by I-type granites: Na2O + K2O values of 7.27 to 7.94 wt%, negative Nb anomalies, positive Pb anomalies, higher initial 87Sr/86Sr ratios (0.710979–0.714841), and much lower ƐNd(t) values (−27.1 to −30.1). The Late Jurassic Zhujiagou complex was derived from partial melting of a thickened low crust, and during the Late Triassic, magmatic rocks were formed under a post-collisional extensional regime. Multiple upwellings of the asthenosphere facilitated the mixing of magmas derived from partial melting of the lithospheric mantle and lower crust. These mixed magmas then ascended to the upper crust after undergoing fractional crystallization, leading to the formation of the YH complex. In the Late Jurassic, the tectonic regime of the NCC shifted from compression to extension. The Late Jurassic intrusion identified in this study developed within a compressional setting linked to the subduction of the Paleo-Pacific Ocean.

The genesis and mineralization potential of the Kunshan pluton in the Xuancheng district, Eastern China

The Xuancheng mineralization district (XMD) is an important part of the Middle-Lower Yangtze River Metallogenic Belt, and hosts a number of porphyry-skarn Cu poly-metal discoveries in recent years. The Kunshan pluton is located in the northeastern sector of the Xuancheng district, and is composed of fine-grained pyroxene diorite, coarse-grained pyroxene diorite and granodiorite. This study reports zircon LA-ICP-MS U-Pb chronology, whole-rock geochemistry and Sr-Nd isotope analysis of the Kunshan Complex pluton. The intrusions young from the fine-grained pyroxene diorite (137.2 ± 2.0 Ma), through the coarse-grained pyroxene diorite (136.1 ± 2.1 Ma) to the granodiorite (134.8 ± 2.2 Ma). All three stages of magmatic samples are high-potassium calc-alkaline to potassium dolerite series, they are enriched in light rare earth elements (LREE) and large ion lithophile elements such as Rb, Sr, and (Ba), but depleted in heavy rare earths elements (HREE), and high field strength elements such as Nb, Ta, Ti, and P. Their (87Sr/86Sr)i and (143Nd/144Nd)i values range from 0.706703 to 0.707618 and 0.512023 to 0.512064 respectively. The samples from the Kunshan complex pluton are consistently plotted the zone between the mantle evolution trending line and MLYB crust, which indicating the three-stage melt are derived from the crust-mantle mixed source, with closer to lithosphere mantle. All phases of the pluton were derived from the same magmatic source, likely the crust-mantle mixed zone between the lower Yangtze crust and enriched lithospheric mantle. The 3 stage melts formed Kunshan complex pluton gradually strengthen fractional crystallization (FC) during their emplacement process, while the late granodiorite melt with highest FC degree was the ore-forming intrusion, which is characterized with oxidized melt with low temperature, and high H2O content. After Compared with the mineralized granitoid intrusive rocks in the Middle-Lower Yangtze River Metallogenic Belt, it suggests that the late highly differentiated granitoids, like Kunshan granodiorite, are probably favorable metallogenic indications.

Geodynamic Evolution of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt, northwest Tibetan Plateau: Implications from the Subarc Crust and Lithospheric Mantle Modification

Abstract Radiogenic isotopes serve as a crucial tool for investigating crustal evolution, playing a pivotal role in revealing magma sources and petrogenesis. However, they can be ineffective in distinguishing between distinct magmatic sources with similar radiogenic isotopic compositions, a common occurrence in nature. Here we addresse this challenge in the Ordovician igneous rocks from the West Kunlun orogenic belt (WKOB), aiming to distinguish between two potential magmatic sources (i.e. the Tarim Craton and the Tianshuihai terrane) with similar isotopic compositions using appropriate thermodynamic and geochemical modeling based on mineral and whole-rock geochemistry. Zircon U–Pb dating yields ages of 483 ± 3 Ma for the Pushou gabbros and 469 ± 2 Ma and 461 ± 2 Ma for the Datong monzogranites and syenites, respectively. The Pushou gabbros exhibit low SiO2 (47.4–49.1 wt %), high MgO (5.5–6.9 wt %), high large-ion lithophile elements (LILEs, e.g. Rb, Ba, Th, and K), and low high field-strength elements (HFSEs, e.g. Nb, Ta, Zr, Hf, P, and Ti), suggesting an origin in subduction-modified mantle. They display high whole-rock (87Sr/86Sr)i ratios (0.7156 to 0.7192), negative whole-rock εNd(t) values (−7.1 to −7.8), as well as high zircon δ18O values (7.6–7.9‰) and enriched zircon Hf isotopic compositions (εHf(t) = −5.3 to −7.7), which are consistent with 1–5% subducted sediments in an enriched mantle source. Trace element models further confirm that the gabbros are most likely derived from low-degree (~15%) partial melting of subduction-modified Tarim mantle in the spinel–garnet facies rather than from the Tianshuihai mantle. The Datong syenites belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt %), relatively low MgO (0.9–1.2 wt %) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (−3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggest that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt %), low MgO (0.6–0.7 wt %), and a typical enrichment in alkalis, Zr, and Nb, with depletion in Sr, P, and Ti, consistent with A-type granites. They are characterized by high whole-rock (87Sr/86Sr)i ratios (0.7321 to 0.7323), negative whole-rock εNd(t) values (−11.3 to −11.8), negative zircon εHf(t) values (−11.0 to −16.5), and high zircon δ18O values (7.2–8.0‰), indicating derivation from the remelting of an ancient crustal source. Thermodynamic, major, and trace element modeling indicate that their parent magma may have been generated by water-deficient (~2 wt %) partial melting of ancient crustal material beneath the SW Tarim Craton rather than that of the Tianshuihai terrane, under high-temperature (T > ~950°C) and low-pressure (P = 5–8 kbar) conditions. Based on the tectonic framework of the WKOB, we propose that the original mantle and crust beneath the southern Kunlun terrane may have been modified or partially replaced by that beneath the SW Tarim Craton during the Ordovician. Therefore, this evidence for Tarim-derived magmatism, when combined with regional sedimentary and structural records, indicates that Ordovician magmatism in the southern Kunlun terrane is most consistent with episodic northward subduction of the Proto-Tethys Ocean, commencing at ~485 Ma. Middle Ordovician slab break-off can explain the formation of the A-type granites, but reinstated northward subduction is required for the generation of late Ordovician Datong syenites.

Formation of high-silica adakites and their relationship with slab break-off: Implications for generating fertile Cu-Au-Mo porphyry systems

In recent years, the characteristics and sources of fertile adakites has received considerable attention. As well, most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation, evolving to slab rollback, then slab break-off after collision (i.e. late- to post-collisional slab failure (arc-like magmatism) and transpression) has gained more recognition, although their relationship to each other has yet to be explored. The geochemical characteristics imply that adakites/adakite-like, in particular high-silica adakites (HSA), can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction, lithosphere thickening, and then failure (all late to post collisional), while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series (ADR) to form intraoceanic island arc to intracontinental margin arc systems, before the collisional stage. HSAs are characterized by high-silica (SiO2 > 67 wt.%), Al2O3 > 15 wt.%, Sr > 300 ppm, Y<20 ppm, Yb < 1.8 ppm, and Nb ≤ 10 ppm, and MgO < 3 wt.%, with high Sr/Y (>50), and La/Yb (>10). Some specific geochemical features, such as high Mg# (ave 0.51), Ni (ave 924 ppm), and Cr (ave 36 ppm), in HSAs are typical, in contrast to calc-alkaline arcs, although both groups display similar but less pronounced negative anomalies of Nb, Ta, and Ti in primitive mantle-normalized trace element spider diagram profiles. These unique geochemical features are likely ascribed to the involvement of garnet, hornblende, and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization (AFC) within the mantle and crustal during ascent in a transpressional collisional environment. Hypotheses for origin of HSA derivative from melting in convergent margins from young, hot oceanic plates subducting into the mantle is applicable to only some adakitic systems. The difference in geochemical characteristics of adakites compared to ADR, such as relative higher MgO, Cr, Cu, and Ni, are due to their slab source, as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle; altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements, as well as sulfates and sulfides. HSA magmas related to slab failure have special geochemical properties, such as Sr/Y > 20, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Gd/Yb > 2, and Sm/Yb > 2.5. Slightly higher Nb + Ta is due to high T melting of rutile. Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure (roll back to break-off). High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O, SO2, HCl, with chalcophile metals that remain incompatible at higher fO2 (low fH2); these situations happen in late- to post-collisional settings where the subducting oceanic crust experienced slab failure, resulting in advective heat addition to the system from upwelling asthenosphere. In such a slab failure setting, transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust. When oxidized slab melts interact with the subduction-modified lithospheric mantle, the resulting magmas stay oxidized, potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.

Carbonatites and related mineralization: an overview

Despite of their scarcity, carbonatites are documented from diverse geological settings, including cratons, continental rifts, orogenic belts and large igneous provinces, which range in age from the Archean to Recent. This study provides a concise overview of the evolution of igneous carbonatites, compiling data on their diverse characteristics and classifying them into mantle- and crustal-derived subtypes. Mantle-derived carbonatites, typically associated with alkaline rocks, are enriched in rare earth elements (REEs) and display geochemical affinities to the mantle. By contrast, crustal-derived carbonatites usually show much lower REE contents and crustal-affinity geochemical features similar to sedimentary carbonaceous rocks. Our study documents the significant economic potential of carbonatites that can host world-class deposits of REE, rare metals, alkaline earth metals and non-metallic resources. Typically, the key processes controlling REE mineralization include the low-degree melting of refertilized mantle, liquid immiscibility, fractional crystallization and hydrothermal fluids. Thus, carbonatites are of significant economic value due to the rapidly increasing global demand for REEs, mainly driven by the ‘green energy transition’.

Petrogenesis of magmatic charnockite-biotite granite suite from parts of the Chotanagpur Granite Gneissic Complex (CGGC), eastern Indian shield: Implication for the break down of the Columbia Supercontinent

A rare porphyritic charnockite that is girdled by and mineralogically grades to biotite granite occurs as a part of the Chotanagpur Granite Gneissic Complex (CGGC) in and around Massanjore, Jharkhand, India. Preservation of certain textural features, including (a) euhedral to subhedral grains of orthopyroxene, (b) low dihedral angle subtended by orthopyroxene and plagioclase grains, and (c) relict intergranular and porphyritic textures, are consistent with the view that orthopyroxene in the studied rocks has a magmatic origin. Preserved magmatic features and other petrological attributes of these rocks do not support any significant mass change beyond a few tens of microns during the overprinting high-grade metamorphism. The geochemical variation of the felsic rock suite (porphyritic charnockite and biotite granite) indicates that they are cogenetic and are derived from a ferroan A-type granitoid magma by crystal fractionation. The observed geochemical trend of the studied felsic rock suites has been simulated by phase equilibria modelling in the open system using the system components Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O2. The observed mineralogical and geochemical attributes and the results of the modelling study are consistent with a petrogenetic process in which high magma temperature (> 900 °C), low pressure, and low water activity in the parental melt favoured the separation of an orthopyroxene bearing cumulate assemblage (orthopyroxene + plagioclase + quartz + K-feldspar + ilmenite + magnetite) in the initial part of the magmatic differentiation. Removal of this anhydrous cumulitic assemblage raised the bulk H2O content in the residual melt. Orthopyroxene became unstable with respect to biotite in the evolved melt that eventually crystallised minerals that formed the biotite granite. An increase in magma fO2 also restricts the orthopyroxene stability in felsic magma. Taken together all the petrological and geochemical attributes, we demonstrate that fractionation of an orthopyroxene-bearing crystal cumulate from the melt is essential to form the charnockites, and that the biotite granite forms from the evolved melt after the fractionation. The charnockite-biotite granite association of the studied area was formed in an extensional tectonic setting, presumably during the breakdown of the Columbia Supercontinent.

Magma storage conditions of Lascar andesites, central volcanic zone, Chile

Abstract. Lascar volcano, located in northern Chile, is among the most active volcanoes of the Andean Central Volcanic Zone (CVZ). Its activity culminated in the last major explosive eruption in April 1993. Lascar andesites which erupted in April 1993 have a phase assemblage composed of plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides, and rhyolitic glass. To better constrain storage conditions and mechanisms of magmatic differentiation for andesitic magmas in a thick continental crust, crystallization experiments were performed in internally heated pressure vessels at 300 and 500 MPa, in the temperature (T) range of 900–1050 °C, at various water activities (aH2O) and oxygen fugacities (logfO2 between QFM+1.5 and QFM+3.3 at aH2O =1; QFM is quartz–fayalite–magnetite oxygen buffer). The comparison of experimental products with natural phase assemblages, phase compositions, and whole-rock compositions was used to estimate magma storage conditions and to reconstruct the magma plumbing system. We estimate that Lascar two-pyroxene andesitic magmas were stored at 975±25 °C, 300±50 MPa, and logfO2 of QFM+1.5±0.5, under H2O-undersaturated conditions with 2.5 wt % to 4.5 wt % H2O in the melt. The geochemical characteristics of the entire suite of Lascar volcanics indicates that a fractionating magmatic system located at a depth of 10–13 km is periodically replenished with less evolved magma. Some eruptive stages were dominated by volcanic products resulting most probably from the mixing of a mafic andesitic magma with a felsic component, whereas compositional variations in other eruptive stages are better explained by crystal fractionation processes. The relative importance of these two mechanisms (mixing vs. crystal fractionation) may be related to the amount and frequency of magma recharge in the reservoir.

Onset and tempo of ignimbrite flare-up volcanism in the eastern and central Mogollon-Datil volcanic field, southern New Mexico, USA

The Cenozoic ignimbrite flare-up (40–18 Ma) generated multiple volcanic fields in the southwestern United States and northern Mexico resulting from asthenospheric mantle upwelling after removal of the Farallon slab. The correlation of tuffs to one another and to source calderas within these volcanic fields is essential for determining spatiotemporal patterns in volcanism and magma geochemistry, which have been used to deduce migration of the Farallon slab at depth and associated mantle melting. However, the correlation of Eocene–Oligocene tuffs in the southwestern U.S. is difficult because of post-emplacement erosion and faulting. This study focuses on spatiotemporal patterns of the initial episode of ignimbrite flare-up activity (ca. 36.5–33.8 Ma) in the Mogollon-Datil volcanic field in south-central New Mexico, USA. We show that alkali feldspar major and trace element geochemistry is an effective tool for correlating tuffs when combined with high-precision, single-crystal 40Ar/39Ar geochronology and bulk-rock geochemistry. Using these data, we correlate several tuff units and differentiate other tuffs that have the same eruption age but very different geochemistry, and we conclude that there was a broadly northwestward migration in volcanism over time. The new tuff correlations are used to investigate spatiotemporal variations in magma geochemistry, erupted volumes, and recurrence intervals during the initial episode of Mogollon-Datil volcanic field volcanism. Early-erupted tuffs restricted to the eastern Mogollon-Datil volcanic field share similarities with western U.S. topaz rhyolites, which suggests that the silicic magmas were generated by partial melting of mafic lower crustal rocks. We also find differences in the compositions, crystallinities, and mineral assemblages between the early- and late-erupted tuffs. The early-erupted tuffs tend to have single-feldspar mineralogies, lower feldspar Or contents, large negative Eu anomalies, and low-whole–rock Ba concentrations. Conversely, late-erupted tuffs have two feldspar plus quartz assemblages, lesser Eu anomalies, higher whole-rock Ba concentrations, and feldspars have higher Or contents. Thus, we suggest that for some of the early eruptions, after magmas underwent crystal fractionation in the crust, the silicic melt largely separated from the crystalline mush prior to eruption, whereas late-erupted tuff magmas underwent crystal fractionation at near the eutectic minimum and were remobilized and erupted with a larger proportion of their crystalline mush. Using our new ages, correlations, and previously published data, we find that the initial phase of Mogollon-Datil volcanic field volcanism produced at least 15 eruptions between 36.5 Ma and 33.8 Ma, with a minimum total erupted volume of ~1350 km3 and an average recurrence interval of 170 k.y. However, eruptions were generally smaller in volume (most &amp;lt;15 km3) than in other coeval fields, and most eruptions (n = 11) occurred in the first 1.2 m.y. (ca. 36.5–35.3 Ma) of activity. Altogether, our work sheds new light on variations in the composition, timing, and migration of volcanism during the initial phase of Mogollon-Datil volcanic field activity and highlights the utility of feldspar geochemistry in both “fingerprinting” tuffs and elucidating magma evolution.

Petrography and geochemistry of aplites from the Seridó Pegmatite Province, NE Brazil: Petrogenetic Implications

In the Seridó Pegmatite Province (SPP) there are thousands of granitic pegmatite intrusions, including unzoned and barren, and zoned and mineralized ones. The whole rock chemical analysis of pegmatite is usually unfeasible because of its large grain size. However, fine-grained aplites syn crystallized with pegmatites should allow them to be used as proxies to assess pegmatite melt differentiation. We selected 15 representative aplite occurrences within unzoned pegmatite intrusions, including 7 intrusions with the grayish muscovite-aplite type and 8 with the reddish magnetite-aplite type. We also used for new trace element analysis samples of medium-to coarse-grained facies of host unzoned pegmatites. These pegmatites occur as NE-SW and N-S dykes and sills intruding mica schist and paragneiss from the Seridó Belt. Both aplite types contain along with quartz and feldspars accessory apatite, zircon, monazite, and xenotime. Tourmaline, muscovite, and garnet occur in the grayish aplites, whereas biotite and opaque minerals occur in the reddish aplites. The grayish aplites are slightly to strong peraluminous, while the reddish aplites are slightly metaluminous to slightly peraluminous. The reddish aplites and their host pegmatites are enriched in Ba, Sr, U and Th, while the grayish aplites and their host pegmatites are enriched in B, Li and Cs. The K/Rb ratios of the aplites varies from 11 to 44 with similar ranges in both reddish and grayish aplites. Normalized to the average upper continental crust composition, both aplite types present positive anomalies of Rb, Pb, U, and Hf, and negative anomalies of Ba, Sr and Ti. K, Nb and Ta show variable enrichment and depletion. They usually show HREE enrichment in relation to LREE and negative Eu anomaly, but positive Eu anomaly and tetrad patterns occur in few samples. In the normative QAP diagram the aplites show a wide range of compositions with a trend from quartz-plagioclase-rich tonalite to quartz-poor K-feldspars-rich alkali granite. Such trend is inverted in relation to the characteristic trends of melt differentiation by crystal fractionation. The albite/K-feldspar ratio of aplites shows a wide distribution range that would be related to the compositional variety of the partially melted protoliths and/or the variation in melting rate during prograde anatexis. The trace element compositions of the medium-to coarse-grained facies of the unzoned pegmatites are very similar to those of the coexisting aplites. The obtained data using aplites as a proxy for their host pegmatites precludes a unique line of melt descent at the origin of the SPP aplites and unzoned pegmatites and thus strongly support an anatectic origin from different sources.

Mechanisms of Co enrichment in the Zhubu Ni–Co–PGE deposit in the Emeishan LIP, SW China: Constraints from mineral geochemistry and Fe–S isotopes of sulfides

The Zhubu Ni–Co–PGE sulfide deposit in the Permian Emeishan Large Igneous Province (LIP), SW China is hosted by a mafic–ultramafic intrusion that comprises two morphologically distinct ore-forming spaces: a central, layered sequence that represents a magma chamber surrounded by a marginal sub-vertical conduit zone. The marginal conduit zone contains significant Ni–Co–PGE sulfide mineralization, suggesting that high volumes of sulfide-bearing silicate melt flowed through the magma conduit system. Determining the hosts of Co is key to understanding the Co enrichment process in the Zhubu deposit, as well as in the Emeishan LIP as a whole. We assess this here using the geochemistry and Fe–S isotope compositions of sulfides in the Zhubu deposit. Cobalt is mainly concentrated in pentlandite sulfide in the websterite, with Co contents increasing in the order pyrrhotite < chalcopyrite < pyrite < pentlandite < violarite; the abundance of violarite, however, is notably lower than that of the other sulfides. The Co contents of pentlandite, pyrite, and chalcopyrite are negatively correlated with their Ni and Fe contents, suggesting that Co substitutes isomorphically for Ni and Fe in the sulfide structure. The δ56Fe values of sulfides increase in the order pyrrhotite < chalcopyrite < pentlandite < pyrite resulted from kinetic fractionation of Fe isotopes. The δ56Fe values of pentlandite and chalcopyrite in lherzolite are higher than those in websterite, implying that lherzolite and websterite formed in compositionally distinct magmas, Fe isotope compositions become heavier with crystal fractionation. The δ34S values of sulfides increase in the order pyrrhotite < pentlandite < chalcopyrite < pyrite, similar to the fractionation sequence of sulfide melts, suggesting that externally derived S was added to the magma and contributed to its sulfide saturation. The Co contents of chalcopyrite and pyrite vary positively with δ56Fe values, and negatively with δ34S values, indicating that the addition of externally derived S and resulting sulfide segregation were critical to Co enrichment.

From melt- to crystal-rich magmatic systems during rift localization: Insights from mineral chemistry in Central Afar (Ethiopia)

Magmatism plays a key role in accommodating and localizing extension during continental breakup. However, how the crustal magmatic systems evolve at the continental-ocean transition is poorly understood. We address these questions by studying the evolution of the magmatic system in the rift of Central Afar (Ethiopia), currently marking the transition from continental rifting to oceanic spreading. We focus on the voluminous and widespread Upper Stratoid Series (2.6–1.1 Ma) and the following Central Afar Gulf Series (1.1–0.6 Ma), the latter corresponding to localization of volcanism in narrow magmatic segments. We carried out the first systematic study of major and trace element mineral chemistry for these two Series and integrated it with geothermobarometry estimates and geochemical modeling, to reconstruct the evolution of the magmatic system architecture during rift localization. The Upper Stratoid magmas evolved by fractional crystallization in a melt-rich, moderately zoned, middle-lower crustal (10–18 km) magmatic system, from where they rose directly to the surface. Polybaric plagioclase convection and dissolution of a plagioclase-rich crystal mush is recorded in the phenocryst texture and chemistry. The Central Afar Gulf magmas evolved at similar depth in a more complex and dynamic storage system, with magma rising and mixing through multiple, relatively small, crystal-rich and interconnected reservoirs. Our study documents the transition during the continental breakup, from an overall stable and melt-rich magmatic system feeding the voluminous and homogeneous Upper Stratoid eruptions to a more dynamic, interconnected and crystal-rich situation feeding small-volume eruption while the rift localizes.

Hydraulically linked reservoirs simultaneously fed the 1975–1984 Krafla Fires eruptions: Insights from petrochemistry

The 1975–1984 Krafla Fires in northeast Iceland was the first plate-boundary rifting episode to be tracked using seismic and geodetic monitoring. Geophysical observations from this episode have inspired conceptual models of magma transport during plate spreading, but a lack of complementary petrologic insights has hindered a holistic understanding of the events. To address this knowledge gap, we studied the petrochemistry of all nine Krafla Fires basaltic eruptions. Our large dataset of new whole-rock, matrix glass and mineral analyses from samples collected during or shortly after each eruption reveal a clear compositional bimodality in the erupted magmas that persisted across the episode, with evolved quartz tholeiite (MgO = 5.7–6.4 wt.%) erupted inside Krafla caldera, and more primitive (usually olivine-normative) tholeiite (MgO = 6.4–8.7 wt%) erupted north of the caldera margin. Barometric calculations indicate tapping of these magmas from distinct reservoirs: a primitive lower-crustal reservoir at a most probable depth of ∼14–19 km, and a more evolved, shallower reservoir at a most probable depth of ∼7–9 km beneath the caldera. These reservoirs were tapped simultaneously in several of the nine eruptions, and in three events the two magma types mixed near the northern caldera margin. Varying levels of trace element depletion in the deep-sourced primitive melts reflect incomplete mixing of diverse mantle-derived melts at depth; the most enriched of these melts could be parental to evolved inside-caldera magma via fractional crystallization. Clinopyroxene rims on gabbroic nodules from primitive September 1984 lavas record lower crustal pressures, while diffusion models suggest that these rims grew up to within a few months before eruption. Ascent of the primitive magma from the lower crust thus occurred over timescales much shorter than eruptive repose periods, without prolonged stalling at shallow depths. These observations are inconsistent with the view that the eruptions were entirely fed by lateral magma outflow from the shallow reservoir. They instead require some decoupling of the flow paths of the two magma types: the primitive magma either bypassed the sub-caldera reservoir laterally or ascended vertically beneath the northern vents. The two reservoirs nonetheless shared a hydraulic connection and jointly responded to rifting. Comparison of the Krafla Fires with other rifting events and eruptions highlights the complexity and diversity of magma transport during plate boundary rifting events, which is not yet captured by a generalizable model. Integration of petrologic, geochemical and geophysical data is essential to provide a holistic view of future rifting events in Iceland and at other spreading centres.

Diverse magmatism along the northern margin of Tarim during the Ediacaran: Transition from Rodinian dispersing to Gondwana assemblage

The Ediacaran igneous rocks and sedimentary sequences play a key role for our understanding the transition from dispersing of the Rodinian continents to the assemblage of the Gondwana. On the other hand, some continental massifs at the marginal area of the Rodinia and Gondwana could preserve solid evidences for deciphering the geodynamic background from the break-up of Rodinia to the assemblage of Gondwana. In this contribution, we report field observations, petrography, ages and systematic geochemistry of the diverse Ediacaran igneous activities in the northern margin of the Tarim Block, which was favored as a marginal continental massif in the Rodinia configuration by most geologists. The Ediacaran igneous rocks along the northern margin of Tarim include the Baicheng and Yangxia granites, Tiemenguan syenite, Kurle mafic dykes and the basalt layers in the Zamoketi Formation (northeastern Tarim) and the Sugetbrak Formation (northwestern Tarim). Precise zircon UPb dating revealed that these distinct rocks were broadly coeval with crystallization/eruption ages of 630–600 Ma. Bulk-rock elemental and SrNd isotope compositions as well as zircon LuHf isotope compositions demonstrate that the Baicheng and Yangxia granites were derived from Neoarchean mafic lower crust and underwent intensive fractionation before their emplacements. The Tiemenguan syenites have potassic andesitic compositions, in combination with their pronounced enriched NdHf isotopes and extremely high Zr saturation temperatures (∼900 °C), we argue that their possible shoshonitic primary magmas were derived from previously metasomatized subcontinental lithospheric mantle source (amphibole-bearing) and then, coupling with assimilation and fractional crystallization (AFC) effects in magma chamber and/or en route to the emplacement space. This can account for their intriguing geochemical features. The ca. 615 Ma Zamoketi and Sugetbrak basalts and the slightly younger porphyrite dykes (ca. 580 Ma) cutting the Zamoketi basalt layer, show typical OIB-like geochemical signatures, they were most likely derived from a depleted asthenospheric mantle source with variable fractionation of olivine and pyroxene and negligible crustal contamination before eruption. On the whole, the geochemistry of the diverse Ediacaran igneous rocks unambiguously demonstrates that they were genetically related to an extensional environment. In combination with the significant passive continental features of the Ediacaran-Cambrian sedimentary sequences, we consider that these Ediacaran igneous rocks were the latest phase of igneous activity related to the dispersing of the Tarim at the margin of the Rodinia supercontinent.The dispersing of the Rodinian supra-continent was diachronic with the assemblage of the Gondwana. Late Tonian to Ediacaran dispersing of the Tarim from northern margin of Australia induced the opening of the Proto-Tethys Ocean. Then the initiation of the southward subduction of the Proto-Tethys Ocean at ca. 530 Ma (Fortunian) led to the Tarim assemblage to Gondwana at ca. 440 Ma.

Revisiting magnesium isotope constraints on the petrogenesis of the Fe-Ti oxide-bearing mafic–ultramafic intrusions

Stable isotopes of metals, such as magnesium (Mg) and iron (Fe), typically undergo minimal fractionation at magmatic temperatures. However, in Fe-Ti oxide-bearing intrusions, significant Mg isotope fractionation (Δ26Mg values of up to 23 ‰) has been observed between silicates and Fe-Ti oxides. The mechanism responsible for this substantial fractionation remains unclear. This study presents Mg isotopic compositions for bulk rocks and mineral separates—including olivine, clinopyroxene, titanomagnetite, ilmenite—from sixteen Fe-Ti oxide ores in both the lower and the upper zones of the Baima layered mafic–ultramafic intrusion, as well as bulk rocks from three high-Ti basalts of the Emeishan large igneous province, SW China. Oxide ores from the lower zone and the lower section of the upper zone of the Baima intrusion are characterized by a low abundance of volatile-bearing minerals, such as apatite and hornblende. These ores exhibit δ26Mg values ranging from −0.30 to −0.14 ‰ for olivine, −0.26 to −0.06 ‰ for clinopyroxene, 0.16 to 0.60 ‰ for titanomagnetite and −0.33 to −0.21 ‰ for ilmenite, indicative of near-equilibrium fractionation. In contrast, oxide ores from the upper section of the upper zone exhibit a high abundance of volatile-bearing minerals and significantly different δ26Mg values, ranging from −0.19 to −0.05 ‰ for olivine, −0.25 to −0.10 ‰ for clinopyroxene, 0.57 to 0.60 ‰ for titanomagnetite and 0.86 to 2.34 ‰ for ilmenite, indicative of disequilibrium fractionation. A negative correlation between Fo content and δ26Mg value in olivine suggests that Mg isotopes fractionate as olivine undergoes fractional crystallization. Trapped liquid fractions, calculated based on Ce contents in apatite, suggest that rocks with high volatile-bearing mineral content have undergone re-equilibration with trapped liquids during cooling. The large disequilibrium Mg isotopic fractionation between silicates and Fe-Ti oxides is likely due to sub-solidus re-equilibration between minerals and trapped liquids. We propose that the extent of sub-solidus, diffusion-driven kinetic Mg isotopic fractionation in Fe-Ti oxide-bearing intrusions likely correlates with the volume of trapped liquids, which may serve as a significant medium accelerating ion exchange between silicates and Fe-Ti oxides. Moreover, mass balance calculations indicate that data of Mg isotopes on their own are insufficient to determine the petrogenesis of Fe-Ti oxides in layered intrusions. This insufficiency is primarily due to the low MgO content in titanomagnetite and ilmenite, where their separation causes minimal Mg isotopic fractionation of the melts.

Deciphering the role of FI catalyst’s C3 substituent on the non-isothermal crystallization kinetics of nascent disentangled UHMWPE

UH1 - UH4 were accessed using tetra-fluorinated phenoxy-imine catalysts (FI catalysts) with linear (allyl, n-propyl, 1-propenyl) or more steric demanding (isopropyl) C3 substituents ortho to the phenoxy-oxygen atom. To get a better understanding of the non-isothermal crystallization kinetics of these nascent ultra-high molecular weight polyethylenes (UHMWPEs), their thermo-rheological properties are investigated. Through the application of annealing-crystallization-melting thermal analysis and isoconversional method, it is verified that the samples are nascent disentangled UHMWPEs with higher crystallization rates compared to the commercial UHMWPE (CUH). The molar fraction of nascent entangled crystals and/or degree of supercooling have a significant influence on the non-isothermal crystallization kinetics of nascent disentangled UHMWPEs. Finally, under a lower supercooling, the electron-donating property and steric hindrance of C3 substituent strongly affect the synthesis and architecture of polyolefin which in turn has a specific non-isothermal crystallization kinetics.

Petrography and Geochemistry of the Farchana-Hadjerhadid Granitoids (Ouaddaï Massif, Eastern Chad)

Work in the Farchana-Hadjerhadid sector is focused on petrography, with the main aim of mapping the geological formations of the eastern Ouaddaï Massif in order to identify petrographic types and gain an understanding of the geodynamic history of this area. The fieldwork showed that the Farchana-Hadjerhadid granitoids consist mainly of granites and diorites and are intersected by numerous pegmatite veins. Microscopic observation shows that the diorite is made up of plagioclase, amphibole, biotite, and quartz. The granite is composed of alkali feldspar, quartz, amphibole, plagioclase, and biotite. The granitoids in the study area are moderately differentiated (SiO2 = 55.62% to 71.67%) and belong to the medium to highly potassic calc-alkaline series. They are ferriferous type I metaluminous granitoids (A/CNK &lt; 1.1). The parent magma was derived from the partial melting of the metabasalts, and the petrographic types evolved by assimilation and fractional crystallization.

Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sri (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sri = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

Occurrence and enrichment of cobalt and nickel in the Yindongshan ultramafic–mafic intrusion-hosted iron deposit, western Hubei Province, China

The Yindongshan iron deposit in western Hubei Province of China is hosted within Ordovician ultramafic–mafic intrusions. Recent field geological surveys have detected cobalt (Co) and nickel (Ni) enrichment within the Yindongshan clinopyroxenite, suggesting good potential for subeconomic Co-Ni mineralization. This study investigated the occurrence modes and enrichment processes of Co and Ni in the Yindongshan deposit through field geology and in situ analysis of the rock texture, geochronology, and geochemistry of sulfides, Fe-Ti oxides, and silicate minerals. The Yindongshan clinopyroxenites were emplaced at 440.2 ± 7.2 Ma via titanite LA-ICP-MS U-Pb dating, and underwent extensive post-magmatic metamorphism at 401.2 ± 8.6 Ma via apatite U-Pb dating. This metamorphism led to the widespread formation of amphibole via the replacement of clinopyroxene. Subsequently, hydrothermal epidote-albite veins, calcite-pyrite veins, and later actinolite veins developed. Pyrites from both clinopyroxenites and calcite veins show limited δ34S variations from −5.4 ‰ to −1.2 ‰, suggesting a magmatic sulfur source. Late-vein actinolites display geochemical characteristics distinct from those of amphiboles, indicating possible involvement of external fluids. LA–ICP–MS trace element results reveal a general increase in Co and Ni contents from silicate minerals and Fe-Ti oxides to pyrite and from earlier-crystallized coarse-grained clinopyroxenite to later medium- to fine-grained clinopyroxenite. The highest Co and Ni contents observed in pyrite from coarse-grained clinopyroxenite suggest their preferential incorporation into early sulfide minerals and then progressively depleted through magmatic evolution such as fractional crystallization. The positive correlation between Co and Ni with Fe in pyrite, along with the consistent parallel time-resolved LA–ICP–MS depth profiles among different mineral phases, indicates that isomorphic substitution occurred in pyrite. In silicate minerals, the Co and Ni contents increase from pyroxene, amphibole, to actinolite, with almost no Co or Ni present in epidote. The elevated Co and Ni contents in metamorphic amphiboles are attributed to their remobilization from magmatic pyrite to newly formed amphiboles. During the late hydrothermal phase, external fluids might transport additional Co and Ni, contributing to higher concentrations in late actinolite veins. In general, Co and Ni in the Yindongshan iron deposit are primarily hosted in sulfide minerals and can be remobilized during regional metamorphism and metasomatism. Thus, magmatic iron or iron–titanium oxide deposits hosted by mafic–ultramafic intrusions with sulfide mineralization may serve as promising targets for further Co-Ni exploration.

In situ geochemistry of apatite: Petrogenetic and tectonic interpretations of Jurassic felsic magmatism in the Yanbian area, NE China

Geochemical analyses of individual minerals provide more detailed insights into the petrogenesis of igneous rocks than whole-rock analyses. This study conducted in situ geochemical and Nd isotope analyses of apatites from 10 Jurassic granitic plutons in the Yanbian area, NE China, to establish the petrogenesis and regional tectonic evolution. The results indicate that the apatite geochemistry of Jurassic granitoids in the Yanbian area was controlled primarily by the composition of parental melt. Post-magmatic alteration may lead to geochemical decoupling between apatite and parental melt, while Nd isotopes exhibit some resilience to such alterations. Apatites from Early Jurassic granitoids display characteristics that are consistent with an I-type origin, whereas those from Middle and Late Jurassic granitoids exhibit an adakitic affinity. Variations in apatite compositions indicate the fractional crystallization of other rare earth element (REE)-bearing minerals during magma evolution. The early crystallization of plagioclase and allanite led to decreases in Sr and Th contents in apatite, respectively, resulting in a negative Eu anomaly and light REE depletion. The fractional crystallization of titanite and hornblende resulted in the depletion of middle REE in apatite. Hornblende is regarded as the main residual phase in the magma source of Middle and Late Jurassic adakitic granitoids in the Yanbian area. Apatite Nd isotopic compositions suggest that the Jurassic granitoids in the Yanbian area originated from two crustal sources: the Central Asian Orogenic Belt and the North China Craton. Additionally, increasing trends in apatite Sr/Y and (La/Yb)N ratios from the Early to Late Jurassic suggest a gradual thickening of the regional crust, which is likely driven by the continentward migration of the subduction zone associated with the Paleo-Pacific Plate.

Non-typical fractionation behaviors in a lepidolite-subtype pegmatite: Implications for the internal evolution of a naturally fluxed system

Despite their relatively rare occurrence, lepidolite-subtype pegmatites host abundant Li–Nb–Ta–Cs–Sn mineralization and represent a high-flux pegmatitic system with abnormally high F and Li activity. Characterization of highly fluxed melts and the impact of fluxes and exsolved fluids on fractionation of peraluminous melts have mainly been studied in experimental systems, with natural system correlations remaining poorly understood. Consequently, we conducted a systematic mineralogical study of a lepidolite-subtype pegmatite in the North Qinling orogenic belt, Central China. An abnormal “concave downward” fractionation trend for primary columbite-group minerals on the quadrilateral diagram is identified, and irregularly zoned columbite crystals coexist with F-rich minerals in one of the core zones have the highest Ta contents (normally 50.17–63.13 wt% Ta2O5) and Ta/(Nb + Ta) ratios (up to 0.65). Despite the consistently Ta-dominated B-site in the crystal lattice of microlite-group minerals, extreme compositional variations at the A- and Y-sites are observed. Compared with microlites in intermediate zones, the abrupt increase in U in microlite crystals in core zones and late units (up to 20.16 wt% UO2), is ascribed to the melt-fluid interaction with exsolved U-rich aqueous fluids. In addition, the fractional crystallization of F-bearing minerals resulted in a gradual decrease in F contents in microlite-group minerals from extremely F-rich (2.68–4.84 wt% F) in intermediate zones to low F species (mainly 0.82–1.71 wt% F) in core and late zones. Moreover, autometasomatism by a late fluxed melt and hydrothermal metasomatism by late aqueous fluids are identified in columbite- and microlite-group minerals. This work highlights that these non-typical fractionation behaviors related to the activity of fluxes (especially F) and the exsolution of aqueous fluids during the internal evolution of pegmatitic melts, are critical for the generation of lepidolite-subtype pegmatites. Fluorine was gradually enriched in the pegmatitic melt, and reached its highest level during crystallization of the (inner) intermediate and core zones. Non-equilibrium crystallization occurred throughout pegmatite evolution, and late units were most probably formed from aqueous fluid-enriched residual melts, rather than by hydrothermal replacement.