Melting Of Crust Research Articles

Ascent-driven differentiation: a mechanism to keep arc magmas metaluminous?

Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

Petrogenesis of early Cretaceous granites in Mufushan complex, northeastern Hunan Province: Insights on middle-late Mesozoic tectonic evolution of eastern South China

We present new field observations, whole-rock major and trace elements, Sr-Nd-Pb-Hf isotopes and zircon UPb ages for the Mesozoic granites in the Mufushan complex, eastern South China. The age determinations by LA-ICP-MS UPb dating of zircons indicate that the porphyritic biotite granites and two-mica monzogranites have ages of ∼136 Ma and ∼ 130 Ma, respectively. These granites display highly evolved S-type granite geochemical characteristics, with high SiO2 (70–74 wt%) contents, high 10000*Ga/Al ratios (2.3–3.2, average of 2.8) and weakly peraluminous (A/CNK > 1.1). Both types of granites are enriched in large ion lithophile elements (LILEs, e.g., Rb, K, Ba, Sr) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti), with negative anomalies of Eu and positive anomalies of Pb, which are akin to continental arc rocks. The two-mica monzogranites have high 87Sr/86Sr(i) (0.71355–0.71535), 206Pb/204Pb(i) (18.105–18.157), low εNd(t) (−9.1 to −9.6) and εHf(t) (−4.1 to −10.4), similar to those of the porphyritic biotite granites which are characterized by high 87Sr/86Sr(i) (0.717337–0.71692), 206Pb/204Pb(i) (18.134–18.580), low εNd(t) (−9.1 to −9.7) and zircon εHf(t) (−6.8 to −3.3). The similar emplacement ages and mineral assemblages as well as Sr-Nd-Hf isotopic signatures indicate their derivations were likely derived from a common magma source. These early Cretaceous granites yield two-stage Nd model ages of 1.6–1.7 Ga, similar to the two-stage Hf model ages of 1.3–1.8 Ga. The presence of Neoproterozoic and subordinate Mesoproterozoic zircons within the granitic rocks suggest a significant contribution of ancient crustal materials involved in the genesis of the magmas. In combination with comprehensive studies on the magmatic rocks, the early Cretaceous Mufushan granitic rocks may have experienced an extensive enrichment process, contemporaneously with partial melting of the Neoproterozoic volcanic-sedimentary sequences that produced the arc-like rocks. Almost simultaneously, an extensional event caused by the subduction and slab roll-back of the Paleo-Pacific plate (Izanagi plate) dominates the middle-late Mesozoic tectonics in eastern South China. This extensional regime was accompanied by the underplating of mantle-derived magma and crustal melting, producing the widespread early Cretaceous granites in adjacent regions. The development of coeval basin-and-range structures with the production of contemporaneous magmatic rocks and related polymetallic deposits in eastern South China are all related to slab roll-back tectonics.

Odyssey of Archaean crustal evolution of the Southern Indian shield: An insight from petrogenesis of distinct Mesoarchaean and Neoarchaean trondhjemite to granite suites from the northwestern margin of the Dharwar Craton, India

Tectonic setting and crustal evolutionary processes recorded in the Meso–and Neoarchaean crust in the northwestern margin of the Dharwar Craton are discussed in this paper based on the geochemistry of 2.97 Ga granite from Kumta-Manjuguni-Kakkalli (KMK) and ≤ 2.65 Ga trondhjemite to granite near Ramnagar-Khanapur (RK). Both the plutonic suites are potassic (total alkalies∼8.9%), high in silica (SiO2 ∼ 71.9 wt%), and metaluminous to weakly peraluminous. The granite from KMK is characterized by moderately fractionated rare earth elements (REE) pattern (Lan/Ybn < 14), unfractionated heavy rare earth elements (HREE) and significant negative Eu/Eu* anomalies (0.2–0.7). The RK granite has fractionated LREE/HREE (20–41) with Eu/Eu* of 0.6–1.07 while the trondhjemite and granodiorite have LREE/HREE of 25–33 and Eu/Eu* between 0.67 and 0.74. The geochemical characteristics of KMK granite suggest derivation from intracrustal melting of TTG type crust and metasedimentary rocks whereas RK granite was derived predominantly from intracrustal melting of felsic crust and high -K basaltic rocks. While the RK trondhjemite was generated from the partial melting of metabasalt.Integration of the present data with those of other granitoids of the WDC indicates a five stage crustal growth, whereby K-rich KMK granite magmatism represented an early crust building cycle with crustal consolidation at ∼3.0 Ga, while the RK granitoids represent the younger and main orogenic cycle at 2.65–2.50 Ga. A similar style of episodic multi-stage crustal growth during the Archaean was common in major cratons globally involving plate convergence and accretion events and supercontinent amalgamation.

Tracking a magmatic arc within a confined orogen: New evidence from the Araçuaí orogen (SE Brazil)

By comparison with igneous rock assemblages of well-exposed modern magmatic arcs and their typical calc-alkaline, magnesian, I-type signatures, many deeply eroded magmatic arcs have been characterized worldwide, unraveling ancient plate margins. Since the 1980's, different assemblages of Ediacaran to Cambrian magmatic rocks have been systematically mapped and characterized according to their distinctive petrographic, structural, lithochemical, geochronologic, and isotopic features, allowing to establish the pre-collisional, collisional, and post-collisional stages of the Araçuaí orogen (SE Brazil). Among them, the tonalitic-granodioritic batholiths and stocks with calc-alkaline, magnesian, I-type signatures, rich in dioritic to mafic enclaves, and related metavolcano-sedimentary successions compose a magmatic arc formed in the early Ediacaran, the Rio Doce magmatic arc. Extending for more than 1000 km along the Araçuaí-Ribeira Orogenic System (AROS), the Rio Doce arc shows a hybrid isotopic signature and crustal inheritance, compatible with modern magmatic arcs built on continental active margins. We present new field, petrographic, and lithochemical studies, coupled with U–Pb (LA-ICPMS) geochronology and isotopic (in-zircon Lu–Hf and whole-rock Sm–Nd) analysis, unraveling the least evolved isotopic signatures ever found for tonalitic-granodioritic plutons of the Rio Doce magmatic arc, and a comprehensive data comparison with modern magmatic arcs. The studied Santa Tereza and Jequitibá suites comprise tonalitic-granodioritic and Opx-bearing charnockitic rocks, with calcic-to-alkali-calcic-magnesian I-type signature, associated with gabbroic rocks. Zircon U–Pb data yielded Concordia ages of 612 ± 3 Ma (Santa Tereza) and 594 ± 3 Ma (Jequitibá) for magmatic crystallization, and 580 ± 5 Ma to 571 ± 3 Ma for metamorphic recrystallization, with associated εNd(612Ma) values from −0.06 to −7.05 (TDM model ages: 1.4–2.04 Ga), and εNd(594Ma) values from −6.67 to −8.8 (TDM model ages: 1.64–1.87 Ga). The zircon εHf(t) (+2.3 to −4.8) and Hf TDM model ages (1.33–1.65 Ga) assigned a moderately juvenile to evolved signature for the Santa Tereza suite, while the Jequitibá suite (εHf(t): −1.7 to −36.4; TDM model ages: 1.33–3.21 Ga) shows a more evolved signature. Altogether, data from the Rio Doce arc are similar to typical continental-margin magmatic arcs with the involvement of mantle melts and/or melting of juvenile crust, suggesting a westward slab retreat during ocean-floor subduction within the Araçuaí confined orogen.

P–T–t conditions of Early Palaeozoic low‐P high‐T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam

AbstractHigh‐grade metamorphic rocks are widely exposed along the SE–NW‐ to E–W‐trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high‐grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse‐grained granulite mineral assemblages, defining a tight clockwise P–T path with near‐isobaric heating to a near ultrahigh‐temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse‐grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K‐feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post‐peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium‐pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine‐grained kyanite‐bearing mineral associations. Petrographic observations indicate that these fine‐grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post‐collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

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

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

Paleoproterozoic U Mineralization in Huayangchuan Deposit, Xiaoqinling Area: Evidence from the U–Rich Granitic Pegmatite

The Huayangchuan uranium deposit, located in the west of the Xiaoqinling belt on the southern margin of the North China Craton, is a large U–Nb–Pb deposit accompanied with rare–earth elements. The Huayangchuan uranium deposit, discovered in the 1950s, has long been known as a carbonatite–type uranium deposit. Recently, new geological work has found uranium mineralization in many granitic pegmatite veins in the Huayangchuan deposit and adjacent areas. Here, we report a systematic investigation of the petrography, whole–rock geochemistry, zircon U–Pb ages, and in situ Lu–Hf isotopic characteristics of newly discovered U–rich granitic pegmatite veins in the west of Huayangchuan deposit. The petrological results showed that the lithology of the samples is granite pegmatite. The U–Pb ages of zircon were 1826.3 ± 7.9 and 1829 ± 11 Ma. Microscopically, the paragenetic characteristics of zircon, betafite, and uraninite exist in the intergranular fissures of K–feldspar and quartz, reflecting metallogenic phenomena in the rock formation process. Almost all whole–rock samples were rich in SiO2 (64.37−70.69 wt.%), total alkalis (K2O + Na2O = 8.50–10.30 wt.%), and Al2O3 (12.20–14.41 wt.%) but poor in TiO2 (0.23–0.73 wt.%), MgO (0.38–0.90 wt.%), CaO (1.23–2.22 wt.%), P2O5 (0.14–0.83 wt.%), and MnO (0.04–0.57 wt.%). Additionally, they showed enrichment of LILEs (such as Rb, Ba, Th, U, and K), depletion of HFSEs (such as Ta, P, Ti, and Hf), and no alkaline dark minerals, and the characteristics are intraplate A1–type granite. The A1–type granite displayed low zircon εHf(t) values (−19.42–−15.02) with zircon two–stage Hf model aged 3.10–2.76 Ga, indicating that the U–rich granitic pegmatite was derived predominantly from partial melting of the ancient continental crust (such as the early Taihua group formed in Archean–Neoarchean). Combined with the above results and regional geological data, the U–rich granitic pegmatite discovered in the Huayangchuan deposit was formed in a post–collisional regime after the Luliang movement in the late Paleoproterozoic. This study suggests that future uranium prospecting work in this area should focus on late Paleoproterozoic U–rich granitic pegmatites.

Granitoid characteristics of basement to the U-mineralized Bhima basin: Implications for crustal epizone processes

The Dharwar Craton (DC) is an archive containing a good proportion of ancient records of crustal evolution in its rocks. The greenstone-gneiss-migmatite and granitoid complex show significant variation in terms of time, space and mechanism of emplacement according to tectonics and geodynamic set up. The plutons of the northernmost part of Eastern Dharwar Craton (EDC) are especially recognizable from typical litho-structural attributes. The intensity of regional/burial metamorphism is negligible in these granitoids with versatile nature. Simple and composite plutons are demarcated on the basis of field association with distinguishable petro-mineralogy and geochemical characters. K-metasomatism in pluton is established from petrography with supportive geochemistry. Simple pluton is syenitic in nature unlike the composite plutons with lithological variation from Tonalite-Trondhjemite-Granodiorite (TTG) to alkali feldspar granite. The granitoids are developed from crustal melting during the plate convergence. Post-orogenic nature of syenite is indicated from geochemical data. The sharp contact between the plutons and country rocks indicates significant temperature difference between country rocks and emplaced magma. The characteristic features like brittle deformation dominated structures, sharp contact between emplaced plutons and country rocks, very low grade of regional metamorphism, intensely differentiated nature and absence of diatexitic migmatite indicate a shallow crustal depth of plutonic emplacement. Higher concentration of large ion incompatible elements (e.g., U, Th, K) and associated high heat production imply shallow crustal segment with rocks derived from evolved differentiated magma. The study indicates a depth less than 8 km and temperature condition less than 250°C which correspond to epizone, where uranium remobilization is facilitated.

Initiation of Mesoarchean nascent subduction zone in the North China Craton

Exploring the nature of Archean plate tectonics is crucial for understanding the geodynamic evolution of early Earth. While previous studies have focused on (ultra-)mafic metavolcanic and tonalite−trondhjemite−granodiorite (TTG) rocks, the crust-mantle interactions of Archean intermediate magmatism have received much less attention. Mesoarchean (ca. 2936−2855 Ma) dioritic and tonalitic-trondhjemitic magmatism occurred in the Jiaobei terrane, North China Craton. While low-silica (65.67−68.82 wt%) TTG magmatism persisted throughout the entire magmatic period, both the high-silica (69.79−74.54 wt%) TTG and biotite dioritic (MgO of 1.52−2.45 wt%) magmatism were transient (ca. 2922−2901 Ma). They were mainly sourced from metabasaltic crust, though the biotite dioritic and low-silica TTG rocks show variable mantle input. In comparison, the hornblende dioritic magmatism (MgO of 2.07−5.00 wt%) occurred later (ca. 2912−2865 Ma) and was derived from melt-metasomatized mantle sources. Spatially, the magmatism became younger from northeast to southwest. Combined with increasing crustal melting pressures after ca. 2920 Ma and mildly elevated zircon δ18O (up to +6.8‰) of TTG rocks, a Mesoarchean nascent subduction zone is proposed for the Jiaobei terrane that is featured by the recycling of supracrustal basaltic rocks to thickened lower crust (forming high-pressure and high-δ18O TTG) and subsequent melt-related mantle metasomatism (forming relatively high-Mg hornblende dioritic rocks). The recycled supracrustal materials may have lubricated the basal part of the thickened crust and then triggered the opening of a nascent mantle wedge. Further compilation of data from global Archean dioritic and TTG rocks reveals chemical discontinuities at ca. 3.6−3.5 Ga and after ca. 3.0 Ga; these periods are marked by the occurrence of both high-Mg (median MgO &amp;gt;3 wt%) dioritic magmatism and high-(La/Yb)N (median &amp;gt;30) and high-δ18O (median &amp;gt;5.9‰) TTG magmatism. These data, along with independent geological evidence, suggest that the nascent subduction processes may have initiated locally at ca. 3.6−3.5 Ga but globally after ca. 3.0 Ga.

Petrogenesis of the late Neoproterozoic Carmo stock, northeastern Brazil: Implications for partial melting of oceanic crust and sediments in a syn-collisional setting

Granitic magmatism is undoubtedly the main geological feature of the Brasiliano Orogeny in the Borborema Province, northeastern Brazil. However, the petrogenesis and tectonic setting of the oldest magmatic pulse (640–620 Ma; Conceição-type granites) in the Central Subprovince is still hotly debated. The Carmo stock consists of calc-alkalic magmatic epidote-bearing Conceição-type porphyritic granodiorite and monzogranites that carry abundant mafic microgranular enclaves (MMEs). LA-ICP-MS zircon U–Pb data indicate crystallization around 615 Ma for these granitoids. Granitoids and MMEs exhibit similar chemical characteristics, both of which show high-K calc-alkalic, metaluminous and magnesian compositions. All studied rocks are enriched in large-ion lithophile elements (LILE) and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE) and heavy rare-earth elements (HREE). Host granitoids are characterized by high initial 87Sr/86Sr ratios (0.707863–0.709677), weakly negative ε Nd(t) values (−2.1 to −3.1), and TDM model ages of 1.31–1.36 Ga. These geochemical and isotopic signatures, together with the low Ba/Th and Sm/La ratios and high La/Sm and Th/La ratios, suggest that granodiorites and monzogranites were generated from the partial melting of altered oceanic crust and subducted sediments. The MMEs have Sr–Nd isotopic compositions almost indistinguishable from their host granitoids, with initial 87Sr/86Sr ratios of 0.708895, ε Nd(t) values of −2.8 and TDM model ages of 1.40 Ga. The similarity in isotopic compositions and the petrographic evidence indicate that magma mixing and mingling played an important role in the formation of MMEs. Based on the new geochemical and isotopic data, together with regional geological features, we infer that the Carmo stock rocks, as well as Conceição-type granites, were most likely formed in a syn-collisional tectonic setting following the early break-off of a relatively weak oceanic slab.

Paleozoic and Mesozoic magmatism in the Gaodi porphyry Mo-Cu deposit: Implications for the evolution of the Mongol-Okhotsk Ocean in the northern Great Xing'an Range, NE China

The newly discovered "460 Gaodi porphyry Mo–Cu deposit" (with Re-Os age of 179.6 ± 3.6 Ma) is located 30 km west of Hanjiayuanzi town in the northern Great Xing’an Range (GXR), NE China. The intrusive rocks in the deposit area can be classified into four periods: (1) Early Ordovician, which includes a high-K calc-alkaline monzodiorite (480 Ma) indicating significant crustal contamination, and a “syntectic” I-type granodiorite (479 Ma) with low Mg# values (34–38) and εHf(t) values (−4.4 to +0.9) show mixing of mantle-crust derived materials, which is related to post-orogenic event between the Ergun and Xing’an blocks; (2) Late Triassic, which includes an adakitic granodiorite porphyry (220 Ma) related to partial melting of the lower part of a thickened crust (C-type adakitic) with low Na2O/K2O ratio (1.27–1.49), Cr and Ni contents, with εHf(t) values of −0.7 to +3.3; (3) early Early Jurassic, which includes a differentiated I-type monzogranite (198 Ma) and syenogranite (196 Ma) with negative Eu anomaly (0.06–0.74), low Mg# (9–37) and high Rb/Sr (0.84–5.07). The εHf(t) values of them are −3.3 to 1.8 and −3.0 to +2.3, respectively, indicating the same source areas originated from partial melting of juvenile lower crust; (4) late Early Jurassic, which includes an ore-related C-type adakitic granite porphyry (182 Ma) with relative low Mg# (36–38), and low Na2O/K2O (0.65–0.81), strongly peraluminous quartz porphyry (181 Ma) with highly differentiated characteristics, and post-collisional gabbroic diorite (180 Ma). The ore-related granite porphyry yields εHf(t) values ranging from −4.1 to −0.4, suggesting partial melting of juvenile lower crust. The 460 Gaodi deposit formed in a late-collisional setting related to the Mongol-Okhotsk Ocean system. A post-collisional stage about 182 to 163 Ma immediately following the main collision is proposed.

Petrogenesis and tectonic setting of Early Permian–Late Triassic granitoids in the Qinling Orogenic Belt: Constraints from petrology, geochemistry and zircon U–Pb–Hf isotopes

The widely distributed Late Hercynian–Indosinian granites in the West Qinling Orogenic Belt are keys to better understand the tectonic–magmatic evolution of the West Qinling Orogenic Belt. In this paper, granitoids from the Zhacanggou area of Guide Basin in the western section of the West Qinling Orogenic Belt were studied on petrography, geochemistry, zircon U–Pb dating, and Lu–Hf isotopes. Zircon U–Pb dating yield granitoids in the Zhacanggou area were emplaced at 228.3 ± 4.4 Ma, 265.2 ± 2.2 Ma and 277.8 ± 2.5 Ma, respectively. The whole-rock geochemical compositions of these granitoids belong to the weakly peraluminous calc-alkaline to metaluminous high-K calc-alkaline series, and exhibit different degrees of negative Eu anomalies, loss of high-field strength elements (Nb, P and Ti), and enrichment of large ion lithophile elements (Rb, Th, U and Sr). The εHf (t) values of Early–Middle Permian and Late Triassic granitoids range from −12.0 to 5.3 and –10.6 to −5.0, and corresponding two-stage model ages (TDM2) of 956 to 2051 Ma and 1581 to 1936 Ma, respectively. A summary of geochronology for granitoids formed in the West Qinling Orogenic Belt during the Late Hercynian–Indosinian indicating magmatic activities concentrated in the Late Triassic (210–230 Ma) and Permian–Middle Triassic (235–277 Ma). These granitoids were both formed by partial melting of ancient crust, which then mixed with enriched lithospheric mantle, and the former has a higher mantle contribution than the latter. The early granitoids were associated to northward subduction of the Mianlue oceanic slab, while the late granitoids were formed in the transition from collision to extension.

Into the High to Ultrahigh Temperature Melting of Earth’s Crust: Investigation of Melt and Fluid Inclusions within Mg-Rich Metapelitic Granulites from the Mather Peninsula, East Antarctica

Abstract Precise constraints on the compositions of melts generated by anatexis under ultrahigh temperature (UHT) conditions are critical for understanding processes of partial melting and differentiation of the Earth’s crust. Here we reveal geochemical and physical signatures of anatectic melts preserved as nanogranitoids (i.e. crystalized melt inclusions) within sapphirine-bearing UHT metapelitic granulites from the Mather Peninsula, East Antarctica. Their coexistence with high−Al orthopyroxene as inclusions in garnets strongly suggests that the investigated melts were at least partially UHT in origin. The nanogranitoids are enriched in SiO2 (69.9–75.6 wt.%), strongly peraluminous (ASI values = 1.2–1.6) and potassic to ultrapotassic (Na2O + K2O = 7.1–9.5 wt.%, K/Na = 2.2–9.3). When compared to the granulitic restite, the melts are enriched in Li, Cs, Rb, Ta, Sm, Nd, Zr, U and Pb, and depleted in Ce, Th, Ba, Sr and Nb. Their geochemical characteristics are consistent with biotite−dehydration melting in the absence of plagioclase. Our calculation results indicate that these hot crustal melts have low densities of 2.47 ± 0.07 g/cm3, low viscosities of 104.9 ± 1.2 Pa·s and high heat production values of ∼2.8 μW/m3. Therefore, such melts are mobile and susceptible to be extracted from the source, and consequently their flow and removal from the deep crust may greatly affect the chemical and thermal structure of the continental crust. Secondary C − O − H fluid inclusions within garnet and orthopyroxene have also been detected. These inclusions contain magnesite, pyrophyllite, corundum, with or without residual CO2. The minerals within the fluid inclusions are interpreted as stepdaughter minerals, which were produced by the reaction of the fluid with its host. The metamorphic timing of the investigated rocks is still a matter of debate. Zircon U–Pb dating results obtained in this study suggest that the metapelitic granulites may have undergone two separated thermal events at ∼1000 and ∼530 Ma, respectively. The presence of fluid inclusions indicates that fluid infiltration and Pan–African reworking may have played an important role in obscuring chronological information of the early thermal scenario in poly-metamorphic terranes.

The permeability of loose magma mush

Abstract Models for the evolution of magma mush zones are of fundamental importance for understanding magma storage, differentiation in the crust, and melt extraction processes that prime eruptions. These models require calculations of the permeability of the evolving crystal frameworks in the mush, which influences the rate of melt movement relative to crystals. Existing approaches for estimating the crystal framework permeability do not account for crystal shape. Here, we represent magma mush crystal frameworks as packs of hard cuboids with a range of aspect ratios, all at their maximum random packing. We use numerical fluid flow simulation tools to determine the melt fraction, specific surface area, and permeability of our three-dimensional digital samples. We find that crystal shape exerts a first-order control both on the melt fraction at maximum packing and on the permeability. We use these new data to generalize a Kozeny-Carman model in order to propose a simple constitutive law for the scaling between permeability and melt fraction that accounts for crystal shape in upscaled mush dynamics simulations. Our results show that magma mush permeability calculated using a model that accounts for crystal shape is significantly different compared with models that make a spherical crystal approximation, with key implications for crustal melt segregation flux and reactive flow.

Melt-fluid interaction in the formation of peralkaline granite: Evidence from the Baiyinwula intrusion, Inner Mongolia, China

The genesis of peralkaline granites (with molar (Na + K)/Al > 1) remains a conundrum, particularly their emplacement accompanied by contemporaneous calc-alkaline rhyolites rather than peralkaline rocks. In this study we present a petrogenetic comparison of calc-alkaline rhyolites (296–293 Ma) and subsequent peralkaline granites (291 Ma) in Baiyinwula, Inner Mongolia of north China. The calc-alkaline rhyolites display a narrow variation of GdN/YbN (1.0–1.4) with increasing Zr/Sm (29–79) and positive correlation of εNd(t) (+ 1.5 to +5.1) versus 1/Nd (0.02–0.08). Accordingly, their formation is interpreted to have involved partial melting of juvenile crust, followed by feldspar-dominant fractionation and crustal contamination. The peralkaline granites exhibit roughly constant variations of MgO (0.06–0.24 wt%), Na2O (4.05–4.89 wt%), K2O (3.77–4.61 wt%) and increasing peralkalinity (0.97–1.12) and Fe-index (0.90–0.97) with increasing Zr (350–1022 μg/g) and Zr/Sm, precluding their genesis from calc- or per-alkaline parental magmas through high degree of fractionation. Interestingly, the mixing hyperbola (R2 = 0.85) of GdN/YbN (0.5–1.2) and Zr/Sm (50–240) for peralkaline granites indicate interaction between magmas and alkali-rich fluids, but is distinctly different from those derived by partial melting of metasomatized sources which exhibit a trend of partial melting. Simulation based on two-component (calc-alkaline rhyolitic melts and alkali-rich fluids) mixing yields highly consistent inference. Both magmatic albite in peralkaline granites and dissolved morphology of zircons from calc-alkaline rhyolites are indicators for onset of melt and fluid interaction. Combining the modelling results with positive correlation between εNd(t) (+ 5.6 to +6.6) and Zr/Sm, we infer that the potential end-member source of alkali-rich fluids is from mantle alkali-magmas or degassing. Considering the petrogenesis of peralkaline granites and their occurrences in the Uliastai Continental Margin (UCM) distributed along the major strike-slip fault, and temporally marking the final stage of the Late Palaeozoic magmatism, we propose a model for the magmatic transition from calc- to peralkaline magmas in an extensional setting during the culmination of the magmatic event. Subsequently, the calcalkaline melts began to wane, corresponding to the gradual weakening of the geothermal gradient in the lowermost crust. In contrast, the activities of mantle degassed fluids became more intense and migrated through the major fault, metasomatizing crustal rocks or injecting into and interacting with the pre-existing calc-alkaline magmas in the roof of the magma chamber.

Possible partial melting and production of felsic melt in a Jurassic oceanic plateau of the Izanagi Plate: Insights from 159 Ma plagiogranites from northern Japan

ABSTRACT A small tonalite – dacite body has been discovered in the Nakanogawa Group, Hidaka Belt, northern Japan, which is a Palaeogene subduction complex formed in the palaeo-Japan trench along the northeastern margin of Eurasia. The tonalites are characterized by extremely low K2O (0.2–0.3 wt.%) and relatively flat chondrite-normalized rare earth element patterns (La/Yb[N] ~2.3), similar to plagiogranites in ophiolites. The major-and trace-element characteristics are consistent with low-pressure partial melting of oceanic crust outside the garnet stability field. Zircon U – Pb dating of the tonalite yielded a weighted-mean 206Pb/238U age of 159.1 ± 1.6 Ma. Late Jurassic oceanic plateau-type basaltic rocks with a small amount of plagiogranitic rocks are distributed sporadically in subduction complexes along the palaeo-Japan arc – trench system. A similar zircon U – Pb age (151.6 ± 1.8 Ma) has been reported for greenstone in the Cretaceous subduction complex along the palaeo-Kuril arc – trench system. The tonalite – dacite block was probably derived from the subduction complex at the junction of the palaeo-Japan and -Kuril arc – trench systems. Thus, the greenstones and the tonalite body were likely part of a large oceanic plateau, which formed at the Izanagi – Pacific–Farallon ridge triple junction during the Late Jurassic – Early Cretaceous.

Lithospheric dripping triggered by slab break-off: A possible mechanism for Late Carboniferous magmatism in the eastern Central Asian Orogenic Belt

In this paper, we present zircon UPb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon UPb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive εNd(t) (+1.88 to +2.00) and εHf(t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted NdHf isotopic compositions (zircon εHf(t) = +12.5 to +18.9, whole rock εNd(t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive εHf(t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off.

Zircon Hf-O and apatite Sr-O isotopes jointly reveal the origin and mineralization of the Miocene adakitic porphyries in the Gangdese belt

The Gangdese belt is a significant belt for exploring the dynamic processes of continental collision orogeny and mineralization. The impact of petrogenesis and chemical heterogeneity of the E-trending Miocene adakitic rocks in Gangdese porphyry deposit belt on mineralization mechanisms has always been controversial. New whole-rock Sr-Nd isotope, zircon U-Pb dating and Hf-O isotope data, especially integrating apatite volatile (Cl and S) and Sr-O isotope compositions, of the Miocene mineralized porphyries from Lakange deposit can provide the development of metallogenic system origin, as well as magmatic and volatile process. The Lakange granites show adakitic geochemical composition affinity with high potassic calc-alkaline. They have high SiO2 content, low MgO and Ni contents, positive εNd(t) value (0.1 ∼ 0.8), and high (87Sr/86Sr)i ratios (0.705088 ∼ 0.705269), positive zircon εHf(t) (7.6 ∼ 11.2), medium δ18O (4.33‰∼7.27‰) values, combining with apatite Sr-O isotopes (87Sr/86Sr = 0.7040 ∼ 0.7169; δ18O = 7.0‰∼8.2‰), implying that the granites are originated from the partial melting of the juvenile lower crust beneath the southern Lhasa subterrane. The Miocene adakitic rocks of the Gangdese metallogenic belt exhibit spatially isotopically heterogeneity in a west to east direction (87°E-93°E). These variations are attributed to the lower crust modification by the oxidized mantle-derived melts/fluids from metasomatic subcontinental lithospheric mantle in Miocene, resulting in the formation of juvenile lower crust, that thickened the crust and shortened the lithospheric mantle. In-situ Sr-O isotope of apatites identified the oceanic components in the volatile sources, indicating that volatiles existed beneath the Lhasa terrane during Neo-Tethyan oceanic slab subduction prior to the Miocene partial melting of the lower crust. The high volatile concentrations (Cl and S) aid in the transport of metals that produce the Gangdese Miocene porphyry deposits.

Mid-Neoproterozoic tectonic switch from orogeny to intraplate extension in NW Tarim Craton: New evidence from a composite dike of the Aksu Group

It is widely acknowledged that the Tarim Craton plays a crucial role in the supercontinent Rodinia. However, the timing and regime of the Neoproterozoic orogeny, as well as the subsequent tectonic evolution along the northern margin of the Tarim Craton, remain contentious. Recent research has revealed a composite dike in the Aksu blueschist terrane located in the NW Tarim, which sheds new light on the pivotal Neoproterozoic tectonic transition. This dike comprises deformed granites and undeformed diabases, resulting from two-episode igneous events shortly after the orogenic metamorphism. The granites exhibit S-type affinities, with high silica, calcic-alkalic, and peraluminous compositions. Enriched isotopic signatures and diagnostic inherited zircons suggest that they derived from the metasedimentary rocks of the accretionary complex. Conversely, the following diabase dike displays tholeiitic affinity, right-sloping REE patterns, enriched LILEs, and a pronounced depletion of high-field-strength elements. Moderately enriched Sr-Nd isotopes and chondrite-akin Hf isotopes of these diabases imply that they were likely derived from partial melting of the metasomatized subcontinental lithospheric mantle. Zircon U-Pb chronological results reveal that the crystallization of these granites occurred at around 780 Ma. When combined with the maximum depositional age of protolith of the Aksu Group metapsammite obtained by detrital zircon dating, the metamorphic age of the Aksu blueschist terrane is limited to a short duration of 800–780 Ma, which is similar to the syn-orogenic metamorphism and crustal melting in the NE Tarim margin constrained to 830–790 Ma. Structural and petrological analysis suggests that the granites underwent deformation and dynamic recrystallization, while the diabases did not. This likely reflects the post-orogenic processes followed by the extrusion of mafic dike swarm associated with lithospheric stretching. The composite dike highlights a rapid tectonic shift from orogeny to intraplate extension during the mid-Neoproterozoic (780–760 Ma). Widespread late Neoproterozoic intraplate mafic and A-type granitic magmatism occurred in the Tarim and its adjacent blocks (Central Tianshan, Yili, and Kazakhstan) along the southwestern Central Asian Orogenic Belt, indicating that these blocks shared a similar geodynamic evolution related to the breakup of Rodinia.

Ore‐forming process within the Hongyuan reduced porphyry Mo–Cu deposit, West Junggar, NW China

The Hongyuan porphyry Mo–Cu deposit (PMCD) is a rare example of a “reduced” deposit, in contrast to classical oxidized porphyry Cu or Mo–Cu deposits. It is characterized by an ilmenite–series I–type felsic granitoid intrusion, reduced ore‐forming fluids with methane‐rich fluid inclusions, and widely distributed primary pyrrhotite. Mo–Cu mineralization in the Hongyuan PMCD occurs as disseminated in porphyry, granular in miarolitic cavity, and vein‐type in the quartz vein. These three types of mineralization are generally produced in the late magmatic period, magmatic‐hydrothermal transition period, and hydrothermal period, respectively. Molybdenite from miarolitic cavities shows a greater contribution from mantle fluid, as evidenced by mantle He proportions of 37.6%–16.8%. Molybdenite in quartz veins has relatively lower mantle He proportions of 1.63%–10.5%, indicating a predominantly crustal fluid source. The 3He/4He variations of these two groups of molybdenite illustrate an increasing input of crustal fluid during the magmatic to hydrothermal transition. Molybdenite samples from miarolitic cavities and quartz veins yield a Re‐Os model age of 300.5 ± 3.7 Ma, which is consistent with the U–Pb ages of zircons in the wallrock porphyries (300–302 Ma). This further indicates simultaneous diagenesis accompanied by mineralization. The Sr–Nd isotopic compositions (initial 87Sr/86Sr: 0.703404–0.703552; initial 143Nd/144Nd: 0.512585–0.512655; εNd[t]: +6.56 to +7.90) and young T2DM model ages (421–531 Ma) of the Hongyuan porphyries suggest that these rocks were derived from partial melting of the basaltic lower crust. We propose that mantle‐derived fluids rose up and mixed with fluids from the lower crust under an extensional tectonic setting, forming mantle‐like Sr–Nd–He–Ar isotopic compositions. The pre‐mineralization stage of the Hongyuan reduced PMCD is marked by quartz‐molybdenite ± K‐feldspar ± pyrite veins or veinlets and miarolitic cavities in the intensively potassic alteration zone. These veins formed at high temperatures (400–476°C) and high salinities (47.5–56.6 wt.%). The syn‐mineralization stage is the main metallogenic stage represented by numerous quartz‐molybdenite ± pyrrhotite veins associated with moderate to high temperatures (248°C–424°C) and low to high salinities (1.57–50.1 wt.%). The post‐mineralization stage is characterized by low temperatures (104–270°C) and low salinities (0.35–8.81 wt.%). Fluid immiscibility, mixing with meteoric water, ore fluid‐wall rock interactions, and decreasing temperature and salinity have played important roles during molybdenum mineralization.