High MgO Contents Research Articles

Diverse middle Neoarchean granitoids and the delamination of thickened crust in the Western Shandong Terrane, North China Craton

Middle Neoarchean granitoids contain crucial clues for understanding crust–mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the Neoarchean. In the Western Shandong Terrane (WST), diverse Neoarchean magmatism is recognized, including ∼2.62 Ga tonalitic gneisses, ∼2.59 Ga granodioritic gneisses and ∼2.61–2.58 Ga monzogranitic gneisses, which generally intruded into ∼2.70 Ga tonalite–trondhjemite–granodiorite (TTG) gneisses. The ∼2.70 Ga TTG gneisses show high SiO2 contents (70.17–72.89 wt%), but low MgO contents (0.58–0.69 wt%) and high (La/Yb)N ratios (30–48) and originated from partial melting of low-K mafic rocks at lower crustal levels. The ∼2.62 Ga tonalitic gneisses exhibit low K2O/Na2O ratios (0.10–0.39), but high (La/Yb)N (7.60–12.7) and Sr/Y (18.8–48) ratios and Mg# (47–57). With high εHf(t2) values ranging from +2.7 to +7.4, these rocks were derived from dehydration melting of subducted oceanic slabs. The ∼2.61–2.58 Ga monzogranitic gneisses show high SiO2 (70.35–75.12 wt%) and K2O (4.35–5.81 wt%) contents but low MgO (0.20–0.77 wt%) contents with positive εHf(t2) values (+2.0 − +6.6), suggesting they were derived from partial melting of juvenile greywackes. The ∼2.59 Ga granodioritic gneisses exhibit high SiO2 (66.34–73.04 wt%), K2O (2.23–4.32 wt%) and MgO (1.39–3.51 wt%) contents with dispersed εHf(t2) values (−0.2 − +5.3), indicating a mixing process between crust- and mantle-sourced magmas. The ∼2.59 Ga granodioritic gneisses show consistent major and trace element compositions (i.e., high Mg#, lack of Eu anomalies and uniform chondrite-normalized REE patterns), and accordingly, their magmatic precursors were derived from partial melting of delaminated lower crustal materials.The lithological assemblages in the WST became increasingly diverse between ∼2.71–2.58 Ga, changing from TTG gneisses to arc-related rock series, and then to delamination-related granodioritic gneisses, crust-derived monzogranitic and trondhjemitic gneisses and mantle-derived tholeiitic basalts and diorites. By integrating these data with a reduction in the continental crustal thickness from ∼2.70 Ga to 2.55 Ga, a subduction–delamination model is proposed in which a long-term lateral accretion beginning at ∼2.7 Ga and persisting through ∼2.68–2.62 Ga led to the delamination of the lower crust at ∼2.61–2.58 Ga, which may have further triggered the transition of the continental crust composition from basaltic to andesitic.

Geochemical constraints on the origin and tectonic setting of the serpentinized peridotites from the Paleoproterozoic Nyong series, Eseka area, SW Cameroon

Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon. Here, we present new field data, petrography, and first comprehensible whole-rock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region. The studied rock samples are characterized by pseudomorphic textures, including mesh microstructure formed by serpentine intergrowths with cores of olivine, bastites after pyroxene. Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite, talc, spinel, and magnetite. Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types. Type 1 has high MgO (> 40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values ( 50 ppm). This similarity, associated with the high Cr contents, spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subduction-related environment. The U-shaped chondrite normalized-REE patterns of serpentinized peridotites, coupled with similar enrichments in LREE and HFSE, suggest the refertilized nature due to melt/rock interaction prior to serpentinization. Based on the results, we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subduction-related environment, especially in Supra Subduction Zone setting. These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the Sao Francisco craton in Brazil.

Neoarchaean subduction tectonics in Western Shandong Province, China: Evidence from geochemistry and zircon U–Pb–Hf isotopes of metabasalts

Neoarchaean supracrustal rocks, which were discovered in the Longhuzhai and Menglianggu areas of the Western Shandong Province (WSP) in the Eastern Block of the North China Craton (NCC), are fine‐ to medium‐grained amphibolites that commonly underwent greenschist‐ to amphibolite‐facies metamorphism. LA‐ICP‐MS zircon U–Pb analyses revealed that these metabasalts erupted at 2,527 ± 5 to 2,514 ± 4 Ma. These supracrustal rocks are tholeiitic basalts in chemical composition and exhibit relatively high SiO2 (49.30–53.34 wt%) and MgO (5.24–11.82 wt%) contents but low TiO2 (0.41–0.75 wt%) contents. They display flat chondrite‐normalized rare earth element (REE) patterns ((La/Yb)N = 1.21–2.60) and negative Nb, Ta, and Ti anomalies ((Nb/La)PM = 0.16–0.54), which are similar to those of typical island arc tholeiites (IATs) in subduction‐related settings. These findings, combined with the positive zircon εHf(t) values (+2.51 to +8.90) and εNd(t) values (+0.19 to +3.65), suggest that these metabasalts were derived from the partial melting of a depleted mantle source that was metasomatized by slab‐derived fluids at shallow levels in the upper mantle. Accordingly, the development of arc‐related tholeiitic volcanic rocks within the interior of the WSP imply that the Neoarchaean (2.6–2.5 Ga) subduction‐related slab–wedge interaction mechanism was widespread in the WSP and played a crucial role in the formation of the continental crust and its growth.

Mafic–intermediate igneous rocks in the East Kunlun Orogenic Belt, northwestern China: Petrogenesis and implications for regional geodynamic evolution during the Triassic

Mafic–intermediate igneous rocks can provide important insights into the geodynamic evolution of orogenic belts. Here, we investigate the petrogenesis of mafic–intermediate intrusive igneous rocks in the East Kunlun Orogenic Belt (EKOB) to constrain regional geodynamic and magmatic evolution. These intrusive rocks are all located in the Permian-Triassic magmatic arc and parallel to the Paleo-Tethys ophiolite belt. Zircon UPb dating results indicate that these various igneous rocks crystallized during the Triassic. The 250–248 Ma appinites display low SiO2 contents, high MgO contents, and marked enrichments in large-ion lithophile elements and depletions in high-field-strength elements; combined with their enriched εHf(t) values, these appinites were considered to be derived from lithospheric mantle modified by subducted slab-derived fluids. The Harizha host granodiorites and MMEs have indistinguishable crystallization ages of 235 Ma but distinct SiO2 contents and δ18OSMOW values, which, together with enriched Sr–Nd–Hf–Pb isotopes, suggest that their primary magmas were derived from the Mesoproterozoic lower crust (MLC) and subduction-modified lithospheric mantle (SMLM), respectively. The 241 Ma Baidungou quartz diorite and the 218 Ma Huanglonggou diorite porphyry show characteristics of medium SiO2 contents, moderate Mg# and εHf(t) values relative to contemporaneous mafic and granitic rocks; therefore they probably represent the mixing products of MLC-derived felsic and SMLM-derived mafic magmas. Different from the Baidungou diorite and Harizha granodiorite, the Huanglonggou diorite porphyries have adakitic characteristics, which suggests crustal thickening during the Late Triassic. A synthesis of these new data with previous researches shows that the EKOB undergone three magmatic peaks corresponding to three extension events (slab-rollback, slab-breakoff, and delamination) during the Triassic evolution.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.

Origins and tectonic implications of Late Cretaceous adakite and primitive high-Mg andesite in the Songdo area, southern Lhasa subterrane, Tibet

Late Cretaceous igneous rocks in the southern Lhasa subterrane, Tibet, include primitive high-Mg andesites and adakites, which provide important constraints on the tectonic evolution of the Neo-Tethys Ocean. Here, we present detailed zircon UPb and Hf isotopic and whole-rock geochemical data for granodioritic and dioritic porphyry samples from the Songdo area in the southern Lhasa subterrane. Zircon UPb dating indicates that the granodiorite crystallized at 88 Ma, whereas the diorite yields ages of 68 and 66 Ma. The granodiorite has adakite-like geochemical characteristics, including high Sr (801–1005 ppm) and low Y (6.8–15.2 ppm) and Yb (0.6–1.3 ppm) concentrations, and high Sr/Y (62–145) and La/Yb (39–93) ratios. We infer that the adakitic granodiorites formed through partial melting of subducted oceanic crust. The dioritic porphyry has intermediate moderate SiO2 (53–58 wt%) and high MgO (5.6–8.2 wt%) contents, and high Mg# (66.4–69.5) values, and is therefore classified as a primitive high-Mg andesite that was derived from interaction between subducted sediment and mantle. The presence of coeval adakite and charnockite, as well as high-temperature granulite-facies metamorphism, indicates that mid-ocean ridge subduction occurred at 100–80 Ma, followed by a 10 Myr hiatus in magmatism and subsequent rollback of the Neo-Tethys slab at 68 Ma. These processes resulted in significant crustal growth within the Lhasa terrane.

Chemistry of chromium spinel in high-Mg rocks from the Morungava Intrusion, Cretaceous Paraná Igneous Province, southernmost Brazil

Mafic-ultramafic intrusions within continental flood basalt terrains are frequently associated with Cu-Ni-PGE mineralization. This study aims to constrain the petrogenesis of early crystal phases in a promising exploration target, the Morungava mafic-ultramafic intrusion that is associated with the Parana flood basalts. Therefore, we analyzed the composition of chromiferous spinel and associated olivine of ultramafic rocks from the sill-like intrusion. Samples were selected from different drill core intervals with high MgO and Cr contents. Two generations of Cr-spinel and olivine were identified in olivine gabbronorite and wehrlite rocks from the intrusion. The first type (Spl1) is enclosed in the core of high-Mg idiomorphic olivine crystals (Ol1) and has higher Cr2O3 (28–35 wt.%), Al2O3 (13–33 wt.%), MgO (12.6–14.6 wt.%), and lower TiO2 (0.5–0.8 wt.%) compared to the second type of spinel (Spl2). Spl2 occurs in the interstitial space between olivine and clinopyroxene and contains higher TiO2 (2.0–15.8 wt.%) and lower Al2O3 (5.2–10.5 wt.%), Cr2O3 (10–25 wt.%), and MgO (2.8–7.6 wt.%) contents. Geothermometric calculations using high-Mg idiomorphic Ol1 - Spl1 pairs resulted in temperatures below the corresponding solidus, indicating subsolidus exchange between these minerals and accounting for the Cr-Al trend in Spl1. The high Mg-contents, normal zoning and high but decreasing Cr contents in Ol1 are consistent with crystallization at an early stage during progressive fractional crystallization. Fractionation of olivine, Cr-spinel, clinopyroxene, and plagioclase deceased Mg, Al, and Cr, and increased Fe and Ti in the residual melt. The compositional hiatus with the low-Mg olivine (Ol2) and especially with Spl2 crystals most likely reflects crystallization from an evolved interstitial liquid at a time where Ol1-Spl1 were relatively isolated from diffusional interaction, in addition to solvus processes in spinel. The Fe-Ti- trend in Spl2 likely reflects varying degrees of evolution of the interstitial melts. The Morungava intrusion thus records a complex history of extensive reactions that started with fractionating Ol1-Spl1 in a magma chamber that experienced periodic magma addition, and finished in trapped, interstitial, in situ intercumulus liquids (Ol2-Spl2). The setting, host rocks, and geochemical characteristics of the Morungava intrusion are reminiscent of the Cu-Ni-PGE mineralized Noril’sk-Talnakh ultramafic complex, confirming the inferred mineralization potential.

Nature and Evolution of Crust in Southern Lhasa, Tibet: Transformation From Microcontinent to Juvenile Terrane

AbstractThe nature and pre‐Cenozoic evolution history of crust in southern Lhasa, which is crucial for our understanding of Indo‐Asian continental collision and Tibetan uplift during the Cenozoic, remains controversial due to a “missing” pre‐Mesozoic magmatic record. In this contribution, we report petrological and geochemical data for newly identified Paleozoic bimodal magmatism in the Zhengga area of southern Tibet. The magmatism comprises Late Devonian‐Early Carboniferous (366–353 Ma) amphibolite and two‐mica gneissic granite. The protoliths of the Zhengga amphibolite were gabbro and diorite with low SiO2 and high MgO, Cr, and Ni contents with high εNd(t) values of +3.3 to +8.0, variable and positive zircon εHf(t) of +0.9 to +11.2, and low zircon δ18O of 5.7 ± 0.2‰. These protoliths are proposed to have formed by decompression melting of asthenosphere during intracontinental back‐arc extension. In contrast, the granite has relatively high SiO2 and low MgO contents with much lower εNd(t) of −8.6 to −7.3, variable and negative zircon εHf(t) of −10.4 to −1.3, and high zircon δ18O of 9.4 ± 0.2‰ values and was most likely derived from an ancient metasedimentary source. This magma subsequently underwent recharge with minor amounts mafic magma followed by fractional crystallization of K‐feldspar in middle‐upper crust (~10–20 km) magma chambers. Using our new data, in combination with Nd‐Hf isotopes, we present the first comprehensive picture of crustal evolution in southern Lhasa. The southern Lhasa sub‐block is likely to have been a microcontinent that underwent extensive Phanerozoic crustal reworking and growth, rather than a Mesozoic‐Early Tertiary juvenile accretionary arc terrane.

Petrogenesis of Permo-Triassic intrusive rocks in Northern Liaoning Province, NE China: implications for the closure of the eastern Paleo-Asian Ocean

ABSTRACT The Changchun-Yanji belt recorded widespread Permo-Triassic magmatism, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Paleo-Asian ocean in the eastern Central Asian Orogenic Belt (CAOB). Here, we present new geochronological, geochemical, and Hf isotopic data for the Permo-Triassic plutons from Northern Liaoning province, NE China. Combined the published ages with our new data, the Permo-Triassic magmatism in the eastern CAOB can be divided into five episodes: early Permian (293–274 Ma), middle–late Permian (270–257 Ma), latest late Permian–Middle Triassic (255–242 Ma), Late Triassic (240–215 Ma), and latest Late Triassic (209–200 Ma). The middle Permian and Late Triassic mafic plutons (i.e. ~266 Ma Mengjiagou gabbro–diorite, ~240 Ma Jiancaicun gabbro and ~224 Ma Shudetun gabbro-diorite) contain relatively high TFe2O3, MgO, Cr and Ni contents with positive εHf(t) values (+1.2 to +7.2), suggesting a depleted mantle origin. These mafic rocks together with the coeval granitoids make up typical bimodal associations, suggesting that they were formed under an extensional environment. The conclusions are also supported by occurrence of A-type granites during 270–257 Ma and 240–215 Ma. By contrast, the granitoids of 255–242 Ma in the eastern CAOB, including the Jianshanzi (~251 Ma) and Daganhe (~242 Ma) monzogranites, show typical geochemical features of adakitic granites, with high Sr/Y ratios and negative εHf(t) values (–8.6 to – 22.0), suggesting that the magmas were generated through partial melting of thickened ancient lower crust. Combined with previous studies, a four-stage tectonic evolution scenario was proposed: (1) active continental margin stage during 293–274 Ma; (2) continuing subduction resulted in the initiation collision, moderate crustal thickening, and slab break-off during 270–257 Ma; (3) final closure of the Paleo-Asian Ocean associated with continued crustal thickening occurred during 255–242 Ma; (4) lithospheric delamination in a post-collisional extensional environment occurred during 240–215 Ma.

Langshan basalts record recycled Paleo-Asian oceanic materials beneath the northwest North China Craton

Crustal recycling is an important cause of mantle heterogeneity and can have significant control on basalt compositions. Recycled components from the subducted (Paleo-) Pacific slab have frequently been recognized in Cenozoic basalts from the eastern North China Craton (NCC). However, it still remains unclear if the subducted Paleo-Asian oceanic slab contributed to intraplate basalts in this Craton.In a search of evidence for the recycled components from this slab, we have studied the ArAr age and elemental and Sr-Nd-Pb isotope compositions of newly-discovered basalts from the Langshan area and compiled a regional synthesis of Cenozoic alkali basalts from the northwest NCC. This region is far from the Pacific domain but near the suture zone of the Paleo-Asian Ocean. With a Late Cretaceous eruption age (~89 Ma), Langshan basalts have low silica and high FeO, MgO and alkali contents, high incompatible elemental concentrations, positive Sr, Eu, Ba, Nb, Ta and negative Pb and Ti anomalies with high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios, superchondritic Zr/Hf ratios, and uniform radiogenic isotopes (ɛNd(t) = 2.20–2.51, 87Sr/86Sri = 0.703807–0.704348, 206Pb/204Pbi = 18.200–18.384, 207Pb/204Pbi = 15.470–15.515, 208Pb/204Pbi = 38.007–39.430). These compositional characteristics are shared by the compiled Cenozoic alkali basalts from northwest NCC.The low silica and high MgO, FeO and alkali contents together with the positive Nb, Ta and negative Ti anomalies were probably controlled by silica-deficient garnet pyroxenite in the mantle source. The partial melting is estimated to have occurred in the asthenosphere under an average mantle potential temperature of 1300–1450 °C and a pressure of ~2.5 GPa. The positive Sr, Eu, Ba, Nb, Ta anomalies and the canonical indices (high Ce/Pb, Nb/U, Ba/Rb and low Rb/Sr ratios) indicate the involvement of a subducted oceanic igneous slab. Considering 1) the particular tectonic locality of the study region, 2) the inferred northward increase of silica-deficient pyroxenite in the mantle source of Cenozoic alkali basalts, 3) the evidence for strong lithospheric modification beneath the northwest NCC induced by slab-derived components, and 4) the longevity of subducted slabs in convecting mantle, the subducted Paleo-Asian oceanic materials could have introduced ubiquitous mantle heterogeneity beneath the northwest NCC and played a significant role in the generation of the Late Cretaceous to Cenozoic intraplate alkali basalts there.

Late early Cretaceous peraluminous biotite granites along the Bangong–Nujiang suture zone, Central Tibet: Products derived by partial melting of metasedimentary rocks?

The petrogenesis of biotite granites, which may be metaluminous or peraluminous in composition, is a subject of debate. In this study, we present new zircon U-Pb geochronology and O-Hf isotopic, and whole-rock major and trace element, and Sr-Nd isotopic data for biotite granites in the Langla–Laqinpu area of northern Lhasa Terrane, central Tibet. The rocks are distributed along the eastern segment of the Bangong–Nujiang Tethys Ocean suture, and were emplaced during the late Early Cretaceous (ca.119 Ma). The biotite granites are peraluminous, and have high SiO2 (67.5–72.4 wt%) and low MgO (0.57–1.20 wt%) and CaO (2.36–4.38 wt%) contents with variable K2O/Na2O ratios (0.77–1.32), and show geochemical affinity with the upper-continental crust, e.g., enrichment of Rb, Th, U, K and Pb, depletion of Nb, Ta, Ti, P, Eu, and Sr. They are characterized by relatively high initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7098–0.7137) and low εNd(t) values (−8.4 to −10.3), and high zircon δ18O values (8.29–13.47‰) and variable εHf(t) values (−0.02 to −21.36), indicating they were derived through partial melting of a crustal source consisting of metagreywackes with or without minor metaigneous rocks. Taking into account all available regional geological data, we suggest that the Bangong–Nujiang Tethys Ocean may undergo diachronous closure–although the eastern segment may have been closed by the late Early Cretaceous, its western and western–middle segments may still have been at least partially open and active at that time. The Langla–Laqinpu biotite granites were probably generated in post-collisional setting, following slab break-off after closure of the Bangong–Nujiang Tethys Ocean.

Zircon U–Pb age and geochemical constraints on the origin and tectonic implication of the Tuotuohe Cenozoic alkaline magmatism in Qinghai–Tibet Plateau

A large number of Eocene–Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau. In this study, we present zircon U–Pb ages, Hf isotope data, and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block. Zircon U–Pb ages were measured via LA–ICP–MS to be 39.6, 37.6 and 32.0 Ma. The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents. The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O, but deficient in MgO. All of the samples belong to the alkaline rocks. These rocks display enrichment in REE, LREE, and LILE, depletion in HFSE, and no obvious Eu anomalies. Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region: (1) the removed mafic lower crust dropped into the asthenosphere, forming the mix magma (Nariniya trachyte); (2) the upwelling asthenosphere triggered the crustal melting (Nariniya and Zamaqu orthophyre). The Eocene–Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic–magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination. This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region (~ 92°E) from Banda Co (~ 82°E).

Mid-Paleozoic ridge subduction in the Central Beishan of the southern Altaids: evidence from geochemical, Sr–Nd and zircon U–Pb–Hf–O isotopic data of Gongpoquan volcanic rocks

The investigation of Mid-Paleozoic tectonics could constrain the relations of early and late Paleozoic subduction events in the southern Altaids and improve our understanding of the architecture of accretionary orogens. The Gongpoquan volcanic rocks are widespread in the Central Beishan and contain important information on the tectonics of the southern Altaids. Geochronological data constrain the eruptive time of the Gongpoquan volcanic rocks as early Silurian–early Devonian (434–411 Ma). The Gongpoquan andesites show similar geochemical characteristics to those of typical sanukitoids, including high MgO and compatible element concentrations, enrichments in large ion lithophile elements and light rare earth elements, and depleted isotopic compositions. The geochemical data show that the Gongpoquan andesites were generated through the partial melting of subducting sediments and subsequent melt–mantle peridotite interactions. The early Silurian basalts are Na-rich and exhibit high Nb/U and Nb/La ratios, an enrichment in high field strength elements and enriched isotopic compositions, making them akin to Nb-enriched basalts. Such Nb-enriched basalts should be derived from the melting of a mantle component similar to ocean island basalt in the mantle wedge. The early Devonian basalts are calc-alkaline, enriched in large ion lithophile elements and depleted in high field strength elements. They yield high Ba/Yb ratios and depleted isotopic compositions, indicating an origin via the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. Our data prove that the Paleo-Asian Ocean may still have been subducted during the Mid-Paleozoic. During this process, a spreading ridge was subducted beneath the Central Beishan, resulting in the opening of a slab window, which generated the observed 431–411 Ma sanukitoids and Nb-enriched basalts. Supplementary Material: Analytical methods, U–Pb geochronological data, zircon Lu–Hf-O isotopic compositions, whole-rock major and trace element data, detailed descriptions of the zircon U–Pb geochronology and Hf–O isotopic compositions for analysed samples, representative field photographs and photomicrographs of samples, and representative cathodoluminescence images of analysed zircon grains are available at: https://doi.org/10.6084/m9.figshare.c.4461062

The early Paleozoic Huangtupo VMS Cu—Zn deposit in Kalatag, Eastern Tianshan: Implications from geochemistry and zircon U—Pb geochronology of volcanic host rocks

The Tianshan in China forms the southern part of the Central Asian Orogenic Belt (CAOB) and also hosts important polymetallic resources. Here we investigate the Huangtupo medium–size copper–zinc deposit, located in the central part of the Kalatag copper Polymetallic Belt with a view to evaluate the nature and genesis of the early Paleozoic volcanogenic massive sulfide (VMS) Cu—Zn deposits. This deposit is correlated to intermediate–felsic volcano–sedimentary sequences of the early Paleozoic Daliugou Formation which are mainly composed of andesite, dacite, and rhyolite with minor basalt, as well as their pyroclastic equivalents. The ore bodies are hosted in the contact zone between andesitic volcanic breccia and tuff representing an eruptive hiatus between the two volcanic cycles with massive sulphide lens at the top and stockwork zone at lower part of the Daliugou Formation. We report new zircon U—Pb data, major and trace element geochemistry and Sr—Nd isotopes from the andesite, dacite and andesitic tuff in the Huangtupo deposit. Zircon from the dacite and the andesite occurring above the ore bodies yield and age of 440.1 ± 1.2 Ma and 439.7 ± 0.95 Ma, and that from the andesitic tuff in footwall shows and age of 442.0 ± 1.1 Ma. All samples show typical geochemical signatures of subduction–related calc–alkaline magmas with slight enrichment in LILE and LREE but depletion in Nb and Ti content relative to primitive mantle. The andesites are characterized by high MgO (7.6–10.8 wt%) contents and Mg# values (64–72) with low Sr/Y and La/Yb ratios, belonging to sanukite. Both dacite and tuff have low MgO (<2.5 wt%) and Al2O3 content (15–16.4 and 11.3–15.3 wt%) with Eu negative anomalies (Eu/Eu* = 0.71–0.75 and 0.44–0.6), showing their low–MgO affinity. All the volcanic rocks display similar Nd and Sr isotopic signature, with εNd(t) of 6.5 to 7.3 and moderately high (87Sr/86Sr)i ratios of 0.705274 to 0.706977. The geochemical characteristics and regional geology indicate that these volcanic rocks were formed in an intra–oceanic arc during southward subduction of the oceanic lithosphere in the northern part of the Eastern Tianshan. They were most likely derived from the partial melting of the mantle wedge with input and interaction with subducted–related melts and/or fluids. We also propose that the Huangtupo VMS deposit is analogous to Kuroko type VMS deposits developed within intra–oceanic rift.

High-alumina pyroxenite xenoliths from Quaternary basalts of NW Spitsbergen - evidence of continental crust delamination

The origin of spinel-garnet pyroxenite from xenoliths in Quaternary basaltoids of Spitsbergen Island (Svalbard Archipelago) is discussed. The rocks have a high concentration of Al2O3 and MgO and low Cr. The primary magmatic association Spl-Opx-Cpx and the high Al content in pyroxenes provide evidence for the formation of these rocks as cumulates of hydrous basaltic melts in the lower crust at a pressure of ~1.2 GPa. Transformation of the texture from the magmatic hypidiomorphic to the metamorphic granoblastic and growth of garnet at the expense of spinel and high-alumina pyroxenes indicate transport of rocks to a depth below the spinel/garnet phase transition boundary in the Cr2O3-free CaO-MgO-Al2O3-SiO2 system. The parameters of the Grt-Opx equilibrium range within 1060-1120 °C and 2.2-2.6 GPa. Thus, magmatic pyroxenite was transported to a great depth into the mantle, which provides evidence for delamination in the region of the lower continental crust containing ultramafic cumulates.

Early Cretaceous adakite from the Atlas porphyry Cu-Au deposit in Cebu Island, Central Philippines: Partial melting of subducted oceanic crust

In the Philippine island arc system, there are widely developed Cenozoic arc volcanic rocks, adakites and large Cu-Au deposits. Besides, only two early Cretaceous adakitic intrusions have been found in the entire Philippines so far, i.e., the Lutopan Diorite and the adjacent Kansi diorites in Cebu Island, Central Philippines. The Lutopan Diorite (diorite to quartz diorite in lithology) is the ore-forming intrusion of the giant Atlas porphyry Cu-Au deposit, which is the only Cretaceous porphyry deposit in the Philippines. In this study, geochronology and geochemistry of the Lutopan Diorite in the Carmen pit of Atlas porphyry Cu-Au deposit are investigated in detail, by comparing them with the previously studied Early Cretaceous Kansi diorites and arc volcanic rocks in Cebu Island, to reveal the petrogenesis and magma sources. The Carmen diorites are characterized by moderate SiO2 and MgO contents, coupled with high Na2O and low K2O contents, LREE-enriched and listric REE distribution patterns, low Yb and Y contents, high Sr contents and thereby high Sr/Y ratios, and low (87Sr/86Sr)i (0.7037–0.7038), typical of modern island arc adakites. Zircon U-Pb ages of the Carmen diorites are 109 Ma − 107 Ma in the Early Cretaceous, which is identical with that of the Kansi diorite in the north within error. Both the Carmen and Kansi diorites were formed in intra-oceanic island arc settings. The Carmen diorites show listric REE patterns and low Nb/Ta, high Zr/Sm ratios, suggesting they are formed by partial melting of subducted oceanic crust in amphibolite facies, which is different from the garnet amphibolite- to amphibole eclogite-melting genesis of the Kansi diorites. There is a small amount of subducted sediments in the magma source of Carmen diorite, as further revealed by εNd(t) (+7.1 to +10.6), zircon εHf(t) (+3.3 to +3.9) values, and low Th/U ratios. The Carmen adakitic magma did not undergo significant interaction with mantle peridotite during magma ascent as indicated by the moderate MgO content and Mg# value, and quite low Cr and Ni contents, distinguished from the Kansi diorite (high-Mg adakite) that have high MgO, Mg#, and Cr and Ni contents. We suggest that the nearly contemporary Lutopan Diorite and Kansi diorite are two independent Early Cretaceous adakitic intrusions in Cebu Island, although they are both closely associated with Cu-Au mineralization. The slab-melting genesis and high oxygen fugacities of the Lutopan Diorite are the most important factors to cause Cu enrichment in the magma, which is necessary to form the giant Atlas porphyry Cu-Au deposit.

Prediction of Flow Characteristics of Al2O3–CaO–MgO–SiO2–TiO2-Type Blast Furnace Slag and Its Evaluation

Regression equations are developed using factorial design technique to predict the flow characteristics of synthetic high-Al2O3 blast furnace (BF) slag in terms of CaO, SiO2, MgO, the main constituents, and TiO2, a minor constituent. The compositions of the synthetic slags are in agreement with the compositional variations of their counterparts in related industry. The regression equations are validated through further experimentation with additional ten numbers of synthetic slags. It is inferred that the CaO/SiO2 (C/S) ratio, MgO and TiO2 jointly affect the flow characteristics of the high-Al2O3 BF slag, granting the BF operator greater flexibilities in burden calculations. It is further concluded that the combination of high C/S ratio and high MgO content irrespective of TiO2 content in the slag is more beneficial from an operation point of view.

Late Cretaceous magmatism in the NW Lhasa Terrane, southern Tibet: Implications for crustal thickening and initial surface uplift

Abstract Crustal thickening and uplift of southern Tibet have been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies indicated that the crust of the northwestern (NW) Lhasa Terrane was thickened during the late Mesozoic. Here we report geochronological and geochemical data for the Gaerqiong diorite porphyries (GPs) and Xiongma plutons (XPs) in the NW Lhasa terrane, southern Tibet. Zircon U-Pb dating suggests that these intrusive rocks were generated at ca. 85 and ca. 88 Ma, respectively. The GPs are characterized by high MgO, Cr, and Ni contents, and they have adakitic affinities. These geochemical features, combined with their depleted εNd(t) (+1.7 to +2.0), 87Sr/86Sr(i) (0.705103–0.705259), and zircon εHf(t) (+5.2 to +10.2) isotopic compositions, indicate that the GPs were produced by partial melting of the delaminated juvenile continental crust. In contrast, the XPs are composed of host granites and mafic microgranular enclaves (MMEs). The MMEs have low SiO2 and high MgO contents, and low εHf(t) (–14.0 to –5.8) values, indicating that their parental magmas were derived from an enriched mantle. The host granites have high SiO2 and low MgO contents, and variable εNd(t) (–7.4 to –6.3) and zircon εHf(t) (–11 to –4.1) values. These observations, combined with the presence of MMEs in the Xiongma granites, suggest that the host granites were the result of mixing of crust- and mantle-derived magmas. Detailed study of these two plutons, combined with the previous researches, suggests that Late Cretaceous (ca. 90 Ma) magmatism in the NW Lhasa Terrane occurred in a post-collisional extensional setting related to delamination of the regionally thickened lithosphere after collision of the Lhasa-Qiangtang Terranes. We propose that the crust of the NW Lhasa Terrane reached a maximum thickness (average of &amp;gt;50 km) before the Late Cretaceous (ca. 90 Ma). This crustal thickening was caused by underplating of mafic magmas during slab roll-back and break-off of the southward-subducting Bangong-Nujiang oceanic lithosphere and subsequent tectonic thrusting during Qiangtang-Lhasa Terrane collision, respectively. Given that crustal thickening generally results in elevated terrain, the regional uplift (driven by isostasy due to crustal thickening) probably commenced before the Late Cretaceous (ca. 90 Ma).

High Mg-glauconite in the Campanian Duwi Formation of Abu Tartur Plateau, Egypt and its implications

This study presents a detailed crystallo-chemical characterization of glauconites occurring at different stratigraphic intervals within the phosphorite-bearing Campanian Duwi Formation. While authigenic glauconites occur within the shallow marine, transgressive deposits of Abu Tartur Phosphate and Maghrabi Shale Members in the Campanian Duwi Formation, allogenic variety occurs within the regressive deposits at the top of the Liffiya Shale Member. Nascent to slightly evolved, shallow-marine originated authigenic glauconites are distinct by a high content of MgO. A good correlation between K2O and Fe2O3(total) contents of glauconites in the Maghrabi Shale supports ‘layer lattice’ theory while a combination of ‘layer lattice’ and ‘verdissement’ theories explain the origin of other glauconites within the Duwi Formation. X-Ray Diffraction studies reveal a ‘true’ di-octahedral nature and 1M ordering of the glauconite pellets with smectite inter-stratification. Mössbauer spectroscopic study reveals Fe3+ as the dominant cation in the octahedral site that preferentially occupies the less distorted cis M(2) sites. Glauconites acquire a high Mg-content because of the passage of Mg- and Ca-rich fluid that leads to extensive dolomitization. Glauconites form at the transition zone between black shale and phosphate, representing oxygen-depleted conditions. Similar shallow marine glauconites occur all along the paleo-Tethyan margin during the Late Cretaceous time.

Water Contents of Early Cretaceous Mafic Dikes in the Jiaodong Peninsula, Eastern North China Craton: Insights into an Enriched Lithospheric Mantle Source Metasomatized by Paleo–Pacific Plate Subduction–Related Fluids

The Early Cretaceous lithospheric mantle beneath the Jiaodong Peninsula has been recognized as ancient and enriched mantle with widespread mafic dike swarms. However, the enriching agent and the process of the lithospheric mantle remain under debate. In this study, we recovered the H2O contents of the primitive magmas of Early Cretaceous mafic dikes in the Jiaodong Peninsula by using the H2O contents of clinopyroxene (Cpx) phenocrysts and H2O partition coefficients DH2Ocpx/melt. We also present whole-rock geochemical data and in situ major- and trace-element contents for Cpx phenocrysts from these mafic dikes. These dikes are characterized by low SiO2 contents, high MgO contents, enrichment of large-ion lithophile elements and light rare earth elements, depletion of high-field-strength elements, high (87Sr/86Sr)i ratios, and low εNd(t) values. These geochemical features imply an origin of enriched subcontinental lithospheric mantle. The calculated H2O contents of the primitive magmas range from 1.38 to 4.45 wt%, with associated H2O/Ce ratios varying from 237 to 322, suggesting that their mantle source was metasomatized by oceanic slab subduction–related fluids. In addition, covariations between H2O/Ce and incompatible elements indicate the addition of a dehydrated marine sediment component in the mantle source of the Jiaodong mafic dikes. In combination with the H2O contents of Early Cretaceous basaltic rocks in the southeastern North China Craton, the lithospheric mantle was metasomatized through fluids related to paleo–Pacific plate subduction.