High MgO Contents Research Articles

Neoarchean-early Paleoproterozoic granitoids, the geothermal gradient and geodynamic evolution in the Hengshan Terrane, North China Craton

The Hengshan Terrane (HST) is a typical Archean terrane in the central North China Craton (NCC) and features Neoarchean-early Paleoproterozoic granitoid rocks and associated supracrustal rocks. Five geological events are identified in the HST, including (1) ~2.71–2.67 Ga tonalites-trondhjemites-granodiorites (TTGs) and monzogranites, (2) ~2.54–2.48 Ga diorites, TTGs and volcanics, (3) ~2.44–2.43 Ga monzogranites, (4) ~2.14–2.04 Ga monzogranites and (5) ~1.92–1.84 Ga granulite- to high-grade amphibolite- facies metamorphism. The ~2.5 Ga diorites exhibit low SiO2 concentrations (55.67–62.61 wt%), high MgO content of 1.39–4.89 wt% with Mg# values (45–62) and positive ɛHf(t2) values of +3.2−+7.8, and the magma originated from a mantle source that had been metasomatized by dehydration fluids and melts from subduction-related sediments and slabs. The ~2.5 Ga TTGs are characterized by variable MgO contents (0.56–2.32 wt%) and Mg# values (44–67), (La/Yb)N values (13.76–98.19) and positive ɛHf(t2) values of +2.8 to +6.5, and derived from the partial melting of tholeiitic basalts with slightly enriched REE patterns, similar to those of back-arc basin basalts (BABBs). And these granitoid melts were contaminated by mantle materials. The ~2.4 Ga monzogranites show high SiO2 (69.09–73.46 wt%) and K2O (3.31–7.52 wt%) contents and low MgO (0.11–1.23 wt%) contents, and their parent magmas originated from the partial melting of metamorphic graywackes.The mantle material that generated the dioritic magmas was enriched by the important metasomatic agent of sediment-derived melts and fluids prior to magmatism. These observations, in conjunction with the occurrence of widespread coeval calc-alkaline volcanic rocks in the adjacent Wutai region, suggest that a slab subduction was the most likely geodynamic regime leading to the late Neoarchean magmatism in the HST. In view of the BABB sources of the TTGs, a Neoarchean back-arc basin slab subduction model may be appropriate, which is supported by several Phanerozoic analogs. The partial melting P-T conditions of the ~2.5 Ga slab-derived TTG magmas are estimated to have been 1.5 ± 0.2 GPa and 823 ± 32 °C on the basis of thermodynamic and trace element simulations, indicating a geothermal gradient of ~17 ± 2 °C/km and a possible Neoarchean hot subduction process in the HST.

Reworking of juvenile crust beneath the Bangong–Nujiang suture zone: Evidence from Late Cretaceous granite porphyries in Southern Qiangtang, Central Tibet

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr)i = 0.7049–0.7050, εNd(t) = 1.2–1.6, εHf(t)whole-rock = 9.9–12.0, εHf(t)zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr)i = 0.7058–0.7067, εNd(t) = −0.9 to −1.1, εHf(t)whole-rock = 7.0–7.4, εHf(t)zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (εNd(t) = −3.0 to −4.3, εHf(t)whole-rock = 5.3–6.6, εHf(t)zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab

Evolution of the Tethyan Bangong-Nujiang Ocean and its SE Asian connection: Perspective from the Early Cretaceous high-Mg granitoids in SW China

High-Mg diorites (HMDs) have important implications on crust-mantle interactions and subduction processes, and are widely used to delineate ancient convergent plate margins. In this study, we described a newly-identified high-Mg diorite-granodiorite suite in the Tengchong terrane (SE Tibetan Plateau), and presented its petrography, whole-rock geochemistry, SrNd isotopes, and zircon U-Pb-Hf-O isotopic compositions. High-precision SIMS zircon UPb dating yielded consistent Early Cretaceous ages (~120 Ma) for all lithologies in this high-Mg suite. The high-Mg diorite is characterized by high MgO (4.7–9.2 wt%), Cr (169–323 ppm) and Ni (32–138 ppm) contents, together with high (87Sr/86Sr)i (0.70867–0.71060) and negative εNd(t) (−4.07 to −6.80) values. Their zircon εHf(t) vary from −5.64 to +4.12, and their δ18O (6.0 to 6.9‰) are slightly higher than the average mantle value. The granodiorite has higher SiO2 and lower MgO contents, and varying zircon εHf(t) (−12.19 to −2.71) and δ18O (6.3 to 8.1‰). The rocks are characterized by enrichments in large ion lithophile elements (LILEs: e.g., Rb, Th, U, K) and light rare earth elements (LREEs), but depleted in high field strength elements (HFSEs), and have also high (87Sr/86Sr)i (0.71057–0.711860) and negative εNd(t) (−8.42 to −9.14). Mafic microgranular enclaves (MMEs) in the granodiorite have intermediate SiO2, MgO, and trace elements and Sr-Nd-Hf-O isotope compositions between the high-Mg diorite and the granodiorite. Based on these geochemical and isotopic features, we interpreted that the high-Mg diorite was generated from the interaction of subducted sediment-derived melt with the overlying mantle wedge, whilst the granodiorite was sourced from the ancient continental crust with minor mantle-derived input. Mixing between the mantle-derived high-Mg dioritic melt and the crustal-derived granodioritic melt may have formed the MMEs. Our new geochemical (incl. isotopic) data, combined with published ones across the region, suggest that these Early Cretaceous plutons were formed by asthenospheric upwelling, which was probably led by slab rollback of the west-dipping subduction of the Luxi-Ruili Ocean between the Baoshan and Tengchong terranes. This implies that the southeastern continuation of the Tethyan Bangong-Nujiang Ocean was located between the Baoshan and Tengchong terranes, instead of between the Tengchong and West Burma terranes as previously suggested.

High-Mg andesitic rocks formed through crustal magmatic differentiation

High-Mg andesitic rocks are enriched in both compatible (Mg, Cr and Ni) and incompatible elements (Si, Ba and Sr), which indicates the complexity of their petrogenesis. Models involving hybridization of the mantle sources and input of adakitic melts have been invoked, but struggle to explain the low Mg# values (<0.65) of some low Sr/Y (<40) high-Mg andesitic rocks, because these rocks cannot represent primary melts (Mg# ≥ 0.72) produced by the partial melting of mantle peridotite. This study presents a petrological and geochemical analysis of the Xinsi diorite (443 ± 3 Ma) and Tiechang gabbro (440 ± 3 Ma) from the Early Paleozoic Wuyi-Yunkai Orogen in South China, and demonstrates the role of magmatic differentiation in the petrogenesis of high-Mg andesitic rocks. These rocks contain high SiO2, MgO, Ba and Sr concentrations with low Mg# values (≤0.69) and Sr/Y ratios (≤16.7), resembling typical low Sr/Y high-Mg andesitic rocks. Amphiboles and clinopyroxenes in the Xinsi diorite contain variable contents of Ba and Sr, both of which negatively correlate with Mg#. This indicates that their parental melts had low Ba and Sr and underwent significant enrichment of Ba and Sr through the fractionation of Mg-rich minerals within upper crust (3.1–4.4 kbar). Clinopyroxene, plagioclase and biotite in the Tiechang gabbro all contain lower Ba and Sr than those in the Xinsi diorite, suggesting that their parental melts were also initially low in Sr and Ba and experienced enrichment of these elements during magma evolution at middle crustal levels (5.7–6.3 kbar). The low Ba and Sr concentrations and Sr/Y ratios for parental melts of the Xinsi diorite and Tiechang gabbro suggest their derivation from the partial melting of mantle sources with input from slab melts at shallow depths without involvement of abundant garnet in petrogenesis. This study demonstrates the importance of fractional crystallization in the genesis of low Sr/Y high-Mg andesitic rocks and is conflict with past views that their geochemical features were related to the involvement of adakitic melts in their mantle sources. Therefore, this study presents an alternative model for the production of low Sr/Y and high Sr/Y high-Mg andesitic rocks, particularly in the Late Archean, which reflects the intense melting of subducted slab at different depths in the hotter mantle regime at that time.

Ultramafic intrusions hosting Ni–Cu sulfide mineralization along a suture zone in the southwestern margin of the Amazonian craton, Brazil: Examples from Morro Sem Boné, Morro do Leme and their satellite intrusions

This research paper describes several medium-to small-sized ultramafic intrusions located along the Mesoproterozoic Guaporé Suture Zone in the southwestern margin of the Amazonian craton. Apart from minor sheared zones, ultramafic rocks have igneous minerals and textures largely preserved. Metamorphic parageneses of metapelites and amphibolites of country rocks, as well as those from sheared zones of the ultramafic intrusions, are indicative of the amphibolite facies of regional metamorphism. Intersections of disseminated to massive sulfides consisting mainly of pyrrhotite and pyrite are common in the metamorphosed volcanic and sedimentary country rocks. The ultramafic intrusions consist mainly of olivine cumulates with high MgO contents (up to 45.9 wt%) and Mg# (up to 0.84), consistent with the composition of their most primitive olivine (Fo93). The crystallization sequence, dominated by olivine, orthopyroxene and chromite, together with ratios and patterns of incompatible trace elements of ultramafic cumulates, support the interpretation that they originated from a high-MgO magma partially contaminated with crustal rocks. Sulfide mineralization in the ultramafic intrusions comprises different styles of magmatic sulfides associated with cumulate rocks, including contact-type, stratabound-type and conduit hosted mineralization. Significant intersections of interstitial to net-textured sulfides consisting of pyrrhotite, pentlandite and chalcopyrire have variable but generally low Ni–Cu-PGE contents (e.g., ~20 m at 0.5 wt% Ni, 0.1 wt% Cu, and 0.5 ppm Pt+ Pd). The range of tenors of Ni, Cu, Pt and Pd, from moderate to very low, is interpreted as resulting from varying efficiency of equilibration of sulfide xenomelts incorporated from the sulfide-bearing country rocks. The Ni–Cu-PGE mineralized ultramafic intrusions are interpreted to belong to a network of magma conduits and staging magma chambers intruding the sulfide-bearing volcanic-sedimentary sequence. Our results indicate an additional prospective belt of high-MgO Ni–Cu-PGE sulfide mineralized intrusions located in a cratonic margin in Brazil.

The Incorporation of Steel Slag into Belite-Sulfoaluminate Cement Clinkers

The potential use of steel slag from treated steel slag in belite-sulfoaluminate cements was investigated in this study. Cement clinkers with two phase compositions were synthesized, allowing the incorporation of different amounts of steel slag. The phase composition and microstructure of cement clinkers at three different sintering temperatures were studied by X-ray powder diffraction and the Rietveld method, as well as scanning electron microscopy with energy dispersive spectrometry. The results showed that the targeted phase composition of clinkers was achieved at a sintering temperature of 1250 °C. However, a higher amount of perovskite instead of ferrite was detected in the clinker with a higher content of Ti-bearing bauxite. Apart from the main phases, such as belite, calcium sulfoaluminate, and ferrite, several minor phases were identified, including mayenite, perovskite, periclase, and alkali sulfates. In both clinker mixtures, a higher content of MgO in the steel slags resulted in the formation of periclase. Furthermore, the hydration kinetics and compressive strength at 7 and 28 days were studied in two cements prepared from clinkers sintered at 1250 °C. As evidenced by the results of isothermal calorimetry, the hydration kinetics were also influenced by the minor clinker phases. Cement with a higher content of calcium sulfoaluminate phase developed a higher compressive strength.

Transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean: constraints from the Jurassic adakitic and I-type granites, and calc-alkaline diorites at the northern margin of the North China Craton

ABSTRACT There is a broad consensus that subduction of the Paleo-Pacific oceanic plate has caused extensive magmatism, lithospheric deformation, and metal mineralization in the eastern margin of the Eurasian continent during the Late Mesozoic. However, the onset and extent of subduction of the Paleo-Pacific Ocean, especially the timing of the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific, and extent of the subduction of the Paleo-Pacific Ocean remain poorly constrained. To address these key issues, we present regional geological observations, zircon U–Pb dating, whole-rock geochemistry, and Hf isotopic compositions of the Jurassic intrusions at the northern margin of the North China Craton, with the aim of constraining their petrogenesis and the tectono-magmatic events related to subduction of the Paleo-Pacific Ocean. The geochronological and geochemical results indicate that these intrusions were emplaced between 177 and 155 Ma, with distinct geochemical features. The Huajianggou pluton (HJG), Tongjiatun pluton (TJT), and Haoguantun pluton (HGT) are characterized by high Al2O3 and Sr contents, and low MgO, TFe2O3, Y, Yb, and heavy rare earth elements (HREEs) contents, coupled with high Sr/Y values (34.10–94.36), showing adakitic affinities. Based on these geochemical characteristics, we suggest that these adakitic magmas (i.e. the HJG, TJT, and HGT plutons) were produced by partial melting of thickened lower crust in response to earlier subduction of the Paleo-Pacific oceanic plate beneath the northern margin of the North China Craton. In contrast, the Baijiagou pluton (BJG) and Sheshangou pluton (SSG) are high-K calc-alkaline rocks with low Sr/Y ratios (0.80–17.77), and they exhibit petrological and geochemical characteristics of I-type granites, indicating an origin from the lower crust, with variable degrees of fractional crystallization. In addition, the Bakeshu pluton (BKS) is mainly comprised of diorite that displays high SiO2, low MgO, Cr, and Ni contents, with Mg# of 39.8–42.9, indicating a lower crustal origin. Despite the range in ages of these intrusions, all the Jurassic calc-alkaline intrusions show typical geochemical characteristics of active continental margins, such as enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs). Combined with spatial-temporal variations of coeval magmatic rocks along the northern margin of the North China Craton, we suggest that the transition in tectonic regime from the Paleo-Asian Ocean to Paleo-Pacific Ocean was initiated in the Early Jurassic. These Jurassic intrusions formed in an active continental margin setting, related to westward subduction of the Paleo-Pacific oceanic plate. Importantly, the Early Jurassic subduction-related calc-alkaline magmatic rocks in northeast (NE) China constitute a NE-trending magmatic arc belt, which is best explained as continental magmatism triggered by the westward subduction of the Paleo-Pacific oceanic plate.

Geochemical, mineralogy, and Sr–Nd–Pb isotopic compositions of the gold-related lamprophyre in the Bengbu–Wuhe Area, southeastern North China Craton: Implications for gold mineralization

Besides granite porphyries, many mafic dikes also have a close temporal and spatial relationship with orebodies in the BWGB. However, compared with the granite porphyries, no detailed attempt has been made to determine the petrogenesis of these mafic dikes or to elucidate their relationships with the gold mineralization. Here, we present comprehensive bulk-rock geochemical data; pyrite, apatite, and amphibole compositions; Sr-Nd-Pb isotopes, and U-Pb zircon data for the lamprophyres to provide constrains on their petrogenesis, magmatic properties, geodynamic setting, and metallogenic implications. We obtained zircon U-Pb ages, which reveal that the lamprophyres were emplaced ~ 130 Ma. The lamprophyres are characterized by pronounced light rare earth element (LREE) enrichment, heavy rare earth element (HREE) depletions ((La/Yb)N > 10). They also have low SiO2 (<51 wt%) and high MgO (>4.1 wt%) contents; low Rb/Sr (0.01–0.04), Th/La (0.1–0.12), and Lu/Yb (0.13–0.16) ratios; high Ba/Rb ratios (>18), low Sr-Nd values (87Sr/86Sr(t) = 0.7060–0.7076; εNd(t) = –8.8 to –12.5); and a wide range of radiogenic Pb isotopic ratios (206Pb/204Pb(t) = 16.37–17.35), indicating an enriched mantle source metasomatized by the upper crust of the subducted South China Block (SCB). Their low apatite MnO contents and high apatite SO3 contents, high apatite molar OH/F ratios, and high amphibole Mg* and VIAl* values suggest that the mafic magmas were more highly oxidized and hydrated. Taken together, the lithospheric mantle source was initially metasomatized by the SCB and was further hydrated by the subducted Paleo-Pacific Plate, resulting in a mafic melt with a high H2O content and oxygen fugacity value, which are favorable conditions for gold transportation.

Intra-continental boninite-series volcanic rocks from the Bangong-Nujiang Suture Zone, Central Tibet

The boninitic volcanic rocks from Rutog, western Bangong-Nujiang Suture Zone (BNSZ), are characterized by high MgO, SiO2 but low TiO2 contents and interpreted as the products of intra-oceanic subduction in earlier studies. Recently, we discovered new breccia-like boninitic volcanic rocks (mainly trachyandesite and trachyte) that overlie the Rutog ophiolitic peridotites in BNSZ. These volcanic rocks not only show typical boninitic petrographical features of porphyritic texture in which euhedral augite phenocrysts (ca. 5 vol%) are embedded in fine-grained groundmass composed of pyroxene, plagioclase, quartz and minor glass, but also display strong boninitic geochemical affinities with high Mg# (61.59–67.90), Al2O3/TiO2 (40.31–55.97) ratios and low TiO2 (0.26–0.35 wt%) contents. In addition, their rare earth elemental patterns show a slight right-dip pattern with (La/Gd)N values of ~1.28 and (Gd/Yb)N values of ~1.31. Given the evolutionary trend from magnesium-rich intermediate to acid series, the boninite-series are therefore used to describe the vlocanic rocks. Zircon U-Pb isotopic dating on these boninite-series volcanic rocks yielded an age of ca. 79.3 ± 2.0 Ma, after the closure of the Bangong-Nujiang Tethyan Ocean (BNTO) that occurred before 96 Ma. The newly recognized boninite-series volcanic rocks probably formed more than 20 Ma after the latest regional magmatism related to subduction process and thus the subduction setting is not suitable for these boninite-series volcanic rocks. The enrichment in compatible elements and depletion in high field elements indicate the Rutog boninite-series originated from a severely depleted mantle while several lines of evidence (e.g., the enrichment in light rare earth elements, large iron lithophile elements) imply that the mantle source has been modified by continental crust. Given the widespread Cretaceous high Sr/Y magmatic rocks within BNSZ, the Rutog boninite-series rocks may provide an example of the crust-mantle interaction as a result of the delamination in a partially thickened and cratonized convergent margin during the late Cretaceous within BNSZ, central Tibetan Plateau.

Construction of a highly silicic upper crust in southeastern China: Insights from the Cretaceous intermediate-to-felsic rocks in eastern Zhejiang

The Cretaceous granitoids in eastern Zhejiang were examined to establish a model for the construction of the highly silicic upper crust in southeastern China. The studied rocks include quartz monzodiorite, quartz monzonite, alkali feldspar granite, and their microgranular enclaves (ME). Zircon UPb dating suggests that the granitic rocks and MEs were emplaced at approximately 102–114 Ma. Geochemical features and isotopic compositions of the quartz monzodiorite, quartz monzonite, and alkali feldspar granite indicate that they are mainly produced by the partial melting of Neoproterozoic to Mesoproterozoic crustal rocks. The quartz monzodiorites, quartz monzonites, and alkali feldspar granite appeared to derive from a similar silicic magma reservoir because they have comparable crystallization age, zircon trace element variation trend, Sr-, Nd-, and Hf-isotopic compositions, and complementary geochemical compositions. The studied quartz monzodiorites, quartz monzonites, and low silica alkali feldspar granites (SiO2 < 75 wt%) are deemed a residual cumulate according to their geochemical compositions and textural features (e.g., aggregates of coarse-grained euhedral plagioclase crystals in “glomeroporphyritic aggregates”), whereas the high silica alkali feldspar granites (SiO2 > 75 wt%) resemble frozen remnants of high silica melts segregated from a crystal mush because they are highly enriched in SiO2 and depleted in Ba, Sr, and Eu. The MEs hosted in the quartz monzodiorites and quartz monzonites have low SiO2 and high MgO contents, high initial 87Sr/86Sr ratio and negative εNd(t) and εHf(t) values, and variable zircon δ18O and εHf(t) values. Petrographic observations and geochemical features suggest that they are formed from solidified mafic magmas and generated by the crystal fractionation of mantle-derived hydrous mafic magma with subsequent crustal contamination. Compared with these mafic MEs, the MEs hosted by the alkali feldspar granites have relatively high SiO2 and low K2O contents, indicating that the felsic MEs are generated by crystal fractionation of the parental magma of the mafic MEs. Since abundant MEs occurred in the quartz monzodiorites, quartz monzonites, and alkali feldspar granites, magma recharge may have promoted the extraction and upward percolation of high silicic melts from the compacting crystal-rich mush in the shallow crust. Thus, the chemical and lithological diversity of the granitic plutons in eastern Zhejiang is generated in the upper crust via crystal-melt segregation. Specifically, the quartz monzonites and quartz monzodiorites are formed as residual cumulate, whereas the high silica alkali feldspar granites are generated as frozen remnants of high silicic melts segregated from a compacting crystal mush. More widely, our work suggests that the formation of the highly silicic upper crust in southeastern China is controlled by crystal-melt segregation processes in the shallow crust.

PETROLOGICAL CONSTRAINTS ON THE ORIGIN OF PYROXENITE DYKES IN THE LITHOSPHERIC MANTLE OF THE CHESHMEH-BID OPHIOLITIC MASSIF, SOUTHERN IRAN

The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.

Petrogenesis and tectonic implications of the ca. 2.5 Ga granitoid assemblage in the eastern Zhongtiao region, southern North China Craton

The key to unraveling the crustal evolution and extensive magmatism of the North China Craton (NCC) at the Archean–Proterozoic boundary (ca. 2.5 Ga) is to investigate the petrogenesis of the volumetrically dominant granitoids. In this contribution, we report new petrological, mineralogical, and geochemical data for the Huping and Luojiahe trondhjemites, which are ca. 2.5 Ga granitoid assemblages from the eastern Zhongtiao region of the southern NCC. The Huping trondhjemites have high SiO2 and Na2O contents and high (La/Yb)N and Sr/Y values, which are consistent with most Tonalite–trondhjemite–granodiorite (TTG) rocks worldwide. Evolved whole-rock Nd and Hf isotopic compositions indicate that the Huping trondhjemites were derived from partial melting of preexisting ca. 2.7 Ga mafic crust, and high MgO, Cr, and Ni contents suggest the possible involvement of depleted mantle in their source region. Integrated petrological, mineralogical, and geochemical data indicate that the Huping trondhjemites were likely formed by partial melting of delaminated lower crust. In contrast, the Luojiahe trondhjemites are Na- and Si-rich with negative Eu and Sr anomalies. Their geochemical characteristics suggest that they were likely formed by partial melting of low-K mafic crust that was shallower than the source of the Huping trondhjemites. The higher LREE content of allanite from the Luojiahe trondhjemites relative to that from the Huping trondhjemites indicates the Luojiahe trondhjemites formed at a higher melting temperature. The occurrence of the two types of coeval granitoids indicates that ca. 2.5 Ga crustal anatexis occurred at different depths. Furthermore, a strong heat flux at the base of the southern NCC at ca. 2.5 Ga is evident, which might have been introduced by mafic magma underplating. Based on our new data and the records of coeval magmatism in the region, this study suggests that lithospheric delamination and large-scale mantle-derived magma underplating beneath the thickened crust could have played important roles in the extensive magmatism of the southern NCC at the Archean–Proterozoic boundary.

U–Pb–Hf–O–Nd isotopic and geochemical constraints on the origin of Archean TTG gneisses from the North China Craton: Implications for crustal growth

Tonalite–trondhjemite–granodiorite (TTG), an Archean dominant lithological assemblage within the cratonic provinces, is believed to have been formed by partial melting of the hydrous basalts in the garnet stability field. The origin of TTG is a window to decipher the evolution of Archean continental crust, whereas the petrogenesis is not well constrained. To solve this issue, we have carried out field observations, zircon U–Pb–Hf–O isotope, as well as whole-rock Nd–O isotope and geochemical analyses on the TTG and associated dioritic gneisses from the North China Craton. U–Pb isotopic dating using the LA–ICP–MS on zircons reveals that the granitoid gneisses in northern Liaoning were contemporaneously generated during ca. 2.56–2.51 Ga and immediately experienced a regional metamorphic event at 2.48–2.45 Ga. The dioritic gneisses occur as quenched inclusions with rounded to ellipsoidal globules in shape rather than lithospheric relics. Such occurrence, together with isotopic and geochemical data, suggests that the mafic enclaves and host TTG represent two co-existing, compositionally distinct magmas. The TTG gneisses are featured by elevated SiO2 (64.72–86.46 wt%) contents, (La/Yb)N (68 on average) and Sr/Y (119 on average) ratios, and low MgO (0.35–2.58 wt%) contents and Mg# values, with radiogenic Hf and Nd isotopes (εHf(t) = +3.5 to +9.0 and εNd(t) = +0.8 to +5.4), indicating the derivation from partial melting of the juvenile basaltic sources at lower crustal levels. Compared with the TTG gneisses, the dioritic gneisses have lower SiO2 contents, higher MgO contents and Mg# values, with similar Hf–Nd isotopes (εHf(t) = +3.6 to +9.5 and εNd(t) = +3.3 to +5.1), implying the depleted mantle source. A model is proposed whereby partial melts from juvenile basaltic sources evolved via mixing with mafic enclaves (viz. dioritic magma) and fractional crystallization, and finally were contaminated by the sedimentary rocks, forming the TTG magma. The granitoid gneisses in northern Liaoning witnessed the major vertical growth of continental crust at the end of the Neoarchean, which is supported by Hf–Nd–O isotopic data and zircon Hf–O isotopic modeling. Our results show that the geochemistry of TTG (such as elevated La/Yb and Sr/Y) might be affected by complex petrogenetic processes, i.e., partial melting of individual sources, magma mixing, fractional crystallization, and crustal contamination, rather than solely controlled by the P–T conditions of crustal anataxis as previously thought, which might have been also overlooked in other Archean cratons.

Oxidation Mechanism of Vanadium Slag with High MgO Content at High Temperature

Oxidation Mechanism of Vanadium Slag with High MgO Content at High Temperature

Petrogenesis of the Early Cretaceous adakitic intrusive rocks in Chuzhou and Guandian, Anhui Province: Constraints from zircon U-Pb geochronology and Sr-Nd-Hf isotopic compositions

本文以华北陆块东部早白垩世滁州闪长玢岩和管店石英闪长岩为研究对象，报道了新的全岩地球化学（主量元素、微量元素以及Sr-Nd同位素）以及锆石U-Pb年代学和Hf同位素的综合研究结果，进而约束其岩石成因和构造意义。滁州闪长玢岩中锆石发育振荡环带，管店石英闪长岩中锆石具条痕状吸收的特点，它们均为岩浆成因。锆石LA-ICP-MS U-Pb定年结果显示，滁州和管店闪长质锆石年龄分别为128±1Ma和130±2Ma，表明二者均形成于早白垩世。地球化学特征显示，滁州和管店闪长质岩石具有类似的主量元素和微量元素组成，整体具中等的SiO₂含量（58.55%~60.12%）和富MgO（4.43%~5.10%，Mg^#=57~61）的特征，Na₂O/K₂O比值为1.20~1.47，属于高钾钙碱性系列岩石；富集轻稀土元素和大离子亲石元素（如Rb、Ba），亏损重稀土元素和高场强元素（如Nb、Ta、Zr、Hf），并具不同程度的Eu负异常（δEu=0.85~0.93）。它们具高的Sr（575×10^-6~1035×10^-6）和低的Y含量（10.2×10^-6~15.3×10^-6）以及高的Sr/Y比值（57~69），属于埃达克质岩石。此外，滁州和管店闪长质岩石具有类似的Sr-Nd以及锆石Hf同位素的特征，滁州闪长玢岩的⁸⁷Sr/⁸⁶Sr初始比值介于0.7061~0.7066之间，ε_Nd（t）值为-17.7~-16.9，锆石ε_Hf（t）值变化于-23.1~-19.1之间，而管店石英闪长岩的⁸⁷Sr/⁸⁶Sr初始比值为0.7046~0.7059，ε_Nd（t）值为-18.7~-15.8，锆石ε_Hf（t）值为-25.6~-20.0。综合上述，滁州和管店闪长质岩石均属于高镁埃达克质岩石，起源于拆沉的加厚下地壳部分熔融的熔体与地幔橄榄岩反应的产物，形成于早白垩世华北东部陆块拆沉作用下的伸展背景。;This paper reports new geochemical data involving whole-rock major, trace and Sr-Nd isotopic compositions, and zircon U-Pb ages with Hf isotopic data for the Chuzhou dioritic porphyry and Guandian quartz diorite in the Early Cretaceous, with the purpose of constraining their petrogenesis and tectonic significance. Zircons in the dioritic porphyry display typical oscillatory zoning and in quartz diorite show clear striped absorption in cathodoluminescence images, both of which have magmatic origin. LA-ICP-MS zircon U-Pb dating yield age of 128±1Ma in dioritic porphyry and 130±2Ma in quartz diorite respectively, indicating that they were both emplaced during the Early Cretaceous. The dioritic rocks show intermediate SiO₂ (58.55%~60.12%) and high MgO contents (4.43%~5.10%, Mg^#=57~61) with Na₂O/K₂O=1.20~1.47, which are classified as high-K cala-alkaline series. In addition, they are enriched in LREE and LILE (e.g., Rb and Ba), depleted in HREE and HFSE (e.g., Nb, Ta, Zr and Hf) with negative Eu anomalies (δEu=0.85~0.93), and exhibit high Sr (575×10^-6~1035×10^-6) and low Y contents (10.2×10^-6~15.3×10^-6) with significantly high Sr/Y ratios (57~69), being typical of adakitic rocks. Moreover, the Chuzhou dioritic porphyry has uniform (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7061~0.7066, ε_Nd(t) values of -17.7~-16.9, and ε_Hf(t) values of -23.1~-19.1, and Guandian quartz diorite has similar (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7046~0.7059 and variable ε_Nd(t) (-18.7~-15.8) and ε_Hf(t) (-25.6~-20.0) values. As a consequence, the adakitic intrusive rocks in Chuzhou and Guandian are derived from the partial melting of delaminated lower crust in contamination with mantle peridotite, and we supposed that the rocks are formed in an extension setting during the Early Cretaceous delamination in eastern North China Block.

Geochemical and geochronological constraints on the origin and emplacement of the Shimo‐ondori diorites in Shikoku, Southwest Japan

AbstractIn this study, the LA‐ICP‐MS zircon U–Pb dating of the Shimo‐ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo‐ondori diorites are suggesstive that they could be influential for the estimate of the early‐Cretaceous tectonic evolution for the eastern Asian margin. Their whole‐rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO2 and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (εHf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge‐mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo‐ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo‐ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.

Tectonic and magmatic evolution of the Aqishan-Yamansu belt: A Paleozoic arc-related basin in the Eastern Tianshan (NW China)

Abstract The Aqishan-Yamansu belt in the Chinese Eastern Tianshan represents a Paleozoic arc-related basin generally accompanied by accretionary magmatism and Fe-Cu mineralization. To characterize the tectonic evolution of such an arc-related basin and related magmatism and metallogenesis, we present a systematic study of the geochronology, whole-rock geochemistry, and Sr-Nd isotopes of igneous rocks from the belt. New zircon U-Pb ages, in combination with published data, reveal three phases of igneous activity in the Aqishan-Yamansu belt: early Carboniferous felsic igneous rocks (ca. 350–330 Ma), late Carboniferous intermediate to felsic igneous rocks (ca. 320–305 Ma), and Permian quartz diorite and diorite porphyry dikes (ca. 280–265 Ma). The early Carboniferous felsic rocks are enriched in large ion lithophile elements (LILEs) and depleted in Nb, Ta, and Ti, showing arc-related magma affinities. Their positive εNd(t) values (3.3–5.9) and corresponding depleted mantle model ages (TDM) of 0.83–0.61 Ga, as well as high MgO contents, Mg# values, and Nb/Ta ratios, suggest that they were derived from lower crust with involvement of mantle-derived magmas. The late Carboniferous intermediate igneous rocks show calc-alkaline affinities, exhibiting LILE enrichment and high field strength element (HFSE) depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg# values with positive εNd(t) values (3.9–7.9), and high Ba/La and Th/Yb ratios, implying a depleted mantle source metasomatized by slab-derived fluids and sediment or sediment-derived melts. The late Carboniferous felsic igneous rocks are metaluminous to peraluminous with characteristics of medium-K calc-alkaline I-type granites. Given the positive εNd(t) values (6.3–6.6) and TDM ages (0.56–0.53 Ga), we suggest the late Carboniferous felsic igneous rocks were produced by partial melting of a juvenile lower crust. The Permian dikes show characteristics of adakite rocks. They have relatively high MgO contents and Mg# values, and positive εNd(t) values (7.2–8.5), which suggest an origin from partial melting of a residual basaltic oceanic crust. We propose that the Aqishan-Yamansu belt was an extensional arc–related basin from ca. 350 to 330 Ma; this was followed by a relatively stable carbonate formation stage at ca. 330–320 Ma, when the Kangguer oceanic slab subducted beneath the Central Tianshan block. As the subduction continued, the Aqishan-Yamansu basin closed due to slab breakoff and rebound during ca. 320–305 Ma, which resulted in basin inversion and the emplacement of granitoids with contemporary Fe-Cu mineralization. During the Permian, the Aqishan-Yamansu belt was in postcollision extension stage, with Permian adakitic dikes formed by partial melting of a residual oceanic crust.

Mantle cooling and cratonization of Archean lithosphere by continuous plate subduction: Constraints from TTGs, sanukitoids, and high-K granites, eastern North China Craton

The evolution of plate subduction styles on the Earth is coupled with its cooling history. Continuous subduction was initiated during the Neoarchean, yet its influence on mantle cooling and the cratonization of Archean lithosphere remains unclear. Successive subduction-related magmatism occurred during the late Neoarchean evolution of the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton (NCC) and preserved crucial clues relating to processes and roles of regional continuous subduction. In this study, petrology, whole rock geochemistry, and zircon U–Pb–Hf isotopes are reported for late Neoarchean TTG gneisses, quartz monzodiorites, and monzogranites from the WSP. Zircon U–Pb dating reveals that the TTG gneisses, quartz monzodiorites, and monzogranites were formed at ~2561–2517 Ma, 2540–2533 Ma, and 2541–2513 Ma, respectively. The TTG gneisses with high (La/Yb)N ratios, low K2O/Na2O ratios, and low MgO contents were mainly derived from partial melting of oceanic slab material. In contrast, the quartz monzodiorites show high K2O and MgO content and high (La/Yb)N ratios, consistent with the characteristics of Archean sanukitoids, suggesting that they were the products of partial melting of mantle source metasomatized by slab-derived melts. The monzogranites with systematically high K2O, SiO2, and low MgO contents were formed by crustal reworking of regional TTGs and sedimentary rocks. Rock assemblages, geochemical features, regional field observation, and structure geology indicate prolonged (~2.56–2.49 Ga) continuous magmatism in the WSP across three northwest-trending belts (Belts A, B, and C) related to continuous subduction and slab rollback. These processes led to a tectonic transition from early compression to late extension. The final stabilization of the continental crust in the late Neoarchean in the eastern NCC was a consequence of successive mantle cooling due to regional continuous subduction recorded by subduction-related rock assemblages (i.e., adakites, sanukitoids, high-Mg basalts-andesites, and Nb-enriched basalts), the paired type of metamorphism, and the rise of alkali basalts. Secular geochemical changes of alkaline basalts outline the role of continuous subduction in dramatic mantle cooling and the final stabilization and cratonization of the eastern NCC.

Neoproterozoic metasomatized mantle beneath the western Yangtze Block, South China: Evidence from whole-rock geochemistry and zircon U-Pb-Hf isotopes of mafic rocks

Mafic rocks contain important information about the nature of the mantle source and tectonic settings. Here we present results of whole-rock geochemistry and zircon U-Pb-Hf isotopes for the Early Neoproterozoic mafic rocks (Ganyuhe hornblende gabbros and Guandaoshan gabbro-diorites) in the western Yangtze Block, South China, in order to constrain the petrogenesis and the mantle metasomatism processes under subduction setting. LA-ICP-MS zircon U-Pb dating results display concordant crystallization ages of 857.4 ± 2.0 Ma for the hornblende gabbros and 856.4 ± 2.8 Ma for the gabbro-diorites, respectively. These mafic rocks possess low SiO2 contents (45.5–53.3 wt%) as well as high MgO contents (3.27–9.20 wt%) and Mg# (48.6–62.6) values, and show characteristics of arc-like trace element patterns with enriched Rb, Ba, Sr, K, and Pb but depleted Nb, Ta, Zr, Hf, and Ti. The prominently positive whole-rock εNd(t) (+4.9 to + 5.8) and zircon εHf(t) (+8.48 to + 20.6) values suggest a depleted lithospheric mantle source. We therefore infer that the mafic rocks investigated here were derived from an amphibole-bearing spinel peridotite source that was affected by metasomatism between the overlying mantle wedge and subduction-related fluids. In conjunction with the compiled data of Neoproterozoic mafic rocks from the western Yangtze Block, we propose that there exist Neoproterozoic metasomatized mantle sources beneath the western Yangtze Block during the subduction process in response to the evolution of the supercontinent Rodinia. The Neoproterozoic metasomatic agents in mantle sources include the slab fluids, sediment melts, and oceanic slab melts.

Control of slag formation in the electric furnace smelting of ferronickel for an energy-saving production

Two-thirds of the global nickel output can be attributed to the rotary kiln-electric furnace process. Due to the high contents of MgO and SiO2 in laterite, a high temperature (∼1600 °C) and corresponding energy (approximately 14 GJ/t-ferronickel) are required during the electric furnace smelting. This work verifies the feasibility of decreasing the smelting temperature by controlling the slag formation for an energy-saving production. The slag regulation method was upgraded through the coordination of the quaternary basicity [ = R4 = m(CaO + MgO)/m(SiO2+Al2O3)] and FeO content. The melting temperature was decreased, and the melting rate of the feed was increased after slag control, owing to the generation of diopside and olivine eutectics instead of olivine and pyroxene. The viscosity of the slag was decreased, thus lowering the required smelting temperature. After regulating the quaternary basicity from 0.58 to 0.61 and the FeO content from 5.0 to 12.5 wt% in the final slag, the smelting temperature decreased by 86 °C. The energy consumption decreased from 13.75 to a minimum of 11.16 GJ/t-ferronickel. Furthermore, a long-term industrial practice (annual output: 0.24 million tons of ferronickel) demonstrated that the total energy consumption could be decreased by 0.16 million GJ a year, which is equivalent to reducing carbon dioxide emissions by 45 kilotons a year.