Low MgO Contents Research Articles

Petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites of the Songnen Block, NE China: Implications for the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans

The petrogenesis of Late Carboniferous A-type granites and Early Cretaceous adakites in NE China offers new insights into the geodynamic evolution of the Paleo-Asian and Paleo-Pacific oceans. This study reports new LA–ICP–MS zircon UPb ages, geochemical data, and zircon Hf isotopic compositions of syenogranites and quartz diorite porphyries exposed on the northern margin of the Songnen Block, NE China. Zircon UPb age data indicate that the syenogranites and quartz diorite porphyries were emplaced at ca. 319 and 120 Ma, respectively. The syenogranites have high K2O contents of 4.15–4.79 wt% and (K2O + Na2O)/CaO ratios of 33–102, and low MgO (0.06–0.20 wt%) and P2O5 (0.01–0.02 wt%) contents. Their remarkably high HREE contents and Y/Nb ratios (2.07–2.40), significantly negative Eu anomalies (Eu/Eu⁎ = 0.15–0.28), and low Sr contents (29.7–85.8 ppm) indicate that the syenogranites are A-type granites. They have positive zircon εHf(t) values of 8.4–12.1 and Neoproterozoic Hf TDM2 ages of 795–561 Ma, suggesting a juvenile lower crustal source. The quartz diorite porphyries have relatively high Na2O and MgO, and moderate K2O contents. Their A/CNK values (0.95–1.18) indicate a transition from being metaluminous to weakly peraluminous. They are enriched in LILEs (e.g., Rb, K, and Ba) and depleted in HFSEs (e.g., Nb, Ta, Ti, and P). Their low Y (11.1–14.4 ppm) and Yb (0.98–1.34 ppm) contents and high Sr (589–1131 ppm), Cr, and Ni contents and Sr/Y ratios indicate that they were generated by partial melting of oceanic crust. Based on these data and regional geological investigations, we propose that the syenogranites on the northern margin of the Songnen Block were formed in a post-collision extensional setting, likely due to closure of the Paleo-Asian Ocean between the Xing'an and Songnen blocks. Formation of the Early Cretaceous quartz diorite porphyries may have been triggered by partial melting of the Paleo-Pacific flat slab. Apart from rollback of the Paleo-Pacific oceanic slab, the main reason for the eastward migration of magmatism during the Cretaceous was the trench-ward dehydration, eclogitization, and sinking of the residual Paleo-Pacific flat slab.

Geochronology, geochemistry, and Hf isotopic compositions of early Permian syenogranite and diabase from the northern Great Xing’an Range, northeastern China: petrogenesis and tectonic implications

Geodynamic evolution in the late Paleozoic is significant for understanding the final amalgamation of the Central Asian Orogenic Belt (CAOB). No consensus has yet been reached regarding the late Paleozoic geodynamic evolution of the northern Great Xing’an Range (GXR) in northeastern China, the eastern CAOB. Furthermore, late Paleozoic syenogranite–diabase dyke association is present in the Xiaokele area in northern GXR. It provides an important opportunity to understand the nature of magmatism and the geodynamic evolution during this period. This paper presents new zircon U–Pb ages, zircon Hf isotopic compositions, and geochemical data of whole rocks for Xiaokele syenogranite and diabase. Zircon U–Pb dating suggests that the Xiaokele syenogranite (292.5 ± 0.9 Ma) and diabase (298.3 ± 1.5 Ma) were emplaced during the early Permian. The Xiaokele syenogranites have high SiO2 contents, low MgO contents, and enriched zircon εHf(t) values, suggesting that their primary magma was generated by the partial melting of the juvenile crustal material. The Xiaokele diabases have low SiO2 contents, high MgO contents, are enriched in large-ion lithophile elements, depleted in high-field-strength elements, and exhibit enriched zircon εHf(t) values. They derived from a lithospheric mantle source that had previously been metasomatized by slab-derived fluids. Combined with previous research results, we believe that the continent–continent collision between the Xing’an and Songliao blocks occurred during the late early Carboniferous – early late Carboniferous (330–310 Ma), and the two blocks were transformed into a post-collisional extensional setting during the latest Carboniferous – early Permian.

Late Paleoproterozoic crustal evolution in the Daqingshan area: Evidences from adakitic and A-type granitoids in the Guyang Changshengqu goldfield, Khondalite Belt, North China Craton

Granitic magmatism related to the collisional orogenic belt provides important information for the evolution of continental crust. We performed petrological, geochemical and zircon U-Pb-Hf isotopic studies on four granitoid samples from the Guyang Changshengqu goldfield, Daqingshan area, Khondalite Belt, North China Craton. The zircon U-Pb dating results indicate that the granitoids emplaced at 1950 ± 11 Ma, 1935 ± 12 Ma, 1846 ± 11 Ma, and 1812 ± 8.6 Ma, represent two periodic granitic magmatism in the Daqingshan area. The 1.95 and 1.93 Ga granites exhibit high SiO2, K2O, and Na2O contents with high Sr/Y and (La/Yb)N ratios, but low MgO, Cr, and Ni contents, indicating that they were produced by the partial melting of a thickened lower crust under relatively high-pressure conditions. The granites show negative-dominated zircon εHf(t) values (−5.7 to 1.9), and their zircon TDM2(Hf) values range between 2.9 and 2.5 Ga, indicating an ancient crust source with minor juvenile crustal material involved. The 1.85 and 1.81 Ga quartz monzonites exhibit high SiO2, K2O, Na2O, Zr, Nb, Ce, and Y but low MgO, Cr, and Ni contents, with high Ga/Al and FeOt/(FeOt + MgO) ratios, indicating an A-type granite affinity. The whole-rock geochemistry suggests that they were formed by dehydration re-melting of TTG and quartz diorite at high temperatures and shallow depths. The quartz monzonites show homogeneous εHf(t) values (−10.6 to −6.1) and the zircon TDM2(Hf) values range between 3.0 and 2.8 Ga, suggesting that they were derived from a Neoarchean ancient crust. The 1.95–1.93 Ga adakitic magmatism is associated with the 1.93–1.92 Ga mafic magmatism and UHT metamorphic in the eastern part of the Khondalite Belt; this suggests a tectonic transition from collisional compression to post-collisional extension. The 1.85–1.81 Ga A-type granitic magmatism indicates evident continental crust thinning and asthenospheric upwelling. The two unrelated magmatism are possibly induced by two collisional events from the Khondalite Belt and the Trans-North China Orogen, respectively. The collision of the western and eastern blocks at ca. 1.85 Ga may have played an important role in the A-type quartz monzonites origin and influenced the crustal evolution as far as the Daqingshan area in the Khondalite Belt.

Geochemical and isotopic compositions of Late Permian metavolcano–sedimentary sequence in the southern Songpan–Ganzi Orogen, eastern Tibetan Plateau

AbstractThe Permian metavolcano–sedimentary sequence of the Jianglang and Taka domes in the southern Songpan–Ganzi Orogen is composed of bedded marble and sandwiched metabasalt. This study presents geochemistry, C–O–Nd isotope systematics and zircon U–Pb geochronology data to explore the formation mechanism and tectonic setting of its protolith. The marble samples have high δ13CV-PDB (4.0 ‰ to −0.1 ‰) and δ18OV-SMOW (16.3 ‰ to 13.6 ‰) values, with similar εNd(t) values (−5.3 to −7.2) to Late Permian conodonts. They display seawater-like features, e.g. superchondritic Y/Ho ratios, negative Ce and positive La, Gd and Y anomalies, indicating a marine carbonate protolith in oxygen-rich palaeoseawater. Their pronounced positive Eu anomalies suggest a contribution (5 % to 1 %) of submarine high-T hydrothermal fluids. The metabasalt samples show low SiO2, high Fe2O3T and MgO contents. They have low La/Sm, (Th/Ta)PM and (La/Nb)PM ratios, without correlations between MgO and other major/trace elements. These signatures could be attributed to insignificant fractional crystallization and crustal contamination. Their rare earth elements (REE) patterns, trace element profiles and trace element ratios are comparable to those of mid-ocean ridge basalt (MORB), suggesting a depleted MORB-type mantle reservoir. Melt modelling illustrates low degrees (<10 %) of partial melting in the spinel stability field at a depth of 30–60 km. Moreover, the metabasalt contains 2453.7–258.9 Ma detrital zircons significantly related to the Palaeoproterozoic crystalline basement, Rodinia supercontinent, Caledonian events and c. 260 Ma Emeishan large igneous province. Combined with previous studies, we advocate that the investigated metavolcano–sedimentary suquence was most likely formed during the Late Permian in a mature back-arc basin of the Palaeo-Tethys ocean.

Syn-kinematic ferroan high-K I-type granites from Dschang in southwestern Cameroon: U–Pb age, geochemistry and implications for crustal growth in the late Pan-African orogeny

Abstract The Dschang granites, SW Cameroon, are post-Panafrican granitoids, hosted in Paleoproterozoic orthogneiss in the Western domain of the Central African fold belt. They include coarse-grained K-feldspar biotite granite and fine-grained magnetite granite. Magnetite forms nodules surrounded by a quartzo-feldsparthic halo. Major and trace element data of both granites show high SiO 2 , Al 2 O 3 , total alkalis and Ba and low TiO 2 , MgO, P 2 O 5 and Nb contents, conferring a fractionated I-type high-K alkali-calcic and weakly peraluminous character. Both granites are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and show a rare earth element pattern with Eu/Eu* = 0.3 to 0.9. Primitive mantle-normalized spidergrams display negative Nb, Ta and Ti anomalies, typical of their derivation from subduction-related post-collisional melt. Laser inductively coupled plasma mass spectrometry U–Pb data on zircons yielded crystallization ages of 578 +7/−11 Ma and 563+8/−3 Ma for the biotite and magnetite granites respectively. Inherited zircon (1991–1442 Ma), whole-rock Nd model ages in the range 2.1–1.9 Ga, highly negative ε Ndt (−16 to −18) and initial 87 Sr/ 86 Sr ratios in the range 0.7075–0.7114, support the remobilization of a Paleoproterozoic crust. The felsic magmas were probably produced by partial melting of Paleoproterozoic orthogneiss host rocks, under oxidizing conditions. Magnetite granite is also known from the Nkambé region. Such granites from both the Dschang and Nkambé regions suggest the presence of a Paleoproterozoic domain rich in iron mineralization. This domain potentially represents a Cu, Pb, Zn and Mo metallogenic province, heavily reworked during the Pan-African orogeny.

Influence mechanism of silicon content in Al-killed steel on compositions of inclusions during LF refining

ABSTRACT To obtain solid inclusions in LF refining process, the influence of silicon content in steel on compositions of inclusions during LF refining were investigated in Al-killed steel. Influence of silicon on compositions of inclusions was found to be great. The [Ca] content of steel was supplied from the reaction of [Al] and CaO. The [Si] content affected the [Ca] content by the reaction (SiO2) + 2[Ca]→[Si]+2(CaO), and [Ca] was consumed more readily by SiO2 in Si-free steel than in Si-containing steel, as a result, Al2O3 and MgO·Al2O3 inclusions could be obtained in Si-free steel, whereas calcium aluminate inclusions were found in Si-containing steel. Moreover, the inclusions were mainly solid Al2O3 with low contents of CaO and MgO when using low-calcium ferrosilicon alloy and adding all low-calcium ferrosilicon alloys required for the steel at the end of LF refining, this ensures high removal efficiency of inclusions in RH degassing.

The formation of the giant Huayangchuan U-Nb deposit associated with carbonatite in the Qingling Orogenic Belt

Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit.The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive δ18OSMOW (7.29 to 15.53‰) and negative δ13CPDB (−5.26 to −10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.

Tectonic evolution of the northeastern North China Craton: Constraints from geochronology and Sr–Nd–Hf–O isotopic data from Late Triassic intrusive rocks on Liaodong Peninsula, NE China

This paper reports new whole-rock major and trace element, SrNd isotopic, and zircon UPb and HfO isotopic data for Late Triassic intrusive rocks on Liaodong Peninsula, NE China, aiming to constrain the petrogenesis of these rocks in terms of lithospheric thinning in the northeastern North China Craton (NCC). New zircon UPb data indicate that the rocks were emplaced during the Late Triassic (220–211 Ma). Based on petrology, petrography and geochemistry, we divided these rocks into three groups. The Group 1 rocks, including hornblende gabbro, hornblende diorite and mafic vein, show low SiO2, Na2O + K2O and high MgO contents, with (87Sr/86Sr)i (0.7066–0.7071), εNd(t) (−10.6 to −5.7), εHf(t) (−16.6 to +1.5), and δ18O (7.06‰–7.82‰) values, indicating that they were formed by partial melting of lithospheric mantle with addition of lower continent crust materials. The Group 2 rocks, including syenites, display high SiO2, Na2O + K2O and low MgO contents, with (87Sr/86Sr)i (0.7064–0.7073), εNd(t) (−14.8 to −14.5), εHf(t) (−11.7 to −5.7), and δ18O (5.49‰–6.37‰) values, indicating they were originated through partial melting of lithospheric mantle with continent crust materials added, coupled by fractional crystallization of mafic minerals. The Group 3 rocks, including monzogranite and syenogranite, show high SiO2, Na2O + K2O and low MgO contents, with different isotopic signatures: the former shows (87Sr/86Sr)i (0.7085–0.7087), εNd(t) (−17.0 to −16.8), εHf(t) (−17.9 to −4.4) and δ18O (6.10‰–6.95‰) values. The latter shows higher εNd(t) (−9.0 to −8.1), εHf(t) (−3.4 to +1.2) and lower δ18O (5.52‰–7.29‰) values. Combined with the adakitic features of monzogranite and different two model ages, we believe they were derived from partial melting of ancient thickened lower crust and juvenile crust, respectively. Together with the spatial and temporal distribution of the Late Triassic rocks, we believe that these rocks were related to the collision between NCC and Yangtze Craton, and the initiation of lithospheric thinning of NCC in eastern segment began in Late Triassic.

Origin of the Triassic Lincang granites in the southeastern Tibetan Plateau: Crystallization from crystal mush

The crustal generation of the Paleo-Tethys orogenic belt in the southeastern Tibetan Plateau is a major unresolved geological problem. The Changning-Menglian suture in the southeastern Tibetan Plateau represents a remnant of the Paleo-Tethys Ocean. The Lincang batholith and associated volcanic rocks (the Manghuai rhyolites and Xiaodingxi basalts) are located to the east of the Changning-Menglian suture, and represent important constraints for the abovementioned geological problem. New zircon UPb ages, together with previously published results, indicate that the Manghuai rhyolites were emplaced at 229–239 Ma and the Lincang batholith yielded longer age span (211–239 Ma). Broad age span of zircons (to 28 million years) from these rocks record a long-lived crustal magmatic system in the Changning-Menglian Paleo-Tethys orogenic belt. These intrusive and associated volcanic rocks have significant differences between them in terms of geochemistry. In this study, these rocks are geochemically classified into three groups: (1) Group 1 comprises biotite granite and granodiorite rocks; (2) Group 2 includes alkali feldspar granite, two-mica granite, and rhyolite, which are high-silica (generally >70 wt%) rocks that have low TiO2, Al2O3, FeOtot, MgO, CaO, P2O5, Eu, and Sr contents, compared to those of Group 1. Based on their geochemical relationships and mineral textures, Groups 1 and 2 include cumulative- and fractionated-type granitoids, respectively; (3) while Group 3 is comprised of dioritic enclaves and the Xiaodingxi basalts. Based on more robust evidence, dioritic enclaves are derived from a mantle source magma. The Lincang granites have high and variable initial 87Sr/86Sr ratios (0.703122–0.728476), significantly negative εNd (t = 220 Ma) values (−13.4 to −7.5), negative εHf (t) values (−17.7 to −6.8), and old TDM2 ages. The Manghuai rhyolites and Xiaodingxi basalts have lower initial 87Sr/86Sr ratios (0.695970–0.709398), and higher εNd (t) values (−3.4 to −0.6) and εHf (t) values (−1.2 to +3.7) than those of the Lincang granites. Geochemical data suggest that the Manghuai rhyolites could be seen as fractionated and erupted melts from crystal mush reservoirs, which later crystallized to form the Lincang granites. The Manghuai rhyolites provide an instantaneous snapshot of the state of the crystal mush at the time of eruption. The Lincang granites are interpreted to record prolonged histories of open-system magmatic evolution, involving magma mixing, wall-rock assimilation, and multiple cycles of crystallization from the crystal mush reservoirs. Magmatic evolution was likely related to mantle upwelling in a post-collisional setting, forming large-volume granites and volcanics in the Changning-Menglian Paleo-Tethys orogenic belt. Thus, this crystal mush model provides an integrated picture of the Triassic silicic magmatism, linking the evolution of the Lincang batholith and Manghuai rhyolites before storage in the Changning-Menglian Paleo-Tethys orogenic belt.

Geochronology and geochemistry of the Late Mesozoic magmatic rocks in the Chizhou Area, Lower Yangtze River Metallogenic Belt: Constraints on petrogenesis and tectonic setting

Late Mesozoic magmatism and its associated polymetallic mineralization are widespread in the Chizhou area, forming one of the ore clusters in the Lower Yangtze River Metallogenic Belt (LYRMB). The mineralization in the Chizhou area, however, is not as same as the other ore clusters (e.g., Tongling, Edong, and Jiurui) in age, source, and mechanism. In this study, we used zircon UPb dating, LuHf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes to investigate six ore-bearing intrusions in the Chizhou area. These intrusions comprise high-K calc-alkaline series pyroxene diorite, quartz diorite (porphyry), and granodiorite (porphyry). LA-ICP-MS zircon UPb dating results indicate that these intrusions formed between 150 and 141 Ma. The mafic intrusion, the Xiaodingchong pyroxene diorite, has low SiO2 and MgO contents, arc-like trace element characteristics, as well as enriched Sr-Nd-Hf isotopic compositions and high radiogenic Pb isotopes, indicating that it was derived from an enriched lithospheric mantle source. The acid intrusion, the Pailou granite, has high SiO2, enriched Sr-Nd-Hf isotopic compositions, continental crust-like geochemical characteristics, and adakitic affinities, such as high Sr, low Y, and low Yb, suggesting that the Pailou granite was derived from partial melting of the Meso-Neoproterozoic thickened accretionary arc crust. The remaining intermediate-acid ore-bearing intrusions were produced by magma mixing of these two end members. The metallogenic deposits in the Chizhou area were mainly controlled by two different magma sources: an enriched mantle and the accreted Meso-Neoproterozoic crust. Furthermore, the low Ti-in-zircon temperatures and the variable amounts of inherited zircons indicate that the Late Mesozoic Chizhou ore-bearing intrusions were formed in hydrous conditions, which is consistent with the tectonic evolution of the low-angle paleo-Pacific plate subduction with slab foundering and roll-back.

Multi-stage crustal melting from Late Permian back-arc extension through Middle Triassic continental collision to Late Triassic post-collisional extension in the East Kunlun Orogen

The East Kunlun Orogen is an important part of the East Tethys region and has received significant attention with regards to the evolution of the Tethys Ocean. This contribution presents geochronological, whole-rock major and trace element geochemical, and Sr-Nd-Hf isotopic data of magmatic rocks within the Kengdenongshe polymetallic deposit in the eastern part of the East Kunlun Orogen. Here, we report zircon UPb ages of ca. 257 Ma and ca. 211 Ma for granite porphyry intrusions, and ca. 240 Ma for the rhyolitic tuff. These rocks are characterized by high SiO2, variable Al2O3 and K2O, low Na2O, MgO and CaO contents, and high A/CNK ratios, which is typical of S-type granitic rocks. They exhibit large-ion lithophile element enrichment, depletion of high field strength elements, have low (La/Yb)N ratios, and negative Eu anomalies. They also display variable (87Sr/86Sr)i ratios (0.709981 to 0.720907), negative εNd (t) values (−8.7 to −5.5), and a wide (enriched) zircon εHf (t) range (−10.1 to −0.8). The geochemical and isotope data indicate magma derivation through dehydration melting of heterogeneous crustal sources including clay-poor meta-sedimentary rocks and amphibolite, which are both parts of the East Kunlun Orogen basement. These results provide evidence for the evolution of the Paleo-Tethys Ocean in the East Kunlun Orogen including Late Permian (266–255 Ma) back-arc extension, Late Permian to Middle Triassic (255–240 Ma) subduction, Middle Triassic (240–225 Ma) continental collision, and Late Triassic (< 225 Ma) post-collisional extension. This study further suggests that the 257 Ma and 211 Ma granite porphyries are related to the asthenosphere upwelling in a back-arc basin and post-collision extensional setting, respectively. The 240 Ma rhyolitic tuff is linked to anatexis associated with crustal thickening during continental collision.

Geochronology, geochemistry, and Sr–Nd–Hf isotopes of the Late Permian–Early Triassic granitoids in Eastern Kunlun Orogen, Northwest China: petrogenesis and implications for geodynamic setting

ABSTRACT The Eastern Kunlun Orogenic Belt (EKOB) in Northwest China, is an ideal area for understanding the tectonic evolution of the Palaeo-Tethys Ocean. The EKOB is marked by widespread Palaeozoic to Early Mesozoic magmatism. However, the Late Permian–Early Mesozoic tectonic evolution of the Palaeo-Tethys Ocean tectonic regime is poorly understood. Here, we report new zircon LA-ICP-MS U–Pb dating and geochemical analyses of Late Permian–Early Triassic intrusive rocks in the Xiangyanggouxi and Dageledong areas in the EKOB, Northwest China, to discuss their petrogenesis and geodynamic setting and reconstruct the tectonic evolution of Palaeo-Tethys oceanic tectonic regime. Zircon U-Pb dating results indicate that these granitoids crystallized during ca. 256–248 Ma, i.e. the Late Permian to Early Triassic. These granitoids are mainly comprised of granodiorites and monzonitic granites that possess high concentrations of SiO2 (66.81–75.98 wt.%), K2O+Na2O (6.21–7.92 wt.%), and low contents of MgO, CaO, Cr, Ni, and Mg# (<0.44). They are classified as medium- to high-K calc-alkaline and metaluminous I-type granitoids. These granitoids are rich in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and are depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) with arc affinity. In addition, combined with the εHf(t) values of −2.4 to +2.4 with two-stage model ages ranging from 1409 Ma to 1027 Ma, and the εNd(t) values from −3.73 to −2.95, it is suggested that the primary magma of these granitoids was derived from the partial melting of the Late Meso–Proterozoic mafic lower crust. These findings, combined with spatio-temporal distributions of regional magmatism, reveal that the Late Permian and Early Triassic granitoids formed in a continental magmatic arc setting related to the northward subduction of the Palaeo-Tethys Ocean plate beneath the EKOB-Qaidam terrane.

Late Triassic back-arc spreading and initial opening of the Neo-Tethyan Ocean in the northern margin of Gondwana: Evidences from Late Triassic BABB-type basalts in the Tethyan Himalaya, Southern Tibet

The tectonic evolution of the Neo-Tethyan oceanic plate between the Indian Plate and the Lhasa Terrane is a key issue for the evolution of Gondwana and formation of the Tibetan Plateau. In this paper we report on the geochemistry, SrNd isotope compositions, and zircon UPb dating of Late Triassic basalts from the Nieru Formation (T3n) in the Kampa region, southern Tibet. The basalts have relatively low contents of MgO, TiO2, and total alkalis (K2O + Na2O), and they have affinities to tholeiitic basalt. They exhibit weakly fractionated rare earth element (REE) patterns with slight depletions in LREEs and slightly negative Eu anomalies on a chondrite-normalized diagram. On a primitive-mantle-normalized spider diagram, they are characterized by slight enrichments in large ion lithophile elements (LILEs) and relatively flat patterns of high field strength elements (HFSEs), except for depletions in Ba, Nb, Ta, and Ti. Their initial 87Sr/86Sr ratios vary from 0.718133 to 0.738977 and εNd(t) values are relatively depleted (4.09–5.22), similar to mid-ocean ridge basalts in the Yarlung Zangbo ophiolite. We propose the T3n basalts were derived from a shallow and depleted mantle source by relatively high-degree partial melting in the spinel stability field. They underwent slight crustal contamination as well as the fractional crystallization of clinopyroxene. The T3n basalts are similar to typical back-arc basin basalts (BABB) such as the Okinawa BABB. This indicates an extensional back-arc-basin setting along the northern margin of Gondwana during the Late Triassic. The initial opening of the Neo-Tethyan Ocean was related to the southwards subduction of the Paleo-Tethyan oceanic plate and back-arc-basin spreading during the Late Triassic. Magmatic activity along the passive continental margin records key information on continental break-up and incipient ocean development.

Geochemistry, geochronology and Hf isotope of granitoids in the northern Alxa region: Implications for the Late Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The tectonic setting of the northern Alxa region during the Late Paleozoic is highly controversial. The key to resolve this controversy is to recognize the Late Paleozoic magmatic processes in the northern Alxa. In this paper, we present new zircon U–Pb ages, Hf-isotopic compositions and whole-rock geochemical data of four granitoids along the Zhusileng-Hangwula Tectonic Belt in the northern Alxa region that could provide critical information about the tectonic evolution of this region. The zircon U–Pb data could be grouped as two phases: Late Devonian granite and diorite (ca. 373–360 ​Ma), and Late Carboniferous granodiorite (ca. 318 Ma). The Late Devonian granites and diorites are metaluminous to slightly peraluminous, with A/CNK and A/NK ratios of 0.90–1.11 and 0.95–2.19, respectively. The Late Devonian diorites are characterized by high MgO, Cr and Ni contents and MgO# values, together with variable εHf(t) values from –1.0 to +1.3 and old TDM2 ages varied from 1283 ​Ma to 1426 ​Ma, indicating the primary magma was potentially derived from magma mixing of depleted mantle with Mesoproterozoic continental crust. Even though the Late Devonian granites yielded most positive and minor negative εHf(t) values between –1.1 to +5.7 (three grains are negative) with two-stage model ages (TDM2) of 1003–1438 Ma, they display low MgO, Cr and Ni contents and MgO# values, suggesting that they were mainly derived from juvenile crustal materials, mixed with a small amount of ancient crust. The Late Carboniferous granitoids are metaluminous and medium-K calc-alkaline series, with A/CNK and A/NK ratios ranging from 0.88 to 0.95 and 1.75 to 1.90, respectively. These rocks were potentially derived from juvenile crustal materials mixed with depleted mantle, as evidenced by their high εHf(t) values (+11.6 to +14.1) and young TDM2 ages (427 ​Ma to 586 ​Ma), as well as high Mg# values, and MgO, Ni and Cr contents. Our data, along with available sedimentary evidence and previous researches, indicate that the Late Devonian and Late Carboniferous rocks are arc-related granitoids under the subduction setting. The identification of arc-related granitoids in the northern Alxa region not only reveals the Late Paleozoic magmatic process in response to the subduction of Paleo Asian Ocean, but also provide significant constrains to the tectonic evolution of the Central Asian Orogenic Belt.

EPMA and LA-ICP-MS Data of Clinopyroxene and Chromite from the Wadi Ghadir Ophiolites, Eastern Desert, Egypt

New EPMA and LA-ICP-MS data of pyroxene and chromite are used to assess the so far published tectonic setting of Wadi Ghadir Ophiolite (WGO) in the Central Eastern Desert of Egypt. The low contents of TiO2 (< 0.46 wt%) and high MgO (13.39-15.17 wt%) concentrations in the analyzed clinopyroxene suggest a depleted residual mantle source material. The trace element composition and REE patterns reflect transitional IAT-boninitic geochemical affinities in a supra-subduction zone setting. Chromite, on the other hand, shows high Cr#(Cr/(Cr+Al)) (0.83-0.88 mol) and relatively constant Mg#(Mg/(Mg+Fe+2)) (0.66-0.70 mol), indicating a high degree of partial melting of the uPP.er mantle source. These features are typical for supra-subduction zone in a back-arc or inter‐arc basin setting. The micro-analytical data of relict magmatic mineral phases in the WGO and other ophiolitic rocks in the Arabian-Nubian Shield can help in interpreting their precise tectonic setting as far as data of the immobile, incompatible elements are used.

Synchronous A-type and adakitic granitic magmatism at ca. 2.2 Ga in the Jiao–Liao–Ji belt, North China Craton: Implications for rifting triggered by lithospheric delamination

The Jiao–Liao–Ji belt (JLJB) underwent back-arc or intra-continental rifting during the early Palaeoproterozoic but the underlying mechanism remains unclear. To address this issue, this study focuses on the petrogenesis of widespread Palaeoproterozoic granitoids on Liaodong Peninsula, China. Zircon U–Pb dating and geochemical and Hf isotopic data suggest that these granitoids were formed at ca. 2.2 Ga and they can be divided into A-type and adakitic granites. The A-type granites have relatively high SiO2, K2O, Zr, Ga, Nb and Y contents, and low CaO, MgO, Al2O3 and Sr contents. Based on these characteristics, εHf(t) values (−0.8 to +2.8) and TDM2 ages (2797–2569 Ma), the A-type granites are inferred to be derived from Neoarchaean tonalite–trondhjemite–granodiorites in a rift setting. In contrast, the coeval adakitic granites have relatively high Na2O, Al2O3 and Sr contents and high Sr/Y and LaN/YbN ratios, but low MgO, Ni, Yb and Y contents, suggesting a magma source with garnet but no plagioclase in the residue beneath thickened lower crust. Their variable εHf(t) values (−8.5 to +6.9) and TDM2 ages (3276–2305 Ma) indicate that the adakitic magmas were mainly generated by the partial melting of Meso–Neoarchaean crust. The co-existence of contemporaneous ca. 2.2 Ga adakitic and A-type granites provides evidence that the JLJB underwent regional rifting in the Palaeoproterozoic, most likely due to delamination of a thickened lithosphere, as follows. (1) In the Neoarchaean, the lower crust of the Longgang Block underwent granulite- and/or eclogite-facies metamorphism due to continuous thickening, which resulted in increasing lower crustal density. (2) The thickened lower crust became unstable and delaminated at ca. 2.2 Ga. Asthenospheric upwelling following this lithospheric delamination triggered the thinning of the overlying lithosphere. The upper crust and remnant thickened lower crust were partially melted to form the A-type and adakitic granites, respectively. The rifting likely resulted in the separation of the Longgang and Nangrim blocks, prior to their amalgamation in the late Palaeoproterozoic to form the JLJB.

Late Devonian to early Carboniferous magmatism in the western Songliao–Xilinhot block, Northeast China: Implications for eastward subduction of the Nenjiang oceanic lithosphere

The timing and direction of narrowing and closure of the Nenjiang Ocean (NJO) by plate subduction remain poorly constrained. This paper presents geochemical, isotopic, and geochronological results for Late Devonian to early Carboniferous intrusive rocks in the western Songliao–Xilinhot block to constrain the evolution of subduction in the NJO region. The Late Devonian syenogranites and monzogranites formed at 366–360 Ma and are classified as A‐type granites. The wide range of εHf(t) values (−17.62 to +8.13) and TDM2 ages (789–2,208 Ma) of the syenogranites indicate that the primary magma was derived from partial melting of multiple sources, including both young accreted continental crust and old lower crust rocks. The early Carboniferous granitic mylonites formed at 342 Ma and are I‐type granites. Their high SiO2 and K2O contents, εHf(t) values of +6.61 to +9.86, and young TDM2 ages of 675 to 857 Ma suggest a preexisting juvenile crustal material source region. The early Carboniferous hornblende gabbros formed at 334 Ma and are part of an appinite suite. The gabbros have low silica and high MgO contents, E‐MORB‐like trace element features, εHf(t) values of +1.34 to +8.97, and young TDM1 ages of 595 to 902 Ma, which together suggest a primitive magmatic source including juvenile MORB and/or enriched lithospheric mantle. Magmatism in the western Songliao–Xilinhot block formed a late Palaeozoic magmatic arc stretching along the eastern Hegenshan–Heihe suture. A new model of the evolution of subduction in the NJO region and associated coeval magmatic events is proposed that includes westward subduction of NJO oceanic crust during the Late Devonian at 366–360 Ma, bidirectional subduction during the early Carboniferous at ~342 Ma, and break‐off/delamination of the eastward subducting slab at ~334 Ma.

Petrochemical and isotopic evidences for multiple sources involving in the generation of post-collisional granitoid complex: A case study of the Mangling complex from the eastern Qinling orogen, China

Granitoids widely occur in collisional orogens, which are a critical target for probing the nature of deep crust and mantle-crust interaction. In this study, we present an integrated study of in situ zircon U–Pb dating and Hf–O isotopes, in combination with whole-rock geochemistry, for the Mangling complex in the eastern Qinling orogen. The Mangling complex is composed mainly of monzogranites with some mafic microgranular enclaves (MMEs) and subordinate diorites. Zircon U–Pb dating yields identical crystallization ages of 141–143 Ma for the diorites and monzogranites, suggesting that they are formed simultaneously and belong to post-collisional magmatism. The Mangling diorites have relatively low SiO2 but high MgO, Cr and Ni contents and Mg# values. They are characterized by enrichment of large ion lithophile elements (LILEs) but depletion of high field-strength elements (HFSEs). They show initial Sr isotope (ISr) of 0.706491 to0.706622, εNd(t) of −10.82 to −10.38, and zircon εHf(t) of −10.8 to −7.9 and δ18O values of 6.48–7.18‰. These geochemical features indicate that the diorites were derived from an enriched lithospheric mantle source. In contrast, the Mangling monzogranites have high silica and low Mg# values. They exhibit more variable isotopic compositions: Isr = 0.707191–0.708310, εNd(t) = −13.74 to −8.10, and zircon εHf(t) = −17.2 to −7.0 and δ18O = 5.83–7.08‰. Some inherited zircon cores have been observed in the monzogranite samples with U–Pb ages of 1773–1791 Ma. These inherited cores have δ18O values of 5.42–5.69‰ and εHf(t) values of −7.8 to −5.9. Their U–Pb ages and Hf–O isotopic compositions are similar to those of the Xiong'er Group in the southern margin of the North China Craton (NCC), which have underthrusted the juvenile (in terms of isotopes) Kuanping unit where the Mangling complex has been emplaced. Isotopic modeling shows that the monzogranites were likely produced by fusion of the Kuanping unit and Xiong'er Group, with the involvement of enriched mantle-derived materials. The crustal rocks as well as the enriched sub-lithospheric mantle were both contributed to the Mangling complex, and some of the crustal components show more juvenile isotopic fingerprints. Therefore, it is not a mandatory to involve a depleted juvenile mantle-derived component to recognize mantle-crust interaction during granitoid generation, especially in which the preceded juvenile arcs were superimposed on ancient crust and/or enriched mantle. The ‘inverse’ isotopic compositions in crust and mantle components imply that a detailed petrochemical and isotopic study should be conducted to address the issue about crust-mantle interaction during granitoid generation.

Magnesium Isotopic Homogeneity of GSR-1 and RGM-2: Two Potential Standards for Mg Isotope Analysis of Low MgO Felsic Rocks

In sample preparation and mass spectrometry analysis, sample dissolution, column chemistry, concentration mismatches, and matrix effects have significant potential for introducing analytical artifacts during Mg isotope analysis. Based on the low MgO content and undesirable matrix elements in felsic rocks, the development of well-characterized felsic standards is essential to reduce inter-laboratory mass bias, enable the assessment of data accuracy, and facilitate the comparison of chemical separation procedures in different laboratories. In this work, the homogeneity and long-term stability of two felsic rock standards, GSR-1 and RGM-2, were evaluated due to their low MgO contents. Furthermore, synthetic solutions with doped matrix elements were used to evaluate potential Mg isotope analytical artifacts using multi-collector inductively coupled plasma mass spectrometry. The accuracy and precision of Mg isotopic compositions in GSR-1 and RGM-2 were assessed by repeated measurements over twelve months. The long-term tests show that the Mg isotopic compositions of the two low MgO felsic rocks (GSR-1 and RGM-2) are homogenous among batches and can be used as low MgO reference materials for accuracy assessments of Mg isotopic analyses. The Mg isotopic compositions (δ26Mg) of GSR-1 and RGM-2 were marked as -0.223‰±0.053‰ (2s, n=50) and -0.184‰±0.058‰ (2s, n=50) respectively.

Genetic links between granitic and related dioritic rocks in Liaodong Peninsula, China: Sr–Nd–Hf–O isotopic evidence

Syenogranite and related dioritic rocks (dioritic microgranular enclaves and quartz diorite) from the Early Cretaceous Shihuzhen–Jiudaogou pluton, NE China, were studied to investigate the formation and evolution of the granitic magma driven by the mantle-derived magma. All selected rocks were formed contemporaneously, with zircon UPb ages of 125–121 Ma. Field and petrographic observations show that magma mixing happened between mafic and felsic magmas represented by dioritic enclaves and syenogranite. The dioritic enclaves with quenching textures have low SiO2 and high MgO contents, variable zircon εHf(t) compositions (−8.7 to +2.4), coupled with low initial 87Sr/86Sr ratios (0.70647–0.70772) and variable εNd(t) values (−8.8 to −3.9) for apatite, suggesting they were derived from an enriched mantle source with contamination of crustal materials. The syenogranites have relatively high 87Rb/86Sr ratios (2.92–22.0), variable zircon εHf(t) compositions (from −11.1 to −3.6), coupled with low initial 87Sr/86Sr ratios (0.70727 to 0.70791) and variable εNd(t) values (from −9.9 to −7.3) for apatite, requiring a juvenile crust origin with participation of mantle-derived magma. They were characterized by high SiO2 (> 74.0 wt.%) and low MgO, P2O5 contents, with conspicuous negative Ba, Sr, and Eu anomalies, implying a late fractionation phase. The quartz diorites have relatively high initial 87Sr/86Sr ratios (0.70781–0.70928) for apatite, with negative zircon εHf(t) values (−11.6 to −7.8) and apatite εNd(t) values (−10.5 to −8.9), which are identical to those of the felsic parental magma of syenogranite. They show complementary geochemical compositions with the syenogranite, including low SiO2 (< 60.0 wt.%), high MgO (Mg# up to 44.2) and Sr (> 744 ppm) contents, with obvious positive Ba and Sr anomalies and weakly positive Eu anomalies. Coupled with petrological observations (e.g., cumulate texture), we argue the crystal-melting segregation model account for the genetic link between the quartz diorite and syenogranite, representing residual crystal accumulations and segregated interstitial melt from a crystal-rich magma chamber.The partial melting of juvenile lower crust and the subsequent crystallization differentiation of intermediate-acid magma developed a crystal-rich magma chamber. The mantle-derived magma then drove the rejuvenation of pre-exist magma chamber, promoted the crystal-melt segregation process and mixed with the felsic magma, and then formed quartz diorite and syenogranite in the Shihuzhen-Jiudaogou pluton. Mantle-derived magma played an important role in the formation of the granitic rocks, providing heat for partial melting, materials, and the driving force for their compositional evolution.