Tracing crustal contamination of the Cenozoic basalts with OIB-affinity in northern marginal region of North China Craton:An Os perspective

The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution.The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources,while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene.However,in this study,the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton.In this model,the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent.The crustal contamination process is supported by(1)the highly radiogenic Os isotopic compositions,and(2)the positive correlation between 187Os/188Os and 1/Os of the Jining basalts.By modeling the Os isotopic composition of the basalts,an incorporation of<10%mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os.In contrast,the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination.Os isotopic data acts as an indicator of crustal contamination during magma evolution,providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.

Crustal Contaminations Responsible for the Petrogenesis of Basalts from the Emeishan Large Igneous Province,NW China:New Evidence from Ba Isotopes

Emeishan large igneous province (ELIP),one of the largest continental volcanic provinces worldwide (e.g.,Karoo),is extensively distributed along the western margin of the Yangtze Block.Even though the consensus that the ELIP is of a plume-related origin has been reached for decades,the role of crustal contamination in the petrogenesis of these basalts is still debatable so far.This paper firstly reports the Ba isotopic compositions of the continental flood basalts from the Lijiang,Miyi and Emeishan regions to discuss the genesis of basalts from the ELIP.According to their TiO_(2) contents and Ti/Y ratios,these basalts are divided into two groups,the low-Ti basalts from Lijiang and the high-Ti basalts from Miyi and Emeishan.The Ba isotopic compositions show that the low-Ti basalts have the δ^(138/134)Ba values from-0.33‰to+0.23‰with an average of-0.02‰±0.40‰,and the high-Ti basalts from -0.38‰to+0.38‰with an average of 0.038‰±0.36‰,all of which show a wider range of Ba isotopes relative to that of the primitive mantle (PM).This is unlikely to be explained by partial melting,fractional crystallization or even chemical weathering owing to little Ba isotopic fractionation during these processes.In contrast,variable extents of crustal contamination into the basaltic magmas more likely resulted in such isotopic diversity.The ratios of incompatible elements (e.g.,Nb/U,La/Nb,and Nb/Y) further suggest that the low-Ti basalts experienced higher degrees of crustal contaminations than those high-Ti basalts,which is well consistent with their spatial distributions.In general,the basaltic magmas in the ELIP were probably contaminated by different degrees of crustal materials during their upwelling to the surface.

Crustal Contamination of the Mantle-Derived Liuyuan Basalts:Implications for the Permian Evolution of the Southern Central Asian Orogenic Belt

The Permian basalts in the Central Asian Orogenic Belt(CAOB)are crucial for constraining the closure of the Paleo-Asian Ocean.However,the origin of these basalts is still under discussion.Here,we present comprehensive bulk-rock geochemical,Sr-Nd-Pb-Hf isotopic,and zircon U-Pb-Lu-Hf isotopic data of the Liuyuan basalts and coexisting gabbros,which are located in the Beishan Orogen in the southern CAOB,to constrain their emplacement setting and tectonic implications.Our new gabbro ages of ca.288–294 Ma are interpreted to represent the formation time of the Liuyuan basaltic belt.The Liuyuan basalts show MORB-like rare earth element(REE)patterns and bulk-rockε_(Hf)(t)andε_(Nd)(t)values of 11.0–15.4 and 4.6–9.2,respectively,suggesting an origination mainly from a depleted mantle source.However,positive Pb anomalies,Nb-Ta depletions,and high Th/Yb ratios as well as evolved Sr-Nd-Pb-Hf isotopic compositions of some samples indicate variable continental crustal contribution.According to the covariation of Pb anomalies(Pb^(*)=2×Pb_(N)/(Ce_(N)+Pr_(N)))with Sr-Nd-Pb-Hf isotopic compositions,we speculate that parent magma of the Liuyuan basalt was contaminated by continental crustal materials during the eruption rather than having been generated from an enriched mantle source.As revealed by mixing modelling,the Liuyuan basaltic magmas would require a minor(<10%)upper continental crustal assimilation to explain the enriched trace elemental and radiogenic Sr-Nd-Pb-Hf isotopic signatures.Consequently,the Liuyuan basaltic belt is believed to have been generated in a continental extensional environment instead of an oceanic setting and does not constitute a Permian ophiolitic suture zone as previously suggested,since the Paleo-Asian Ocean was already closed in the southern Beishan Orogen in the Early Permian.

Geochronology, Geochemistry and Sr-Nd-Pb-Hf Isotopes of No. I Complex from the Shitoukengde Ni–Cu Sulfide Deposit in the Eastern Kunlun Orogen, Western China: Implications for the Magmatic Source, Geodynamic Setting and Genesis

The Shitoukengde Ni-Cu deposit, located in the Eastern Kunlun Orogen, comprises three mafic-ultramafic complexes, with the No. I complex hosting six Ni-Cu orebodies found recently. The deposit is hosted in the small ultramafic bodies intruding Proterozoic metamorphic rocks. Complexes at Shitoukengde contain all kinds of mafic-ultramafic rocks, and olivine websterite and pyroxene peridotite are the most important Ni-Cu-hosted rocks. Zircon U-Pb dating suggests that the Shitoukengde Ni-Cu deposit formed in late Silurian （426-422 Ma）, and their zircons have ~Hf（t） values of-9.4 to 5.9 with the older TDMm ages （0.80-1.42 Ga）. Mafic-ultramafic rocks from the No. I complex show the similar rare earth and trace element patterns, which are enriched in light rare earth elements and large ion iithophile elements （e.g., K, Rb, Th） and depleted in heavy rare earth elements and high field strength elements （e.g., Ta, Nb, Zr, Ti）. Sulfides from the deposit have the slightly higher ~34S values of 1.9-4.3%o than the mantle （0 ~ 2%o）. The major and trace element characteristics, and Sr-Nd-Pb and Hf, S isotopes indicate that their parental magmas originated from a metasomatised, asthenospheric mantle source which had previously been modified by subduction-related fluids, and experienced significant crustal contamination both in the magma chamber and during ascent triggering S oversaturation by addition of S and Si, that resulted in the deposition and enrichment of sulfides. Combined with the tectonic evolution, we suggest that the Shitoukengde Ni-Cu deposit formed in the post-collisional, extensional regime related to the subducted oceanic slab break-off after the Wanbaogou oceanic basalt plateau collaged northward to the Qaidam Block in late Silurian.

Geochemistry and Petrogenesis of Tertiary Volcanic Rocks of the Eastern Roodbar, Alborz Mountain, North of Iran

In the Alborz Mountains of the eastern Roodbar (north of Iran), Tertiary volcanic rocks have a variety of composition between olivine basalt, basaltic andesite, pyroxene andesite and andesite. The presence of different xenoliths and xenocrysts is among the evidence of crustal contamination of these rocks. The positive correlations of CaO/Al2O3 vs to MgO and Al2O3 vs. SiO2 are of signs of the olivine and clinopyroxene fractionation in the variation trend of the area rocks. Positive correlations K2O/P2O5 vs with SiO2 and La/Sm vs. K2O/P2O5 demonstrate contamination of magma with the continental crust. The incompatible trace element patterns and their comparison with crustal contents indicate contamination of the rocks of the area with the lower and upper continental crust. Linear trends in the variation diagram of Nb/Y vs. Zr/Y, introduce two different source regions: a MORB source and the other continental crust for the rocks which are the genesis. The variations of Y/Nb vs. Zr/Nb and Rb/Y vs. Nb/Y reveal a crustal contamination of the magma originated from the MORB source. Geochemical studies represent that the area’s rocks were derived from the 15% melting of a mantle source of MORB type with spinel facies within a continental environment, which was contaminated by varying degrees of continental crustal rocks.

Petrogenesis of Volcanic Rocks in Alasht, Mazandaran, Iran

Area studied in this paper is located in the northwest of city of Alasht. The most important varieties of studied rocks are volcanic and semi-volcanic rocks of basic nature. These rocks, which form a distinguishable solid mass on 1:100,000 geological map, belong to Cretaceous period. According to results of petrographic study, rocks of this area include olivine basalt, dolerite, gabbro and microgabbro, and all have almost similar chemical compositions. According to chemistry-based naming process carried out through TAS method, these rocks are entirely basalt and basic and are all products of Sub-alkaline-Tholeiitic magma. According to Harker diagrams, a magmatic differentiation can be observed in all volcanic rocks;furthermore, basaltic magma erupted in this area has undergone an alteration and possibly a crustal contamination. The spider diagrams plotted in this paper show that basic rocks of Alasht area (basalts, andesite basalts, dolerites, gabbros and microgabbros) are all mostly similar to ocean island basalts (OIB). Once field study, sampling and microscopic study stages were complete, ten samples were selected for chemical analysis and were analyzed by XRF method.

Geochemistry of Mafic Rocks from the Birimian Basement of Doropo (Northeast of Côte d’Ivoire): Petrogenetic and Geodynamic Implications

The northeastern region of Côte d’Ivoire is characterised by a granitic basement mainly composed of biotite granite rocks. According to mapping work in the Gbabédjou and Doropo areas, these Birimian granitoids are cut by gabbro dykes and amphibolite enclaves, which are the subject of this study. In order to better understand the role and the implication of mafic rocks in the Doropo basement emplacement, a multidisciplinary methodology integrating microscopic observations and geochemical analyses of major and trace elements was carried out on 4 samples considered representative of the outcrops studied. Green hornblende, clinopyroxene, and accessory sphene minerals are found in mafic mineral phases, according to petrographic research. Whole-rock analyses reveal that mafic samples with TiO2 contents 1.1 ratios giving them an orogenic granite nature (I-type). Their REE patterns are moderately fractionated (La/Sm)N = 2.66 - 6.13 and (La/Yb)N = 11.17 - 43.70) with a very negative Eu anomaly (Eu/Eu* = 0.75 - 0.97). The multi-element diagrams are characterized by negative Nb-Ta anomalies and geotectonic studies have identified them as volcanic arc formations. All these characteristics allowed us to distinguish the Doropo mafic rocks as formations originating from the juvenile continental crust, emplaced under the Archean tectonics model with significant crustal contamination in the source. Magma driven by mantle diapir has been injected at the base of the continental crust and the heat induces the partial melting of the overlying crust giving rise to mixed liquids. This magma now enriched in LILE was immediately drained to the upper crust to form the mafic rocks from the studied area.

Magma Evolution Processes in the Southern Okinawa Trough:Insights from Melt Inclusions

The Okinawa Trough is an initial back-arc basin that is influenced by the subduction of the Philippine Sea Plate and develops on the continental crust.The Okinawa Trough is a natural laboratory for the study of basin evolution,magmatism,and crustmantle processes in the early stage of back-arc spreading.Melt inclusions are small droplets of magma that are captured during the mineral crystallization process and can record the geochemical composition changes during magma evolution.In this study,the geochemical compositions of melt inclusions in host plagioclases of two volcanic rock samples at Station nos.9-1 and 9-2 from the southern Okinawa Trough are systematically analyzed.Based on previous studies,the origin and evolution of magma and the introduction of subducting materials in the study area are discussed.Results show that melt inclusions are characterized by the relative enrichment of large-ion lithophile elements,depletion of high-field-strength elements,and slight enrichment of rare earth elements.Indeed,the subduction of the Philippine Sea Plate introduced sediment-derived melts and fluids into the magma source area of the southern Okinawa Trough.Subsequently,4%to 5%partial melting of the hydrated mantle produces basaltic magma.The melt inclusions of andesite and dacite investigated in this study were formed by fractional crystallization of basaltic magma.Finally,the crystallization of plagioclase,pyroxene,and magnetite occurred during the late stage of magma evolution.The temperature-pressure data show that the melt inclusions in plagioclase have two capture periods:one is at temperatures above 1250℃and the other is at temperatures between 1180℃and 1200℃.The capture pressure of the inclusions at temperatures between 1180℃and 1200℃is between 5.6 kPa and 6.1 kPa,corresponding to the depth of 15–17 km below the seafloor.The geochemical characteristics of major and trace elements in inclusions show that the samples from two stations(i.e.,9-1 and 9-2)have similar or identical magma source areas.However,the crystallization differentiation reflected by inclusions in sample 9-1 is more obvious than that in sample 9-2.The inclusions were captured during magma evolution and were not contaminated by crustal materials.