Early Cretaceous Metasomatized Lithospheric Mantle beneath the Central Jiangnan Orogen in South China:Geochemical and Sr-Nd Isotope Evidence from the Tuanshanbei Dolerite

It is well established that Cretaceous magmatism in the South China Block(SCB)is related to the Paleo-Pacific subduction.However,the starting time and the associated deep crust-mantle processes are still debatable.Mafic dike swarms carry important information on the deep earth(including mantle)geodynamics and geochemical evolution.In the Jiangnan Orogen(South China).there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not.In this study,we present detailed zircon U-Pb geochronological,wholerock element and Sr-Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes,and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time.LA-ICP-MS zircon U-Pb dating suggests that this dolerite erupted in the Early Cretaceous(~145 Ma).All samples have alkaline geochemical affinities with K_(2)O+Na_(2)O=3.11-4.04 wt%,K_(2)O/Na_(2)O=0.50-0.72,and Mg^(#)=62.24-65.13.They are enriched in LILE but depleted in HFSE with higher initial^(87)Sr/^(86)Sr ratio(0.706896-0.714743)and lower ε_(Nd)(t)(-2.61 to-1.67).They have high Nb/U,Nb/La,La/Sm and Rb/Sr,and low La/Nb,La/Ta,Ce/Pb,Ba/Rb,Tb/Yb and Gd/Yb ratios.Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution.Tuanshanbei dolerite were most likely derived from low-degree(2%-5%)partial melting of a phlogopite-bearing mantle material consisted of~85% spinel peridotite and~15% garnet peridotite previously metasomatized by asthenospherederived fluids/melts with minor subduction-derived fluids/melts.Slab-rollback generally lead to the upwelling of the hot asthenosphere.The upwelling of asthenosphere consuming the lithospheric mantle by thermo-mechanical-chemical erosion.The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension.It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca.145 Ma.

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle:1.Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite+K-rich terite,some including 5%of accessory phases,have been conducted at 15 and 50 kbar.The assemblages represent probable source components that contribute to melts in cratonic regions,but whose melt compositions are poorly known.A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene(CPX),phlogopite(PHL)and K-richterite(KR)with or without 5%ilmenite,rutile or apatite.Additional experiments were run without KR and with higher proportions of accessory phases.Melt traps were used at near-solidus temperatures to facilitate accurate analysis of wellquenched melts,for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50°C of the solidus,so that melts reflect the composition of the amphibole and its melting reaction.Melts have high SiO_(2) and especially K_(2)O but low CaO and Al_(2)O_(3) relative to basaltic melts produced from peridotites at similar pressures.They have no counterparts amongst natural rocks,but most closely resemble leucite lamproites at 15 kbar.KR and PHL melt incongruently to form olivine(OL)and CPX at 15 kbar,promoting SiO2 contents of the melt,whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures,leading to an increase in Mg O and decrease in SiO_(2) in melts,which resemble olivine lamproites.Melts of mica pyroxenites without KR are richer in CaO and Al_(2)O_(3) and do not resemble lamproites.These experiments show that low CaO and Al_(2)O_(3) in igneous rocks is not necessarily a sign of a depleted peridotite source.Accessory phases produce melts exceptionally rich in P_(2)O_(5) or TiO_(2) depending on the phases present and are unlike any melts seen at the Earth’s surface,but may be important agents of metasomatism seen in xenoliths.The addition of the 5%accessory phases ilmenite,rutile or apatite result in melting temperatures a few ten of degrees lower;at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX+PHL+KR mixtures are close to cratonic geotherms at depths>130 km:minor perturbations of the stable geotherm at>150 km will rapidly lead to 20%melting.Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth,but will change if reaction with wall-rocks occurs,leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite.At higher temperatures,extensive melting of peridotite will dilute the initial alkaline melts:this is recognizable as alkaline components in basalts and,in extreme cases,alkali picrites.Hydrous pyroxenites are,therefore,components of most mantle-derived igneous rocks:basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.

Early Devonian Ultrapotassic Magmatism in the North China Craton: Geochemical and Isotopic Evidence for Subcontinental Lithospheric Mantle Metasomatism by Subducted Sediment–Derived Fluid

The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

Geochemical characteristics and tectonic setting of the Tuerkubantao mafic-ultramafic intrusion in West Junggar,Xinjiang,China

Mineral chemistry, whole-rock major oxide, and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion, in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt. The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite, olivine pyroxenite, gabbro, and diorite. The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks. The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB. In addition, the Tuerkubantao intrusion displays relatively low Th/U and Nb/U （1.13-2.98 and 2.53-7.02, respectively） and high La/Nb and Ba/Nb （1.15 4.19 and 37.7-79.82, respectively）. These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting. The trace element patterns of peridotites, gabbros, and diorite in the Tuerkubantao intrusion have sub-parallel trends, suggesting that the different rock types are related to each other by differentiation of the same primary magma. The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite. However, the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic setting, internal lithological zoning, and geochemistry. The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions. In combination with the Devonian magmatism and porphyry mineralization, we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.

Early Indosinian Weiya Gabbro in Eastern Tianshan, China: Elemental and Sr-Nd-O Isotopic Geochemistry, and Its Tectonic Implications

The Weiya gabbro in eastern Tianshan was formed during the early Indosinian. This rock, with low ratios of Ce/Pb （5.74-10.16）, is notably characterized by enrichment in large ion lithophile elements （LILE）, such as Rb, K, Ba and Pb, and in high field strength elements （HSFE）, such as U and Th, but depletion in Nb and Ta. All samples of the Weiya gabbro display similar chondrite-normalized patterns with moderate enrichment in LREE （72.58-135.61ppm）, moderate depletion in HREE （15.26-25.31ppm） and mild fractionation between LREE and HREE （L/ H=4.09-5.98）. The average initial Sr value of the rock is 0.7069, and δ18O values of the rock range from 5.67‰-8.04‰. In terms of Nd isotope ratios, the Weiya gabbro is characterized by positive eNd（t） values （0.52-0.76）. All these characteristics indicate that the source region of the Weiya gabbro was metasomatized by fluids released from subducted young continental crust, with limited crustal contamination during magma ascent and emplacement. Continental （A-type） subduction was induced by northward subduction of the Paleo-Tethyan oceanic plate during the latest Permian to Triassic. From this point of view, it is supposed that tectonic conversion from the Paleo-Asian to the Paleo-Tethys regime occurred during the latest Permian or earliest Triassic.

Ancient deep roots for Mesozoic world-class gold deposits in the north China craton:An integrated genetic perspective

The North China Craton(NCC)hosts some of the world-class gold deposits that formed more than 2 billion years after the major orogenic cycles and cratonization.The diverse models for the genesis of these deposits remain equivocal,and mostly focused on the craton margin examples,although synchronous deposits formed in the interior domains.Here we adopt an integrated geological and geophysical perspective to evaluate the possible factors that contributed to the formation of the major gold deposits in the NCC.In the Archean tectonic framework of the NCC,the locations of the major gold deposits fall within or adjacent to greenstone belts or the margins of micro-continents.In the Paleoproterozoic framework,they are markedly aligned along two major collisional sutures-the Trans North China Orogen and the Jiao-Liao-Ji Belt.Since the Mesozoic intrusions hosting these deposits do not carry adequate signals for the source of gold,we explore the deep roots based on available geophysical data.We show that the gold deposits are preferentially distributed above zones of uplifted MOHO and shallow LAB corresponding to thinned crust and eroded sub-lithospheric mantle,and that the mineralization is located above regions of high heat flow representing mantle upwelling.The NCC was at the center of a multi-convergent regime during the Mesozoic which intensely churned the mantle and significantly en riched it.The geophysical data on Moho and LAB upwarp from the centre towards east of the craton is more consistent with paleo-Pacific slab subduction from the east exerting the dominant control on lithospheric thinning.Based on these results,and together with an evaluation of the geochemical and isotopic features of the Mesozoic magmatic intrusions hosting the gold mineralization,we propose a genetic model that invokes reworking of ancient Au archives preserved in the lower crust and metasomatised upper mantle and which were generated through multiple subduction,underplating and cumulation events associated with cratonization of the NCC as well as the subduction-collision of Yangtze Craton with the NCC.The heat and material input along zones of heterogeneously thinned lithosphere from a rising turbulent mantle triggered by Mesozoic convergent margins surrounding the craton aided in reworking the deep roots of the ancient Au reservoirs,leading to the major gold metallogeny along craton margins as well as in the interior of the NCC.

Sheared peridotite xenolith from the V.Grib kimberlite pipe,Arkhangelsk Diamond Province,Russia:Texture,composition,and origin

The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V.Grib kimberlite pipe（Arkhangelsk Diamond Province,Russia） was studied.Based on petrographic characteristics,the peridotite xenolith reflects a sheared peridotite.The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages：（1） a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains（Gar1） with sinusoidal rare earth elements（REE） chondrite C1 normalized patterns;（2） a neoblastic olivine and orthopyroxene assemblage;（3） the last assemblage associated with the formation of clinopyroxene and garnet marginal zones（Gar2）.Major and trace element compositions of olivine,orthopyroxene,clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent.The nature of the metasomatic agent was estimated based on high field strength elements（HFSE） composition of olivine neoblasts,the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene.All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.

Lithium elemental and isotopic disequilibrium in minerals from peridotite xenoliths from Shangzhi, NE China： products of recent melt/fluid-peridotite interaction

Lithium elemental and isotopic disequilibrium has frequently been observed in the continental and oceanic mantle xenoliths, but its origin remains controversial. Here,we present a combined elemental and Li isotopic study on variably metasomatised peridotite xenoliths entrained in the Cenozoic basalts from Shangzhi in Northeast (NE) China that provides insight into this issue. Li concentration (0.3–2.7 ppm) and δ7 Li (mostly 2‰–6‰) in olivine from the Shangzhi peridotites are similar to the normal mantle values and show roughly negative correlations with the indices of melt extraction(such as modal olivine and whole rock MgO). These features are consistent with variable degrees of partial melting. In contrast, clinopyroxene from the Shangzhi xenoliths shows significant Li enrichment (0.9–6.1 ppm) and anomalously light δ7 Li (-13.8‰ to7.7‰) relative to normal mantle values. Such features can be explained by Li diffusion from silicate melts or Li-rich fluids occurring over a very short time(several minutes to several hours). Moreover, the light Li isotopic compositions preserved in some bulk samples also indicate that these percolated melts/fluids have not had enough time to isotopically equilibrate with the bulk peridotite. We thus emphasize that Li isotopic fractionation in the Shangzhi mantle xenoliths is mainly related to Li diffusion from silicate melts or Li-rich fluids that took place shortly before or coincident with their entrainment into the host magmas.

Formation of Melt Pocket in Mantle Peridotite Xenolith from Western Qinling, Central China: Partial Melting and Metasomatism

Two types of melt pockets, closed melt pocket （CMP） and open melt pocket （OMP）, are recognized from the peridotite xenoliths entrained in the Cenozoic kamafugites in western Qinling （秦岭）, Central China. The Haoti （好梯） CMPs have a mineral assemblage of olivine＋ clinopyroxene＋amphibole＋K- feldspar, whereas the Baiguan （白关） CMPs are composed of olivine＋clinopyroxene＋Umenite＋carbonate. The components of the OMPs are more complicated. In the Haoti OMPs, there are olivine, ciinopyroxene, glass, low modal abundances of amphibole, K-feldspar （Kfs）, Umenite, sulfide, chlorite, perovskite, chromite and phlogopite. The Baiguan OMPs contain olivine, clinopyroxene, glass, chlorite and chromite. Compositionally, ofivines in the CMPs and OMPs are both apparently depleted in Ni, and those in the OMPs are also depleted in Fe and Mg, and enriched in Ca compared to the primary ones. Ciinopyroxenes display large and systematical compositional variations between the CMPs and OMPs, particularly in Al, Cr, Na, Ca and Ti. Glasses are generally depleted in Si compared to the worldwide glasses in melt pockets, although they still have large variations. Amphiboles and K-feldspars have relatively restricted compositional variations. The petrographical observations and mineral chemistry suggest that the Haoti and Baiguan CMPs were generated by the in-sitn decompression melting of orthopyroxenes, olivines and clinopyroxenes, and by the addition of minor external K-rich and Ca-rich melt/fluids. The OMPs formed during the latest metasomatic event in the lithospheric mantle beneath the western Qinling.

Carbonate metasomatism in the lithospheric mantle: Implications for cratonic destruction in North China

The activity of melts and fluids may have played a key role in inducing the destruction of the eastern North China Craton in the early Cretaceous. Carbonate melts are important agents in mantle metasomatism and can significantly modify the physical and chemical properties of the subcontinental lithospheric mantle. Carbonate metasomatism can be identified by specific geochemical indices in clinopyroxene, such as high Ca/Al and low Ti/Eu ratios. This study presents the spatial and temporal variations of carbonate metasomatism in the lithospheric mantle beneath the eastern North China Craton. Three types of carbonate metasomatism are classified based on the geochemical compositions of clinopyroxene in mantle peridotites. Clinopyroxene formed by Type 1 carbonate metasomatism is characterized by very high Ca/Al ratios(15–70) and^(87)Sr/^(86)Sr ratios(0.706–0.713). Clinopyroxene derived from Type 2 carbonate metasomatism shows relatively high Ca/Al ratios(5–18) and^(87)Sr/^(86)Sr ratios(0.703–0.706). However, clinopyroxene resulting from Type 3 carbonate metasomatism has low Ca/Al ratios(5–9) and^(87)Sr/^(86)Sr ratios(0.702–0.704). Deep(garnet-bearing) and shallow(spinel-bearing) lithospheric mantle beneath the Sulu orogen and surrounding areas in the eastern North China Craton were affected by intense Type 1 carbonate metasomatism before the late Triassic. The deep subduction of the South China Block with its accompanying carbonate sediments was the trigger for Type 1 carbonate metasomatism, which reduced strength of the lithospheric mantle and provided a prerequisite for the destruction of the eastern North China Craton in the early Cretaceous. After the destruction of the eastern North China Craton, the ancient relict lithospheric mantle, represented by spinel harzburgite xenoliths hosted in the late Cretaceous to Cenozoic basalts,only recorded Type 2 carbonate metasomatism. This implies that the lithospheric mantle experienced the intense Type 1 carbonate metasomatism was completely destroyed and not preserved during decratonization. Spinel lherzolite xenoliths hosted in the late Cretaceous to Cenozoic basalts represent the young, fertile lithospheric mantle formed after the cratonic destruction and only a few samples record Type 2 and 3 carbonate metasomatisms. We suggest that carbonate melts derived from the subduction-modified asthenospheric mantle with variable proportions of recycled crustal material was responsible for the Type 2 and 3 carbonate metasomatisms. The carbonate metasomatism of the lithospheric mantle beneath the Jiaodong Peninsula and surrounding areas is very pervasive and is spatially consistent with the remarkable thinning of lithospheric mantle and giant gold deposits in this region. Therefore, we conclude that carbonate metasomatism in the lithospheric mantle played a crucial part in the modification, destruction and gold deposits in the eastern North China Craton.

Metallogenetic model of Jiaodong-type gold deposits,eastern China

The genesis of giant gold provinces is an international scientific frontier,in which the source of a huge amount of gold and the drive for mineralization are key challenges.The mineralization intensity of the Jiaodong gold province in eastern China is a rare occasion in the world,because it owns gold reserves of~5500 tons within an area of less than 10,000 km^(2).The Jiaodong gold province formed in the superimposed domain of diverse tectonic regimes in an intracontinental setting.Paleoproterozoic regional peak metamorphism and Triassic continental collision are followed by the tectonic transition and gold mineralization at ca.120 Ma with time intervals of 1.9 billion years and about 100 million years,respectively.The Jiaodong gold deposits are different from orogenic and intrusion-related gold deposits in terms of the tectonic background,types of host rocks,and oreforming mechanism.These gold deposits show close spatial-temporal and genetic relationships to mafic igneous rocks,implying the derivation of ore-forming fluids from the metasomatic mantle domains.Mafic dykes in Jiaodong have negativeε_(Hf)(t)values of-29.9 to-9.1,Os content of 0.002-0.16 ppb,heavyδ^(18)O up to 8.23‰,and high initial^(187)Os/^(188)Os ratios of 0.1352-0.8858.These indicate that the lithospheric mantle was metasomatized by ancient crust-derived components.The lithospheric mantle in the western Jiaodong shows generally more enriched isotope features than that in the eastern part,which is explained to be an important reason for its huge gold resources.The mafic dykes show lighter Mg isotope characteristics(averageδ^(26)Mg of-0.33‰,n=50)and high Ca O content(overall greater than 6.5 wt%),indicating that the lithospheric mantle would also record the metasomatism by the carbonate rocks from the subducting oceanic slab.Under the background of the rollback of the subducting paleo-Pacific slab and the destruction of the North China Craton,partial melting of the lower crust would produce granitic magmas that led to the enrichment of gold in the residual crust.The syn-mineralization asthenosphere upwelling would promote the recycling of the lower crust and the partial melting of the metasomatic mantle domains.Basic magmas,produced by the partial melting,and the mantle itself would degas to form ore fluids.The ore fluids would further leach gold in the lower crust to increase its fertility.The auriferous fluids were transported to the middle to upper crust along the detachment and strike-slip faults.Water-rock interaction and fluid immiscibility,which occurred in and above the ductile-brittle transition zone to induce gold precipitation,formed the Jiaodong gold deposits.Given the unique geological features and genetic model of the Jiaodong gold deposits,they can be defined as“Jiaodong-type”gold deposits.

The distribution,characteristics and fluid sources of lode gold deposits:An overview

Lode gold deposits are among the most economically important types of gold deposits in the world.Globally,they formed mainly in three time intervals,2.8 to 2.5 Ga,2.1 to 1.8 Ga,and 700 Ma to the present.Sources of ore-forming fluids and other components are of critical importance in a better understanding of the genesis and the geodynamic controls of these deposits.Although ore-forming fluids were mostly derived from devolatization of sedimentary and/or volcanic sequences during greenschist to amphibolite facies metamorphism associated with orogenic deformation,magmatic hydrothermal fluids have been increasingly shown to be important in many gold deposits in various regions.In this review paper,we summarize the major features of lode gold deposits,possible sources of ore-forming fluids,and mechanisms of gold mineralization.While we acknowledge the critical role of metamorphically derived fluids in the genesis of such deposits worldwide,we emphasize that mantle-or basaltic magma-derived fluids may have been much more important than commonly thought.We use the Liaodong peninsula of the North China Craton as an example to demonstrate the significance of mantle-derived fluids.Integrating earlier studies and new data,we show that some of the late Mesozoic lode gold deposits in the North China Craton may have formed from magmatic hydrothermal fluids due to the extension and partial melting of the hydrated,metasomatized subcontinental lithosphere mantle,as best exemplified by the Wulong gold deposit.

The origin of arc basalts:New advances and remaining questions

Whether arc magmatism occurs above oceanic subduction zones is the forefront of studies on convergent plate margins.The most important petrologic issue related to the evolution of arc systems is the origin of arc magmatism,among which arc basalts are the most important one because they provide insights into mantle enrichment mechanism and crust-mantle interaction at oceanic subduction zones.Fluids or melts released either by dehydration or by melting of subducting oceanic slab infiltrate and metasomatize the overlying mantle wedge at varying depth,leading to the formation of source regions of arc basalts.Such processes make most of arc basalts commonly enriched in large ion lithosphile elements and light rare earth elements,but depleted in high-field strength elements and heavy rare earth elements.Small amounts of arc basalts are characterized by relatively high Nb contents or by Nb enrichment.Rare basalts with compositions similar to ocean island basalts or mid-ocean ridge basalt also occur in arc systems.For these peculiar rocks,it remains debated whether their source is affected by subduction-related components.During their ascent and before their eruption,arc basaltic magmas are subjected to crystal fractionation,mixing and crustal contamination.In addition to the contribution of subducting slab components to the mantle source of arc basalts,the materials above the subducting slab at forearc depths would have been transported either by drag or by subduction erosion into the subarc mantle and into the source of arc magmas.Heats and materials brought by corner flows also play important roles in the generation of arc basalts.Despite the important progresses made in recent studies of arc basalts,further efforts are needed to investigate subarc mantle metasomatism,material recycling,the formation of arc magma sources,geodynamic mechanism in generating arc basalts,and their implicationd s for the initiation of plate tectonics on Earth.

Geochemical characteristics and geological significance of Cretaceous phonotephrite from the Mid-Pacific Mountains

Systematic analyses were conducted including the petrographic features, major and trace elements, Sr and Nd isotopic compositions, and mineral structure and compositions of whole rocks. Mid-Pacific Mountain volcanic rocks are mainly phonotephrite with a porphyritic texture. Phenocrysts are mainly composed of Ca-rich plagioclase, clinopyroxene and nepheline.These volcanic rocks are significantly rich in large-ion lithophile and light rare earth elements, without obvious Eu anomalies(δEu=0.99–1.03), and with relatively enriched^(87) Sr/^(86) Sr(0.703829–0.704313) and^(143) Nd/^(144) Nd isotopic ratios(0.512857–0.512871), suggesting that they have similar but more enriched features than the OIB magmatic source. These volcanic rocks may originate from relatively deep magma source with the existence of spinel-garnet Iherzolites, and have undergone partial melting at a low degree of 1–3%. In addition, The residual Nb-Ta minerals(such as sphene, rutile, perovskite) may remain in the mantle source, and the magma components have undergone metasomatism by carbonate melt/fluid or alkali-rich fluid, causing high contents of incompatible elements and significant loss of Nb, Ta and Ti in these volcanic rocks. There are many similarities between the phonotephrites in the Mid-Pacific Mountain and the volcanic rocks in the Line Islands based on the tectonic settings and the geochemical characteristics. We thus speculate that Site 313 volcanic rocks in the Mid-Pacific Mountain is most likely to be a continuation of the Line Islands.

Source of ore-forming substances and theoretical problems of metallogeny relevant to the Bayan Obo Fe-REE ore deposits in Inner Mongolia,China

The source of rare earth elements (REE) ore-forming substances is identified to be extremely distinct from that of iron ores.The Bayan Obo Fe-REE ore deposits were generated by a composite process of both crustal and mantle source mineralization.The original iron bodies are of a sedimentary deposit from supergenesis,while the REE ores have been formed by mantle fluid metasomatism superimposed upon the pre-existing iron bodies.It is believed that the REE ore deposit would be controlled by intracontinental hot spot.The H_8 dolomite in mine regions belongs to normal sedimentary carbonate rock,its C and O isotopic composition rules out the possibility comparable with magrnatic carbonatite.The Sm-Nd isochrons of separated REE minerals have shown two REE peak mineralization periods:early-middle Proterozoic (1 700 Ma±480 Ma) and Caledonia (424-402 Ma).