The Fanshan Apatite-Magnetite Deposit in the Potassic Ultramafic Layered Intrusion, North China

Abstract. This article reviews work carried out on the Fanshan layered apatite (-magnetite) deposit, which occurs in the Fanshan complex, a potassic ultramafic-syenite layered intrusion located in North China. The Fanshan complex intruded into Proterozoic sedimentary rocks in the early Triassic. The complex comprises First- to Third-Phase Intrusives. The First-Phase Intrusive includes rhythmic Units 1 to 9, which mainly consist of clinopyroxenite and biotite clinopyroxenite. The apatite (-magnetite) deposit is situated near the top of rhythmic Unit no. 6 of the First-Phase Intrusive.
The economical deposit comprises Orebody II and Orebody III. Orebody II, situated at the top of Unit no. 6, consists of two kinds of layer: one magnetite-apatite rock layer at the top and a few apatite rock layers at the bottom. Orebody III is situated several meters below Orebody II, and consists of an apatite rock layer.
There is controversy over the ore formation mechanism, the temperature and pressure of the magma intrusion, and the origin of the magma (including the possibility of magma immiscibility). We review these problems of this unique apatite (-magnetite) magmatic deposit.     

Minerogenic Model of Chrysotile Deposits in Ultramafic Rocks

Most chrysotile deposits occur in ultramafic rocks of the ophiolite suite. The chrysotile deposits dis-cussed in the present paper were formed through metasomatism and infilling-crystallization in a continentalserpentinization environment after plate convergence, where ultramafic rocks were replaced byhydrothermal solutions consisting mainly of deep-circulating heated water derived from atmospheric precip-itation. The critical state for the formation of asbestos in ultramafic rock bodies might be reached bysuperposition of multiple stages of serpentinization. Favourable fracture systems and relatively stable geo-logical environment are important conditions for forming chrysotile deposits. Three subtypes of chrysotiledeposits could be formed in different tectonic settings and under different minerogenic geochemical condi-tions.     

The evolution of the Precambrian Shield of Greenland

The Archaean gneiss block of Greenland is made up of gneisses, amphibolites, anorthositic rocks and minor supracrustals. It contains the oldest crustal rocks yet recorded on earth. The Archaean gneiss block is bordered to the north and to the south by Proterozoic mobile belts. The Nagssugtoqidian and Rinkian mobile belts to the north, differentiated on the basis of differences in the tectonic development, consist mainly of reworked Archaean rocks. Early Proterozoic supracrustal rocks are prominent in the Rinkian mobile belt, where they overlie the Archaean basement. The Ketilidian mobile belt to the south consists mainly of Proterozoic supracrustal rocks and granites. After renewed denudation late Proterozoic supracrustal rocks were deposited in North and South Greenland where they are associated with large amounts of late Proterozoic intrusive rocks.

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Zusammenfassung Das Archaische Kraton Grönlands ist aus Gneisen, Amphiboliten, anorthositischen und untergeordneten Suprakrustal-Gesteinen aufgebaut. Es enthält die ältesten bis jetzt gefundenen krustalen Gesteine. Das Archaische Kraton ist gegen Norden und gegen Süden von Proterozoischen Orogenen begrenzt. Die Nagssugtoqidischen und Rinkischen Orogene gegen Norden, die sich durch ihre verschiedene tektonische Entwicklung unterschieden, bestehen hauptsächlich aus aufgearbeiteten Archaischen Gesteinen. Früh-Proterozoische Suprakrustal-Gesteine spielen eine wichtige Rolle im Rinkischen Orogen, wo sie das Archaische Grundgebirge überlagern. Gegen Süden besteht das Ketilidische Orogen hauptsächlich aus Proterozoischen Suprakrustal-Gesteinen und Graniten. Nach erneuerter Denudation wurden spätproterozoische Suprakrustal-Gesteine in Nord- und Südgrönland abgelagert. Diese sind assoziiert mit bedeutenden Mengen von spätproterozoischen Intrusivgesteinen.

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Résumé Le socle archéen du Groenland est composé principalement de gneiss, d'amphibolites et d'anorthosites avec accessoirement des roches supracrustales. Dans ce socle se trouvent les roches les plus âgées de l'écorce terrestre trouvées jusqu'à présent. Au nord et au sud, le socle archéen est flanqué par des ceintures orogéniques protérozoïques. Au nord on trouve le Nagssugtoqidien et le Rinkien qui ont des styles tectoniques différents, et sont composés principalement de roches archéennes transformées. Dans le Rinkien les roches supracrustales du début du Protérozoique jouent un rôle important; elles y recouvrent les gneiss archéens. Au sud du socle archéen, la ceinture orogénique du Kétilidien est composée principalement de roches supracrustales et de granite protérozoïques. Après une période de dénudation intense, des sédiments et des laves d'âge protérozoïque tardif se sont déposées gans le nord et le sud du Groenland en association avec d'abondantes roches intrusives.

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Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqen Basin

The Nangqen basin is one of the Tertiary pull-apart basins situated in the east of the Qiangtang block.Similar to the adjacent Dengqen basin and Baxoi basin, there occurred a series of potassic volcanic and sub-volcanic rocks, ranging from basic, intermediate to intermediate-acid in lithology.Based on the study of petrology, mineralogy and geochemistry, including REEs, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in the Nangqen basin were formed in the post-collisional intraplate tectonic settings. The relations between the basic, intermediate and intermediate-acid rocks are neither differentiation nor evolution, but instead the geochemical variability is mainly attributable to the different partial melting degrees of the mantle sources formed at depths of 50-80 km.The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. The recycling model can b     

云开地块晚二叠世富Fe-Ti-P超镁铁质岩的年代学、地球化学与岩石成因

为了更好地理解华南板块南缘二叠纪与三叠纪之交复杂的构造格局,通过岩石学、地球化学和年代学的方法,对粤西云开地区新发现的3处晚二叠世超镁铁质侵入岩进行了研究.这些岩石位于云开地块东缘的阳春三甲和地块中部的高州大井、东岸等地,呈小岩株、岩脉产出于云开岩群的片岩、变粒岩中.它们的主要岩性为辉石岩、角闪石岩、角闪岩和斜长角闪岩...     

The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

Textural and mineral chemistry data for the ultramafic sequenceof the Hartley Complex are presented with the object of evaluatingemplacement mechanisms, crystallization history and sub-solidusre-equilibration processes for the Great ‘Dyke’.Mineral chemistry indicates in situ crystallizaration for theultramafic sequence, whereas textural evidence suggests thatlimited crystal settling of chromite took place. It is concludedthat crystallization of cumulus phases occurred at or near thefloor of the magma chamber. The mineral chemistry indicates that the volume of magma fromwhich each unit crystallized was significantly smaller thanthat represented by the stratigraphic succession of the HartleyComplex. The magma chamber may effectively have been part ofan open system during the crystallization of the ultramaficsequence. The results are consistent with the concept of a stratifiedmagma chamber and the process of double-diffusion convection. Modelling of the liquid line of descent and crystallizationsequences indicate that none of the previously proposed initialliquid compositions are likely to have constituted the parentalmagma of the Great ‘Dyke’. Rather than komatüticor exceptionally high magnesium liquids, as previously suggested,a parental magma with about 15 per cent MgO, similar to thecomposition of the chill phase of a dyke parallel to and inclose proximity to the East Dyke is in closest agreement withthe observed and modelled results. Chromite compositions are strongly related to textural and mineralogicalenvironments. Seam chromitites are higher in Cr, Mg and Fe³⁺than chromites enclosed in silicates. Chromite enclosed in cumulusolivine is higher in Fe²⁺ than that in coexisting pyroxenesbut there is little difference in the proportions of the trivalentcaptions. Seam chromitites are considered to have precipitatedin response to increases in foi associated with periodic influxesof magma into the magma chamber. The higher ferric iron contentof the seam chromitites compared with the chromite enclosedin the silicates is consistent with such a mechanism. Compositional zoning in olivine and pyroxene adjacent to enclosedchromite grains is interpreted as reflecting subsolidus re-equilibrationwith cooling. Zoning profiles exhibit strong crystallographiccontrol. Computer modelling using finite difference approximationshas allowed controlling factors to be assessed by optimizationof the modelled parameters to give closest agreement to themeasured results. Interdiffusion coefficients and distributioncoefficients for Fe²⁺ and Mg for olivine and pyroxene with chromiteare modelled and compared with published data. Indicated blockingtemperatures for olivine are of the order of 600 °C to 700°C and 750 °C to 850 °C for orthopyroxene. Thuschromites enclosed in orthopyroxene are more Mg-rich than thoseenclosed in olivine. Coarse-grained seam chromitites have beenlittle modified subsequent to crystallization but the compositionsof the associated silicates have been influenced by the modalabundance of the chromite. Geothermometers based on chromite-silicate equilibria are probablynot applicable to layered intrusions, but information on thermalhistories may be provided by evaluation of the diffusion profiles.     

The Age of Ultramafic Rocks of the Main Ural Fault

The results of U–Pb zircon age dating of ultramafic massifs occurring as a chain along the Main Ural Fault zone (MUF) are given. Three groups of ages were obtained (Ma): 2500?2800, 600?2100, and 430?440. The first age group represents the time of origin of the ultramafic rocks in the Earth’s mantle. The second age group records the time of metamorphism of these rocks prior to intrusion into the host rocks. The third age group determines the time of the intrusion of ultramafic magma into the host rocks. It was established that the intrusion of ultramafic magma along the entire length of the Ural ultramafic belt occurred in the age range of 430?440 Ma.     

Genetic Implications of Two Different Ultramafic Rocks from Hongseong Area in the Southwestern Gyeonggi Massif, South Korea

Two distinct ultramafic bodies occur in Baekdong and Bibong in the Hongseong area within Gyeonggi massif of South Korea. The Hongseong area is now extensively documented as an extension of the Dabie-Sulu collision belt in China. The Baekdong ultramafic body has a NWW elongation direction. This elongation trend is similar to the general trend of the Dabie-Sulu collision belt. The Bibong ultramafic body is elongated in a NNE direction and runs parallel to the direction of the main fault in the study area. The Baekdong ultramafic bodies show porphyroclastic and mylonitic textures while those at Bibong exhibit a mosaic texture. Both were grouped into peridotite and serpentinite based on their modal abundance of serpentine. In the olivine (Fo) vs. spinel [Cr# = Cr/ (Cr+Al)] diagram, both ultramafic rocks fall with in olivine spinel mantle array. The compositions of olivine, orthopyroxene and spinel indicate that the Baekdong ultramafic rock formed in deeper parts of the upper-mantle under passive margin tectonic setting. The SREE content of Baekdong ultramafic rock vary from 0.19 to 5.7, exhibits a flat REE pattern in the chondrite-normalized diagram, and underwent 5% partial melting. Conversely, large variation in SREE (0.5 21.53) was observed for Bibong ultramafic rocks with an enrichment of LREE with a negative slope and underwent 17 24% partial melting. The Baekdong ultramafic rocks experienced three stages of metamorphism after a high pressure residual mantle stage. The first stage of metamorphism occurred under the eclogite-granulite transitional facies (1123 911°C, >16.3 kb) the second under the granulite facies (825 740°C, 16.3 11.8 kb) and the third is the retrogressive metamorphism under amphibolite facies (782 718°C, 8.2 8.7 kb) metamorphism. The Baekdong ultramafic rocks had undergone high-P/T metamorphism during subduction of the South China Block, and experienced a fast isothermal uplift, and finally cooled down isobarically. Evidences for metamorphism were not identified in Bibong ultramafic rocks. Hence, the Baekdong ultramafic rocks with in the Hongseong area may indicate a link on the Korean counterpart of Dabie-Sulu collision belt between North and South China Blocks.     

20 Years Advances in Preparation of Potassium Salts from Potassic Rocks: A Review

Long-term research on key techniques of clean utilization of potassic rocks from over twenty localities has been performed to settle the shortage of soluble potassium resources in China. The results show that K-feldspar as the principal mineral enriched in potassium could be decomposed in the media of Ca(OH)2, NaOH, KOH-H2O solution via hydrothermal treatment, into tobermorite, hydroxyl- cancrinite, and kalsilite respectively. By further processing, these compounds are feasible for being as slow-release carrier of potassium nitrate, extracting alumina, and preparing farm-oriented fertilizers of potassium sulphate and nitrate. Correspondingly, the filtrate is KOH, (Na, K)2SiO3, and K2SiO3 solution, from which potassium carbonate, sulphate, nitrate, and phosphate could be easily fabricated. As NaOH and KOH are recycled in the processing chains by causticizing sodium and/or potassium metasilicate solutions, the hydrothermal alkaline techniques as developed in this research have several advantages as lower consumption of disposable mineral resources and energy, maximized utilization of potassic mineral resources, as well as clean productions etc. Based on the approaches presented in this paper, the technical system of efficiently utilizing insoluble potassium resources has been established. The hydrothermal alkaline methods are feasible to be industrialized on a large scale, thus resulting in decreasing imports of potash fertilizers, improving the pattern of potassium fertilizer consumption, and enhancing the supplying guarantee of potassium resource in China.     

Petrogenesis of Cenozoic Potassic Volcanic Rocks in the Nangqên Basin

The Nangqên basin is one of the Tertiary pull‐apart basins situated in the east of the Qiangtang block. Similar to the adjacent Dengqên basin and Baxoi basin, there occurred a series of potassic volcanic and sub‐volcanic rocks, ranging from basic, intermediate to intermediate‐acid in lithology. Based on the study of petrology, mineralogy and geochemistry, including REEs, trace elements, isotopic elements and chronology, the authors concluded that the Cenozoic potassic volcanic rocks in the Nangqên basin were formed in the post‐collisional intraplate tectonic settings. The relations between the basic, intermediate and intermediate‐acid rocks are neither differentiation nor evolution, but instead the geochemical variability is mainly attributable to the different partial melting degrees of the mantle sources formed at depths of 50–80 km. The sources of the potassic rocks are enriched metasomatic mantle that has experienced multiple mixing of components mainly derived from the crust. The recycling model can be described as follows: after they had subducted to the mantle wedge, the crust‐derived rocks were metasomatized with the mantle materials. In view of the fact that the ratio of crust‐derived rocks increases by the age of volcanism, it can be concluded that the sources of the potassic rocks moved upwards progressively with time. The underplating of small scattered magmas upwelling from the asthenosphere may have induced partial melting of the sources of the volcanic rocks in some pull‐apart basins in the Hengduanshan area and the intense tectonic movements of large‐scale strike‐slip belts provided conduits for the ascending melts.     

苏鲁造山带中胡家林超镁铁质岩岩石地球化学特征

苏鲁造山带中胡家林超镁铁质岩地块主要由两部分组成:南部滑石山以蛇纹岩和蛇纹石化橄榄岩为主,夹薄层状石榴橄辉岩-(石榴)单斜辉石岩;北部胡家林主要由(石榴)单斜辉石岩组成,夹厚层状蛇纹岩。蛇纹岩-蛇纹石化橄榄岩低Al2O3、低Ca O和高Mg O,REE含量低,但LREE稍富集。石榴橄辉岩和(石榴)单斜辉石岩低Mg O和Ca O,高REE含量高,其稀土配分曲线均表现出单斜辉石单矿物的配分特征。这些超镁铁岩块中不同岩石的微量元素均具有Pb的正异常,弱的Nb、Ta的负异常,显示地壳流体交代信息。Nd、Sr、Pb同位素特征显示存在亏损地幔与地壳之间的混合作用。亲石元素含量最低的蛇纹石化石榴橄辉岩的Sr、Nd、Pb同位素组成受交代流体控制明显,而石榴橄辉岩和(石榴)单斜辉石岩不明显。胡家林和滑石山超镁铁岩中所含的交代地壳成分不同,胡家林样品受到含水熔体和富水流体的双重交代,滑石山样品主要受富水流体的交代。     

The Nature and Origin of Ultramafic Lamprophyres: Aln?ites and Allied Rocks

Ultramafic lamprophyres (UML) are rare but widespread, hypabyssalrocks, rich in K, Mg, Cr, Ni, Sr, Ba, REE, and volatiles, containingless Si and more Ca than other silicate igneous rocks, and gradinginto carbonatites. They carry phenocryst combinations of olivine(Fo_92–72), phlogopite (rich in Ti, Fe³⁺ , Ba or F), Al-Ti-(Fe³⁺and richteritic to eckermannitic amphiboles. Groundmasses includeCa-Fe-Mg-carbonates (?partly primary), feldspathoids, Ca-Fe-Ti-Zr-garnets,soda-melilite, monti cellite, Mg-Mn-ilmenite, perovskite, serpentine,Fe- Mg-Ti-Mn-Cr-Al-spinels ? glass. Megacrysts include (?cognate)salitic pyroxenes, phiogopites or Ti-hastingsites, and morerarely, xenocrystic Cr-Ti-pyrope, orthopyroxenes and diopsides.Xenoliths include spinel and/or garnet-lherzolites, dunites,and phlogopite?amphibole-bearing pendotites. UML are readilydistinguished from kimberlites by petrological association (e.g.with ijolites), geochemistry (e.g. lower Mg, higher Ca, P),texture (e.g. lack of two olivine generations) and mineralogy(e.g. presence of groundmass feldspathoids ? melilite). UMLare distinguished also from melilitites by lower Si, Al, Na,higher Ca, K, P; more abundant primary carbonate, phiogopiteand amphibole; and by lack of groundmass olivine and phenocrystmelilite. Closer extrusive equivalents of UML may however occuramong ankaratrites. The most common UML types are aln?ite (melilite-rich)and aillikite (melilite-free, carbonate-rich). Aillikites arecompositionally closer to carbonatites than alnOites. Rarertypes include poizenite (melilite + feldspathoids), ouachitite(feldspathoids + carbonates) and damkjernite (feldspathoids+ carbonates+ < 10 per cent alkali feldspars). UML form localizeddyke-swarms or diatreme-clusters, mainly related to continentalrifting, and may represent parent magmas for coeval carbonatitecomplexes. Their additional occurrence in an oceanic setting,their mantle xenolith content, and their high mg, Cr and Ni,together suggest that many of them are primary, mantle-derivedmagmas, generated at depths between those of melilitites andkimberlites (c. 100–150 km), but at higher CO₂ pressuresthan melilitites. Other UML, however, have been extensivelymodified from primary compositions by fractionation, accumulation,or interaction with alkali+volatile-rich fluids.     

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO₂ vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe²⁺/(Mg + Fe²⁺) (the Cr–Al trend); increasingFe³⁺, Fe²⁺/(Mg + Fe²⁺) and T iO₂ at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe²⁺/(Mg + Fe²⁺); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel     

Potassic Volcanic Rocks in NE China: Geochemical Constraints on Mantle Source and Magma Genesis

Potassic volcanic rocks from the Wudalianchi, Erkeshan and Keluo(WEK) fields in NE China are located between the Mesozoic SongliaoBasin and the Palaeozoic Xing'am Mountains fold belt. Theserocks erupted during three main eruptive episodes-Miocene (9•6–7•0Ma), Pleistocene (0•56–0•13 Ma) and Recent (AD1719–1721)-and are subdivided into three types-olivineleucitite, leucite basanite and trachybasalt—on the basisof modal composition. In comparison with Cenozoic alkaline basaltsfrom East China that are similar to oceanic island basalts (OIBs),WEK volcanic rocks are lower in Al₂O₃, CaO, Fe₂O3 and Sc, buthigher in K₂O (3•5–7•1 wt %), K₂O/Na₂O (>1)and incompatible elements. High ⁸⁷Sr/⁸⁶Sr (0•7050–0•7056),low ¹⁴³Nd/¹⁴⁴Nd (0•51238–0•51250) and ²⁰⁶Pb/²⁰⁴Pb(17•06–16•61) ratios also distinguish them fromoceanic and Chinese basalts. Trace element and isotope dataindicate that a post-Archaean subcontinental lithospheric mantlesource similar to the postulated EM1 component (enriched mantlewith low ^l43Nd/¹⁴⁴Nd and moderate high ⁸⁷Sr/⁸⁶Sr) must haveplayed a significant role in magma generation. The source rockis considered to be refractory phlogopite-bearing garnet peridotiteheterogeneously enriched in both large ion lithophile elementsand light rare earth elements by ancient metasomatism duringProterozoic times. This source may have mixed recently withOIB-like melts, but has not been modified by subduction of theKula-Pacific plate. Primitive WEK potassic magma was generatedby a low degree of partial melting, initiated by an extensionalphase beginning in the late Tertiary, at pressures of 20–45kbar and in the presence of mixed volatile components of H₂O,CO₂ and halogens. KEY WORDS: potassic volcanic rocks; NE China; geochemistry; montle sourc ^*Corresponding author. Present address: Centre for Petrology and Lithoipheric Studies, School of Earth Sciences, Macquarie University, NSW 2109, Australia     

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR₂) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.     

Geochemistry of the Eocene Felsic Porphyric Rocks and High-Mg Potassic Rocks along JARSZ: Implication for the Tectonic Evolution in Eastern Tibet

<正>Eocene felsic porphyric rocks and the high-Mg potassic volcanic rocks(HMPR) occur along the Jinshajiang-Ailao Shan-Red River shear zone(JARSZ) in eastern Tibet.Compared with the HMPR,which are generally believed to be sourced from an enriched mantle,the felsic porphyric rocks show similar K_2O contents,enrichment in LREE and LILE,particularly radiogenic isotope(e.g.Sr and Nd) features much similar to the former,implying generation of the felsic porphyric rocks most likely related to the HMPR,although they both have clearly different major and trace element compositions. The close relationship in spatial-temporal distribution and similar Sr-Nd characteristics between the felsic porphyric rocks and HMPR in eastern Tibet indicate that both of them were possibly formed by a similar tectonic process(event).Combining the basic dikes in southern and eastern Tibet,we suggest that the break-off of north-dipping Neo-Tethyan slab in southern Tibet during 50-40 Ma,triggered formation of high-Mg potassic magma.This led to developing felsic porphyric magma production by partial melting of underplating HMPR in the lower crust,or fractionation crystallization of the high-Mg potassic magmas.The break-off of slab in the Eocene may also have contributed to the abundant ore-forming material related to earlier subduction events,resulting in formation of the porphyric deposits along JARSZ in eastern Tibet.     

Contact Metamorphosed Ultramafic Rocks in the Western Sierra Nevada Foothills, California

Within the western Sierra Nevada metamorphic belt, linear bodiesof alpine-type ultramafic rock, now composed largely of serpentineminerals, parallel the regional strike and commonly coincidewith major fault zones. Within this metamorphic belt, east ofSacramento, California, ultramafic rocks near a large maficintrusion, the Pine Hill Intrusive Complex, have been emplacedduring at least two separate episodes. Those ultramafic rocks,evidently unaffected by the Pine Hill Intrusive Complex andcomposed largely of serpentine minerals, were emplaced alonga major fault zone after emplacement of the Pine Hill IntrusiveComplex. Those ultramafic rocks, contact metamorphosed by thePine Hill Intrusive Complex, show a zonation of mineral assemblagesas the igneous contact is approached: olivine+antigorite+chlorite+tremolite+Fe-Cr spinel olivine+talc+chlorite+tremolite+Fe-Crspinel olivine+anthophyllite+chlorite+tremolite+Fe-Cr spinel olivine+orthopyroxene+aluminous spinel+hornblende+Fe-Cr spinel.Superimposed on these mineral assemblages are abundant secondaryminerals (serpentine minerals, talc, chlorite, magnetite) whichformed after contact metamorphism. Correlation of observed mineralassemblages with the experimental systems, MgO-SiO₂-H₂O andMgO-Al₂O₃-SiO₂-H₂O suggests an initial contact temperature of775±25 °C for the Pine Hill Intrusive Complex assumingPtotal P_fluid PH₂O. The pressure acting on the metamorphic rockduring emplacement of the intrusion is estimated to be a minimumof 1.5 kb.     

Highly refractory Archaean peridotite cumulates:Petrology and geochemistry of the Seqi Ultramafic Complex,SW Greenland

This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km~2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite(s.s.),peridotite(s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust.Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian(Mg# ~ 80), near-anhydrous magma, as olivinedominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting(40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle(SCLM) in this region,which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.     

中国西北地区超基性岩封存CO2潜力研究

超基性岩可通过碳酸盐化生成稳定的碳酸盐矿物,它是一种以地球化学手段有效且永久封存CO2的矿物。在自然界中矿物封存CO2可通过风化作用自发发生,人工干预能进一步提升碳酸盐化反应效率,促进工业化进程。笔者基于最新1∶100万西北地质图及数据库,试图对西北地区分布的超基性岩的封存潜力进行理论评估。结果表明,西北地区超基性岩封存CO2量可达963.23亿t,其中新疆超基性岩CO2封存量最大,可达613.52亿t,占西北地区总封存量的63.69%。西北地区超基性岩封存CO2量大致相当于全国2021年CO2排放量的10倍,在完全释放其固碳潜力的情况下,初步静态估算可封存全国CO2排放量约10年。因此,西北地区超基性岩封存CO2潜力巨大。未来,应针对单个超基性岩体收集已有大比例尺精细基础地质调查数据,并补充性开展调查及研究工作,进一步圈定CO2地质封存的有利靶区,促进超基性岩封存CO2的地质解决方案成为未来碳中和目标在西北地区落地实现的最优方案之一。     

Tale-Carbonate Alteration of some Serpentinized Ultramafic Rocks south of Timmins, Ontario

Serpentinized peridotites in an area south of Timmins, Ontario,have been extensively altered to tale and carbonate. In someplaces, rocks altered in this way have subsequently been decarbonatizedand converted back to a serpentine-magnetite assemblage. Chemicalanalyses of variably altered rocks indicate that the bulk chemicalchanges involved in the tale-carbonate alteration have beenthe addition of CO₂, and removal of H₂O and a very small amountof O₂. Little or no magnesium, silicon, iron, or nickel metasomatismhas occurred. Consequently the relative partial pressures ofH₂O and CO₂ in solutions passing through the rocks are likelyto have been controlling factors for the alteration and subsequentdecarbonatization.