Petrographical and mineral chemistry evidence to delineate the source/sources of the Central Indian Ocean Basin pumices

We present data pertaining to mineral assemblages and composition of the Central Indian Ocean Basin(CIOB)pumices.Eight groups of pumices were identified considering the presence of phenocrysts of plagioclase,clinopyroxene,orthopyroxene,hornblende and biotite together with the occurrence of quartz and glass.Pigeonite,fayalite and ulvospinelare reported for the first time from these pumices.In the eight groups,the modal percentage of the constituents are phenocrysts 3%to 19%(avg 9.6%),silicic glass 33%to 54%(avg 43%)and the rest is vesicles.Based on the above factors we have identified the possible sources of the CIOB pumices.The mineral compositions of plagioclase,pyroxenes,and biotite of the CIOB pumices were compared with those of Krakatau and Toba.Most of the plagioclase and pyroxene compositions resemble the Haranggoal Dacite Tuff of Toba and Krakatau.Considering the mineral assemblages and compositions,there are pumices which do not correlate to any of the above eruptions and are probably from yet unidentified source/sources.These sources could either be from nearby terrestrial volcanoes or intraplate seamounts present in the CIOB.In a global context,it is viable that petrological characteristics could be used as initial criteria to determine the source of pumices that occur at abyssal depths in the world ocean.

Advances in Research of Mineral Chemistry of Magmatic and Hydrothermal Biotites

Biotite is an important hydrated ferromagnesian silicate mineral in igneous rocks and porphyry deposits.The determination of chemical compositions of biotite plays an important role in both igneous petrology and ore forming processes.This paper summarizes research results of magmatic and hydrothermal biotites exemplified by the Lakange porphyry Cu–Mo deposit and the Qulong porphyry Cu deposit in the Gangdese porphyry–skarn metallogenic belt,Tibet.Biotite mineral chemistry can provide critical insights into classification,geothermometer,geothermobarometry,oxygen fugacity,petrogenesis and tectonic setting,evaluating magmatic-hydrothermal process by halogen and halogen fugacity ratios,and distinguishing between barren and mineralized rocks.Biotite provides the latest mineralogical evidence on metallogenic prognosis and prospecting evaluation for porphyry Cu polymetallic deposits or magmatic hydrothermal deposits.

Geology and Mineral Chemistry of Gold Mineralization in Mirge-Naqshineh Occurrence(Saqez,NW Iran):Implications for Transportation and Precipitation of Gold

The Mirge-Naqshineh gold district is situated at northwest of Iran with a NW-trending brittle- ductile shear zone. It is hosted by Precambrian meta-sedimentary and meta-volcanic units traversed by mineralized quartz veins. In terms of cross-cutting relationships and sulfide content three types of quartz veins are identified in the region. Among those, parallel to bedding quartz vein （type I） is the main host for gold mineralization. Gold is found in three different forms： 1） submicrometer-size inclusions of gold in arsenian pyrite, 2） as electrum and 3） in the crystal lattice of sulfides （pyrite, galena and chalcopyrite）. Six types of pyrite （Pyl-Py6） were identified in this ore reserve. Py3 coexists with arsenopyrite and contains the greatest As-Au concentrations. There is a negative correlation between the As and S contents in Py2 and Py3, implying the substitution of sulfur by arsenic. Pyrites and mineralized quartz veins were formed via metamorphic-hydrothermal fluid and reflect the gold-transportation as Au（HS）2 under reducing and acidic conditions. The gold precipitation mainly controlled by crystallization of arsenian pyrite during fluid/rock interactions and variation offO2. The volcanic host rock has played an important role in gold concentration, as Py3 in this rock contains inclusion of gold particles, but gold is within the lattice of pyrite in phyllite or other units.

Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts:Compositional Diversity and Petrogenetic Significance

The glass and mineral chemistry of basaits examined from the northern central Indian ridge （NCIR） provides an insight into magma genesis around the vicinity of two transform faults： Vityaz （VT） and Vema （VM）. The studied mid-ocean ridge basalts （MORBs） from the outer ridge flank （VT area） and a near-ridge seamount （VM area） reveal that they are moderately phyric plagioclase basalts composed of plagioclase （phenocryst [An60-90] and groundmass [An35-79]）, olivine （Fo81-88）, diopside （Wo45-51, En25-37, Fs14-24）, and titanomagnetite （FeOt -63.75 wt% and TiO2 -22.69 wt%）. The wholerock composition of these basalts has similar Mg# [mole Mg/mole（Mg＋Fe2＋）] （VT basalt： -0.56-0.58; VM basalt： -0.57）, but differ in their total alkali content （VT basalt： -2.65; VM basalt： -3.24）. The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the ∑REE of magma also increased with fractionation, without any change in the （La/ Yb）N value. Glass from the VM seamount shows more fractionated characters （Mg#： 0.56-0.57） compared to the outer ridge flank lava of the VT area （Mg#： 0.63-0.65）. This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure 〉10 kbar, followed by Fe-rich olivine at 〈10 kbar pressure.

Zircon U-Pb-Hf isotopes and mineral chemistry of Early Cretaceous granodiorite in the Lunggar iron deposit in central Lhasa, Tibet Y, China

The Lunggar iron deposit belongs to the Bangong-Nujiang metallogenic belt and is located in central Lhasa on the Tibetan Plateau.In the Lunggar deposit,iron mineralization formed in the skarnization contact zone between the Early Cretaceous granodiorite and the late Permian Xiala Formation limestone.In this study,we achieved detailed zircon U-Pb-Hf isotopes and mineral chemistry for the Early Cretaceous granodiorite.Zircon U-Pb dating results indicate that the Early Cretaceous granodiorite emplaced at ca.119 Ma.Based on the trace elements in zircons and the mineral chemical composition of amphibole and biotite,the Early Cretaceous granodiorite was believed to form under condition of high temperature(>700°C),low pressure(100400 MPa),and relatively high oxygen fugacity(lgfO2)(13.6 to 13.9)and H2O content(4%8%).Zircon trace elements,Hf isotope and biotite chemistry collectively reveal that significant juvenile mantle-derived magmas contributed to the source of the granodiorite.The relatively high logfO2 and shallow magma chamber are beneficial for skarn iron mineralization,implying remarkable potential for further prospecting in the Lunggar iron deposit.

Petrography and mineral chemistry of 4 carbonaceous chondrites from the Grove Mountains,Antarctica

Petrography and mineral chemistry of four carbonaceous chondrites （ GRV 020017, GRV 020025, GRV 021579, GRV 022459 ） collected from the Grove Mountains （ GRV）, Antarctica, were reported here. All four chondrites are unequilibrated, as indicated by well shaped chondrules and the chemical variations of olivine and low-Ca pyroxene. The modal abundance ratio of matrix/chondrule are 2 （ GRV 020017）, 2. 8 （ GRV 020025 ）, l. 2 （ GRV 021579 ）, 1 （ GRV 022459 ）. GRV 022459 has the largest chondrules （0.6--2.0 ram）. A total of 30 Ca-Al-rich inclu- sions were found in the four meteorites. Most inclusions were highly altered, with a- bundant phyllosilicates in the inclusions of GRV 020017 and GRV 020025. On the base of petrography and mineral chemistry, these chondrites are classified as CM2 （GRV 020017 and 020025）, CO3 （GRV 021579） and CV3 （GRV 022459）.

Mineral Chemistry of Wehrlite Xenoliths Hosted in Basalts from the SW of Hossere Dammougalre(Adamawa Plateau,Cameroon):Thermobarometric Implications

Wehrlite samples (size: ~4 cm) hosted in basaltic lavas from the SW of Hosséré Dammougalré are located in the western Adamawa Plateau. Porphyritic and allotriomorphic texture characterize respectively host Basalt and wehrlite xenoliths. The phenocrysts of olivine (Fo68&minus;74), and Ti-magnetite are scattered in host basalt. Wehrlite xenoliths (~4 cm size) contain Cr-rich clinopyroxene (diopise-augite), olivine (Fo76&minus;88) and chromiferous spinel. Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/spinel vary between 944&#176;C and 1102&#176;C. The wehrlite olivine crystals with low Fo (<90) indicate a re-equilibration of Fe-Mg in the host basalt at low temperatures. All the analyzed wehrlite clinopyroxenes have crystallized at high pressures as evidenced by the Alvi and Aliv contents. The studied spinel-bearing wehrlite xenoliths represent probably the residual portions of the upper mantle, which are an important source of information about lithospheric composition and thermal evolution beneath the Adamawa Plateau.

Evolution of Mount Cameroon volcanism: Geochemistry, mineral chemistry and radiogenic isotopes(Pb, Sr, Nd)

Mount Cameroon volcano has erupted several times in the 20 th Century with documented eruptions in 1909,1922,1954,1959,1982,1999 and 2000.Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano.This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical,whole rock geochemical and Pb,Sr,Nd isotope data generated from historic and recent lava flows.In this study,samples were collected from the 1959,1982,1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings.More than 90%of the studied historic lavas(n=29)classify as tephrites and basanites whereas the modern lavas(n=38)are predominantly trachybasalts,demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization.Petrographically,the lavas are porphyritic with main mineral phases being olivine,clinopyroxene,plagioclase feldspars and Fe-Ti-Cr oxides.The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass.Olivine chemistry shows variable forsterite compositions from Fo60-89.Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo45En32Fs7 to Wo51En47Fs17.Plagioclase feldspars vary from labradorite(An567 O)to bytownite(An80-87).For the Fe-Ti-Cr oxides,calculated ulvospinel component shows a wide variation from ulv38-87.CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative(Ne ranges from4.20 wt.%to 11.45 wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU(or high μ=238U/204Pb),characterized by 206Pb/204Pb=20.19-20.46,207Pb/204Pb=15.63-15.69,208Pb/204Pb=40.01-40.30,87Sr/86Sr=0.70322-0.70339(εsr=-21.37 to-18.96)and 143 Nd/144 Nd=0.51276-0.51285(εNd=+2.29 to+4.05).The historic lavas show stronger HIMU signature relative to the modern lavas,suggesting evolution towards less HIMU signatures over time.This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high 206/204Pb and 208/204Pb isotopes,low SiO2 and high Mg,Ni,Cr content towards lower HIMU signatures with relatively higher SiO2,lower Mg,Cr and Ni compositions.The geochemical and isotopic changes,which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous,with very limited variation in SiO2 compositions.

Geodynamic significance and genesis of chromitites from the Islahiye ophiolite(Gaziantep,SE Anatolia)as constrained by platinum group element(PGE)compositions and mineral chemistry characteristics

Chromitites associated with intensely altered dunites and harzburgites from fourteen dif ferent localities in the Islahiye ophiolites(SE Anatolia)is reported here for the first time.These chromitites were observed as lenticular and banded bodies with disseminated and massive textures and containing magnesiochromite grains with the following composition:CrO=58.91–59.74 wt%,Al_(2)O_(3)=10.85–11.20 wt%,and TiO=0.09–0.13 wt%.The Mg#[Mg/(Mg+Fe)]values of magnesiochromite from the Islahiye ophiolite range between 0.52 and 0.60 and their Cr#[Cr/(Cr+Al)]values vary from 0.7802 to 0.7844.These contents vary with a constant pattern,coincident with the estimated parental liquids that have originated from the derivative of a single bulk of boninitic magma together with Al,Ti-poor,and Cr-rich initial contents.The chromitites are serpentinised in almost all parts of the study area,and harzburgite and dunite can be observed in different locations.Although the overall composition of platinum group elements(PGE)in most examined chromitites varies between 97 and 191 ppb,three chromitites from the Islahiye region present enrichments in overall PGE(up to 214 ppb).The mineralogical and geochemical features of chromitites from the Islahiye region exhibit a robust similarity to podiform chromitites in the mantle fragment of supra–subduction zone type ophiolitic bodies.The estimated parental magmas of the investigated chromites are consistent with the diff erentiation of arcrelated melts and do not suggest an oceanic spreading centre tectonic environment.The Islahiye chromites are enriched in IPGE(Ir,Os,Ru),with the occasional presence of Ru and Ir and higher Os contents in chromite.Furthermore,we did not find any platinum group minerals(PGM)associated with the serpentine silicate sample matrix,which would have stated a secondary enrichment in PGEs.All chromitites in the investigated region have high Cr and low Ti values,are defined as magnesiochromite and were crystallised from a characteristic boninitic magma.

Mineral Chemistry, Geobarometry and Oxygen Fugacity of the Granitic Rocks from the Itremo Domain, Central Madagascar

Major and accessory minerals from the Ibity granite, Tsarasaotra monzonitic and granite dykes, and Antsahakely granite of the Itremo domain in the Precambrian basement of Madagascar were characterized by using microscopic observations and chemical analyses with the aim of understanding their chemical characteristics and estimating the crystallization pressure and oxygen fugacity of their host rocks. Plagioclases in these rocks are albite and oligoclase, while alkali feldspars are orthoclase. For the phlogopite-micas, Fe-biotite and Li-phengite are common for the Ibity and Antsahakely granites, Mg-biotite is common for the Ibity granite and the Tsarasaotra monzonitic and granite dykes, and siderophyllite and Zinnwaldite are specific to the Ibity granite. Phlogopite-micas in the studied rocks are mainly primary, accessorily re-equilibrated, and rarely secondary. Calcic amphiboles distributed in the Magnesio-and Ferro-hornblende are identified in the Tsarasaotra monzonitic, whereas amphibole is rare and absent in the other rocks. Igneous titanite is observed in the Ibity granite and in the Tsarasaotra monzonitic rocks, which have similar compositions to some REE oxide-rich titanites. Concerning the Fe-Ti oxide phases, the rhombohedral and spinel/trifer tetroxide phases are found in both the Tsarasaotra monzonitic and the Tsarasaotra granite dyke, the trifer tetroxide and spinel + wüstite phases are found only in the Ibity granite, and the pseudobrookite + rhombohedral phase is found only in the Tsarasaotra granite dyke. The epidote mineral, rarely found in the Antsahakely granite, could be an indicator of metamorphism or hydrothermal activity involved during the emplacement of this rock. Aluminum in hornblende geobarometer gave pressure ranges of around 5 kbar for the Tsarasaotra monzonitic rocks. The Titanite geobarometer gave pressures of 2.5 - 3.2 kbar for the Ibity granite, 2.9 kbar for the Tsarasaotra monzonitic, and 7.1 kbar for the Antsahakely granite. Both amphibole and Fe-Ti oxide-base oxygen fugacity reveal high oxygen fugacity conditions for the Tsarasaotra monzonitic and granite dyke emplacements, which might have a relationship with a porphyritic environment.

Geochemistry, Mineral Chemistry and Thermobarometry of Boneh-Shorou Amphibolites in Gelmandeh Massive (Saghand-Central Iran)

The Gelmandeh Massive is located in the Central part of Iranian Microcontinent. Amphibolitic rocks occupy the main portion of Gelmandeh complex, which consists of hornblendite, garnet amphibolites, and gneiss. Geochemical investigations indicate that the ratio of Eu/Eu* separates samples into two categories: first category with ratio of Eu/Eu* > 1 and the second with ratio of Eu/Eu* < 1, which the former indicates an enriched mantle origin while the latter points to crustal contamination. Generally, amphibolites can be differentiated into two separate series. First series is characterized with calc-alkaline to alkaline composition with enriched mantle origin, and the second one has calc-alkaline to tholeiitic nature and it is of continental type that underwent crustal contamination within subduction zone. The microprobe electron analysis shows that the amphiboles were a member of the calcic group and hastingsite-tschermakite series. Geo-barometric studied and mineral paragenesis show that the metamorphism is of Barovian type with PT condition upto 7 K bar and 580°C.

Fe-Ti Oxide Mineralization in the XV Intrusion,Bafq Mining District,Central Iran:Insights from Mineralogy,Mineral Chemistry and S Isotopic Data

The mafic-ultramafic intrusion in the XV anomaly area,contains magmatic Fe-Ti oxides±(p)ore,is located in the Bafq mining district in the Central Iran.It consists of cumulate and layered Fe-Ti-bearing gabbros and pyroxenites.The mineral assemblages include clinopyroxene,Fe-Ti oxides,plagioclase,amphibole,apatite and sulfides(pyrite and chalcopyrite).The Fe-Ti oxides mainly consist of magnetite-titanomagnetite and ilmenite,which occurred as disseminated,intergrowth,lamellae(trellis and sandwich textures)and inclusions.Magnetite in the gabbroic rocks is from the near end-member of Fe_(3)O_(4)(<1 wt.%TiO_(2))to titanomagnetite containing up to 8 wt.%TiO_(2)(about3.73 wt.%to 26.84 wt.%Ulvospinel(X_(Usp))).Magnetite in pyroxenite rocks is characterized with TiO_(2)range from 0.46 wt.%to 3.14 wt.%(X_(Usp)varied from 1.76 wt.%to 10.46 wt.%).The abundances of V_(2)O_(3)range from 0.03 wt.%to 1.29 wt.%and 0.24 wt.%to 1.00 wt.%for gabbro and pyroxenite,respectively.X_(Usp)contents of magnetite show insignificant correlations with Al_(2)O_(3)and MgO.The average XIlmin the ilmenite of gabbro is 92%,whereas it is 90.37%in the pyroxenite rocks.The MgO and V_(2)O_(3)contents show a slightly positive correlation with TiO_(2)in ilmenite.The composition of clinopyroxenes in gabbro and pyroxenite rocks fall in the diopside to augite field with Mg#ranging from 67 to 98 and 74 to 96,respectively.In both rock types,amphiboles are mainly pargasite and rarely actinolite.Plagioclase in pyroxenite rocks is clustered in the labradorite to andesine fields with a compositional ranges of An46-69and in gabboic rocks fall in two fields with compositional ranges of albite with An0.65-5.95and labradorite with An50-63.Theδ34S isotopic values cover a limited range from+3.15‰to+4.10‰V-CDT consistent with magmatic origin.Fe-Ti mineralization is formed in two stages,minor inclusions of Fe-Ti oxide minerals in the pyroxene and plagioclase crystallized in the early magmatic stage,whereas interstitial oxides formed by fractional crystallization processes that accumulated by gravitational settling in the later stage as intercumulus phase.Gravitational settling process is supported by the observation of decreasing the amount of Fe-Ti oxides from Fe-Ti oxide-rich pyroxenite to weak mineralized gabbro(base to top).The high contents of H2O,phosphorate and high initial Ti-Fe in parental magma are the crucial factors controlling the Fe-Ti oxides enrichment and mineralization.

Mineral Chemistry of Biotite and Its Petrogenesis Implications in ca.2.5 Ga Wangjiazhuang Granitic Pluton,North China Craton

The Wangjiazhuang granitic pluton is located in the central Zanhuang Domain,the central part of the North China Craton,which is mainly composed of biotite monzogranite with few mafic microgranular enclaves.Biotite is an important ferromagnesian mineral in most of the intermediatefelsic igneous rocks,and its mineral chemistry can record the properties of magma and the petrogenetic physicochemical conditions.In this study,we carried out a detailed petrographic study by electric probe microanalysis on biotite for the biotite monzogranite and mafic microgranular enclaves,to discuss the source,physicochemical conditions,and the magma mingling/mixing processes of the Wangjiazhuang granite.The results show significantly different chemical compositions from the biotite monzogranite and mafic microgranular enclaves.The crystallization of these biotite grains from the biotite monzogranite and mafic microgranular enclaves all occurred in low oxygen fugacity.The biotite grains in biotite monzogranite are rich in Fe,poor in Mg,which belong to siderophyllite.The ratios of[(Fe^(3+)+Fe^(2+))/(Fe^(3+)+Fe^(2+)+Mg2+)]are between 0.78 and 0.86.The average of FeOT(total FeO)/MgO of biotite grains in biotite monzogranite is 9.02.The MF values[2×Mg/(Fe^(2+)+Mg+Mn)]of biotite monzogranite are between 0.31 and 0.47,suggesting biotite monzogranite derived from crustal source rocks(metasedimentary rocks).The formation of granitic rocks including the Wangjiazhuang granite was related to the subduction event at ca.2.5 Ga which resulted in the melting event,and then induced the early partial melting of TTGs and metasedimentary rocks.The biotite in mafic microgranular enclaves varies from siderophyllite to ferrobiotite,and MF values range from 0.63 to 1.06,suggesting that magma of mafic microgranular enclaves had experienced magma mixing/mingling in various degrees.Biotite monzogranite and parts of mafic microgranular enclaves have a similar crystallized condition,while other mafic microgranular enclaves are different from biotite monzogranite.The differences between biotite monzogranite and mafic microgranular enclaves may be a consequence of continuous interaction between granitic and mafic magmas.

Fall,mineralogy and chemistry of Nathdwara H6 chondrite

We report here for the first time the composition and mineralogical studies on a new meteorite, which fell in Dhayala ki Chappar （24°58°N, 73° 48°27°E） 5 km NW of Nathdwara in south Rajasthan, India, on Dec. 25th, 2012. Mineralogical and compositional studies were carried out on a representative piece of the Nathdwara meteorite sample. The mineralogical composition of the meteorite has been found to be olivine （42 45 vol.%）, feldspar （10-15 vol.%）, orthopyroxene （23-25 vol.%）, troilite （6 8 vol.%）, and titanium bearing minerals （6-8 vol.%）. Our investigations show that the Nathdwara meteorite belongs to H6 group of ordinary chondrites.

Solution chemistry of carbonate minerals and its effects on the flotation of hematite with sodium oleate

The effects of carbonate minerals（dolomite and siderite） on the flotation of hematite using sodium oleate as a collector were investigated through flotation tests, supplemented by dissolution measurements, solution chemistry calculations, zeta-potential measurements, Fourier transform infrared（FTIR） spectroscopic studies, and X-ray photoelectron spectroscopy（XPS） analyses. The results of flotation tests show that the presence of siderite or dolomite reduced the recovery of hematite and that the inhibiting effects of dolomite were stronger. Dissolution measurements, solution chemistry calculations, and flotation tests confirmed that both the cations（Ca^（2＋） and Mg^（2＋）） and CO_3^（2＋）ions dissolved from dolomite depressed hematite flotation, whereas only the 23CO-ions dissolved from siderite were responsible for hematite depression. The zeta-potential, FTIR spectroscopic, and XPS analyses indicated that Ca^（2＋）, Mg^（2＋）, and CO_3^（2-）（HCO_3^-） could adsorb onto the hematite surface, thereby hindering the adsorption of sodium oleate, which was the main reason for the inhibiting effects of carbonate minerals on hematite flotation.

A Textural and Mineralogical Study of the Shanzhuang Banded Iron Formation, Southeastern Margin of the North China Craton: Implications for the Overprint History of Hydrothermal Alteration and Supergenesis after Mineralization

The newly discovered Shanzhuang BIF is hosted in the Shancaoyu Formation of the Taishan Group within the Eastern Block,southeastern margin of the North China Craton.The ores can be subdivided into three types in terms of mineral assemblages,corresponding to three types(Ⅰ,Ⅱ,Ⅲ).The element concentration of the type Ⅰ magnetite is similar to that of the type Ⅱ magnetite,while the type Ⅲ magnetite is similar to that of the schist.In general,magnetite and hemat ite grains from the ores show high concentrations of Mn(1317,1162 ppm),Co(787,1023 ppm),Al(2224,2435 ppm)and Ti(540,300 ppm),Whereas magnetite is depleted in Si(420 ppm)and hematite enriched in Si(1690 ppm).Detailed petrographic and mineral chemical analysis of magnetite,hematite,amphibole/hornblende and pyroxene,reveals that almost all the minerals occur as subhedral-anhedral grains with pits and fractures,and the BIF is recrystallized to metamorphic assemblages of high amphibolite facies.Hornblende is highly enriched in Fe,Mg and Ca,but depleted in K and Na,mostly belonging to magnesiohornblende.In addition,the ratios of Mg/(Mg+Fe^2+),Fe^3+/(Fe^3++Fe^2+),Si/(Si+Ti+Al)and Al/Si are 0.48-0.64,0.17-0.36,0.79-0.88 and 0.14-0.27,respectively.It is suggested that hornblende is neither a typical magmatic origin nor a typical metamorphic.Pyroxene has the characteristics of high Ca and Fe,but low Ti and Al,with end-member components En,Wo and Fs in the ranges of 25.22-28.64 wt%,43.71-46.40 wt%and 24.51-27.62 wt%,respectively,belonging to clinopyroxene,and mostly diopside,might be formed during the prograde metamorphism in the absence of H2 O.The carbonate such as dolomite-ankerite series is probably a precursor mineral of the BIF deposit.Mass mineral chemical and structural characteristics indicate that the Shanzhuang iron deposit has been subjected to varying degrees of oxidized hydrothermal superimposed reformation,metamorphism,and supergenesis after mineralization,during which some elements have been migrated in some degree.

Origin of iron-rich minerals,ooids and pisoids in the Jurassic ooidal ironstones of the Labino-Malkin region(Caucasus)

This study investigates the Middle Jurassic rocks of the Labino-Malkin zone(Caucasus),represented by ooidal ironstones,biogenic limestones,and nodular phosphorites.All rocks contain ooids/pisoids.Based on the mineral composition,nine types of ooids and three possible scenarios for their formation have been identified.The mineral composition of ooids indicates the variable conditions of their deposition and repeated changes in physical and chemical conditions of the environment,including fluctuations in theredox regime.The access of oxygen in the bottom environment as a factor of increasing oxic conditions was associated with the regression of the sea and/or the inflow of meteoric waters.The main sources of ooids are the drift of material from the continent and convecting metal-bearing meteoric and/or deep solutions.

Petrology, Metamorphic Process and Genesis of The Dabie--Sulu Eclogite Belt, Eastern--Central China

The Qinling-Dabie-Sulu high-pressure and ultra-high pressure metamorphic belt wasformed by subduction and collision between the North China and Yangtze plates. The study ofthe eclogite belt is very important in understanding the evolution of the Qinling Dabie orogen. Inthe present paper the geology, petrology, minerology and chronology of the eclogites in the Dabieand Sulu areas are described. The principal conclusions of this work are as follows: (1) Based up-on the field occurrence and the P-T conditions of the eclogites, two types of eclogite can be dis-tinguished: Type 1—the low-temperature and high-pressure eclogite in the mid-late Proterozoicmetamorphic series, and Type 2—the ultra-high pressure eclogite in the late Archaean to earlyProterozoic metamorphic complex. In the Dabie area, the ultra-high-pressure eclogite,high-pressure eclogite and epidote-blueschist units are nearly parallel to each other and stretchintermittently from north to south. (2) The P-T conditions of the high-pressure eclogites and ul-tra-high pressure eclogites have been estimated. The former are formed at 450-550℃ and1.4-1.6 GPa; while the latter at 650-870℃ and >2.7-2.9 GPa in the Dabie area and at820-1000℃ and >2.8-3.1 GPa in the Sulu area. The metamorphic temperatures of the eclogitesincrease progressively from west to east. (3) The ultra-high pressure eclogites were subjected to 5stages of metamorphism: pre-eclogite epidote amphibolite facies, peak coesite eclogite facies,post-eclogite amphibolite facies, epidote-blueschist facies or epidote amphibolite facies andgreenschist facies. The general features of the PTt path of the ultra-high pressure eclogite are:clockwise pattern, progressive metamorphism being a process of slow increasing temperature andrapid increasing pressure, and the retrogressive section with nearly isothermal decompression atthe early stage, isobaric cooling at the middle stage and nearly isothermal decompression at thelate stage. (4) At least two stages of high-pressure metamorphism occurred in the orogenic belt:the high-pressure eclogite and ultra-high pressure eclogite were formed by the subduction of theoceanic crust northward beneath the North China plate or the Dabie block during theCaledonian; while the epidote-blueschist belt came into being by subdution and collision be-tween the two continental plates during the Indosinian. (5) Due to the continuous sequentialsubduction of the cold plate, the ultra high-presssure metamorphic rocks were uplifted to thecrust by the underplating processes. They can be preserved just because of the 'frozen effect' re-sulting from the continuous subduction of the cold plate. (6) The carbonates, such as magnesite,breunnerite, aragonite and dolomite, and the H_2O-bearing minerals, such as phengite, epidoteand zoisite, were stable during the high-pressure and/or ultra-high pressure metamorphism.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone:A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths.The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions.Eclogite facies conditions,better recorded in mafic protoliths,are revealed by relic lawsonite and phengite,bleb-to worm-like diopside-albite symplectites,as well as garnet core composition.Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole,augite,and oligoclase recrystallization,as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature(P-T)path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2-24.5 kbar and 465-580℃,followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5-14.2 kbar and 580-720℃.Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286-400℃at pressures below 6.5-11 kbar.The obtained clockwise P-T path,the garnet zonation pattern revealing a decrease in X_(grs)/X_(prp)related to Mg#increment from core to rim,the presence of partial melting veins,as well as regional constraints,document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous.Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered.This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period.The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms,representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.

Geochemistry and geochronology of multi-generation garnet:New insights on the genesis and fluid evolution of prograde skarn formation

Origin of garnet in skarn(magmatic vs.hydrothermal)and the prograde skarn fluid evolution are still controversial.Two generations of garnet(Grt1,Grt2)were identified at the Tongshankou deposit:Grt1 is anisotropic with oscillatory zoning and resorbed boundary,whilst Grt2 grew around Grt1 and formed oscillatory rims.In-situ LA-ICP-MS U-Pb dating of Grt1 and Grt2 yielded a lower intercept^(206)Pb/^(238)U age of 142.4±2.8 Ma(n=57;MSWD=1.16)and 142.3±9.6 Ma(n=60;MSWD=1.06),respectively,coeval with the ore formation and ore-related granodiorite emplacement.Positive Eu anomaly,non-CHARAC Y/Ho value and low TiO_(2)content,together with the mineral assemblages indicate that both Grtl and Grt2 have a hydrothermal origin.The existence of melt and melt-fluid inclusions in Grt1,together with similar LREE-enriched patterns to the granodiorite,further indicate that Grt1 may have formed in the magmatic-hydrothermal transition.Higher U contents and LREE-enriched patterns of Grt1 indicate that fluid I is mildly acidic pH and low fO_(2).The inner gray Grt2 rims(Grt2A)is HREE-enriched with low U contents,indicating that fluid II has nearly neutral pH and high fO_(2).The wider Y/Ho range and LREE-enriched patterns of the outer light-gray Grt2 rims(Grt2B)show that the evolved magmatic fluidⅡhad mixed with an external fluid,characterized by being mildly acidic pH and with high fO_(2)·Our results suggest that the prograde skarn-forming fluids can be multistage at Tongshankou,and the mixing of meteoric water may have been prominent in the prograde skarn stage.