Kimberlitic sources of super-deep diamonds in the Juina area,Mato Grosso State,Brazil

The Juina diamond field, in the 1970–80s, was producing up to 5–6 million carats per year from rich placer deposits, but no economic primary deposits had been found in the area. In 2006–2007, Diagem Inc. discovered a group of diamondiferous kimberlitic pipes within the Chapadão Plateau (Chapadão, or Pandrea cluster), at the head of a drainage system which has produced most of the alluvial diamonds mined in the Juina area. Diamonds from placer deposits and newly discovered kimberlites are identical; they have super-deep origins from the upper-mantle and transition zone. Field observations and petrographic studies have identified crater-facies kimberlitic material at seven separate localities. Kimberlitic material is represented by tuffs, tuffisites and various epiclastic sediments containing chrome spinel, picroilmenite, manganoan ilmenite, zircon and diamond. The diamond grade varies from 0.2–1.8 ct/m³. Chrome spinel has 30–61 wt.% Cr₂O₃. Picroilmenite contains 6–14 wt.% MgO and 0.2–4 wt.% Cr₂O₃. Manganoan ilmenite has less than 3 wt.% MgO and 0.38–1.41 wt.% MnO. The ¹⁷⁶Hf/¹⁷⁷Hf ratio in kimberlitic zircons is 0.028288–0.28295 with ε_Hf = 5.9–8.3, and lies on the average kimberlite trend between depleted mantle and CHUR. The previously known barren and weakly diamondiferous kimberlites in the Juina area have ages of 79–80 Ma. In contrast, zircons from the newly discovered Chapadão kimberlites have a mean ²⁰⁶Pb/²³⁸U age of 93.6 ± 0.4 Ma, corresponding to a time of magmatic activity related to the opening of the southern part of the Atlantic Ocean. The most likely mechanism of the origin of kimberlitic magma is super-deep subduction process that initiated partial melting of zones in lower mantle with subsequent ascent of proto-kimberlitic magma.     

Alteration of the Damiao anorthosite complex in the northern North China Craton: Implications for high-grade iron mineralization

The Damiao type iron deposit is hosted in a typical Proterozoic anorthosite complex in the northern North China Craton. The types of ores in Damiao mainly comprise massive Fe ores, massive Fe–P ores, and disseminated Fe and Fe–P ores. The disseminated Fe and Fe–P ores formed by fractional crystallization are generally hosted in oxide-apatite gabbronorite and account for 70% of the proven reserve of the Damiao type iron ore. The massive Fe and Fe–P ores account for 30% of the proven reserve of the Damiao type deposit iron ore and generally occur as irregular dykes or veins filling vertical fractures of the previously consolidated anorthosite, showing typical features of hydrothermal mineralization. The contact between the massive orebodies and wall rocks is sharp and straight. The anorthosite comprises white and dark varieties, with the former resulted by the alteration of the latter that occurs as relicts. Petrographic observation and electron microprobe analyses show abundant Fe–Ti oxide inclusions in plagioclase which impart the dark color to the rock. The similar spider diagram patterns between fresh and altered plagioclase and between dark- and white-colored anorthosite imply a genetic relationship between the dark and white types. During the alteration of anorthosite, CaO and MgO were slightly decreased, the SiO₂, Al₂O₃ and Na₂O were significantly increased, and the TFe₂O₃ and TiO₂ were significantly decreased. The TFe₂O₃ and TiO₂ in the dark-colored anorthosite have a range of 4.86–12.18 wt.% and 0.37–1.65 wt.%, respectively. However, The TFe₂O₃ and TiO₂ in the white-colored anorthosite have a range of 1.67–3.1 wt.% and 0.14–0.31 wt.%, respectively. These features suggest that the alteration of the anorthosite led the Fe element by leaching from the dark-colored anorthosite at highly oxidized condition, and then precipitated within the fractures of the anorthosite, thus forming the massive Fe and Fe–P orebodies. Because the estimated amount of transported Fe is much more abundant than the proven ore reserve, we infer that there should be huge potential for prospecting Damiao type iron ores.     

Geochronology and geochemistry of submarine volcanic rocks in the Yamansu iron deposit,Eastern Tianshan Mountains,NW China: Constraints on the metallogenesis

The Yamansu skarn iron deposit is hosted in Early Carboniferous submarine lava flow and volcaniclastic rocks of the Yamansu Formation in Eastern Tianshan Mountains, NW China. The lava flows are predominantly basaltic, with minor andesites. Laser ablation inductively coupled plasma mass spectrometry (LAICP-MS) U–Pb zircon dating of the basalts and skarns yields almost coeval ages of 324.4 ± 0.94 and 323.47 ± 0.95 Ma, respectively. The basalts contain clinopyroxene and plagioclase phenocrysts with a considerable amount of Fe–Ti oxide minerals in the groundmass as interstitial phases, probably suggesting that olivine–, clinopyroxene- and plagioclase fractionated within the magma chamber. Geochemically, the basalts are characterized by slight variations in SiO₂ (42.90–46.61 wt.%), P₂O₅ (0.08–0.12 wt.%), MnO (0.35–0.97 wt.%) and TiO₂ (0.74–0.82 wt.%), and relatively large variations in CaO (6.93–15.13 wt.%), Al₂O₃ (14.71–19.93 wt.%), total Fe₂O₃ (8.14–12.66 wt.%) and MgO (4.96–8.52 wt.%). They possess flat to light rare earth element (REE)-depleted patterns and display variable degrees of depletions in high field-strength elements (HFSE), suggesting a transitional feature between MORB and arc volcanic rocks, and indicating a back-arc tectonic setting. Furthermore, the geochemical signature also suggests that the volcanic rocks of Yamansu Formation were produced by partial melting of the spinel-facies, asthenospheric mantle peridotite which had been metasomatized by slab-derived fluids. The broadly overlapping ages of the basalts and skarn mineralization suggests that the skarn formation in the Yamansu deposit is related to subaqueous volcanism. In combination with the available information including fluid inclusions and stable isotope data, we infer that the hydrothermal fluids that generated the skarns could be a mixture of evolved magma-derived fluids and convecting sea water driven by the heat from the shallow active magma chamber. The Yamansu basalts provided the source of iron for the skarn mineralization. We envisage the submarine volcanism, skarn alteration and iron mineralization in the Yamansu iron deposit as a continuous process, different from either conventional intrusion-related skarn type or submarine volcanic exhalation sedimentation type.     

Fractional crystallization and the formation of thick Fe–Ti–V oxide layers in the Baima layered intrusion,SW China

The Baima layered intrusion is located in the central part of the Emeishan Large Igneous Province (ELIP). The N–S striking intrusion is ~ 24 km long and ~ 2 km thick and dips to the west. Based on variations in modal proportions and cumulus mineral assemblages, the intrusion from the base to the top is simply subdivided into a lower zone (LZ) with most of the economic magnetite layers, and an upper zone (UZ) with apatite-bearing troctolite and gabbro. The rock textures suggest crystallization of the Fe–Ti oxide slightly later than plagioclase (An_67-54) but relatively earlier than olivine (Fo_74-55), followed by clinopyroxene and finally apatite.Relatively low olivine forsterite content and abundant ilmenite exsolution lamellae in clinopyroxene indicate that the Baima parental magma is a highly evolved Fe–Ti-rich magma. Via MELTS model, it demonstrates that under a closed oxygen system, extensive silicate mineral fractionation of a picritic magma might lead to Fe and Ti enrichment and oxygen fugacity elevation in the residual magma. When such Fe–Ti-rich magma ascends to the shallower Baima intrusion, the Fe–Ti oxides may become an early liquidus phase. Well-matched olivine and plagioclase microprobe data with the results of MELTS calculation, combined with relatively low CaO content in olivine (0.02–0.08 wt.%) indicate that wall-rock contamination probably plays a weak role on oxygen fugacity elevation and the early crystallization of Fe–Ti oxides. Several reversals in whole-rock chromium and plagioclase anorthite contents illustrate that multiple recharges of such Fe–Ti-rich magma mainly occurred along the lower part of the Baima magma chamber. Frequent Fe–Ti-rich magma replenishment and gravitational sorting and settling are crucial for the development of thick Fe–Ti oxide layers at the base of the Baima layered intrusion.     

Crustal contamination of Late Neogene basalts in the Dien Bien Phu Basin,NW Vietnam: Some insights from petrological and geochronological studies

Early Pliocene (Zanclean) basalts in the Dien Bien Phu pull-apart basin in NW Vietnam, associated with the presently sinistral Dien Bien Phu Fault Zone, have been dated by the K–Ar method at 4.4–4.9 and 5.4–5.2 Ma. Rapid migration of basaltic magma to the surface in the Dien Bien Phu Fault Zone may be due to Pliocene transtension of the crust in this region, resulting from asthenospheric upwelling induced by lateral displacement of the mantle. The basalts are moderately phyric ( < 10%) and consist of olivine (hyalosiderite), plagioclase (bytownite–labradorite) and orthopyroxene (bytownite–labradorite) phenocrysts, and a fine-grained crystalline matrix (olivine–hortonolite, plagioclase–labradorite, clinopyroxene–pigeonite and augite, K-feldspar). The presence of Fe-rich olivine and orthopyroxene phenocrysts indicates that the basalts are SiO₂-saturated/oversaturated olivine tholeiites which formed under water-undersaturated conditions. The Dien Bien Phu basalts contain both mantle-derived (pyroxenites, dunites, gabbros) and crustal (sillimanite/mullite + Mg–Fe spinel), wallrock xenoliths, indicative of crustal contamination during the ascent of the basaltic magma. The basalts show selective enrichment in some mobile elements (K, Rb, Sr and Th), a feature considered to be a result of metasomatism. These rocks, classified on the basis of their normative composition as quartz tholeiites, could represent primary olivine tholeiites/basalts, in which the geochemical signatures were modified by the processes of contamination.     

Compositions of biotite,amphibole, apatite and silicate melt inclusions from the Tongchang mine,Dexing porphyry deposit,SE China: Implications for the behavior of halogens in mineralized porphyry systems

The Dexing deposit, located in the Circum-Pacific ore belt, is the largest porphyry copper deposit in eastern China. It is composed of 3 separate plutons, which host three mines: Tongchang, Fujiawu and Zhushahong mines. The porphyritic granodiorite samples studied in this investigation were collected from the Tongchang ore-forming pluton of this giant deposit. This paper presents electron microprobe analyses of biotite, apatite, amphibole, plagioclase, potassium feldspar and rehomogenized glassy melt inclusions from the Tongchang porphyritic granodiorites. Petrographic observations of the samples are consistent with portions of the granodioritic magma represented by our samples being overprinted by potassic hydrothermal fluid which variably altered these minerals.All of the studied micas are Mg-rich biotites. The biotites are separated into altered magmatic and secondary types based on their petrographic and geochemical characteristics. The phlogopite components of the secondary biotites are typically higher than those of the altered magmatic biotites, and the X_Mg values of all biotites correlate negatively with Cl contents, consistent with the Mg–Cl avoidance principle. The X_Mg values also correlate negatively with (K₂O + Na₂O + BaO), FeO and TiO₂ for both generations of biotites. The calculated log (fH₂O/fHCl) values (for 690 K) of the coexisting potassic fluids, which are determined from the altered magmatic biotite compositions, range from 4.43 to 4.67, and are very similar to those of other major porphyry deposits. However, the log(fH₂O/fHF) and log(fHF/fHCl) values for the same batch of hydrothermal fluids are significant higher and lower than those of these other porphyry deposits, respectively.The Cl concentrations of amphiboles and melt inclusions range from 0.18 to 0.32 wt.% and 0.15 to 0.44 wt.%, respectively. Most apatites trapped in biotite and plagioclase phenocrysts display a bimodal Cl distribution: 0.19 to 1.35 wt.% and 1.48 to 3.73 wt.%. Similarly, the S contents of the apatite also show a distinct bimodal distribution reflecting the effects of variable anhydrite saturation during evolution of the Tongchang melt and variable dissolution of anhydrite by saline aqueous fluids. The Cl contents of the apatites from the Tongchang system are typically higher than those of other studied porphyry deposits. Furthermore, the Cl contents of the melt inclusions are at or very near the Cl saturation levels (0.36 to 0.46 wt.% at 850 °C and 50 MPa and 0.42 to 0.54 wt.% at 850 °C and 200 MPa) for these melt compositions at shallow crustal pressures. These findings suggest that the area of the granodioritic magma represented by our samples, and perhaps the bulk of the Tongchang granodioritic magma was rich in Cl. The melt inclusion compositions are consistent with a high-salinity, hydrosaline liquid being exsolved directly from the granodioritic melt directly. This high-salinity hydrosaline liquid was likely very efficient at dissolving, transporting and precipitating ore metals in the mineralizing magmatic–hydrothermal system.     

Using platinum-group elements and Au geochemistry to constrain the genesis of podiform chromitites and associated peridotites from the Soghan mafic–ultramafic complex,Kerman, Southeastern Iran

The podiform chromite deposit of the Soghan mafic–ultramafic complex is one of the largest chromite deposits in south-east Iran (Esfandagheh area). The Soghan complex is composed mainly of dunite, harzburgite, lherzolite, pyroxenite, chromitite, wehrlite and gabbro. Olivine, orthopyroxene, and to a lesser extent clinopyroxene with highly refractory nature, are the primary silicates found in the harzburgites and dunites. The forsterite content of olivine is slightly higher in dunites (Fo₉₄) than those in harzburgites (Fo₉₂) and lherzolites (Fo₈₉). Chromian spinel mainly occurs as massive chromitite pods and as thin massive chromitite bands together with minor disseminations in dunites and harzburgites. Chromian spinels in massive chromitites show very high Cr-numbers (80–83.6), Mg-numbers (62–69) and very low TiO₂ content (averaging 0.17 wt.%) for which may reflect the crystallization of chromite from a boninitic magma. The Fe^3 +-number is very low, down to < 0.04 wt.%, in the chromian spinel of chromitites and associated peridotites of the Soghan complex.PGE contents are variable and range from 80 to 153 pbb. Chromitites have strongly fractionated chondrite-normalized PGE patterns, which are characterized by enrichments in Os, Ir and Rh relative to Pt and Pd. Moreover, the Pd/Ir value which is an indicator of PGE fractionation ranges from < 0.08 to 0.24 in chromitite of the Soghan complex. These patterns and the low PGE abundances are typical of ophiolitic chromitites and indicating a high degree of partial melting (about 20–24%) of the mantle source. Moreover, the Pd_N/Ir_N ratios in dunites are unfractionated, averaging 1.2, whereas the harzburgites and lherzolites show slightly positive slopes PGE spidergrams, together with a small positive Ru and Pd anomaly, and their Pd_N/Ir_N ratio averages 1.98 and 2.15 respectively.The mineral chemistry data and PGE geochemistry, along with the calculated parental melts in equilibrium with chromian spinel of the Soghan chromitites indicate that the Soghan complex was generated from an arc-related magma with boninitic affinity above a supra-subduction zone setting.     

Mafic and silica-rich glasses in mantle xenoliths from Wau-en-Namus,Libya: Textural and geochemical evidence for peridotite–melt reactions

Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤ 2 GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43–67 wt.% SiO₂) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe–Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr–Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ = ²³⁸Pb/²⁰⁴Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa.     

Ultramafic xenoliths from Lake Nyos area,Cameroon volcanic line,West-central Africa: Petrography,mineral chemistry,equilibration conditions and metasomatic features

Spinel-bearing mantle xenoliths have been recovered in the pyroclastic breccia surrounding the Lake Nyos maar. These include spinel lherzolites, spinel harzburgites and olivine websterites. They exhibit coarse granular or protogranular to weakly porphyroclastic textures, and show variations in mineral chemistry, modal compositions and equilibrium temperature. The xenoliths consist of four mineral phases typical of upper mantle origin: olivine (Fo₈₉–Fo_91.5, NiO = 0.29–0.38 wt%, CaO = 0.02–0.17 wt%), enstatite (Mg# = 90–92, Cr₂O₃ = 0.35 ± 0.04 wt%), Cr-diopside (Mg# = 92–98, Cr₂O₃ = 0.7–1.65 wt%, TiO₂ = 0.26–0.6 wt%) and spinel (high Mg# of 70–80, low TiO₂ ≤ 0.4 wt%). Spinels are aluminous (Cr# = 9.7–11) in most lherzolites, and become increasingly chromiferous from websterites (Cr#_Sp = 15.3–19.8) to harzburgites (Cr#_Sp = 19–33.6). The lherzolites are composed of olivine (48–58%), orthopyroxene (22–30%), and clinopyroxene (8–15%). The harzburgites modes are olivine (60–81%), orthopyroxene (11–29%), and clinopyroxene (<5%). The websterites are mainly composed of pyroxene (～62%) with variable amounts of olivine (23–31%). Temperatures of mineral equilibration in the xenoliths have been estimated from the two-pyroxene thermometer of Wells (1977) and range between 850 and 1050 °C, corresponding to about 10–30 kbar at a depth mantle of 30 km at least. These P–T conditions show significant variations between different petrographical types, the maximum conditions being recorded in two spinel lherzolites (NY-05 and NY-23) that have atypical chemical compositions and textures suggesting that they were initially formed in an environment close to the garnet stability field, then re-equilibrated within the spinel stability field prior to their incorporation in the host magma. With the exception of minerals from these two lherzolite nodules, all the minerals exhibit depletion of light REE, a typical feature of abyssal peridotites implying that some xenoliths from the Cameroon volcanic line were probably sampled in a part of the sub-continental mantle that is chemically similar to sub-oceanic mantle. The variations observed in the mineral chemistry and modal compositions of xenoliths suggest that the spinel harzbugite nodules which represent residues of a significant degree of partial melting of lherzolitic mantle were affected by infiltration of alkali-enriched metasomatizing melts (or fluids) within the uppermost mantle to produce pargasitic amphiboles prior to their sampling by the host lava. The features of this metasomatism event occur in the rocks of all three petrographical facies xenoliths from Lake Nyos.     

Mineral chemistry of high-Mg diorites and skarn in the Han-Xing Iron deposits of South Taihang Mountains,China: Constraints on mineralization process

The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Early Cretaceous high-Mg diorite complex, including gabbro diorite, hornblende diorite, diorite, diorite porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the clinopyroxenes in the diorite and monzonite are diopside. Amphiboles in the high-Mg diorites show compositional range from magnesiohornblende to magnesiohastingsite, with minor pargasite and tschermakite. Most plagioclase in the high-Mg diorite is andesine and oligoclase. The magnesio-biotite in gabbro diorites shows chemical characteristics of re-equilibrated primary biotites and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (< 9 wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly > 25 wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the early stage shows high temperature (~ 900 °C–950 °C), pressure (~ 300 MPa–500 MPa), relatively high logfO₂ (NNO–NNO + 2), and H₂O content in melt (4%–8%). During the late stage, the magma temperature dropped to about 750 °C, and pressure came down to less than 100 MPa, with the logfO₂ rising to NNO + 1–NNO + 2.The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection of mafic magma into the felsic magma chamber. The relatively high logfO₂ and H₂O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate wall rocks. The high logfO₂, Fe^3 + rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively.     

Thermal and redox equilibrium conditions of the upper-mantle xenoliths from the Quaternary volcanoes of NW Spitsbergen,Svalbard Archipelago

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 °C, 13–27 kbar, and oxygen fugacity of − 1.5 to + 0.3 log. un.; in the pyroxenites—1100–1310 °C, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 °C higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe^2 + between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH⁻, crystal hydrate water H₂O_cryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH– + H₂O_cryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 °C. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (> 80% H₂O) to aqueous–carbonic (~ 60% H₂O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.     

Geochronology and geochemistry of the major host rock of the Dong’an gold deposit,Lesser Khingan Range: Implications for petrogenesis and metallogenic setting during the Early–Middle Jurassic in northeast China

The Dong’an gold deposit is a large-sized epithermal gold deposit recently discovered in the Lesser Khingan Range, NE China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of a medium–coarse grained alkali-feldspar granite, the major host rock of the Dong’an gold deposit. The LA–ICP–MS zircon U–Pb dating of the medium–coarse grained alkali-feldspar granite yields an early Jurassic age of 176.3 ± 1.1 Ma (MSWD = 0.62). The whole-rock geochemical data indicate that the samples are felsic, ferroan, alkali-calcic and peraluminous with relatively high alkali (K₂O + Na₂O) content. They are enriched in LREEs and LILEs (e.g., Rb, Ba, K), but are depleted in HFSEs (e.g., Nb, Ta, P, Ti), especially in P and Ti, showing characteristics of volcanic arc magmas and similarities with the Early–Middle Jurassic granitic rocks in Xing’an Mongolian orogenic belt. Meanwhile, the negative Eu, Nb, Ta, Ti, and P anomalies are consistent with fractional crystallization of plagioclase, Ti-bearing phases (rutile, ilmenite, titanite, etc.) and apatite during magma evolution. The samples have low Nb/Ta ratios (8.65–14.91) and low Mg^# values (18–36), which are indicative of crustal derived magmas and no interaction between source magmas and the mantle. In-situ Hf isotopic analyses of the zircons from the medium–coarse grained alkali-feldspar granite yield εHf(t) values of +3.38–+5.68 and two-stage model ages (T_DM2) of 772–900 Ma, indicating the magmas formed this intrusion were generated by partial melting of Neoproterozoic basaltic materials in the young lower crust, and the magma source could be derived from a depleted mantle. The medium–coarse grained alkali-feldspar granite most likely formed in the late stage of Toarcian subduction of the Pacific plate, which can be identified on the tectonic setting discrimination diagrams, and the formation of this intrusion was associated with underplating of mantle-derived magmas, which provided heat for crustal partial melting. Similar to the medium–coarse grained alkali-feldspar granite, large amounts of granitic rocks and a series of nonferrous metal hydrothermal deposits (Mo, Cu, Au) formed in northeast China as results of magmatic activities triggered by subduction of the Pacific plate during the Early–Middle Jurassic.     

Manganoan ilmenite as kimberlite/diamond indicator mineral

Manganoan ilmenite was identified in Juina, Brazil kimberlitic rocks among other megacrysts. It forms oval, elongated, rimless grains comprising 8–30 wt.% of the heavy fraction. Internally the grains are homogeneous. The chemical composition of Mn-ilmenite is almost stoichiometric for ilmenite except for an unusually high manganese content, with MnO = 0.63–2.49 wt.% (up to 11 wt.% in inclusions in diamond) and an elevated vanadium admixture (V₂O₃ = 0.21–0.43 wt.%). By the composition, Mn-ilmenite megacrysts and inclusions in diamond are almost identical. The concentrations of trace elements in Mn-ilmenite, compared to picroilmenite, are much greater and their variations are very wide. Chondrite-normalized distribution of trace elements in Mn-ilmenite megacrysts is similar to the distribution in Mn-ilmenites included in diamond. This confirms that Mn-ilmenite in kimberlites is genetically related to diamond. The finds of Mn-ilmenite known before in kimberlitic and related rocks are late- or postmagmatic, metasomatic phases. They either form reaction rims on grains of picroilmenite or other ore minerals, or compose laths in groundmass. In contrast to those finds, Mn-ilmenite megacrysts in Juina kimberlites are a primary mineral phase with a homogeneous internal structure obtained under stable conditions of growth within lower mantle and/or transition zone. In addition to pyrope garnet, chromian spinel, picroilmenite, chrome-diopside, and magnesian olivine, manganoan ilmenite may be considered as another kimberlite/diamond indicator mineral.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

The Neoproterozoic (∼ 820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ∼ 35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ∼ 150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite–biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age.     

Melt impregnation phases in the mantle section of the Ślęża ophiolite (SW Poland)

The ultramafic member of the Variscan Ślęża Ophiolite (SW Poland) consists of heavily serpentinised, refractory harzburgites. Those located down to 1.5 km below paleo-Moho contain scarce grains or aggregates of olivine, clinopyroxene and spinel. Non-serpentine phases occur in various assemblages: M1—olivine (Fo 90.2–91.0%, NiO 0.38–0.47 wt.%) and rounded or amaeboidal aluminous chromite, rimmed by Al poor chromite and magnetite; M2—olivine (Fo 90.5–91.5, NiO 0.32–0.44 wt.%), olivine with magnetite inclusions (Fo 87.1–92.5, NiO 0.01–0.68 wt.%), rounded, cleavaged clinopyroxene I (Mg# 91.1–93.2, Al₂O₃ 3.00–4.00 wt.%, Cr₂O₃ 1.00–1.40 wt.%) and elongated clinopyroxene II and clinopyroxene from symplectites with magnetite (Mg# = 92.2–94.1, Al₂O₃ 2.20–3.20 wt.% and Cr₂O₃ 0.80–1.20 wt.%). Clinopyroxene is depleted in REEs relative to chondrite. The M3 assemblage consists of intergrown olivine (Fo 90.8–92.7, NiO 0.20–0.38 wt.%) and clinopyroxene (Mg# = 96.0–98.1, Al₂O₃ 0.00–1.00 wt.% and Cr₂O₃ 0.20–0.60 wt.%).The M1 assemblage contains chromite which records greenschist-facies metamorphism. Textural relationships and chemical composition of clinopyroxene occurring in the M2 assemblage are similar to those formed in oceanic spreading centres by LREE depleted basaltic melt percolation. Olivine occurring in M1 assemblage and part of that from M2 have composition typical of residual olivine from the abyssal harzburgites and of olivine formed in those rocks by melt percolation. The olivine with magnetite inclusions (M2 assemblage) and that from M3 record later deserpentinization event, which supposedly produced also M3 clinopyroxene. The non-serpentine phases from the Ślęża ophiolite mantle member, albeit very poorly preserved, document depleted basaltic melt percolation in the Variscan oceanic spreading centre.     

Weathering of ilmenite from Chavara deposit and its comparison with Manavalakurichi placer ilmenite,southwestern India

The magnetic fractions of ilmenite from the beach placer deposit of Chavara, southwest India have been studied for mineralogical and chemical composition to assess the range of their physical and chemical variations with weathering. Chavara deposit represents a highly weathered and relatively homogenous concentration. Significant variation in composition has been documented with alteration. The most magnetic of the fractions of ilmenite, separated at 0.15 Å, and with a susceptibility of 3.2 × 10^?6 m³ kg^?1, indicates the presence of haematite–ilmenite intergrowth. An iron-poor, titanium-rich component of the ilmenite ore has been identified from among the magnetic fractions of the Chavara ilmenite albeit with an undesirably high Nb₂O₅ (0.28%), Cr₂O₃ (0.23%) and Th (149 ppm) contents. The ilmenite from Chavara is compared with that from the nearby Manavalakurichi deposit of similar geological setting and provenance. The lower ferrous iron oxide (2.32–14.22%) and higher TiO₂ (56.31–66.45%) contents highlight the advanced state of alteration of Chavara. This is also evidenced by the relatively higher Fe³⁺/Fe²⁺ ratio compared to Manavalakurichi ilmenite. In fact, the ilmenite has significantly been converted to pseudorutile/leucoxene.     

Neoarchean suprasubduction zone arc magmatism in southern India: Geochemistry,zircon U-Pb geochronology and Hf isotopes of the Sittampundi Anorthosite Complex

The Sittampundi Anorthosite Complex (SAC) in southern India is one of the well exposed Archean layered anorthosite-gabbro-ultramafic rock associations. Here we present high precision geochemical data for the various units of SAC, coupled with zircon U-Pb geochronology and Hf isotopic data for the anorthosite. The zircon ages define two populations, the older yield a concordia age of 2541 ± 13 Ma, which is interpreted as the best estimate of the magmatic crystallization age for the Sittampundi anorthosite. A high-grade metamorphic event at 2461 ± 15 Ma is suggested by the upper intercept age of the younger zircon population. A Neoproterozoic event at 715 ± 180 Ma resulted in Pb loss from some of the metamorphic zircons. The magmatic age of the anorthosite correlates well with the timing of crystallization of the arc-related ~ 2530 Ma magmatic charnockites in the adjacent Salem Block, while the metamorphic age is synchronous with the regional metamorphic event. The geochemical data suggest that the rocks were derived from a depleted mantle source. Sub-arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous, aluminous basalt magma. The magma fractionated at depth to produce a variety of high-alumina basalt compositions, from which the anorthositic complex with its chromite-rich and amphibole-rich layers formed as cumulates within the magma chamber of a supra-subduction zone arc. The coherent initial¹⁷⁶Hf/¹⁷⁷Hf ratios and positive εHf values (1.7 – 4.5) of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source. Our study further confirms that the southern part of the Dharwar Craton was an active convergent margin during the Neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth.     

Petrology of mafic and ultramafic layered rocks from the Jaboncillo Valley,Sierra de Valle Fértil,Argentina: Implications for the evolution of magmas in the lower crust of the Famatinian arc

This work presents the field setting, petrography, mineralogy and geochemistry of a gabbroic and peridotitic layered body that is lens-shaped and surrounded by gabbronorites, diorites, and metasedimentary migmatites. This body exposed at Jaboncillo Valley is one among several examples of mafic and ultramafic layered sequences in the Sierras Valle Fértil and La Huerta, which formed as part of the lower crust of the Ordovician Famatinian magmatic arc in central-western Argentina. The layered sequence grew at deep crustal levels (20–25 km) within a mafic lower crust. The base of the layered body was detached during the tectonic uplift of the Famatinian lower crust, whereas the roof of the layered body is exposed in the eastern zone. In the inferred roof, olivine-bearing rocks vanish, cumulate textures are less frequent, and the igneous sequence becomes dominated by massive or thinly banded gabbronorites. Mainly based on the petrographic relationships, the inferred order of crystallization in the gabbroic and peridotitic layered sequence is: (1) Cr–Al-spinel + olivine, (2) Cr–Al-spinel + olivine + clinopyroxene + magnetite, (3) Cr–Al-spinel + olivine + plagioclase + magnetite ± orthopyroxene, and (4) Al-spinel + orthopyroxene + amphibole. A strong linear negative correlation between olivine and plagioclase modal proportions combined with field, petrographic and geochemical observations are used to demonstrate that the physical separation of olivine and plagioclase results in rock diversity at scales of a few centimeters to tens of meters. However, the composition of olivine (Fo ～ 0.81) and plagioclase (An > 94%) remains similar throughout the layered sequence. Spinels are restricted to olivine-bearing assemblages, and display chemical trends characteristic of spinels found in arc-related cumulates. Gabbroic and peridotitic layered rocks have trace element concentrations reflecting cumulates of early crystallizing minerals. The trace element patterns still retain the typical features of subduction-related arc magmatism, showing that the process of cumulate formation did not obscure the trace element signature of the parental magma. Using the composition of cumulus minerals and whole-rock chemical trends, we show that the parental magma was mafic (SiO₂ ～ 48 wt.%) with Mg-number around 0.6, and hydrous. The oxygen fugacity (fO₂) of the parental magma estimated between +0.8 and ?0.6 log fO₂ units around the fayalite–magnetite–quartz (FMQ) buffer is also characteristic of primitive hydrous arc magmas. The initially high water content of the parental magma allowed amphibole to crystallize as an interstitial phase all over the crystallization evolution of the layered sequence. Amphibole crystallization in the inter-cumulus assemblage gives rise to the retention of many trace elements which would otherwise be incompatible with the mineral assemblage of mafic–ultramafic cumulates. This study shows that there exist strongly mafic and primitive magmas that are both generated and emplaced within the lower crustal levels of subduction-related magmatic arc. Our findings together with previous studies suggest that the Early Ordovician magmatic paleo-arc from central-northwestern Argentina cannot be regarded as a typical Andean-type tectono-magmatic setting.     

Geochemical modelling of the Chilas Complex in the Kohistan Terrane,northern Pakistan

The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma.     

Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: Subduction and ultrahigh-pressure metamorphism of lower crustal cumulates

Three distinct groups of eclogites (low-Mg–Ti eclogites, high-Ti eclogites and Mg-rich eclogites) and ultramafic rocks from the depth interval of 100–680 m of the Chinese Continental Scientific Drill Hole were studied. The low Mg#s (= 100^?molar Mg/(Mg + Fe)) (81–84%) and low Ni (1150–1220 ppm) and high Fe₂O₃^total (13–15 wt.%) contents of ultramafic rocks suggest a cumulate origin. Mg-rich eclogites show middle and heavy REE enrichments, which could not be produced by metamorphic growth of garnet. Instead, if the rocks formed from a light REE enriched magma, there may be an igneous precursor for some garnets in their protolith. Alternatively, perhaps they formed from a light REE depleted magma without garnet. The high-Ti eclogites are characterized by unusually high Fe₂O₃^total contents (up to 24.5 wt.%) and decoupling of high TiO₂ from low Nb and Ta contents. These features cannot be produced by concentration of rutile during UHP metamorphism (even for samples with TiO₂ > 4 wt.%) of high-Ti basalts, but could be attributed to crystal fractionation of titanomagnetite (for those with TiO₂ < ～ 4 wt.%) or titanomagnetite + ilmenite (for those with TiO₂ > ～ 4 wt.%). Thus, we suggest that protoliths of the high-Ti eclogites were titanomagnetite/ilmenite-rich gabbroic cumulates. As a whole, the low-Mg–Ti eclogites are geochemically complementary to the high-Ti eclogites, Mg-rich eclogites and ultramafic rocks, and could be metamorphic products of gabbroic/dioritic cumulates formed by high degree crystal fractionation. All these observations suggest that parental materials of the ultramafic rock-eclogite assemblage could represent a complete sequence of fractional crystallization of tholeiitic or picritic magmas at intermediate to high pressure, which were later carried to ultrahigh-pressure conditions during a continental collision event.