Geology and the origin of the corundum-bearing skarn at Bakamuna,Sri Lanka

A corundum-bearing skarn deposit was discovered in the Bakumuna area. The geology, petrology, and geochemistry of the deposit were studied. The skarn deposit comprises three mineralogically different zones, and the corundum is chiefly mineralized in the outermost zone. The deposit formed by the reaction of late magmatic fluids with the marble which eventually enriched the fluids with Al, causing Al₂O₃ to precipitate as corundum.     

Devonian rodingite from the northern margin of the North China Craton: mantle wedge metasomatism during ocean–continent convergence

The northern margin of the North China Craton (NCC) was an active convergent margin during Palaeozoic and preserves important imprints of magmatic and metasomatic processes associated with oceanic plate subduction. Here, we investigate the mafic–ultramafic rocks in the Xiahabaqin–Sandaogou complexes from the northern NCC including pyroxenite, hornblendites, hornblende gabbro, and their rodingitized counterparts within a serpentinite domain. We present petrological, zircon U–Pb geochronological, and geochemical data to constrain the nature and timing of the magmatic and metasomatic processes in the subduction zone mantle wedge. The rock suites investigated in this study are characterized by low contents of SiO₂, Na₂O, and K₂O, with high CaO, FeO, Fe₂O₃, and MgO. The rodingitized rocks show markedly high CaO and lower MgO compared to their ultramafic protolith, suggesting extensive post-magmatic infiltration of Ca-rich, Si-poor fluids derived by serpentinization of mantle peridotite. The enrichment of large ion lithophile and light rare earth elements such as Ba, Sr, K, La, and Ce with relative depletion of high field strength elements like Nb, Ta, Zr, and Hf in the ultramafic rocks collectively suggest metasomatism of a fore-arc mantle wedge by fluids released through dehydration of subducted oceanic slab and subduction-derived sediments. Dehydration and decarbonation leading to metasomatic fluid influx and serpentinization of mantle wedge peridotite account for the enriched geochemical signatures for the rodingitized rocks. The zircon grains in these rocks show textures indicating magmatic crystallization followed by fluid-controlled dissolution–precipitation. Magmatic zircons from altered pyroxenite, hornblendite, and rodingitized pyroxenite in Xiahabaqin yield protolith crystallization ages peaks at 396 Ma and 392 Ma and metasomatic grains show ages of 386 Ma, 378 Ma, and 348 Ma. The zircons from hornblendite and basaltic trachyandesite indicate protolith emplacement during 402–388 Ma. Metasomatic zircon grains from rodingitized hornblende gabbro in Sandaogou complex show a wide range of ages as 412 Ma, 398 Ma, 383 Ma, and 380 Ma. The common magmatic zircon ages peaks at 398–388 Ma in most of the rocks suggest a similar time for magma crystallization in the Xiahabaqin and Baiqi during Middle Devonian. Subsequently, repeated pulses fluids and melts resulted in metasomatic reactions in mantle wedge until early Permian. The Lu–Hf analysis of the zircon grains from these rocks display markedly negative εHf(t) values ranging from ?22.4 to ?7.7, suggesting magma derivation from an enriched, hydrated lithospheric mantle through fluid–rock interaction and mantle wedge metasomatism. Rodingitization processes are associated with exhumation of ultramafic mantle wedge rocks within a serpentinized subduction channel close to the subducted slab in response to slab roll back in a long-lasting subduction regime. This study offers insights into magmatic and metasomatic processes of ultramafic rocks in the fore-arc mantle wedge which were exhumed and accreted to an active continental margin during the southward subduction of the Palaeo-Asian oceanic lithosphere beneath the NCC.     

攀枝花岩体钛铁矿成分特征及其成因意义

峨眉大火成岩省是全球最大的钒钛磁铁矿床聚集区,攀枝花岩体是其中的典型代表。根据岩性特点,攀枝花岩体主体可划分为上、中、下三个岩相带,其中中部岩相带和下部岩相带岩性旋回非常发育,每个旋回从下向上铁钛氧化物和暗色硅酸盐矿物逐渐减少,块状铁钛氧化物矿石或磁铁矿辉长岩都出现在每个旋回的底部和下部。然而,尽管钛铁矿固相线以下固溶体出溶远弱于磁铁矿,从而能更好地保留成因信息,但其成分变化的成因意义没有受到足够重视。本次研究发现作为主要金属氧化物之一的钛铁矿的成分不仅在不同岩性中有明显差异,同时,中、下部岩相带的各岩性旋回中钛铁矿成分也具有周期性变化。例如,块状矿石中钛铁矿具有最高的MgO和TiO2及最低的FeO、Fe2O3和MnO,而辉长岩中钛铁矿则具有相反的成分特征。同时,钛铁矿的MgO含量与磁铁矿的MgO含量及橄榄石的Fo牌号具有显著的正相关关系。这种规律性变化说明每个旋回可以代表一次比较明显的岩浆补充,每次新岩浆补充后,钛铁矿和磁铁矿及橄榄石都是结晶较早的矿物。与Skaergaard岩体相比,攀枝花岩体钛铁矿的MgO含量较高,表明攀枝花岩体分离结晶过程中铁钛氧化物结晶较早;与挪威Tellnes斜长岩套铁钛矿床中的钛铁矿相比,攀枝花岩体的钛铁矿不仅具有较高的MgO和FeO,还具有极高的TiO2和MnO,但Fe2O3却很低,说明地幔柱背景下形成的钛铁矿与斜长岩套中钛铁矿的成分有显著的区别。     

Age and geochemistry of zircons from the ancient granitoids of the Vyg River,Southeastern Karelia

The structure and composition of accessory zircons from the tonalites of the Vyg River, southeastern Karelia, were investigated. Their local U-Pb SHRIMP dating yielded ages between 3127±15 and 3146±25 Ma. It was shown that the zircons consist of three zones, a central part containing solid and melt inclusions and zoned magmatic and metasomatic shells. The obtained ages correspond to the magmatic and metasomatic stages of zircon crystallization. In general, the zircons have elevated contents of LREE (up to 867 ppm La), which were mainly accumulated in the outer metasomatic shell. Apatite and CO₂ inclusions are widespread. Orthoclase, orthopyroxene, ilmenite, galena, quartz, and bastnaesite were identified in a solid inclusion in one zircon core using a CAMSCAN MX 2500 electron microscope. The presence of bastnaesite accentuates the relation of LREE with a CO₂-rich fluid. It was shown that REE content is not correlated with U, Th, and U/Th ratio.     

FeO*-Al2O3-TiO2-Rich Rocks of the Tertiary Bana Igneous Complex, West Cameroon

FeO*‐Al₂O₃‐TiO₂‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al₂O₃‐TiO₂‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al₂O₃‐TiO₂‐rich rocks are characterized by low concentrations of SiO₂ (25–54.2 wt%), Na₂O + K₂O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al₂O₃ (20–42%), TiO₂ (3–9.2%), and P₂O₅ (0.26–1.30%). TiO₂ is positively correlated with Al₂O₃ and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al₂O₃)?(CaO + Na₂O + K₂O)?(FeO*+ MnO + MgO) and (SiO₂)?(FeO*)?(Al₂O₃), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al₂O₃‐TiO₂‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.     

Corundum formation by metasomatic reactions in Archean metapelite,SW Greenland:Exploration vectors for ruby deposits within high-grade greenstone belts

Corundum (ruby-sapphire) is known to have formed in situ within Archean metamorphic rocks at several localities in the North Atlantic Craton of Greenland. Here we present two case studies for such occurrences: (1) Maniitsoq region (Kangerdluarssuk), where kyanite paragneiss hosts ruby corundum, and (2) Nuuk region (Storø), where sillimanite gneiss hosts ruby corundum. At both occurrences, ultramafic rocks (amphibole-peridotite) are in direct contact with the ruby-bearing zones, which have been transformed to mica schist by metasomatic reactions. The bulk-rock geochemistry of the ruby-bearing rocks is consistent with significant depletion of SiO₂ in combination with addition of Al₂O₃, MgO, K₂O, Th and Sr relative to an assumed aluminous precursor metapelite. Phase equilibria modelling supports ruby genesis from the breakdown of sillimanite and kyanite at elevated temperatures due to the removal of SiO₂. The juxtaposition of relatively silica- and aluminum-rich metasedimentary rocks with low silica ultramafic rocks established a chemical potential gradient that leached/mobilized SiO₂ allowing corundum to stabilize in the former rocks. Furthermore, addition of Al₂O₃ via a metasomatic reaction is required, because Al/Ti is fractionated between the aluminous precursor metapelites and the resulting corundum-bearing mica schist. We propose that Al was mobilized either by complexation with hydroxide at alkaline conditions, or that Al was transported as K-Al-Si-O polymers at deep crustal levels. The three main exploration vectors for corundum within Archean greenstone belts are: (1) amphibolite- to granulite-facies metamorphic conditions, (2) the juxtaposition of ultramafic rocks and aluminous metapelite, and (3) mica-rich reactions zones at their interface.     

Corundum-bearing metasomatic rocks in the Central Pamirs

The Muzkol metamorphic complex in the Central Pamirs contains widespread occurrences of corundum mineralization, sometimes with gem-quality corundum. These occurrences are spatially related to zones of metasomatic alterations in calcite and dolomite marbles and crystalline schists. The calcite marbles contain corundum together with muscovite, scapolite, and biotite; the dolomite marbles contain corundum in association with biotite; and the schists bear this mineral coexisting with biotite and chlorite. All these rocks additionally contain tourmaline, apatite, rutile, and pyrite. The biotite is typically highly aluminous (up to 1.9 f.u. Al), and the scapolite is rich in the marialite end member (60–75 mol %). The crystallization parameters of corundum were estimated using mineral assemblages at T = 600–650°C, P = 4–6 kbar, X _{CO 2} = 0.2–0.5 at elevated alkalinity of the fluid. The Sr concentration in the calcite and dolomite marbles is low (345–460 and 62–110 ppm, respectively), as is typical of recrystallized sedimentary carbonates. The variations in the ⁸⁷Sr/⁸⁶Sr ratio in the calcite and dolomite marbles (0.70852–0.70999 and 0.70902–0.71021, respectively) were controlled by the introduction of radiogenic ⁸⁷Sr during the metasomatic transformations of the rocks. The isotopic-geochemical characteristics obtained for the rocks and the results of numerical simulations of the fluid-rock interactions indicate that the corundum-bearing metasomatic rocks developed after originally sedimentary Phanerozoic carbonate rocks, with the desilication of the terrigenous material contained in them. This process was a manifestation of regional alkaline metasomatism during the closing stages of Alpine metamorphism. In the course of transformations in the carbonate reservoir, the juvenile fluid flow became undersaturated with respect to silica, which was a necessary prerequisite for the formation of corundum.     

Geochemistry of granitic aplite-pegmatite dykes and sills and their minerals from the Gravanho-Gouveia area in Central Portugal

Two distinct series of Variscan granitic rocks have been distinguished in the Gravanho-Gouveia area of Portugal, based on field work, variation diagrams for major and trace elements, rare earth patterns and δ¹⁸O versus total FeO diagram of rocks, anorthite content of plagioclase, BaO and P₂O₅ contents of feldspars and Al^VI versus Fe²⁺ diagram for magmatic muscovite. One series consists of a late-orogenic porphyritic biotite > muscovite granite (G1), less evolved beryl-columbite pegmatites and more evolved beryl-columbite pegmatites showing gradational contacts. The other series consists of post-orogenic porphyritic muscovite > biotite granodiorite to granite (G2), slightly porphyritic muscovite > biotite granite (G3) and lepidolite pegmatites. In each series, pegmatites are derived from the parent granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Some metasomatic effects occur like muscovite replacing feldspars, chlorite in pegmatites of the first series and a late muscovite in pegmatites of the second series, probably due to hydrothermal fluids. The lepidolite pegmatites contain cassiterite and two generations of rutile. The first magmatic generation consists of homogeneous crystals and the second generation occurs as heterogeneous zoned crystals derived from hydrothermal fluids. The beryl-columbite pegmatites and lepidolite pegmatites also contain the first magmatic generation and the late hydrothermal generation of zoned columbite-group minerals. More evolved beryl-columbite pegmatites were converted into episyenite by intense hydrothermal alteration and regional circulation of fluids in the granitic rocks.     

Formation of sapphirine during retrogression of a basic high-pressure granulite,Roan, Western Gneiss Region,Norway

Sapphirine-bearing rocks occur in the northern part of the Western Gneiss Region, Vestranden, central Norway. The sapphirine-bearing rocks are characterized by a high MgO/(MgO + FeO) ratio, high Al₂O₃, MgO and CaO, and low SiO₂ contents. These rocks form layers within larger complexes which originated as layered magmatic rocks. High PT-metamorphism produced a cpx+ky+gt assemblage. The P and T estimates are P = 14.5±2 kbar and T= 870±50° C. During retrogression, the high-P granulite assemblage broke down to form an intermediate-P granulite mineralogy comprising orthopyroxene, spinel, anorthite, andesine, sapphirine and corundum. Textural relationships suggest that sapphirine formed by the reaction: spinel+kyanite sapphirine+corundum, and probably also by a reaction between corundum, spinel and orthopyroxene. All reactions took place within the stability field of kyanite. Textural and micro-chemical relationships indicate equilibrium, conditions during the peak metamorphism, whereas pronounced disequilibrium characterizes the mineral associations formed during the early retrogression at low P _H2O. The investigation shows that parts of the northern segment of the Western Gneiss Region underwent a metamorphic evolution similar to the Caledonian one recorded from eclogite/granulite terrains further south.     

Crystallization of titaniferous chromite,magnesian ilmenite and armalcolite in tholeiitic suites in the Karoo Igneous Province

Titaniferous chromite (up to 8 wt% TiO₂) and magnesian ilmenite (up to 10 wt% MgO) coexist at the base of the differentiated tholeiitic Mount Ayliff Intrusion in the Karoo Province of southern Africa, suggesting that the original magma was TiO₂-rich. Picritic lavas with 3% TiO₂ from the Lebombo monocline of the Karoo Province also contain microphenocrysts of magnesian ilmenite (up to 6 wt% mgO) and armalcolite (up to 7 wt% MgO). These oxide mineral associations and compositions are atypical of tholeiitic magmas, in which chromite usually has less than 1 wt% TiO₂, ilmenite less than 3 wt% MgO and armalcolite is rarely a primary mineral. Experiments have been conducted at one atmosphere pressure on a range of compositions to determine the effect of TiO₂ on the crystallization and composition of chromite, ilmenite and armalcolite. The results indicate that increasing the TiO₂ content of picritic magmas increases the TiO₂ content of the spinel, mainly at the expense of Al₂O₃, whereas Cr₂O₃ is not affected. Spinel compositions in the Mount Ayliff Intrusion (with over 45 wt% Cr₂O₃, less than 10 wt% Al₂O₃ and 8 wt% TiO₂) were duplicated in experiments on a picrite at temperatures of about 1,200°C at the Ni/NiO buffer. Increasing fO₂ from fayalite-magnetite-quartz to Ni/NiO buffer is shown to increase the crystallization temperature of armalcolite and to decrease that of ilmenite. The total FeO content of the liquid has little influence on the crystallization temperature of these phases. The TiO₂ content of the liquid, when either ilmenite or armalcolite crystallizes, varies inversely with SiO₂ content. The MgO content of the liquid at which ilmenite or armalcolite crystallizes depends upon the TiO₂ content of the starting composition, with naturally occurring and experimetally determined saturation being demonstrated for liquids with 5 wt% MgO and 5.5 wt% TiO₂. The partition coefficent for MgO between armalcolite or ilmenite and liquid is about 1.5. Observed magnesian armalcolite and ilmenite compositions in picrite lavas (both minerals) and in the Mount Ayliff Intrusion (ilmenite only) are consistent with crystallization from a TiO₂-rich magma with approximately 5 wt% MgO. The Fe ₂ ³⁺ TiO₅ component of armalcolite in the picrite lavas matches those formed experimentally at temperatures of 1,150–1,110°C and fO₂ of the Ni/NiO to Ni/NiO+1 log unit. Similarities also exist between the compositions of chromite, ilmenite and armalcolite and liquid fraction-ation trends of some Hawaiian high-TiO₂ lavas and the experimental studies presented here.     

山阳-柞水矿集区李家砭Ti-Fe矿床成矿构造背景研究

李家砭Ti-Fe矿床位于南秦岭山阳-柞水矿集区内南侧山阳-凤镇构造混杂带内,主要表现为钛磁铁矿和钛铁矿化,含矿围岩为堆晶辉长岩。矿物组合特征及结构分析表明,该矿床为典型的岩浆分异型矿床。矿物成分和岩石地球化学分析结果共同表明李家砭杂岩体具有岛弧钙碱性和大洋板内玄武岩双重岩浆性质。辉长岩SHRIMP锆石U-Pb测年结果表明,李家砭辉长岩和Ti-Fe矿床形成于621±6Ma,其中所含有的继承性锆石来自新元古代晚期岛弧中酸性岩浆岩,以上特征表明李家砭Ti-Fe矿床形成于新元古代晚期弧后盆地构造背景下。     

Petrology of corundum-spinel-sapphirine-anorthite rocks (sakenites) from the type locality in southern Madagascar

‘Sakenites’ constitute a unique association of corundum‐, spinel‐ and sapphirine‐bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum‐, spinel‐ and sapphirine‐bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite‐sillimanite‐cordierite gneisses of the Ihosy granulite unit in the NW of the Pan‐African Bongolava‐Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite‐rich sediments or calcareous bauxites that were altered by Ca or Si infiltration‐metasomatism prior to or coeval with the development of the anorthite‐corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak‐conditions around 800 °C and 6–7 kbar for the regional high‐grade metamorphism and deformation in the NW part of the Bongolava‐Ranotsara shear zone. The well‐annealed granoblastic‐polygonal textures indicate complete chemical and textural re‐equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg‐, Si‐ and K‐bearing fluids into the anorthite‐corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg‐Al minerals, thereby producing corundum‐, spinel‐ and sapphirine‐bearing phlogopitites. Further advance of the resulting ‘residual’ Mg‐ and Si‐bearing fluids into anorthite‐corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava‐Ranotsara ductile shear zone c. 530–500 Ma ago.     

Relative contributions of crust and mantle to generation of Oligo-Miocene medium-K calc-alkaline I-type granitoids in a subduction setting—a case study from the Nabar Pluton,central Iran

The Nabar pluton with the age of Oligo-Miocene located northwest of Isfahan, the Urumieh-Dokhtar magmatic belt, is composed of gabbro, gabbro diorite, diorite, quartz diorite, tonalite, and quartz monzonite. These rocks contain plagioclase, quartz, alkali-feldspar, magnesiohornblende, actinolite, tremolite-hornblende, actinolite-hornblende, anthophyllite, biotite, and Na-poor pyroxene. Application of the Al-in-hornblende barometry indicates pressures of 2–2.15 kbar, whereas the clinopyroxene barometry shows a pressure of 5 kbar. The temperature (i.e., 750–800°C) is estimated using the amphibole-clinopyroxene thermometry in a dioritic sample. Magmatic water content was greater than 10% at the time of formation of dioritic rocks in the Nabar pluton. Based on chemistry of mafic minerals and geochemical data, the Nabar plutonic complex comprises medium-K, calc-alkaline, and I-type granitoid. The rocks are characterized by enrichment of lithophile elements (LILEs) and depletion of high-field-strength elements (HFSEs). The Nabar rocks have weak concave-upward rare earth element (REE) patterns, suggesting that amphibole played a significant role in their generation during magma segregation. Low (Al₂O₃/(FeO + MgO + TiO₂) and (Na₂O + K₂O)/(FeO + MgO + TiO₂) ratios, and the patterns of trace and rare earth elements suggest that these rocks formed along a destructive plate margin and were derived from a lower crustal source. The magma probably formed by partial melting of lower crustal protoliths (amphibolites). Lower crust contamination with magma derived from partial melting of the upper mantle has an important role in the formation of this intrusive body, and a fractional crystallization of melts in higher crustal levels generated this spectrum of rock types. Mantle-derived gabbroic magmas emplaced into the lower crust are the most likely heat sources for partial melting.     

New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

The Hongge magmatic Fe-Ti-V oxide deposit in the Panxi region, SW China, is hosted in a layered mafic–ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr₂O₃ and 3.20 to 16.2 wt% TiO₂, whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr₂O₃ (<0.4 wt%) but more variable TiO₂ (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the net-textured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.     

磁铁矿和钛铁矿成分对四川太和富磷灰石钒钛磁铁矿床成因的约束

产于层状镁铁质-超镁铁质岩体中的太和岩浆型Fe-Ti氧化物矿床是峨眉山大火成岩省内带几个超大型Fe-Ti氧化物矿床之一。太和岩体长超过3km,宽2km,厚约1.2km。根据矿物含量和结构等特征,整个岩体从下向上可划分为下部岩相带、中部岩相带、上部岩相带。下部岩相带主要以(橄榄)辉长岩和厚层不含磷灰石的块状Fe-Ti氧化物矿层组成。中部岩相带韵律旋回发育,(磷灰石)磁铁辉石岩主要位于旋回的底部,旋回上部为(磷灰石)辉长岩。上部岩相带主要是贫Fe-Ti氧化物的磷灰石辉长岩。太和中部岩相带磷灰石磁铁辉石岩含有5%~12%磷灰石、20%~35%Fe-Ti氧化物、50%~60%硅酸盐矿物,且硅酸盐矿物与磷灰石呈堆积结构。磷灰石磁铁辉石岩中磁铁矿显示高TiO2、FeO、MnO、MgO,且变化范围与趋势接近于攀枝花岩体。钛铁矿FeO分别与TiO2、MgO显示负相关,而FeO分别与Fe2O3、MnO显示正的相关,且TiO2、FeO、MnO、MgO含量变化较大,这些特征都暗示磁铁矿和钛铁矿是从富Fe-Ti-P岩浆中分离结晶。因此,可以推断太和磷灰石磁铁矿辉石岩形成于矿物重力分选和堆积。太和下部岩相带包裹在橄榄石中磁铁矿含有相对较高Cr2O3(0.07%~0.21%),而中部岩相带包裹在橄榄石中磁铁矿Cr2O3(0.00%~0.03%)显著降低,且这些磁铁矿Cr2O3含量变化与单斜辉石Cr含量和斜长石An牌号呈正相关。这些特征印证了形成中部岩相带的相对演化的富Fe-Ti-P母岩浆可能是源自中部岩浆房的混合岩浆。上部岩相带磁铁矿和中部岩相带顶部少量磁铁矿显示较低Ti+V可能是由于岩浆房中累积的岩浆热液对磁铁矿成分进行了改造。     

The interpretation of reaction textures in Fe‐rich metapelitic granulites of the Musgrave Block,central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

Fe‐rich metapelitic granulites of the Musgrave Block, central Australia, contain several symplectic and coronal reaction textures that post‐date a peak S2 metamorphic assemblage involving garnet, sillimanite, spinel, ilmenite, K‐feldspar and quartz. The earliest reaction textures involve spinel‐ and quartz‐bearing symplectites that enclose garnet and to a lesser extent sillimanite. The symplectic spinel and quartz are in places separated by later garnet and/or sillimanite coronas. The metamorphic effects of a later, D3, event are restricted to zones of moderate to high strain where a metamorphic assemblage of garnet, sillimanite, K‐feldspar, magnetite, ilmenite, quartz and biotite is preserved. Quantitative mineral equilibria calculations in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (KFMASHTO) using Thermocalc 3.0 and the accompanying internally consistent dataset provide important constraints on the influence of TiO₂ and Fe₂O₃ on biotite‐bearing and spinel‐bearing equilibria, respectively. Biotite‐bearing equilibria are shifted to higher temperatures and spinel‐bearing equilibria to higher pressures and lower temperatures in comparison to the equivalent equilibria in K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The sequence of reaction textures involving spinel is consistent with a D2 P–T path that involved a small amount of decompression followed predominantly by cooling within a single mineral assemblage stability field. Thus, the reaction textures reflect changes in modal proportions within an equilibrium assemblage rather than the crossing of a univariant reaction. The D3 metamorphic assemblage is consistent with lower temperatures than those inferred for D2.     

Genesis of a low-grade apatite-ilmenite-magnetite deposit in the Kauhajärvi gabbro, western Finland

The Paleoproterozoic Kauhajärvi gabbro is one of several Fe-, Ti-, and P-rich mafic intrusions associated with granitoids in the Fennoscandian shield in western Finland. The gabbro is cut by the late-orogenic Lauhanvuori granite (ca.1870?Ma), whereas the surrounding area is composed of synorogenic, collision-related granitoids and calc-alkaline volcanic rocks (ca. 1890?Ma) belonging to the Mid Finland Granitoid Complex. The mafic intrusions were probably emplaced into a Svecofennian rift zone. They are characterized by a high phosphorus content; the common occurrence of ilmenite as separate grains; and the coeval crystallization of apatite, Fe-Ti oxides, and Fe-Mg silicates. The Kauhajärvi gabbro is composed of two geochemically and structurally distinct zones. The basal zone is composed of poorly-layered, fine- to medium-grained gabbro, which represents an early intrusion of tholeiitic magma, and has rather high concentrations of chromium, magnesium and silica. Typically, the concentrations of iron, titanium and phosphorus are low, except for the top that is enriched in apatite and ilmenite. During most of the crystal-liquid fractionation of the basal zone magma, low f _O2 limited the crystallization of Fe-Ti-oxides. Instead, titanium became enriched in the uppermost layer of the basal zone. The main zone represents a later injection of more evolved tholeiitic magma and makes up 80 to 90%of the total intrusion volume. Peridotite is common, along with gabbro and gabbronorite, in the lower and middle parts of the main zone, and anorthosite is common near the top of the main zone. The Mg:Fe ratio in mafic minerals and vanadium concentrations in magnetite decrease upwards. The variation within the main zone can be explained by crystal-liquid fractionation of a single batch of a parental magma under conditions of relatively high f _O2. Titanium is not progressively enriched. The ratio of titanium to iron (TiO₂/Fe₂O₃ = 0.16 to 0.20; Fe total as Fe₂O₃) is constant in the main zone and normal for mafic intrusions. Titanium is sited in separate ilmenite grains and in lamella within ilmenomagnetite (Ti-bearing magnetite). The high phosphorus content in the main zone is interpreted to result in crystallization of ilmenite and ilmenomagnetite instead of Ti-rich magnetite under relatively high f _O2 conditions. High concentrations of titanium, iron and phosphorus in rocks of the main zone can be explained by pre-emplacement crystal-melt fractionation in a deep magma reservoir and/or contamination of mantle-derived mafic magmas by granitic magmas from partial melting of crustal rocks. A low-grade Fe-Ti-P resource at Kauhajärvi consists of layers with as much as 20 wt. % combined ilmenite (usually 8 to 11 wt. %), apatite (1 to 8 wt. %) and magnetite (1 to 9 wt. %). Mineralized layers are of variable thickness (2?m to 30?m) and occur in variable host rocks (peridotite or gabbro). The Fe-Ti oxides are most abundant in peridotite and pyroxene- or olivine-rich gabbronorite within the main zone. The contact between mineralized rocks (4%TiO₂) and non- or slightly-mineralized rocks is gradual. The deposit as a whole consists of three to five mineralized layers with maximum combined thickness of 70?m. Apatite is most abundant in the oxide-rich layers, but is locally also concentrated in anorthosite with low Fe-Ti oxide contents. The weight ratio of ilmenite to magnetite is 3:2. The ratio of total Ti-Fe-oxides to apatite averages 4.0, with the range of 1.5 to >15.     

Provenance of the Sumian basal schists and age of the Lopian metavolcanic rocks at the Archean-Proterozoic boundary in the Kukasozero structure,North-Karelian zone of the Karelides,Baltic Shield

This paper presents the results of a study of the Paleoproterozoic basal garnet-kyanite-staurolite-two-mica paraschists from the Kukasozero structure of the Karelides of Northern Karelia, Baltic Shield, underlying Neoarchean acid metavolcanic rocks, and schists with quartz, phengite, kyanite, staurolite, garnet, and tschermakite located in the Paleoproterozoic rocks and considered to be metasomatic in origin. It was established that the sedimentary protolith of the Paleoproterozoic paraschists contains detritus of Neoarchean igneous rocks as follows from detrital 2737 ± 11 Ma zircons with oscillatory magmatic zoning. Metavolcanic 2757 ± 13 Ma rocks, close in age and composition, are known directly in the framework of the Kukasozero structure and are considered to be the most likely source of the sedimentary schist protolith. The coincidence of the Nd-model ages of paraschists (t _DM is 2.73–2.76 Ga) with the age of detrital zircons indicates no contribution of older rocks to the protolith composition. The age of magmatic crystallization of metavolcanic rocks directly underlying the Paleoproterozoic paraschists is 2681 ± 18 Ma and coincides with the age of porphyry granites in the western framework of the structure (2680.3 ± 3.6 Ma). No detrital zircons of similar age were found in basal paraschists, but the restricted amount of dated zircons does not allow us to draw a final conclusion about the absence of detritus of the underlying metavolcanic rocks in the paraschist protolith. It was confirmed that phengite-bearing schists are the products of acid metasomatism of the Paleoproterozoic amphibolites and amphibole schists (metavolcanic rocks). The metasomatic features were revealed in garnet-kyanite-staurolite-two-mica paraschists, so the strict identification of their sedimentary protolith is impossible. The paraschists do not represent metamorphosed weathering crust, because acid metasomatism gives a false impression of the greater maturity of the primary sedimentary rocks.     

Oxide minerals in lithic fragments from Luna 20 fines

One hundred and seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvöspinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO₂-rich phase were also found.Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The $\text{Fe}\text{(Fe + Mg)}$ ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains.Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinzet al. (1973a).Sub-micron wide rods of metallic Fe occur in plagioclase grains and may have formed by sub-solidus reduction processes.     

REE distribution in corundum-bearing and other metasomatic rocks during the exhumation of metamorphic rocks of the belomorian belt of the Baltic Shield

The paper reports data on the distribution of REE in metasomatic rocks, including those with corundum, that were formed during the exhumation of the rocks of the Belomorian Belt at 1.9–1.75 Ga. This process is thought to have occurred concurrently with the horizontal extension and tectonic denudation of the upper crust, which, in turn, induced the massive release of fluids. The latter formed two major groups of silicic metasomatic rocks. The deepest-sitting corundum-bearing and other mafic metasomatic rocks are enriched in REE, alkalis, and alumina compared to the host rocks. The coeval acid metasomatic rocks such as orthotectites are, conversely, depleted in REE (with positive Eu anomalies), mafic elements, and HFSE but are also enriched in alumina. These complimentary rocks are thought to have been produced early during the exhumation of deep rocks under the effect of reduced fluids whose genesis was related to decompression. The silicic metasomatic rocks of the second group (with muscovite) have elevated REE concentrations (with negative Eu anomalies) and were formed by already oxidized fluids at shallower depths. The fact that the corundum-bearing metasomatic rocks are enriched in REE (in spite of the ultrabasic composition of these rocks) suggests that they were generated in an extensional environment with the participation of deep fluids, which enriched the rocks in Al, Na, K, Ba, Sr, Zr, and LREE.