Crystallization Of Ilmenite Research Articles

Origin of the giant Allard Lake ilmenite ore deposit (Canada) by fractional crystallization, multiple magma pulses and mixing

The late-Proterozoic Allard Lake ilmenite deposit is located in the Havre-Saint-Pierre anorthosite complex, part of the allochtonous polycyclic belt of the Grenville Province. Presently the world's largest Fe–Ti oxide deposit, it had a pre-mining amount in excess of 200 Mt at grades over 60 wt.% hemo-ilmenite. The main ore body is a funnel-shaped intrusion, measuring 1.03 × 1.10 km and 100–300 m-thick. Two smaller bodies are separated by faults and anorthosite. The ore is an ilmenite-rich norite (or ilmenitite) made up of hemo-ilmenite (Hem 22.6–29.4, 66.2 wt.% on average), andesine plagioclase (An 45–50), aluminous spinel and locally orthopyroxene. Whole-rock chemical compositions are controlled by the proportions of ilmenite and plagioclase ± orthopyroxene which supports the cumulate origin of the deposit. Ore-forming processes are further constrained by normal and reverse fractionation trends of Cr concentration in cumulus ilmenite that reveal multiple magma emplacements and alternating periods of fractional crystallization and magma mixing. Mixing of magmas produced hybrids located in the stability field of ilmenite resulted in periodic crystallization of ilmenite alone. The unsystematic differentiation trends in the Allard Lake deposit, arising from a succession of magma pulses, hybridisation, and the fractionation of hemo-ilmenite alone or together with plagioclase suggest that the deposit formed within a magma conduit. This dynamic emplacement mechanism associated with continuous gravity driven accumulation of Fe–Ti oxides and possibly plagioclase buoyancy in a fractionating ferrobasalt explains the huge concentration of hemo-ilmenite. The occurrence of sapphirine associated with aluminous spinel and high-alumina orthopyroxene (7.6–9.1 wt.% Al 2O 3) lacking exsolved plagioclase supports the involvement of a metamorphic overprint during the synchronous Ottawan orogeny, which is also responsible for strong textural equilibration and external granule of exsolved aluminous spinel due to slow cooling.

Discoveries of nanostructures of natural mercury-bearing gold at the surface of ilmenite crystals from the Fadeevskii ore-and-placer cluster (Primor’ye)

1310 Recently, the volume of publications dedicated to the nanomineralogy of gold has increased significantly in the geological literature [1, 2]. Although there is wide coverage of the problem as a whole, we note that there is a lack of information on the natural nano structures of gold. Investigations of ultrasmall amounts of rock material sampled from the occur rences of rocks and deluvial sediments of the Fadeevskii ore and placer cluster (Fig. 1) located at the boundary between the Khankayskii and Laoelin Grodekovskii territories [3] may make up for this lack of information. In the lower part of the stratified section of the investigated area, basalt–siliceous–tuffaceous, sili ceous–clayey, argilliferous, and tuffaceous–sandstone sediments (S1–2) occur. They are overlapped by volca nogenic–sedimentary strata of Permian age (P1–2) and acid effusive rocks (P2). The rocks of the volcano genic–terrigenous complex, including black shales, are intruded in the south by large massifs of granitoids. In the north they are intruded by small bodies of gab broids and syenites [4]. Silver bearing gold, minerals of platinum group metals (first of all, isoferroplatinum) as well as sulfides (pyrite, arsenopyrite, sphalerite, galenite, molibden ite), kassiterite, volframite, and minerals of bismuth were found among the minerals of heavy crop schlichs of loose sediments of the Faddevka (watershed system of Razdol’naya River) and Zolotaya (basin of the Nes terovka River) rivers draining granitoids and black shale stratum in the southern and northern parts of the area, respectively. A significant part of black schlich consists of ilmenite, magnetite, chromespinellides, garnets, rutile, and sphene. The presence of the unique association of natural mercury bearing gold, cinnabar, native metals, and carbides is a unique fea ture of northern placers [5].

Progressive Formation of Halloysite From the Hydrothermal Alteration of Biotite and the Formation Mechanisms of Anatase in Altered Volcanic Rocks From Limnos Island, Northeast Aegean Sea, Greece

AbstractOccurrences of halloysite-rich material in altered volcanic rocks, principally trachyandesites, dacites, and tuffs, extend over an area of ∼1 km2 in the southwestern part of Limnos, Island, northeast Aegean Sea, Greece. The present study was designed to investigate the alteration processes which acted on the biotite in these volcanic rocks, to describe in detail the mechanism of formation of the halloysite, and to specify the mechanisms of formation of anatase during the alteration processes. Samples were examined using polarized-light microscopy, X-ray powder diffraction, scanning electron microscopy, scanning electron microscopy-energy dispersive spectroscopy, and Fourier-transform-Raman techniques. The extensive alteration of the parent rocks, triggered by the circulation of hydrothermal fluids through faults and fractures, resulted in the alteration of biotite to halloysite. Six stages of alteration were recognized. Nanoparticles of halloysite were initially formed on the mica layers, which progressively grew through short-tubular to well formed tubular halloysite, with increasing alteration. In the most altered samples, laths and interconnected laths with the composition (Al3.96Fe0.04)Si4O10(OH)8, were the dominant halloysite morphologies. Anatase was encountered as an alteration product of both ilmenite and biotite. Ilmenite was altered to anatase and Fe oxides. The altered ilmenite crystals constrained most of the newly formed anatase within the space occupied previously by ilmenite, leading to the formation of skeletal anatase. The layered structure of the micas was the main factor governing the morphology of newly formed anatase developed outside ilmenite margins in the form of layers parallel to those of mica. An unusual ring-like structure of anatase was thought to be the result of the uncommon alteration of inner parts of mica folia to tubular halloysite oriented perpendicular to the mica layers. The detachment of the halloysite tubes by circulating hydrothermal fluids was considered to be the reason for the creation of holes which were subsequently surrounded by the anatase ring forms.

Geochemical and isotopic constraints on the petrogenesis of Early Ordovician granodiorite and Variscan two-mica granites from the Gouveia area, central Portugal

A biotite granodiorite and seven Sn-bearing two-mica granites crop out in the Gouveia area, central Portugal. A SHRIMP U–Th–Pb zircon age from the granodiorite, and monazite ages from four of the two-mica granites, show that they are of Early Ordovician (~ 480 Ma) and Permo-Carboniferous, i.e. Variscan (~ 305 and 290 Ma) age respectively. The Variscan two-mica granites are late- and post-D3. Major and trace element variation in the granitic rocks and their biotite and muscovite indicate mainly individual fractionation trends. The granitic rocks are mostly depleted in HREE relative to LREE. The biotite granodiorite is probably derived from igneous lower crust, as evidenced by low initial 87Sr/ 86Sr (0.7036), high εNd T (+ 2.5) and moderate δ 18O (8.8‰). The two-mica granites are probably derived by partial melting of heterogeneous mid-crustal metasediments, mainly metapelite and some metagraywacke, as evidenced by their high initial 87Sr/ 86Sr (0.7076–0.7174), δ 18O (10.7–13.4‰) and major element compositions. However, variation diagrams for major and trace elements from two of the muscovite > biotite granites and their micas define fractionation trends. Rb–Sr whole-rock analyses from the two granites are perfectly fitted to a single isochron and the rocks have sub-parallel REE patterns; the younger granite is derived from the older by fractional crystallization of quartz, plagioclase, biotite and ilmenite (tested by modelling major and trace elements). Most of the Sn-bearing granites are derived from distinct magma batches. They result from partial melting of a heterogeneous mid-crustal metasediment. They do not represent a crustal anomaly in tin. Fractional crystallization is responsible for the increase in the Sn contents of the granites and their micas. Muscovite has a higher Sn content than coexisting biotite and is the principal host mineral for Sn in these rocks.

The genesis of I- and S-type granitoid rocks of the Early Ordovician Oledo pluton, Central Iberian Zone (central Portugal)

The Early Ordovician Oledo pluton consists of four distinct granodioritic to granitic phase (G1–G4), which intruded a Cambrian schist-metagraywacke complex, but were themselves intruded by a Late Carboniferous pluton. ID-TIMS U–Pb ages for zircon and monazite from these granitic rocks indicate emplacements within a short period of time at 479–480 Ma. Granodiorite G1 is the most deformed rock with shear zones and deformation at the border. G1 and G3 contain fine-grained biotite tonalite and biotite granodiorite microgranular enclaves, which are darker and richer in mafic minerals than the host granodiorites. The geological, mineralogical, geochemical and Sr, Nd and O isotopic data show that tonalitic and granodioritic enclaves and host G1 are of I-type and were related predominantly by a fractional crystallization process. Least-square analysis of major elements and modelling of trace elements indicate that granodioritic enclaves and host G1 could be derived from the tonalitic enclave magma by fractional crystallization of plagioclase, grunerite, biotite and ilmenite. Granodiorite G2 is of hybrid origin. Most variation diagrams for granodioritic enclaves and host G3 granodiorite and their biotites show linear trends. Modelling of major and trace elements of granodioritic enclaves indicate that they result from mixing of relatively primitive granodiorite magma with magma derived from crustal melting. Tonalitic enclaves correspond to globules of a more mafic relatively primitive magma. Granite G4 has the most pronounced crustal signature and is of S-type.

Petrological evidence for crustal melting, unmixing, and undercooling in an alkali-calcic, high-level intrusion: the late Sveconorwegian Vinga intrusion, SW Sweden

The subvolcanic Vinga intrusion is a quartz jotunite to monzogranite located in the outermost part of the Goteborg archipelago, SW Sweden. It is one of a number of post-kinematic, late Sveconorwegian intrusions in the region. Rocks in the intrusion display complex, but well preserved igneous textures, including complexly zoned plagioclase megacrysts (cellular) and skeletal/acicular crystals of apatite, zircon, ilmenite and pyrite. The intrusion has high contents of Σ Fe, Ti, P, K, F and high field strength elements, and contains abundant and diverse enclaves (xenolithic, restitic and co-mingled). The cellular plagioclase may represent partially melted and re-equilibrated restitic plagioclase. The skeletal crystals formed due to undercooling and rapid crystallisation in the magma when it was brought to upper crustal levels. Ion-microprobe analyses of skeletal zircons from the intrusion yield a concordia age of 951 ± 7 Ma. The protolith of the parental magma is inferred to have been intermediate, meta-igneous and was partially molten under high temperature conditions as revealed from pyroxene thermometry (>1000 °C) on restitic pyroxenes. Miarolitic cavities, in combination with pressure estimates of ≤2 kbar from Al-in-hornblende barometry, indicate an epizonal emplacement depth at final crystallisation temperatures of around 750–800 °C. Zircon saturation occurred late, during rapid cooling below 900 °C, resulting in skeletal morphologies. The magma evolution started with early separation of restitic pyroxenes, plagioclase and Fe–Ti oxides at midcrustal depths, followed by fractionation. The heat source of melting was related to mafic underplating. Evolved parts of this mafic magma were co-mingled with the Vinga magma.

Chronological and geochemical studies of granite and enclave in Baimashan pluton, Hunan, South China

Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low ɛ Nd(t) values but high (87Sr/86Sr)i ratios, and old T 2DM (ca. 1.9–2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.

Geochemistry of S-type granitic rocks from the reversely zoned Castelo Branco pluton (central Portugal)

The zoned pluton from Castelo Branco consists of Variscan peraluminous S-type granitic rocks. A muscovite>biotite granite in the pluton's core is surrounded successively by biotite>muscovite granodiorite, porphyritic biotite>muscovite granodiorite grading to biotite=muscovite granite, and finally by muscovite>biotite granite. ID-TIMS U–Pb ages for zircon and monazite indicate that all phases of the pluton formed at 310 ± 1 Ma. Whole-rock analyses show slight variation in 87Sr/ 86Sr 310 Ma between 0.708 and 0.712, ɛNd 310 Ma values between − 1 and − 4 and δ 18O values between 12.2 and 13.6. These geological, mineralogical, geochemical and isotopic data indicate a crustal origin of the suite, probably from partial melting of heterogeneous Early Paleozoic pelitic country rock. In detail there is evidence for derivation from different sources, but also fractional crystallization linking some of internal plutonic phases. Least-squares analysis of major elements and modelling of trace elements indicate that the porphyritic granodiorite and biotite=muscovite granite were derived from the granodiorite magma by fractional crystallization of plagioclase, quartz, biotite and ilmenite. By contrast variation diagrams of major and trace elements in biotite and muscovite, the behaviours of Ba in microcline and whole-rock δ 18O, the REE patterns of rocks and isotopic data indicate that both muscovite-dominant granites were probably originated by two distinct pulses of granite magma.

Extreme geochemical heterogeneity in Afro-Arabian Oligocene tephras: Preserving fractional crystallization and mafic recharge processes in silicic magma chambers

Four Indian Ocean deep-sea tephras can be correlated to individual Oligocene on-land silicic pyroclastic units found in the Afro-Arabian flood volcanic province (Yemen and Ethiopia), providing valuable stratigraphic marker horizons. They also preserve among the largest geochemical heterogeneities observed in individual eruption events (SiO 2: 43 to 75 wt.% and 56 to 79 wt.%). The major, trace element and isotopic variations preserved in individual shards in these ash layers provide a unique series of snapshots of magma chamber processes that are used to elucidate the formation and petrogenetic evolution of large volume, chemically zoned silicic magmatic systems. Banded shards within individual ash layers represent up to 85% of the SiO 2 variation observed in the entire tephra on the scale of < 1 mm 3, clearly demonstrating that the observed compositional variations found within individual layers represent individual eruptive events. The silicic magmas are compositionally related to the underlying Main Flood Basalt phase of volcanism in Yemen, whereas the basaltic end-member found in the tephras is compositionally distinct and related to the Upper Series mafic lavas, found intercalated with the Main Silicic Series on-land ignimbrites and tuffs. The intermediate to silicic component of the tephras was generated by extreme fractional crystallization ( F = ca. 60%) of plagioclase, anothoclase, augite, magnetite and ilmenite, as observed in on-land phenocryst assemblages.

The origin of geochemical diversity of lunar mantle sources inferred from the combined U–Pb, Rb–Sr, and Sm–Nd isotope systematics of mare basalt 10017

The results of our combined U–Pb, Rb–Sr, and Sm–Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U–Pb isotope systematics of the Moon. The Rb–Sr, Sm–Nd, and 238U– 206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U– 207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb– 207Pb system nor U–Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm–Nd, Rb–Sr, and 238U– 206Pb systems. The initial 87Sr/ 86Sr of 10017 is 0.69941 ± 7 and the initial ε Nd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U–Pb isochrons, are 206Pb/ 204Pb i = 31 ± 11 and 207Pb/ 204Pb i = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100–600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U–Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.

Collisional metamorphism as a result of thrusting in the Transangara region of the Yenisei Ridge

The Yenisei Ridge is one of the most interesting regions in the southwestern folded framework of the Siberian Craton with respect to geodynamics. The regional structure of the Transangara sector of the Yenisei Ridge is traditionally displayed as a NW-trending system of tectonic sheets divided by faults characterized by the collision of blocks and thrusting. Therefore, this region was subject to pressure-variable regional metamorphism expressed in the juxtaposition of lowand moderate-pressure metamorphic facies. Collisionrelated moderate-pressure metamorphism is locally superimposed on the low-pressure (presumably, younger) metamorphic rocks. As a result, andalusite is replaced with kyanite with the formation of new mineral assemblages and deformational structures. The prograde replacement of andalusite with kyanite in the Yenisei Tange is a rare phenomenon, because the stationary continental geotherm commonly does not intersect the andalusite‐kyanite equilibrium line. Such replacements are usually referred to the retrograde stage of metamorphism, but this interpretation comes into conflict with the regional geological situation. Only a few examples are known in the literature (Northwest Cordillera in the United States and Canada, Dalradian in Scotland, central and northwestern Appalachians in the United States, and the Kola Peninsula and Yenisei Ridge in Russia), where prograde transformation of andalusite into kyanite is assigned either to the metastable state of andalusite in the PT stability field of kyanite or to an increase in pressure as a result of thrusting or magmatic loading characterized by different PT trends. While studying collisional metamorphism in the Transangara sector of the Yenisei Ridge, we selected three (Chapa, Mayakon, and Angara) areas composed of Paleoproterozoic, Middle Riphean, and Upper Riphean rocks (Fig. 1). The Chapa and Mayakon areas are located in the Central uplift between the Ishimbino and Tatarka deep faults. The Angara area covers the junction of the Transangara structural units and the Angara‐ Kan block. The Angara area is situated at the interfluve of the Angara, Belokopytovka, and Malaya Sploshnaya rivers. The reference sections are exposed in the Tatarka shear zone along the right bank of the Angara River between the mouths of the Babkin and Polovinkin creeks. The area is composed of the Upper Riphean low-pressure metasedimentary rocks (rhythmic intercalation of quartzites and phyllites of the Sukhoi Ridge Formation). In the study area, this sequence is largely made up of phyllites of the greenschist facies represented by quartz (Qtz), muscovite (Ms), chlorite (Chl), and ilmenite (Ilm). These rocks underwent high-pressure collisional metamorphism with the formation of new (kyanite-bearing) mineral assemblages. Metamorphism occurred simultaneously with the development of steep (80 ° ‐85 ° NW and SE) near-meridional cleavage. The increase in the metamorphic grade in the near-latitudinal direction is marked by the successive formation of chloritoid (apparent thickness 0.5‐0.8 km) and kyanite (~1.5‐1.7 km) zones. These minerals correspond to the conditions of kyanite schist facies. The eastern boundary of these rocks is hidden beneath unmetamorphosed Paleozoic rocks of the Pogromnino basin. In the Kulakovo uplift on the left bank of the Angara River, collisional metamorphism is expressed in the crystallization of kyanite (Ky), chloritoid (Cld), and ilmenite in metapelites consisting of staurolite (St), plagioclase (Pl), Ms, biotite (Bt), Qtz, and garnet (Grt) [2].

The 1362 AD Öræfajökull eruption, Iceland: Petrology and geochemistry of large-volume homogeneous rhyolite

The ice-covered Öræfajökull stratovolcano is composed mostly of subglacial pillow lava and hyaloclastite tuff, ranging from basalt to rhyolite. A large devastating plinian eruption in 1362 AD produced 10 km 3 (2 km 3 DRE) rhyolitic ash and pumice from a vent within the summit caldera, with fallout mainly towards ESE. The ejected rhyolite magma with 0.5–1% crystals of oligoclase, fayalite, hedenbergite, ilmenite and magnetite was remarkably homogenous throughout the eruption. A 1.8 m thick tephra section on the SE flank of the volcano has 14 recognizable units. The tephra is dominated by fine-grained vesicular glass with bubble wall thickness of 1–5 μm. The high and even vesiculation of the glass indicates fast magma ascent and explains the extreme mechanical fragmentation within the eruptive column. The grain-size distribution indicates time-variable intensity of the plinian eruption with three evenly spaced phases of maximum fragmentation. An initial vent-clearing explosion produced phreatomagmatic debris with up to 35% lithic fragments. The low abundance (< 3%) of lithic fragments during the subsequent eruption indicates that the conduit and vent remained stable. The tephra fallout deposit is characterized by upwards increasing pumice dimensions and occasional bomb-like pumice blocks, indicating less mechanical fragmentation during contraction and lowering of the plinian column. A conservative estimate of 20–40 km 3 for the total volume of the magma reservoir is based on the erupted volume of highly differentiated and homogeneous rhyolite. The 365-year period between 1362 and a minor benmoreitic eruption in 1727, and the absence of currently detectable magma reservoirs in the crust below Öræfajökull show that differentiated crustal magma chambers feeding large plinian eruptions can be established and disappear on a 100–500 year timescale.

U-Pb and 207Pb- 206Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites. The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The 207Pb- 206Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged 207Pb- 206Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon ( T closure = ∼1050°C with a cooling rate of 0.2°C/yr), the 207Pb- 206Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas. This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7–20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.

Geochemistry and zircon U-Pb ages of granulite xenolith from Tuoyun basalts, Xinjiang: Implications for the petrogenesis and the lower crustal nature beneath the southwestern Tianshan

The granulitic xenoliths discovered in the late Cretaceous basaltic rocks from Tuoyun basin, western Xinjiang Province contain the assemblage of Opx + Cpx + Pl ± Grt ± Qtz ± Kfs. Mineral chemistry, petrochemistry and zircon U-Pb ages from the xenoliths have been reported in the paper. Petrogenesis, the lower crustal nature and the mantle-crust interaction reflected by the granulites are discussed as well. Tuoyun granulites are mainly mafic with few intermediate components, and represent the magmatic products underwent granulite-facies metamorphism in lower crustal condition. Equilibrium temperature and the maximum pressure estimates of the granulites are 910 ± 35 °C and 13.5 × 108 Pa respectively, indicating that the crust beneath southwestern Tianshan had the thickness of less than 44 km and high geotherm (e.g. > 80 mW/m2) when the host magma erupted. During their formation, the fractional crystallization of olivine, pyroxene, plagioclase and ilmenite, plagioclase accumulation and the contamination of felsic components or fluid metasomatism processes had happened. Metamorphic zircon U-Pb age of 253 Ma may record the crust-mantle interaction caused by the orogenic root delamination beneath the southwestern Tianshan.

Perovskites in the comb roof base of hornets: Their possible function

On the ceiling of the Oriental hornet comb cell, there are mineral granules of polycrystalline material known to belong to the group of perovskites. In a comb cell intended to house a worker hornet, the roof base usually carries one or several such perovskite granules containing titanium (Ti), whereas in the roof base of a cell housing a developing queen, there are usually several granules containing a high percentage of silicon (Si), aluminum (Al), calcium (Ca), and iron (Fe), but very little if any Ti. In worker comb cells, Ti usually appears as ilmenite (FeTiO3). Besides documenting the above-mentioned facts, this report discusses possible reasons for the appearance of ilmenite crystals in worker cells only and not in queen cells.

Compositional variation of Fe−Ti oxides from the Sokli complex, northeastern Finland

The phoscorite-carbonatite complex at Sokli, northeastern Finland, is composed of five stages of intrusions of phoscorites and carbonatites (P1-C1, P2-C2, P3-C3 phoscorites and calcite carbonatites, D4 and D5 dolomite carbonatites) which are cut by numerous lamprophyric dikes. Magnetite is ubiquitous in all constituent rock units of the complex and frequently associates with ilmenite. Most ilmenite intergrowths from the Sokli phoscorite-carbonatite complex occur as internal and/or external granules (composite type exsolutions) or sandwich type exsolution lamellae in the host magnetites. Discrete ilmenite crystals are found only in P3 and C3. On the other hand, ilmenites in the ultramafic lamprophyres occur as discrete crystals as well as trellis and sandwich type exsolution lamellae or composite type exsolutions in magnetite. Electron microprobe analyses reveal that magnetite of the Sokli complex belongs essentially to the magnetite-ulvospinel solid solution series with a small proportion of magnesioferrite. The compositions of magnetite from the Sokli phoscorite-carbonatite complex vary systematically from stage to stage with increasing Fe2+/(Fe2++Mg) ratio, and decreasing Al, Mn and Ti contents. Magnetite from the ultramafic lamprophyre is characterized by large compositional ranges owing to the large amount of mantle-derived xenocrysts. Some grains are particularly high in Cr (up to 21.0 wt.% Cr2O3). Aluminous magnesian titanomagnetites are also found, but most magnetites in the ultramafic lamprophyres are pure magnetite. The compositional variation of ilmenite from the Sokli complex is mainly caused by the substitution of Fe2+ into Mg and Mn, and partly Ti into Nb. Mg-and Mn-rich ilmenites in the early stage P1-C1 rocks evolve towards pure FeTiO3 composition in the latest D5 dolomite carbonatite. Ilmenites from the ultramafic lamprophyres are relatively poor in Mn compared to those from the phoscorite-carbonatite complex. In the coexisting magnetite and ilmenite, Mg and Mn cations preferentially partition into ilmenite rather than magnetite. In spite of the mineralogical and geochemical contrasts between the phoscorites and associated carbonatites, compositions of Fe−Ti oxide minerals from the two paired rocks at a given stage are basically the same, and evolve systematically from stage to stage. This supports the hypothesis that the phoscorites and conjugate carbonatites are derived from common parental melts. Furthermore, the intrusions of the Sokli phoscorite-carbonatite complex are considered to have either crystallized from successive batches representing a continuum in time and magmatic evolution, or that the P1-C1 parental magma has simply differentiated to form the later stage rocks.

Geochemistry of granitoids and their minerals from Rebordelo–Agrochão area, northern Portugal

Deformed Hercynian peraluminous granitoids ranging from tonalite to granite crop out in the Rebordelo–Agrochão area, northern Portugal and some of them contain tonalitic and granodioritic enclaves. Variation diagrams of major and trace elements of the rocks, biotites and sphenes show fractionation trends. The most- and the least-deformed samples of granite and their biotites also define fractionation trends. There is decrease in all rare earth element (REE) contents and increase in the Eu anomaly in REE patterns from the most- to the least-deformed samples of granite. All the granitoids define a whole-rock Rb–Sr errorchron. A whole-rock Rb–Sr isochron for the least-deformed samples of granite yields an age of 357±9 Ma and an initial 87Sr/ 86Sr ratio of 0.7087±0.0007. Geochemical modelling suggests that the tonalitic magma evolved by AFC (fractional crystallization of magnesiohornblende, plagioclase, quartz, biotite and ilmenite, and assimilation of metasediments) to originate tonalitic and granodioritic enclaves, granodiorite and granite. δ 18O values support this mechanism. The tonalite is hybrid and derived by interaction of a mantle-derived magma and crustal materials.

Chronostratigraphy of the key Upper Miocene (Lower Turolian) sequence of la Montagne d’Andance (Ardèche, France). Implications of new 40Ar/39Ar laser fusion and unspiked K-Ar dating of trachytic tephra and basalts

Abstract A trachytic tephra layer deposited in the lower part of the Upper Miocene (Turolian) diatomite sequence at la Montagne d’Andance (Coiron basaltic province, France) contains glass shards (73 % SiO2), plus crystals of sanidine, ilmenite, biotite, amphibole, clinopyroxene, zircon, sphene and apatite. This composition and other volcanological data suggest a correlation to the Cantal stratovolcano, 150 km distant, better than to the nearer (50 km) Mézenc area. The 7.54 ± 0.06 Ma (± 2 σ) isochron defined by eight 40Ar/39Ar laser fusion analyses of sanidine gives a precise age for the beginning of the fossiliferous sequence, known for its rich flora and fauna. Moreover, unspiked K-Ar ages determined from three basaltic lava flows that pre- and post-date the lacustrine sedimentation are 7.37 ± 0.06, 7.17 ± 0.06 and 7.13 ± 0.06 Ma (± 2σ), thereby bracketing the sequence to a 400 kyr period between ca. 7.5 and 7.1 Ma. These are the first radioisotopic constraints for the lower part of the Turolian continental stage. They imply an age about 20 % younger for the fauna than proposed by previous biostratigraphic correlation. This new chronology is consistent with correlation of the lower limit of the Turolian to the Tortonian-Messinian boundary (7.30 ± 0.15 Ma) of the global marine stages.

The Neoproterozoic Dubr intrusives, Central Eastern Desert, Egypt: petrological and geochemical constraints on the evolution of a mafic-felsic suite

The Neoproterozoic magmatism in the Dubr area took place in two suites namely: (1) a mafic suite of a gabbro-diorite complex and (2) a felsic suite of a granodiorite and monzogranite composition. Pressures estimated for the gabbro and diorite rocks show that they have originated over a wide range of pressures (1.5-6.5 kbar). The gabbro and diorite rocks predict crystallization temperatures ranging from 820 to 940 °C and 660 to 710 °C, respectively. The rare-earth element abundances of the Fe--tholeiite gabbro-diorite complex display fractionated {(La/Lu) n = 1.53-2.96} and LREE-enriched {(La/Sm) n = 1.12-2.62} patterns. The REE patterns of the gabbro-diorite complex suggest that it can be generated from a spinel-bearing mantle source. Transition metal contents of the gabbro-diorite complex are variable and low (Ni, 9-133 ppm; Cr, 20-325 ppm; Sc, 17.7-34 ppm), indicating that none of the samples are primitive. The gabbro-diorite complex may have formed via relatively high degree partial melting (25-30 %) of an enriched mantle source followed by fractional crystallization of olivine, Cr-spinel with minor clinopyroxene and plagioclase. Chemical evidences imply that crustal contamination is likely to be a minor factor in trace element variation of the gabbro-diorite complex. The granodiorite and monzogranite are calc-alkaline showing metaluminous affinity for the granodiorite and a slightly peraluminous nature for the monzogranite. They display trace element and REE characteristics of I-type granites formed in a subduction-related arc environment. The hornblende of the granodiorite and monzogranite yielded pressures of an average 3 kbar and 2.5 kbar, respectively. Amphibole-plagioclase geothermometer yielded crystallization temperatures ranging from 680 to 700 °C and 650 to 675 °C for the granodiorite and monzogranite, respectively. The co-linear trends on variation diagrams for the granodiorite and monzogranite indicate a genetic link between them through fractional crystallization. It appears likely that the restite-unmixing model is responsible for the petrological and chemical features of the granodiorite. The most probable source for the granodiorite is a dioritic rock. The monzogranite was derived as a result of crystallization of plagioclase, biotite, hornblende, quartz, ilmenite and magnetite phases from a granodioritic melt.

A sulphide-bearing orthopyroxenite layer in the Bjerkreim–Sokndal Intrusion, Norway: implications for processes during magma-chamber replenishment

The Bjerkreim–Sokndal Layered Intrusion contains an up to 3-m-thick layer of sulphide-bearing orthopyroxenite or melanorite that can be followed for about 30 km along the boundary between megacyclic units II and III. The layer of orthopyroxenite is developed within a sequence of ilmenite leuconorites and defines the base of zone IIIa. The leuconorites are generally succeeded by ilmenite–magnetite leucotroctolite, which defines the base of zone IIIb. The orthopyroxenite layer has an initial Sr-isotopic ratio similar to the leucotroctolites (ca. 0.7051) and lower than the enveloping ilmenite leuconorites of zones IIc and IIIa (ca. 0.7054). Variations of tetrahedrally coordinated Al in orthopyroxene and elevated Cr/TiO 2 in the orthopyroxenite layer and the leucotroctolite imply that the orthopyroxenite is genetically more related to the leucotroctolite than the enveloping leuconorites. On the basis of field observations and analytical results, we conclude that the orthopyroxenite crystallised as a result of mixing between relatively primitive magma and differentiated resident magma, both of which were sulphide-saturated, during chamber replenishment. The injected magma formed a buoyant plume that spread out laterally at its level of neutral buoyancy within the compositionally zoned resident magma, some distance above the magma-chamber floor. Mixing in the plume resulted in a hybrid saturated in orthopyroxene, ilmenite and sulphide melt. Batches of relatively dense magma containing crystals of orthopyroxene and ilmenite and droplets of sulphide melt sank to the floor of the chamber from the hybrid magma layer and formed a layer of orthopyroxenite on the magma-chamber floor. Removal of orthopyroxene from the hybrid melt resulted in the sporadic crystallisation of plagioclase at the orthopyroxene–plagioclase cotectic. Plagioclase joined orthopyroxene and sulphide droplets in the dense melt batches which sank to the floor, resulting in the local development of melanorite instead of orthopyroxenite. Crystallisation of ilmenite leuconorite from the resident magma below the hybrid magma layer resumed after formation of the orthopyroxenite layer. This continued as the hybrid layer thickened and eventually came into contact with the chamber floor when leucotroctolites of zone IIIb started to crystallise on the deeper parts of the floor. Elsewhere, lower-temperature cumulates formed on elevated portions of the floor from magma higher in the stratified column. The sulphide-bearing orthopyroxenite layer is not associated with PGE mineralisation because both the replenishing and differentiated resident magmas had lost PGE through earlier sulphide saturation. Zone IIIb is followed by a sequence of lithologies that reflect progressive fractional crystallisation with the appearance of cumulus orthopyroxene at the expense of olivine (zone IIIc), and the successive entries of magnetite (zone IIId) and clinopyroxene plus apatite (zone IIIe). A new major influx of magma took place at this time to produce megacyclic unit IV.