Fe-Ti Oxide Gabbros Research Articles

Nano-Micron Exsolved Spinels in Titanomagnetite and Their Implications for the Formation of the Panzhihua Fe-Ti-V Oxide Deposit, Southwest China.

The nano-micron exsolved spinels with various mineralogical characteristics in titanomagnetite from Fe-Ti oxide gabbros in the Panzhihua Fe-Ti-V oxide deposit, SW China, have been studied by field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) based on comparisons of physical and chemical conditions at different stratigraphic heights to investigate the compositional inheritance between titanomagnetite and exsolved spinel and further explore the relationship between the morphology and growth of exsolved spinels. Restored chemical data for titanomagnetite combined with evidence from petrography and whole-rock geochemistry imply fractional crystallization of the Panzhihua Fe-Ti-V oxide deposit, where the titanomagnetite of thick massive oxides at the bottom of the No. VIII orebody represents the early crystallizing phase characterized by high temperature and oxygen fugacity. The chemical variation in the exsolved spinel, which has the same trend as the restored composition of titanomagnetite, represents inheritance from the parent rock within the Panzhihua deposit. Exsolved spinel continuously adjusts morphology and grain size to decrease the total energy of the manganate-spinel system from fine-grained spinels parallel to the {100} plane of titanomagnetite to spinels with complex stellate morphology to bulky granular spinels with high degrees of idiomorphism. The unusual multiple magma replenishment during the mineralizing process and at different stratigraphic heights in the Panzhihua intrusion had an important influence on the thermal evolution history of the orebody, resulting in the identifiable spatial distribution patterns of spinel morphology and grain size. Using spinel exsolution as a discriminator for the provenance of magmatic ore deposits may provide intuitive and easy mineralogical evidence to qualitatively discuss the evolution of the metallogenetic environment and the ore-forming conditions for similar large mafic intrusions.

Physicochemical Control of the Early Permian Xiangshan Fe-Ti Oxide Deposit in Eastern Tianshan (Xinjiang), NW China

The Xiangshan mafic-ultramafic complex is one of the major Early Permian mafic-ultramafic intrusions in eastern Tianshan (Xinjiang, NW China), and consists of two major intrusive phases. The first intrusive phase is mainly gabbroic rocks hosting ilmenite mineralization, while the second intrusive phase is mainly lherzoilite associated with Ni-Cu sulfide mineralization. The Xiangshan ilmenite orebodies hosted in the Fe-Ti oxide-bearing gabbro occur along the contact between hornblende gabbros and leucogabbros. The hornblende gabbros and Fe-Ti oxide rich gabbros at Xiangshan are newly dated to be Early Permian (280.1 and 279.2 Ma, respectively). Major and trace element compositions of zircons and whole rocks from Xiangshan hornblende gabbro and Fe-Ti oxide gabbro have been measured by in situ excimer laser ablation ICP-MS. Zircon Ce4+/Ce3+ ratios based on lattice-strain model and Ti-in-zircon temperatures of hornblende gabbro and Fe-Ti oxide gabbro of the Xiangshan complex are calculated to evaluate the physicochemical variations during the ilmenite mineralization. Whole-rock geochemistry and zircon trace element geochemistry suggest that Fe-Ti oxide gabbros were formed from a basaltic parent magma which had undergone a transfromation from being H2O-rich to H2O-poor. During the magmatic evolution, primitive, H2O-poor basaltic melts may have been replenished into the system, increasing its solidus temperature and decreasing its oxygen fugacity and H2O contents. This may have supperessed the Ti-rich poikilitic hornblende fractionation and promoted the plagioclase fractionation, which consequently concentrated the ore-forming components in the residual melts and generated the ilmenite mineralization.

Magnetite exsolution in ilmenite from the Fe-Ti oxide gabbro in the Xinjie intrusion (SW China) and sources of unusually strong remnant magnetization

This study investigates magnetite exsolution in ilmenite from Fe-Ti oxide gabbro in the Xinjie intrusion, SW China. Exsolved magnetite lamellae in ilmenite contain nearly pure Fe3O4 with ~1 wt% TiO2. EBSD-based analyses indicate that the magnetite lamellae have close-packed oxygen planes and directions parallel to those in the host ilmenite with {111}Mag//(0001)Ilm and ![Formula][1] The Fe2+ in the magnetite lamellae is probably derived from adjacent titanomagnetite by sub-solidus inter-oxide cation repartitioning of Fe2+ + Ti4+ = 2Fe3+ during cooling. It is thus suggested that only Fe3+ cations in the magnetite lamellae should be included into the composition of the Ilm-Hemss precursor for the Fe-Ti oxide oxy-thermometer. The existence of magnetite exsolution in ilmenite also provides an alternative explanation for unusually strong natural remnant magnetization in natural ilmenite. [1]: /embed/mml-math-1.gif

Hydrogen incorporation and charge balance in natural zircon

The water and trace element contents of natural igneous zircons were determined to constrain the mechanism of hydrogen incorporation. The low radiation-damage zircons were derived from Fe–Ti oxide gabbros from the Vema Fracture Zone (11°N, Mid-Atlantic Ridge). They contain up to 1212ppmw H2O, 1.9wt.% Y2O3 and 0.6wt.% P2O5 and are generally strongly zoned. REE+Y are partially charge-balanced by P (Y, REE3++P5+=Zr4++Si4+), but a large REE excess is present. On an atomic basis, this excess is closely approximated by the amount of H present in the zircons. We therefore conclude that H is incorporated by a charge-balance mechanism (H++REE3+=Zr4+). This interpretation is consistent with FTIR data of the Vema zircons, which shows a strongly polarised main absorption band at ca. 3100cm−1, similar to experimentally grown Lu-doped hydrous zircon. The size of this 3100cm−1 band scales with H and REE contents. Apart from a small overlapping band at 3200cm−1, no other absorption bands are visible, indicating that a hydrogrossular-type exchange mechanism does not appear to be operating in these zircons. Because of charge-balanced uptake of H, P and REE in zircon, the partitioning of these elements into zircon is dependent on each of their concentrations. For instance, DREEzrc/melt increases with increasing H and P contents of the melt, whereas DHzrc/melt increases with increasing REE content but decreases with increasing P content. In addition, H–P–REE systematics of sector zoning indicate kinetic effects may play an important role. Hence, using H in zircon to determine the water content of melts is problematic, and REE partitioning studies need to take into account P and H2O contents of the melt.

Prograde P–T Evolution of a Lawsonite Eclogite from the Monviso Meta-ophiolite (Western Alps): Dehydration and Redox Reactions during Subduction of Oceanic FeTi-oxide Gabbro

Lawsonite eclogites represent fossil records of processes occurring during subduction of cold- and fast-subducting slabs, and provide an opportunity to investigate the thermal and metamorphic evolution of palaeosubduction zones. Occurrences of lawsonite eclogites are rare because lawsonite is often replaced during exhumation. We report here, for the first time, the occurrence of a lawsonite eclogite from the Monviso meta-ophiolite (Western Alps), representing the product of the Alpine metamorphism of a FeTi-oxide gabbro. The prograde metamorphic evolution of this lawsonite eclogite has been investigated using the petrological approach of pseudosections. The reconstruction of its prograde P-T evolution allowed us to monitor changes in chemical and physical properties (mineral assemblages and compositions, density, H2O content, oxygen fugacity) during subduction. The pseudosection modelling suggests peak metamorphic conditions of T epsilon 550 degrees C, P = 25-26 kbar, and a decrease in the thermal gradient during subduction from about 9 degrees C km(-1) to less than 7 degrees C km(-1), which may be interpreted as related to an increase in the subduction rate. During its prograde evolution, at a depth of about 65-70 km, the eclogite-facies metagabbro experienced a significant dehydration passing from the Grt(1) + Omp(1) + Lws + Chl + Qtz + Rt assemblage (stage I) to the Grt(2) + Omp(2) + Tlc + Qtz + Rt assemblage (stage II). The breakdown of lawsonite and chlorite caused the release of up to 3 wt % of H2O and also of oxygen, as modelled by the redox equilibrium Lws + Qtz + Chl + Omp(1) = Grt + Omp(2) + H2O + O-2. This redox-equilibrium represents the boundary between an earlier, more oxidized assemblage (stage I), stable at lower T, and a later, more reduced assemblage (stage II), stable at higher T. These results have possible implications for the understanding of the complex interactions between crust and mantle in subduction zones, especially in clarifying the processes locally involved in the oxidation of the mantle wedge overlying the subducting slab.

Geochemical implications of gabbro from the slow-spreading Northern Central Indian Ocean Ridge, Indian Ocean

AbstractGabbro samples (c. < 0.4 Ma old) dredged from close to the ‘Vityaz Megamullion’ on the slow-spreading Northern Central Indian Ridge (NCIR, 18–22 mm yr−1) include mostly olivine gabbro and Fe–Ti oxide gabbro. The cumulate olivine gabbro shows ophitic to subophitic texture with early formed plagioclase crystals in mutual contact with each other, and a narrow range of compositions of olivine (Fo80–81), clinopyroxene (magnesium number: 85–87) and plagioclase (An67–70). This olivine gabbro could be geochemically cogenetic with the evolved oxide gabbro. These gabbro samples are geochemically distinct from the CIR gabbro occurring along the Vema, Argo and Marie Celeste transform faults and can further be discriminated from the associated NCIR basalts by their clinopyroxene (augite in gabbro, and diopsidic in basalts) and olivine (gabbro: Fo80–81, basalts: Fo82–88) compositions. Our major oxide, trace element and REE geochemistry analyses suggest that the gabbro and the NCIR basalts are also not cogenetic and had experienced different trends of geochemical evolution. The clinopyroxenes of the present NCIR gabbros are geochemically similar to primitive melt that is in equilibrium with mantle peridotite, and do not show any poikilitic texture with resorbed plagioclase; these results negate the possibility of these gabbros being a pre-existing cumulate that has been brought up to the shallower oceanic crust and interacted with the NCIR basalt. The Sr, Pb and Nd isotopic data of the gabbro substantially differ from those of the NCIR basalts and suggest significant contamination of the depleted mantle source of the gabbro, most likely by the Indian Ocean pelagic sediments. The Pb-isotope data suggest that the proportion of pelagic sediment that mixed in the depleted mantle source of the NCIR gabbro is much higher than the level of contamination observed for the Indian Ocean MORBs.

Mesoarchean subduction processes: 2.87 Ga eclogites from the Kola Peninsula, Russia

The nature of tectonic processes on the early Earth is still controversial. The scarcity of high-pressure metamorphic rocks such as eclogite (the high-pressure equivalent of basalt) in Archean cratons has been used to argue that plate tectonics did not operate until Earth had cooled to a critical point, perhaps around the 2.5 Ga Archean-Proterozoic transition. However, eclogites occur as meter- to kilometer-sized lenses enclosed in Archean gneisses of the Belomorian Province of the Fennoscandian shield. Geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros, troctolites, and Fe-Ti oxide gabbros. Greenschist facies mineral parageneses are enclosed in prograde-zoned eclogite garnets, and peak metamorphic conditions define an apparent thermal gradient (12–15 °C/km), consistent with metamorphism in a warm Archean subduction zone. We show here that these eclogites represent the oldest known high-pressure metamorphic rocks. U-Pb dating and Hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2.87 Ga for the Uzkaya Salma eclogite; a 2.70 Ga age for the Shirokaya Salma eclogite is interpreted as the age of a granulite facies overprint. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9–1.8 Ga) orogeny. These new data imply that plate tectonic processes operated at least locally in late Mesoarchean time. The adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce Archean tonalite-trondhjemite-granodiorite–type magmas.

RARE EARTH AND TRACE ELEMENTS IN AMPHIBOLES OF OCEANIC GABBROS (MARK AREA, MID-ATLANTIC RIDGE) AT MEDIUM- TO LOW-TEMPERATURE SEAFLOOR ALTERATION

The gabbro pile cored at the ODP sites 921-924 (Mid-Atlantic Ridge) underwent subseafloor alteration under polyphase retrograde recrystallization from very high temperatures in a ductile deformation regime to amphibolite and greenschist facies under transitional to brittle regimes (Gaggero and Cortesogno, 1997). To complement the study on the highest grade conditions (Cortesogno et al., 2000), amphiboles developed in different textural sites during the amphibolite to greenschist facies alteration of an olivine gabbro and an Fe-Ti oxide gabbro were analysed for REE, HFSE, LILE, LLE (Li, Be, B) and volatile elements (H, F, Cl) using SIMS. Brown, amphibolite facies, Mg-hornblendes grading to actinolitic hornblendes (Ti 16795-8645 ppm), which had replaced pyroxenes or filled veins, were analysed. The normalised C1 patterns of the pseudomorphic grains were, on the whole, comparable to those of the higher-temperature titanian amphiboles (Cortesogno et al., 2000), whereas the hornblendes filling the veins showed abundances of normalised Nb, La, Ce, Nd, Sm, Gd, Dy, Y, Er, Yb about 10 times higher with moderately increased Eu. On the contrary, Sr, Zr, Ti, Cr were lower. The REE and trace elements were largely controlled by heritage from clinopyroxene, water–amphibole partitioning and water–rock interactions. Green hornblendes (Ti 3492-207 ppm) that had filled the veins within the amphibolite facies, at lower temperatures, under the brittle regime, had C1 normalised patterns parallel to those of the brown hornblendes filling the fractures; the element contents being controlled by microsite equilibria. On the whole, the late actinolite filling of the fractures had low trace and RE element contents, and a positive Eu anomaly; the trace and REE element level decreased in the pseudomorphic grains. The tremolite pseudomorphic on olivine had a C1 normalised pattern parallel to, but with lower values than, the veinfilling hornblendes, except for very low Nb. The pattern was also comparable to the vein-filling actinolites, except for higher Dy, Y, Er, Yb and a negative Eu anomaly. On the whole the H2O content of the amphiboles decreased and F increased with the metamorphic temperatures, whereas Cl increased in the greenschist facies actinolites.

Petrogenesis of the Canindé de São Francisco complex: A major Late Proterozoic gabbroic body in the Sergipe Foldbelt, northeastern Brazil

The large Canindé de São Francisco gabbroic complex, with its associated volcanic units, has been widely interpreted as a Late Proterozoic island-arc sequence emplaced onto the margin of the São Francisco craton during the development of the Sergipe Foldbelt. Whole-rock major, trace, and rare earth element data and mineral chemistry data are presented here for the Canindé gabbroic complex and the associated Gentileza and Novo Gosto volcanics. These data show that both the gabbros and the volcanics have compositions that bear little resemblance to island-arc sequences. Instead, much of the complex has trace element characteristics that resemble those of continental flood basalts, such as the Paraná. It is more likely that the Canindé Complex has resulted from remobilization of the subcontinental lithosphere during some thermal event in the Late Proterozoic. The Sergipe Foldbelt, therefore, may not necessarily represent a zone of continental collision. Two distinct magma types are recognized in both the gabbro complex and the volcanic sequence. The dominant leucogabbros and the Gentileza basaltic volcanics are relatively enriched in incompatible trace elements and have multi-element patterns that are similar to those Paraná basalts. However, the Novo Gosto volcanics and the FeTi oxide gabbros are much more depleted in light rare earth and other incompatible trace elements, and have more refractory major element compositions. These rocks cannot be related to the rest of the complex by any conceivable scheme involving fractional crystallization. They have more likely resulted from melting of a more refractory mantle component in the subcontinental lithosphere, or they represent second-stage melting of mantle from which a basalt component had already been removed.