Serpentinization and alteration in an olivine cumulate from the Stillwater Complex, Southwestern Montana
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Extensive lenticular-shaped ultramafic complexes that are composed of dominant amount of dunite associated with harzburgite and irregular-shaped pyroxenite cumulates, occur in the basal part of the Salahi mantle section in the northern Oman ophiolite. Petrologic characteristics such as rock types, textures and mineral compositions were examined to understand the origin of the ultramafic complex. Peridotites exhibit coarse-granular to very coarse-granular texture with grain size greater than five millimeters. The Cr#[=Cr/(Cr+Al) atomic ratio] of spinels in harzburgites ranges from 0.56 to 0.72, and is most frequent at 0.64-0.66 while spinel Cr# of dunites ranges from 0.61 to 0.84, and is most frequent at 0.76-0.82. The origin of voluminous dunite with high Cr# spinel can be explained by flux melting of residual harzburgite as a result of infiltration of large amount of fluid from the base of the ophiolite during oceanic thrusting. Dunites with very coarse-granular texture also support this hypothesis. The dunites in the central part of the complex tend to have both olivine Fo content and spinel Cr# greater than those in the host harzburgites. We speculate that not only orthopyroxene but also olivine were consumed during flux melting in this region resulted in the formation of voluminous dunites in the core of the complex. On the other hand, the dunites from the periphery of the complex have olivine with Fo content lower than those in the host harzburgites indicating precipitation of olivine from the melt. Moreover, crystallization of pyroxene formed numerous pyroxenites in this region. The southern part of the Salahi mantle section has been considered as a paleo-ridge segment end region. Our results support that the occurrence of highly refractory peridotites is spatially related to the segment end region in the northern Oman ophiolite.
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Felsic
Mantle plume
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1984)
The mathematical background has been established to evaluate the equilibrium pressure and temperature of ultramafic xenolith by the application of least squares technique to the equilibrium among olivine, orthopyroxene and garnet. Estimated pressures in the approximated system SiO2-Al2O3-FeO-MgO-CaO are slightly greater than those in the additional correction of Mn-Ni-Cr-Ti for the granular type of lherzolites and are shifted to lower pressure sides for the sheared lherzolites. The inflection in the paleogeotherm beneath northern Lesotho is observed at the depths 150 km.
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Xenolith
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Least-squares function approximation
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Relevance of the research. Permanently identified signs of plastic flow in ultramafic rocks predetermined an approach to their study as metamorphic rocks. This approach uses non-traditional method of the petrofabric analysis. This method allows reconstructing the chronological sequence of formation and plastic deformation of ultramafic rocks in the upper mantle-crust, revealing the general trend of structural and matter evolution, and solving a number of urgent problems of the ultramafic mineral genesis. The aim of the study is to develop a scenario of the structural evolution of the Paramsky and Shamansky ultramafic massifs of Baikal-Muya ophiolite belt at the upper mantle-crust levels through the dunite and harzburgite microstructural heterogeneity, typing their microstructures, calculating olivine-chromospinelide thermal equilibrium, and olivine petrofabric analysis. Methods. The detailed petrographic characteristics of ultramafic rocks were performed by the polarizing microscope AxioScope-40 (Carl Zeiss). This study allowed carrying out microstructural typification of dunites and harzburgites. It was based on the morphological features of olivine and it was developed by many researchers. Quantification of deformation microstructures in olivine of ultramafic rocks was studied using the stereometric metallography. Microstructural analysis of olivine is an integral part of the petrofabric study of the ultramafic rocks. It allows determining the preferred orientation of minerals in the internal structure, which in their turn is a reflection of the thermodynamic conditions of their plastic deformation. The olivine and chromospinelide matter composition was determined using micro X-ray spectrum, obtained by microanalyser «Camebax» in the Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences (Novosibirsk). The thermal equilibrium was calculated using olivine-chromospinelide Fabry geothermometer on the matter composition of coexisting olivine and chromospinelide. Results. The research provided the identification of the deformation microstructures of olivine for dunites and harzburgites of the Paramsky and Shamansky ultramafic massifs of the Baikal-Muya ophiolite belt. They are grouped into six types: protogranular, mesogranular porphyroclastic, porphyrolath, mosaic (mosaic-lath), mosaic-parquet-like. The spatial distribution of the allocated microstruct ures in the studied massifs from the сenter to the periphery is characterized by reducing the grain size in the rocks. This feature shows the increase in their plastic deformation degree and reflects the dynamometamorphiс zoning. The petrofabric analysis of the olivine in the identified microstructural types allowed determining thermodynamic conditions of their realization. These microstructures reflect a regressive trend of metamorphic transformations of the ultramafic rocks by high-temperature plastic deformations, syntectonic recrystallization, an d secondary annealing recrystallization in formation, transformation and consolidation in the upper mantle - earthґs crust. The identified evolution of the olivine microstructures reflects the limits of repeatedly overlaid plastic deformations, which can be decisive in l ocalization of chromite, chrysotile asbestos, jade, and jadeite within ultramafic rocks of ophiolite complexes under favorable conditions.
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Four ultramafic lamprophyre to kimberlite intrusive sites have been recognized in the Archean Nain Province of northern Labrador. Two sites are located within the Hopedale Block, at (1) capes Aillik–Makkovik and (2) Ford's Bight, and two are located within the Saglek Block, in the (3) Saglek and (4) northern Torngat Mountains areas. To date, no diamonds have been found in either intrusive site, but geochemical analysis has been, at best, rudimentary. Group 1 has been previously described as kimberlitic, at least in part, but, to date, no detailed geochemical evaluation of diamond indicator minerals has been completed. The intrusive rocks of Group 2 have only recently been identified as part of a possible kimberlite diatreme; there are no whole-rock or mineral geochemical data yet derived for this suite. The Group 3 suite has been defined as kimberlitic for some time, but spatial and geochemical data have been restricted to unpublished reports. Group 4 dykes were originally classified as ultramafic lamprophyres, but mineral assemblages of some dykes indicate that they are macrocrystal hypabyssal phlogopite–perovskite archetypal kimberlites, whereas the remainder are olivine–phlogopite–calcite ultramafic lamprophyres, possibly melilitites. Sample size, however, was very small for definitive determinations, and one kimberlite has a strike extension classified as lamprophyre. Within the kimberlites, the following minerals were identified: (1) lherzolitic, eclogitic and megacrystal garnets (megacrystal garnets were also found in some of the lamprophyres), (2) clinopyroxenes, which plot in Fipke's diamondiferous CPX domain (as does one lamprophyre), (3) orthopyroxenes with < 1% to 1.5 % Al2O3, and (4) olivines with Fo contents of ~ 88-92 (some of the lamprophyres have two populations of olivine with clusters of similar high Fo values). Orthopyroxene and clinopyroxene geothermobarometers suggest a possible harzburgite chemistry for some mineral separates, and also that the postulated geotherm for the intrusive history of the dykes is permissive of diamond stability.
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