Two types of reaction rims occur between olivine and plagioclase in ultramafic rocks from the Sefuri Mountains, NW Kyushu, Japan, which were metamorphosed under granulite-facies conditions. One occurs as a thin film of orthopyroxene along the boundary between olivine and plagioclase (orthopyroxene zone). The other is composed of two zones: symplectite of calcic amphibole and spinel on the plagioclase-side (symplectite zone) and calcic amphibole with sporadic orthopyroxene on the olivine-side (tremolite zone). In the tremolite zone, calcic amphibole shows a systematic decrease in Al content and increase in Mg/(Fe +Mg) with decreasing distance from olivine. Local equilibria maintained during the diffusion-controlled corona-forming reaction enable us to apply equilibrium thermodynamics to calcic amphibole and adjacent orthopyroxene. An integrated formulation of the Gibbs method for an Fe–Mg exchange reaction constrains the equilibrium temperature recorded in the tremolite zone to be 600–710 °C. It is significantly lower than the temperature of the granulite-facies metamorphism (800–900 °C) estimated using conventional geothermobarometry. Except for H2O, the association of calcic amphibole and spinel in the symplectite zone is chemically equivalent to the association of olivine, plagioclase and orthopyroxene that was stable before the corona formation. This suggests that the following orthopyroxene-consuming reaction describes the paragenetic change taking place between 800–900 °C and 600–710 °C, olivine + plagioclase + orthopyroxene + aqueous fluid = calcic amphibole + spinel. In contrast, the overall reaction inferred from microstructures produces orthopyroxene as well as calcic amphibole and spinel at the expense of olivine and plagioclase. This reaction requires removal mainly of MgO that is also responsible for destabilizing the local association of olivine and plagioclase. These features suggest that the presence of orthopyroxene as a product in the corona is not always indicative of an orthopyroxene-producing reaction being responsible for the change of paragenetic relation. Microstructural features should be carefully applied to infer the reaction describing paragenetic change by which we argue the P–T path of the rocks. 相似文献
Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin.
Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface. 相似文献
Clinopyroxene + plagioclase (±Hbl ± Qtz) symplectites after omphacite are widely cited as evidence for prior eclogite-facies
or high-pressure (HP) metamorphism. Precursor omphacite compositions of retrograde eclogites, used for reconstructing retrograde
P–T paths, are commonly estimated by reintegrating symplectite phases with the assumption that the symplectite-forming reactions
were isochemical. Comparisons of broadbeam symplectite compositions to adjacent unreacted pyroxene from various symplectites
after clinopyroxene from the Appalachian Blue Ridge (ABR) and Western Gneiss Region (WGR) suggest that the symplectite forming
reactions are largely isochemical. Endmember calculations based on reintegrated symplectite compositions from the ABR and
WGR suggest that a minor Ca-Eskola (CaEs) component (XCaEs = 0.04–0.15) was present in precursor HP clinopyroxene. WGR symplectites consist of fine-grained (∼1 μm-scale), vermicular
intergrowths of Pl + Cpx II ± Hbl that occur at grain boundaries or internally. ABR symplectites contain coarser (∼10 μm-scale)
planar lamellae and rods of Pl + Cpx II + Qtz + Hbl within clinopyroxene cores. The contrasting textures correlate with decompression
and cooling rate, and degree of overstepping of the retrograde reaction (lamellar: slow, erosionally controlled exhumation
with slow/low overstepping; fine-grained, grainboundary symplectite: rapid, tectonic exhumation with rapid/high overstepping).
Variations in XCaEs, Xjd, and XCaTs of precursor HP omphacite are related to the symplectic mineral assemblages that result from decompression. Quartz-normative
symplectities indicate quartz-producing retrograde reactions (e.g., breakdown of precursor CaEs); quartz-free symplectities
(e.g., diopside + plagioclase after omphacite) indicate quartz-consuming reactions (jd, CaTs breakdown) outpaced quartz-producing
reactions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献