Trace-element mobilization during dehydration reactions in slabs: constraints from eclogite-facies veins (Tianshan, China)
Final Report Abstract
The Tianshan (U)HP Akeyazi metamorphic terrane in NW China represents a former oceanic subduction zone complex, in which eclogite-facies rocks and related high-pressure veins are commonly observed. Interconnected high-pressure (HP) veins in eclogites provide insight into element mobility during fluid-rock interaction in subduction zones. This case study focuses on a sulfide-bearing HP vein and its massive lawsonite eclogite host-rock from the Chinese Tianshan (ultra-)high-pressure/low-temperature metamorphic belt. A sulfide-bearing eclogite-facies reaction selvage envelops the omphacite-dominated vein. Lawsonite, garnet, omphacite and glaucophane inclusions in selvage pyrite porphyroblasts suggest that the selvage formed under prograde eclogite-facies conditions. Relics of wall-rock garnet cores in the recrystallized selvage garnet close to the wall rock, which images the overgrown selvage matrix, suggest that the selvage formed due to dissolution-precipitation processes as a direct consequence of fluid-rock interaction of the wall-rock eclogite with the vein-forming fluid. Mass-balance calculations indicate that the reaction selvage underwent: (1) a depletion of the large-ion lithophile elements (K-Rb-Ba) of up to 100% relative to their concentrations in the wall-rock eclogite; (2) a moderate depletion of the HREE and transition metal elements such as Fe, Cu, Ni, Zn, Co, Cr and Mn (10-40%); (3) a significant enrichment of CaO (up to 50-80%), Sr (up to 200%), Pb (up to 170%) and S (up to 210%); (4) a slight to moderate enrichment of the LREE (10-20%) and MREE (0-40%), whereas (5) the HFSE show no significant variations. The chemical changes in the selvage suggest that the fluid, which caused the dissolution of the wall-rock and the precipitation of the selvage assemblage while the vein formed, was probably a mixture of an "internal" fluid derived from the prograde dehydration (e.g. lawsonite breakdown) of the wall-rock and an "external" fluid most likely derived from dehydrating altered oceanic crust located in stratigraphically lower units of the subducting slab. The external fluid introduced Ca, S, Sr, Pb and at least parts of the LREE and the MREE into the selvage, whereas some elements, such as the remaining LREE and MREE, may have been derived from the wall-rock eclogite via diffusional transport into the reaction selvage. Significant amounts of transitional metal elements were released into the fluid during the dissolution of white mica and the dissolution-precipitation behavior of garnet, omphacite, dolomite and sulfides. Thus the LILE and HREE along with some transition metal elements were mobilized during the dissolution-precipitation processes that led to the selvage formation. Accordingly the slab fluids are not only strongly enriched in LILE and depleted in HFSE, but also carry significant amounts of transition metals. It is most likely that slab fluids strongly contribute to the metal flux into the arc magma systems finally resulting in arc-related ore deposits.
Publications
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2011. Fluid-mediated metal transport in subduction zones and its link to arc-related giant ore deposits: Constraints from a sulfide-bearing HP vein in lawsonite eclogite (Tianshan, China). Geochimica et Cosmochimica Acta 12, 326-362
Li, J.L., Gao, J., John, T., Klemd, R., Su, W.
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2012. Coexisting carbonate-bearing eclogite and blueschist in SW Tianshan, China: Petrology and phase equilibria. Journal of Asian Earth Sciences 60, 174-187
Li, J.L., Klemd, R., Gao, J., Meyer, M.
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2013. High-pressure mafic oceanic rocks from the Makbal Complex, Tianshan Mountains (Kazakhstan & Kyrgyzstan): Implications for the metamorphic evolution of a fossil subduction zone. Lithos 177, 207-225
Meyer, M., Klemd, R., Konopelko, D.
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2013. Metasomatism During High-Pressure Metamorphism: Eclogites and Blueschist- Facies Rocks. In: D.E. Harlov ; H. Austrheim (Eds.): Metasomatism and the Chemical Transformation of Rocks. Berlin Heidelberg : Springer-Verlag, 2013, (Lecture Notes in Earth System Sciences), 351-413
Klemd, R.
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2014. Compositional zoning in dolomite from lawsonite-bearing eclogite (SW Tianshan, China): Evidence for prograde metamorphism during subduction of oceanic crust. American Mineralogist 99, 206-217
Li, J.L., Klemd, R., Gao, J., Meyer, M.
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2014. Eclogitization of continental crust of the Aktyuz Complex during Late Palaeozoic plate collisions in the Northern Tianshan of Kyrgyzstan. Gondwana Research 26, 925-941
Klemd, R., Hegner, E., Bergmann, H., Pfänder, J.A., Li, J.L., Hentschel, F.
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2014. Subduction and exhumation mechanisms of ultrahigh and high-pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan). Journal of metamorphic Geology 32, 861-884
Meyer, M., Klemd, R., Hegner, E., Konopelko, D.
