Two stages of fluid-rock interaction in UHP marbles (Dabie Shan, China): grain-scale processes and map-scale metasomatism
Abstract
Fluid-mediated element mobility during ultra-deep subduction and exhumation of continental crust results in characteristic isotope signatures of UHP rocks. In the Dabie UHP complex large volumes of meta-carbonates show unusually unradiogenic 87Sr/86Sr ratios, as low as 0.7037 (Romer et al. 2003). The mineral reaction history, combined with the initial Sr isotopic record of prograde and retrograde phases of the meta-carbonates provide evidence for two stages of fluid-rock interaction during exhumation along the subduction zone. To constrain (i) the mechanisms of fluid transport through the rocks, (ii) the source of the fluid(s) and (iii) the timing of fluid-rock interactions, a calcsilicate marble has been investigated at the grain-scale. A crucial advantage of the studied sample is the record of the initial Sr isotopic signature of the carbonate rock preserved in the core of a large pre-UHP titanite (U-Pb crystallization age of 244±2 Ma, Wawrzenitz et al., 2006). Based on the results of microdrilling, ID TIMS and mineral chemical investigations, two pulses of infiltration of external fluids distinct in their Sr isotopic composition are inferred. During the first stage, fluids with very low 87Sr/86Sr values induced dissolution-precipitation reactions resulting in the isotopically homogeneous phases clinozoisite, titanite, amphibole and calcite, that replace the UHP assemblage (garnet, omphacite, rutile, phengite, aragonite, coesite) in the marble. The source of the very unradiogenic fluids may be the dehydrating young mafic rocks from the downgoing slab. Dissolution-precipitation reactions resulted in a high porosity, and efficiently supported material transport through the carbonate rock and isotope chemical exchange among fluids and rocks. This first stage of fluid-infiltration is recorded by a U-Pb isochron age (224±2 Ma, Wawrzenitz et al., 2006) of titanite, clinozoisite, feldspar, epidote from a marble from the same unit, assuming a common pressure-temperature-time history. During the second stage, the interacting fluid had a radiogenic initial Sr isotopic composition and may be derived from dehydrating subducted crustal rocks that were juxtaposed to the marble during a later stage of exhumation. At this second stage, allanite precipitated locally at calcite grain boundaries and in veins related to brittle deformation of the large pre-UHP titanite. Grain boundaries and mineral cleavage served as pathways for the fluid. In the calcite marble matrix, dissolution-precipitation led to Mg-enrichment at the rim of calcite grains and to precipitation of small rectangular and vermicular dolomite in calcite. This texture is commonly interpreted as the result of exsolution of the Mg-component of UHP-calcite during decompression. Associated with the recorded fluid-rock interactions, complementary isotopic (e.g. high 87Sr/86Sr) and geochemical signatures (e.g. enrichment of alkali elements) in the mantle-derived rocks, now exhumed as mafic eclogites within the Dabie UHP complex, are expected, similar to the phlogopite-bearing rocks of the Finero body (Ivrea zone, Italy). Romer, R.L., Wawrzenitz, N., Oberhänsli, R., 2003. Terra Nova 15, 5, 330-336. Wawrzenitz, N., Romer, R.L., Oberhänsli, R., Dong, S., 2006. Lithos 89, 1-2, 174-201.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V43A2817W
- Keywords:
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- 1031 GEOCHEMISTRY / Subduction zone processes;
- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 3653 MINERALOGY AND PETROLOGY / Fluid flow;
- 3654 MINERALOGY AND PETROLOGY / Ultra-high pressure metamorphism