INTERFACE-COUPLED DISSOLUTION-REPRECIPITATION TEXTURES WITH MAJOR ELEMENT ZONATION PRESERVED IN GRENVILLIAN GARNET

Interface-Coupled Dissolution-Reprecipitation (ICDR) pseudomorphously alters mineral compositions with the facilitation of external fluid infiltration. As the ICDR reaction front propagates in a crystal, chemical zones with sharp boundaries and irregular shapes develop. In garnets from high-grade metamorphic terranes, sharp zonation of major elements is erased rapidly by diffusional relaxation, so the ICDR textures are typically only preserved by trace elements zonation. In this study, we report garnet ICDR textures preserved in major element zoning found in felsic gneiss in the Mattawa domain, which is part of the Central Gneiss Belt in the western Grenville Province. Mattawa domain is mainly comprised of felsic orthogneiss hosting enclaves of retrograded eclogites. Evidence from the metabasites shows that the Mattawa domain underwent a pressure peak ~2 GPa at 850 °C. The profiles of the garnets from the metabasites yield short durations of isothermal decompression and cooling (within a couple of hundred kyrs; Fan et al., 2022). The felsic host gneisses are tonalitic to granodioritic of arc origins, and consist of quartz + plagioclase + K-feldspar + biotite + garnet ± hornblende. In situ zircon U-Pb geochronology revealed both early Ottawan (1095 ± 4 Ma) that overlaps the ages of eclogitization in the Central Gneiss Belt, and late Ottawan (1024 ± 4 Ma) metamorphic ages. The garnet porphyroblasts appear patchy, with irregular-shaped darker red patches hosted in pale red garnet. Fractional mass balance calculation shows depletion of major elements except Ca, and enrichment of Ti and LREE in the patches. The characteristic diffusion length scale across the zonation boundary is <10 µm, indicating an extremely short timescale of post-ICDR cooling. Diffusion simulation yields an unrealistic cooling rate > 20,000 °C/kyr if it was cooled down from 850 °C, in stark contrast to ~2 °C/kyr calculated from the metabasite. Thus, we suspect that the ICDR took place during the late Ottawan, in response to the widespread post-orogenic pegmatitic intrusion. The cooling rate would be 2-4 °C/kyr from 650 °C, equivalent to the cooling of a km-scale thermal anomaly in the orogenic crust.