A GARNET AND LAWSONITE RECORD OF HYBRID ROCK FORMATION IN THE SIVRIHISAR (TURKEY) SUBDUCTION COMPLEX

Chlorite (Chl) ± talc-rich hybrid rocks, which form as a result of fluid-mediated tectonic mixing and/or metasomatic interactions between ultramafic rocks and the mafic and sedimentary rocks of the subducted slab, are valuable records of the physical and chemical interactions that take place during subduction. However, many hybrid rocks are mono- or bimineralic, so most prior studies have been conducted at the bulk rock scale, making it challenging to identify contributions from mixed fluid sources, as well as the extent and relative timing of fluid-rock interaction(s). In the Sivrihisar subduction complex, some hybrid rocks also contain garnet (Grt) and lawsonite (Lws). As Grt and Lws both preserve a record of metamorphic reaction history and fluid-rock interactions in their composition and zoning patterns, micro-scale geochemical studies of these minerals were used to better resolve the petrogenesis of hybrid rocks.

Grt records two growth stages, each with distinct inclusion assemblages, major and trace element (TE) compositions, and δ¹⁸O values. Euhedral Mg-poor inner Grt (Grt I) contains Chl and rutile (Rt) inclusions and records prograde growth zoning in both major and TE. Grt I δ¹⁸O values are relatively homogeneous and vary from ~9 ‰ (core) to ~10 ‰ (rim). These δ¹⁸O values, along with Rt inclusion Nb/Cr ratios, indicate the hybrid rock was derived from a mafic protolith that experienced low-T seafloor alteration prior to subduction. The outer Grt (Grt II) contains epidote (Ep) and titanite (Ttn) inclusions and records a sharp increase in Mg, V, Cr, and Sc. Grt II δ¹⁸O values are higher (~10-18 ‰) and vary cyclically, consistent with episodic infiltration of high δ¹⁸O fluids and subsequent mixing with lower δ¹⁸O fluids. Grts record up to four oscillations, each indicating a pulse of high δ¹⁸O (possibly sediment-derived) fluids during later stages of subduction. Lws includes Ep, Ttn, and (rare) Grt and is zoned in both major and TE, with Fe-poor cores and Fe-rich rims and oscillations in Sc, Y, and REE. Intragrain Lws Sr/Pb ratios vary from 18–42 and show a rimward decrease, also consistent with late interactions with sediment-derived fluid. These results highlight the utility of micro-scale investigations of zoned Grt and Lws in unravelling the multi-stage metamorphic evolution of high variance metasomatic rocks.