Controls on development of different mineral assemblages in gabbro and basalt during subduction metamorphism

Abstract

Coexisting fine-grained (meta-volcanic) and coarse-grained (meta-plutonic) mafic rocks in a high-pressure (P)/low-temperature (T) complex (Sivrihisar, Turkey) preserve different prograde, peak, and retrograde mineral assemblages, providing an opportunity to evaluate controls on mineral assemblages in metabasites that experienced the same P-T conditions. Finegrained metabasalts are garnet-bearing lawsonite blueschist and eclogite with similar assemblages that vary on a mm-to cm-scale in mode of glaucophane vs. omphacite. In contrast, metagabbro consists of a disequilibrium mineral suite of relict igneous clinopyroxene partially replaced by omphacite or hydrous phases (lawsonite + tremolite or glaucophane) in a matrix of fine-grained lawsonite, omphacite, tremolite, white mica, very rare garnet, and retrograde minerals (e.g., epidote, albite, and titanite), with later chlorite and calcite. Pseudosection modeling predicts similar peak P-T conditions (490-530 °C, 1.8-2.0 GPa) for both glaucophane-rich (blueschist) and omphacite-rich (eclogite) layers of the metabasalt and similar to slightly higher conditions (490-600 °C, 1.9-2.5 GPa) for metagabbro. The range of modelled H 2 O content at peak P-T conditions in metabasalt (2.0-5.4 wt%) is significantly lower than in metagabbro (6.4-8.7 wt%) due to the higher modal abundance of hydrous minerals in the latter. At the relatively similar peak P-T conditions, metagabbro experienced different reaction histories from coexisting metabasalt, thereby developing distinctive HP/LT mineral assemblages and modes (e.g., scarce garnet) owing to its more Mg-rich bulk composition (X Mg = 0.58-0.84 vs. 0.50), higher H 2 O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H 2 O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world.

Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

“…Lawsonite also occurs in chlorite ± garnet‐rich metasomatite at the contact of serpentinite lenses with metabasite and metasediment. The HP/LT rocks record prograde‐to‐peak P‐T conditions of 380–550°C, 1.2–2.4 GPa (Davis & Whitney, 2006, 2008; Kang et al., 2020; Pourteau et al., 2019) with ages of 105–83 Ma (Fornash et al., 2016; Mulcahy et al., 2014; Pourteau et al., 2019); eclogites are older than the dated blueschists, indicating that at least some blueschists are retrograde (Fornash et al., 2016; Mulcahy et al., 2014).…”