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

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. Fine-grained 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.