Controls on development of different mineral assemblages in gabbro and basalt during subduction metamorphism
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ORIGINAL PAPER
Controls on development of different mineral assemblages in gabbro and basalt during subduction metamorphism Patricia Kang1 · Katherine F. Fornash2 · Donna L. Whitney1 Received: 13 May 2020 / Accepted: 20 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
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 2O 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 (XMg = 0.58–0.84 vs. 0.50), higher H2O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H 2O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world. Keywords Blueschist · Lawsonite · Eclogite · Metabasalt · Metagabbro · Subduction · Turkey
Introduction Basalt and gabbro represent different parts of an oceanic crustal section. Both experience a metamorphic transformation into blueschist and eclogite during subduction, and this transformation is controlled by various factors. The Communicated by Daniela Rubatto. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00410-020-01753-6) contains supplementary material, which is available to authorized users. * Patricia Kang [email protected] 1
Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455, USA
Department of Geological Sciences, Ohio University, Athens, OH 45701, USA
2
role of H 2O has bee
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