Phase relations in metabasic rocks: constraints from the results of experiments, phase modelling and ACF analysis

Abstract

Phase relations in metabasic rocks are documented from the results of phase modelling, experiments and ACF analysis. A P–T pseudosection for a mid-ocean-ridge basalt (MORB) composition was calculated in the P–T range of 1–32 kbar and 400–1100°C using THERMOCALC 3.45. Phase relations in this pseudosection are mostly consistent with the results from experiments but complicated. ACF analyses suggest that the complicated phase relations can be simplified into 10 invariant assemblages involving orthopyroxene, clinopyroxene, hornblende, glaucophane, actinolite, garnet, chlorite, epidote, lawsonite, kyanite and plagioclase, assuming quartz, NaAlO2 and H2O/melt being are excess. Phase relation analyses in the P–T projections that are constructed according to Schreinemakers’ rules and qualitative P–T pseudosections that are obtained for a MORB composition indicate that 22 subfacies assemblages are recognized for metabasic rocks. A four-fold classification of metamorphism is proposed on the basis of the phase relations for metabasic rocks, including low-, medium-, high- and very-high-P/T types, with apparent geothermal gradients >110°C/kbar, between 110 and 55°C/kbar, between 55 and 28°C/kbar, and <28°C/kbar, respectively. This four-fold classification of metamorphism is better able to match various tectonic settings.

The Exhumation of Subducted Oceanic‐Derived Eclogites: Insights From Phase Equilibrium and Thermomechanical Modeling

“…As for the stabilities of talc and glaucophane, there are few experiment constrains. Instead, the calculated stabilities of glaucophane and talc are generally consistent with the phase equilibrium modeling results of Wei and Duan () and Chen et al ().…”

“…The model and density calculations are also affected by the uncertainties of predict mineral assemblages and stability field of minerals. The general consistency of our results with experiments works (Schmidt & Poli, ; Sen & Dunn, ; Wyllie & Wolf, ) and previous pseudosection calculations (Chen et al, ; Wei & Duan, ) indicates the validity of our calculations. Besides, the limitation of activity‐composition model would influence the modeling phase relationship.…”

“…Compared with the stabilities of chlorite, epidote, and lawsonite experimentally constrained by Schmidt and Poli () for a H 2 O‐saturated MORB system, the calculated results are generally consistent for chlorite, slightly larger for lawsonite, but much smaller for epidote (Figure S8). This discrepancy also appears in the phase equilibrium modeling of Wei and Duan () and Chen et al (), which might be caused by different bulk‐rock compositions used in pseudosection calculation and experimental starting material (Chen et al, ). As for the stabilities of talc and glaucophane, there are few experiment constrains.…”

“…Therefore, the dehydration of talc is equally important to the density increase of eclogites along cold P‐T paths surpassed only by chlorite and lawsonite. Glaucophane appears under low temperature (<660 °C) and high pressure (>9 kbar) in Wei and Duan () and is predicted to be stable below 630 °C in Chen et al (). The stability of glaucophane (<600 °C) in our results (Figure c) is consistent with the result of both (<600 °C).…”

HP–UHP metamorphism and tectonic evolution of orogenic belts: introduction

“…The second paper focuses on mineralogical phase relations in metamorphic rocks, which is key to deciphering the metamorphic development during the tectonic evolution of an orogenic belt. Wei & Duan (2018) present an overview of the phase relations in metabasic rocks based on the results of phase modelling, experiments and ACF analysis. They propose a four-fold classification of metamorphism, including low-, medium-, high-and very high-P-T types, with apparent geothermal gradients at >110, 110-55, 55-28 and <28°C/kbar, respectively.…”