Metamorphic evolution of lawsonite eclogites from the southern Motagua fault zone, Guatemala: insights from phase equilibria and Raman spectroscopy

Abstract

Lawsonite eclogite (metabasalt and metadolerite) and associated metasedimentary rocks in a serpentinite me´lange from an area just south of the Motagua fault zone (SMFZ), Guatemala, represent excellent natural records of the forearc slab-mantle interface. Pseudosection modelling of pristine lawsonite eclogite reproduces the observed predominant mineral assemblages, and garnet compositional isopleths intersect within the phase fields, yielding a prograde P-T path that evolves from 20 kbar, 470°C (M1) to 25 kbar, 520°C (M2). The dominant penetrative foliation within the eclogite blocks is defined by minerals developed during the prograde evolution, and the associated deformation, therefore, took place during subduction. Thermometry using Raman spectra of carbonaceous material in metasedimentary rocks associated with the SMFZ eclogites gives estimates of peak-T of $520°C. Barometry using Raman spectroscopy shows unfractured quartz inclusions in garnet rims retain overpressures of up to $10 kbar, implying these inclusions were trapped at conditions just below the quartz ⁄ coesite transition, in agreement with the results of phase equilibrium analysis. Additional growth of Ca-rich garnet indicates initial isothermal decompression to 20 kbar (M3) followed by hydration and substantial cooling to the lawsonite-blueschist facies (M4). Further decompression of the hydrated eclogite blocks to the pumpellyite-actinolite facies (3-5 kbar, 230-250°C) is associated with dehydration and veining (M5). The presence of eclogite as m-to 10 m-sized blocks in a serpentinite matrix, lack of widespread deformation developed during exhumation and derived prograde P-T path associated with substantial dehydration of metabasites within the antigorite stability field suggest that the SMFZ eclogites represent the uppermost part of the forearc slab crust sampled by an ascending serpentinite diapir in an active, moderate-T subduction zone.

Quartz‐in‐garnet and Ti‐in‐quartz thermobarometry: Methodology and first application to a quartzofeldspathic gneiss from eastern Papua New Guinea

“…For example, Raman measurements can be performed on a quartz inclusion in garnet, and the same inclusion can be subsequently exposed and polished for Ti-in-quartz measurements. Most previous applications of elastic thermobarometry have generally used a temperature estimate obtained from an external method or an assumed constraint to determine the P-T of mineral entrapment conditions on an isomeke (Anzolini et al, 2019;Ashley et al, 2014Ashley et al, , 2016Barkoff et al, 2017;Bayet, John, Agard, Gao, & Li, 2018;Castro & Spear, 2017;Enami et al, 2007;Endo et al, 2012;Kouketsu, Hattori, Guillot, & Rayner, 2016;Kouketsu, Enami, & Mizukami, 2010;Nestola et al, 2018;Soret et al, 2019;Spear, Thomas, & Hallett, 2014;Taguchi, Enami, & Kouketsu, 2016Wolfe & Spear, 2018;Zhong et al, 2019). For example, pseudosection modelling uses bulk rock geochemistry and mineral chemical compositions to reveal the ranges of stable P-T conditions where possible mineral assemblage can crystallize at chemical equilibrium.…”

“…Previous applications of elastic thermobarometry have combined elastic models with a Zr‐in‐rutile solubility model (Castro & Spear, ; Wolfe & Spear, ), mineral assemblage equilibrium thermobarometry (Ashley, Caddick, Steele‐MacInnis, Bodnar, & Dragovic, ; Barkoff et al, ), Fourier transform infrared spectroscopy (Nestola et al, ), Raman spectroscopy of carbonaceous material (Endo, Wallis, Tsuboi, Torres De León, & Solari, ), and isomekes of two coexisting inclusions (e.g. QuiG and zircon‐in‐garnet; Zhong, Andersen, Dabrowski, & Jamtviet, ) to estimate unique P–T conditions.…”

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

“…Late-stage veins including epidote-or Fe-rich prehnite-bearing veins are also related to fluid flow during decompression and exhumation. Basaltic rocks require high H 2 O contents for H 2 O-saturated phase equilibria at low temperatures and it is likely that subducting oceanic crust can act as an H 2 O sink at shallow depths (Clarke, Powell, & Fitzherbert, 2006;Endo, Wallis, Tsuboi, Torres de Le on, & Solari, 2012). However, this may not be the case if the reaction system is localized and deviates from an ideal basaltic composition.…”

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

“…Besides, the ultimate self‐exhumation depth, an overwhelmingly concerned issue, is the maximal self‐exhumation depth for oceanic‐derived eclogites. Those realistic geothermal gradients of oceanic subduction zones are hotter than 5 °C/km (Agard et al, ; Endo et al, ; Erdman & Lee, ), such as northwest Turkey and Guatemala (near 5 °C/km; Okay, ; Tsujimori et al, ). Thus, the ultimate self‐exhumation depth corresponds to the depth at which the 5 °C/km geothermal gradient and the isoline of Δ ρ MORB = 0 intersect, which predicts 107 km (~3.2 GPa; Figure ).…”