Two-Stage Garnet Growth and Polyphase Metamorphism in Eclogites From the Serkout Area (Central Hoggar, Algeria): Constraints From Petrology and Thermodynamic Modelling

Abstract

Garnet in eclogites is routinely analysed to reconstruct detailed P–T paths, as its chemical zoning records multiple metamorphic stages. In this aim, eclogites from the Serkout area (Aleksod terrane, Central Hoggar, Southern Algeria) have been petrologically investigated and thermodynamically modelled to trace their P–T history. Our study reveals that the samples have preserved the eclogite-facies paragenesis, characterised by garnet, omphacite, quartz, clinozoisite and amphibole, indicative of prograde to peak metamorphism. However, these minerals have been partially replaced during retrogression by clinopyroxene–plagioclase and amphibole–plagioclase symplectites. Coarse garnet crystals have highly heterogeneous, almandine-dominated compositions (X_Alm ranging from 0.30 to 0.56), whereas omphacite has a maximum jadeite content (X_Jd) of 0.31. Coarse amphibole, assumed to be of eclogite facies, is magnesio-hornblende with Na_M4 = 0.31–0.47. Secondary plagioclase shows a wide range of compositions (X_An = 0.20–0.99), from albite-rich in plagioclase–diopside symplectites after omphacite to anorthite-rich in plagioclase coronae around clinozoisite. The garnet grains in the Serkout eclogites display clear core–rim zoning, which, together with the evolution of mineral inclusions, allows the garnet growth to be correlated with different metamorphic events. Garnet cores formed under low-pressure, relatively high-temperature conditions (first M1 event), whereas garnet inner to outer rims developed during the prograde path from M1 to M2 stages. The late symplectites and coronas are related to retrogression (M3 stage). The combination of detailed mineralogical analysis, Ti-in-amphibole thermometry and thermodynamic modelling (P–T-M_H2O pseudosections), using THERMOCALC-345, has revealed the existence of two distinct events: (a) an older high-temperature event (718°C) most likely Paleoproterozoic in age, and (b) a younger eclogitic event characterised by (i) a prograde P–T path starting at 9 ± 1 kbar and 610 ± 50°C under H₂O-undersaturated conditions, progressing to peak conditions of 21.5 ± 1 kbar and 720 ± 50°C under H₂O-saturated conditions, and (ii) a retrograde path characterised by nearly isothermal decompression towards 5.5 kbar and 685°C.