Zircon and Titanite U-Pb geochronology, petrology and P-T path of garnet-bearing amphibolites from Tadoumet area (Assodé-Issalane terrane, Central Hoggar, Algeria)

Abstract

The garnet-bearing amphibolites of the Tadoumet area (Assodé-Issalane terrane, Orosirian Stripe, Hoggar), show three principal metamorphic stages. (i) A pre-peak metamorphic stage, characterized by mineral inclusions of quartz, plagioclase, epidote, rutile, and titanite encapsulated within garnet porphyroblasts (M0); (ii) A relatively high-grade stage, represented by a coarse-grained paragenesis comprising garnet porphyroblasts, amphibole, clinopyroxene, quartz, titanite, and rutile (M1); (iii) A retrograde stage, marked by the development of amphibole‒plagioclase‒K-feldspar coronae assemblages around garnet with titanite (M2).Microprobe analysis indicates that garnet compositions range between X_Alm = 0.47–0.58, X_Gr = 0.25–0.35, and X_Py = 0.06–0.16. Clinopyroxenes exhibit zoning, with cores depleted in Mg relative to the rims (X_Mg core = 0.57, X_Mg rim = 0.65). Furthermore, amphibole associated with the M1 stage displays compositions indicative of magnesio-hornblende, whereas the M2 stage is characterized by magnesian-hastingsite coronae (Si < 6.25 and X_Mg = 0.4–0.6). Tschermakite, with X_Mg values of 0.51–0.60, is found in association with clinopyroxene and plagioclase (X_An = 0.34–0.48).Calculated P-T pseudosections suggest peak P–T conditions of 13.5 ± 1 kbar and 700 ± 50 °C (M1), characterizing a mineral assemblage comprising hornblende, diopside, garnet, K-feldspar, plagioclase, rutile, titanite, and melt, based on garnet core compositions. Additionally, decompression stage corresponds to high temperature conditions (750 ± 50 °C, M2a) followed by cooling (600 ± 50 °C, M2b) at 6.5 ± 1 kbar, using the compositions of garnet rims, secondary diopside, amphibole, and plagioclase isopleths and Ti in amphibole geothermometer. This study constitutes the first geochronological investigation of metamorphism in the Tadoumet area, utilizing in situ U–Pb dating of zircon and titanite. The findings indicate a protolith age of 712 ± 20 Ma (derived from zircons) and an amphibolite-facies metamorphic age of 599 ± 4 Ma (obtained from titanite). The P–T trajectory recorded in the garnet-bearing amphibolites is interpreted as reflecting a distinct sequence of continental collision, in which crustal thickening denotes the peak of metamorphism, followed by decompression driven probably by extensional processes.