Neoproterozoic subductions and differential exhumation of western Hoggar (southwest Algeria): new structural, petrological and geochronological evidence

Abstract

The western terranes exposed east of the Pan-African suture in western Hoggar (southwest Algeria), are reexamined in the light of new structural, petrologic and by the 40Ar/ 39Ar laser probe data on metamorphic micas and amphiboles. To the north, the Tassendjanet nappe includes the Paleoproterozoic basement, its Mesoproterozoic cover and mafic rocks representing the roots of a ca. 680 Ma arc overlain by Late Neoproterozoic andesites and volcanic greywackes. The nappe preserved at rather shallow crustal level in the east was emplaced southward (D 1a) to southeastward (D 2). In the south, two metamorphic suites are distinguished. The Tideridjaouine–Tileouine high-pressure metamorphic belt ( T=550–600 °C, P=1.4–1.8 GPa) represents a slab of subducted continental material exposed along the western edge of the In Ouzzal granulite unit interpreted as a microcontinent. Differential exhumation of tectonic slices from the high-pressure belt occurred around 615–600 Ma through a system of west-directed recumbent folds (D 1b). The Egatalis high grade belt in the west was intruded by syn-metamorphic gabbro–norite bodies. It includes unretrogressed low-pressure granulite facies rocks ( T around 750–800 °C, P⩽0.45 GPa) cooled at a rate of 15°/m.y. between 600 and 580 Ma, and followed by the emplacement of several late-kinematic granitic plutons. Final exhumation of the low-pressure, high-temperature metamorphic rocks, that are not found as pebbles in the molasse, took place in the Late Cambrian. The early and relatively fast cooling of the high-pressure and high-temperature metamorphic rocks of the southern part of the Tassendjanet terrane is at variance with the slow cooling of central Hoggar where repeated magmatic activity as young as Late Cambrian occurred [Lithos 45 (1998) 245].