The Torak granitic massif: structure, petrology, and mineralogy (Tefedest, Central Hoggar, Algeria)

Abstract

The Torak massif located to the west of the Atakor volcanic district is a granitic pluton NNE-SSW elongated and crosscut by several NNW-SSE lineaments. It is intrusive within the S-W Tefedest terrane composed of Eburnean (c. 2 Ga) gneisses and of Pan-African (c. 615 Ma) syntectonic granitic batholiths. New field mapping, petrographic, and mineralogical data (feldspars and mica chemistry) and geochemical data (major and trace elements, REE) reveal the following: the Torak granitic massif shows a fairly simple and homogenous mineralogy dominated by feldspars (orthoclase and microcline, albite, and oligoclase) and lithium micas (siderophyllite to protolithionite and phengite to Li-phengite). The Torak granite chemistry is remarkably homogeneous both in major elements (73.95 ≤ SiO ≤ 76.85 wt.%; 7.68 ≤ (NaO + KO) ≤ 9.55 wt.%) and trace elements (500 ≤ Rb ≤ 780 ppm; 9.774 ≤ La ≤ 45.267 ppm; 0.316 ≤ (La/Yb) ≤ 2.418; 0.027 ≤ (Eu/Eu*) ≤ 0.064). This data indicates that Torak rocks are highly fractionated calk-alkaline (HFCA) and co-genetic granites having evolved by assimilation-fractional crystallization (AFC). Similar to the surrounding GIIb granites of the Taourirt province (Azzouni-Sekkal & Boissonnas in Bulletin Socièté Géologique France 164: 597–608, 1993; Azzouni-Sekkal et al. in J Afr Earth Sci 37: 337-350, 2003), a similar mixed deep source (asthenosphere + old lower crust) can be described here. The country rocks could correspond to the old Archaean–Paleoproterozoic LATEA metacraton. The emplacement of the Torak pluton could be linked to the Murzukian intracontinental orogenic phase that occurred in the eastern part of the Tuareg Shield (Fezaa et al. in Precambrian Res 180: 299-327, 2010).