Geochemical comparison of biotite from TTG batholiths and A-type complexes on either side of the Tin-Dahar fault: Geodynamic considerations (Silet region, Western Hoggar, Algeria)

We studied the chemical composition of primary biotite (n = 265 chemical analyses, performed by electron microprobe) from some acidic plutonic rocks in the central part of the Silet terrane (Hoggar, Algeria). Which is considered an island arc accreted onto the metacraton LATEA during the climax of the Pan-African orogeny at around 650 Ma, accompanied by northward migration of LATEA, following final clamping between the West African craton and the Saharan metacraton. The plutonic acidic rocks constitute nearly 70 % from study area, essentially represented by Tonian-Cryogenian TTG batholiths and Ediacaran-Fortunian A-type granitic complexes. They cross-cut Neoproterozoic volcano-sedimentary series and are organized into two narrow stripes with opposite isotopic signatures, separated by the N-S Tin-Dahar fault: the Juvenile Western Stripe (J-WS) and the Contaminated Eastern Stripe (C-ES). We compared biotite of the western stripe (TTG + type A granitic batholiths) with that of the same rocks of the eastern stripe (C-ES). So, we selected three TTG batholiths (Tin-Tekadiouit, Ahambatou and Silet) + two A-type granite complexes (Tin-Erit and Tioueïne) from the J-WS and one TTG type (Eheli) + two A-type granite (Teg-Orak and Inedjaren) from the C-ES. The petrographic observation shows in all studied rocks of the western stripe (J-WS) that biotite is often associated with amphibole, whereas in the eastern stripe rocks (C-ES), biotite is only. Consequently, biotite from both TTG and A-type granitoids from J-WS differ in chemical compositions from biotite from C-ES. Three biotite types were determined in the studied felsic rocks: 1- Mg-biotite in TTG batholiths of the two stripes (Tin-Tekadiouit, Ahambatou, Silet and Eheli) and in the A-type granite complex of Inedjaren, located in the C-ES; 2- Fe-biotite–annite of A-type Tioueine and Tin-Erit complexes (J-WS) and 3- Fe-biotite–siderophyllite of A-type Teg-Orak (C-ES). In detail, Mg-biotite in TTG of the J-WS reflects calc-alkaline, orogenic and peraluminous host rocks, whereas, in the Eheli TTG and the Inedjaren A-type of C-ES, it reflects calc-alkaline, orogenic host rocks. Fe-biotite–annite of the A-type granite of J-WS reflects alkali-calcic to alkaline anorogenic host rocks when Fe-biotite–siderophyllite from Teg-Orak complex, located to the north of C-ES, reflects calc-alkaline to alkali-calcic rocks.

Crystallization temperatures and pressures of Mg-biotite in TTG batholiths of J-WS are lower (750–800 °C, 3–6 kbar) than those recorded in the Eheli TTG batholith (850–900 °C, 4–8 kbar) of C-ES. Mg-biotite of the Inedjaren A-type complex (800–900 °C, 3–6 kbar) of C-ES shows higher temperature than biotite of the other A-type complexes (Fe-biotite–annite trend; Fe-biotite–siderophyllite trend), yielding variable pressure values (700–750 °C, 3–10 kbar).

The three different types of biotite are emphasized by estimates of thermodynamic conditions (T, P, fO₂), which reflect the sensitivity of biotite to conditions that have controlled its crystallization and distinguish different geodynamic contexts. Our data support the subdivision of the Silet terrane (based on isotopic data) into two stripes, J-WS and C-ES, and identify two compartments within the second stripe, geographically separated by the Tahalgha Cenozoic basaltic plateau: 1- The contaminated north-east stripe (C-NES) and 2- The contaminated south-east stripe (C-SES).