Syn-kinematic emplacement of granitic batholith and leucogranite along the extensional detachment shear zone system of the Tin Begane area, Laouni terrane (LATEA metacraton, Central Hoggar, Algeria)

Abstract

The vast Tin Begane batholith (5000 km2) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 ± 3 Ma, U–Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 ± 4, U–Pb zircon) and a fine-grained monzogranite (591 ± 6 Ma, U–Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 ± 6 Ma, U–Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleoproterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 °C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 °C) not far from the emplacement of the Tin Begane batholith.A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.