THE TESSALIT RING COMPLEX (ADRAR DES IFORAS, MALIAN TUAREG SHIELD): A PAN-AFRICAN, POST-COLLISIONAL, SYN-SHEAR, ALKALINE GRANITE INTRUSION

摘要

The Tessalit ring complex (600 km~2) belongs to the Adrar des Iforas late Pan-African alkaline province. It is located near the shear zone delimiting the western border of the Kidal terrane, one of the 23 terranes so far recognized in the Tuareg shield. Intruded beneath a thick rhyolitic cover and alternating with N-S striking dykes, it is composed of four main intrusive centres (Wartoradj, Tin Edemba, Tin Atiwen and Tiraouanine). Each centre comprises several phases, metaluminous or peralkaline, subsolvus or hypersolvus. Composed as a mean by more than 50 % of perthitic K-feldspar and 20 to 40 % of quartz, nearly all these facies are alkali feldspar granites. Clinopyroxene displays compositions from augite to aegirine through hedenbergite showing typical enrichment in Fe followed by enrichment in Na. Amphibole displays a complete range of composition between early hornblende and actinolite to late arfvedsonite and riebeckite. The Tessalit complex is dated by the Rb-Sr method at 553 ±8 Ma (Sr_i= 0.7059 ±0.0004) which is in the middle of the Iforas alkaline province ages. This indicates a late Pan-African age and precludes any spatial age trend. The Sr_i is also within the Iforas alkaline values (mantle origin with lower erust contamination). Major and some trace elements geochemistry allows to charaeterize both metaluminous and peralkaline facies, both belonging to the alkaline family. The Tessalit ring complex, an early post-collisional Pan-African pluton, possesses different structural features recording the effects of the stiil mobile, recently amalgamated terranes of the Tuareg shield, in particular movements along mega-shear zones. The local extension shown by the permissive type of emplacement can easily be integrated in the regional transpressive context recently proposed for the Tuareg shield formation.