On unifying of the Lut and Afghan blocks which was occurred along the Sistan suture zone at late Eocene (Tirrul et al.1983), Birjand-Zahedan marginal sea basin was closed (McCall 1985) and subsequently the Paleogene terrigenous marine sedimentary rocks of Sefidabeh forearc basin were highly defbrmed and uplifted (Tirrul et al. 1983). After this event, during 32 to 27 Ma ago, a potassic magmatism appeared along transcurrent faults. The results of this magmatism are exposed in some regions (Camp & Griffis 1982)Such as: kuh-e Lar,kuh-e Malek Siah, Kuh-e Assagi. Hormak region and some other places.
Petrographic and geochemical investigations have shown that these various rock types are petrogenitically related and a shoshonitic, lamprophyric magmatism (Rock 1987) can be infirred (Bagheri & Bakhshi 2001. Results also provide role of fractional crystallization, magma mixing. contamination and degassing processes during shallow level evolution of the magmatic system (O’Brien et al.1991 ) in this region. On the other hand, geochemical evidences such as enrichment in K. Ba. Sr. LREI and volatiles and depletion in Rb, Tb, U, Nb. Ta, and I IFSE suggest a replenished. subconti nental, lithospheric mantle wedge dun ng suhd uction process before late Eocene.This postcollisional magmatism has tightly relations to middle Alpine transtentional tectonism and shows which subduction has not directly rolled in the genesis of this group of Shoshonites.