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In Central Iran, the mixed siliciclastic-carbonate Nakhlak Group of Triassic age is commonly seen to have a Cimmerian affinity, although it shows considerable resemblances with the Triassic Aghdarband Group in far northeastern Iran, east of Kopeh-Dagh area, with Eurasian affinity. The Nakhlak Group is composed of the Alam (Late Olenekian-Anisian), Baqoroq (Late Anisian-?Early Ladinian), and Ashin (Ladinian-?Early Carnian) formations consisting mainly of volcanoclastic sandstone and shale and fossiliferous limestone. The Baqoroq Formation contains also metamorphic detritus. Sandstone petrofacies reflect the detrital evolution from active volcanism to growing orogen and again active volcanism. Textural and modal analyses of volcanic lithic fragments from the Alam Formation reflect the eruption style and magma composition of a felsic to intermediate syn-sedimentary arc activity. The

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Com bined petrographic and geo chem i cal data of the siliciclastic sed i men tary rocks from the Shemshak Group in the northeast ern Alborz Moun tains, north of Iran are de scribed, to gether with their im pli ca tions for palaeoweathering, their prov enance, and tec tonic set ting. Based on field ob ser va tions and modal com po si tion, the sand stones are clas si fied as litharenites. The chem i cal in dex of al ter ation (CIA) in di cated that the source ter rains un der went a mod er ate in ten sity of chem i cal weath er ing. The in dex of chem i cal vari a tion (ICV) val ues in di cated that the Shemshak Group rocks were im ma ture and re lated to a source area with an ac tive tec tonic re gime. Ma jor, trace and rare earth el e ment (REE) data sug gested the dom i na tion of mixed sed i men tary (re cy cled) and ig ne ous rocks in the source area of the Shemshak Group. Petrographic and geo chem i cal char ac ter is tics of Shemshak Group rocks sug gest an ac tive con ti nen tal mar gin (ACM), which cor re sponds to the col li sion of the Iran plate with the Turan plate.