Writers : Samani, Bahram
Refference : Geosciences Scientific Quarterly Journal,No.:6,P.:32
Publishing Year : 1370

Abstract :
The Chapedony,Boneh Shurow, Posh-e-Badam and Sarkuh Complexes, as welt as Tashk 1 and Tashk
2 formations and Morad Series have been considered as the oldest rock units in Central lran,regardless of paleontological or isotopic age determinations formation time of the aforesaid rock units and tectonic events were considered to be early to late Precambrian period.
From integration of different data sourcers such us analysis of MSS and TM images, isotopic age measurements,stable isotope analyses,airborne and ground gamma spectrometric and magnetic surveying,and field observations, the following points are concluded and a new stratigraphic subdivision is established.
The oldest exposed unit is a sequence of submature, clastic deposits with intracalations of carbonate layers, resembling continental slope facies.The best exposure of this unit is located on Kuh-e-Natk, northwest of Saghand Uranium district from where the name of Natk formation is obtained for this sequcnce.The metamorphosed and non-metamorphosed units of Bonehshurow-Chapedony complexes, Tashk 1. and Tashk 2 formations also represent the Natk formation that got deformed,intensely metamorphosed and granitized by Cimmerian and early Alpine orogenies.
The Natk formation is overlain by a riftogenic sequence with an angular unconformable contact. This sequence is named Saghand formation, after its exposure locality in Saghand Uranium disti-ict.Saghand formation consists mainly of riftogenic sequence commencing from basal conglomerate as debries flow (accumulation facies),bimodal volcanic lava followed by exhalative hydrothermal sediments and evaporite deposition at the end. This formation was subjected to intense Na-metasomatism, hydrothermal alterations, alkali magmatism, REE,U,Th,Fe and apatite ore formation; related to enriched- mantle- type magmatic activities.According to the isotopic age measurements, the Saghand formation has been dated 750-580 my old. This formation is overlin by non-metasomatized sequences of clastic rocks,carbonatic beds, pyroclastics, K- Lava flows, and evaporite deposits which hosted Pb-Zn and Fe-Mn ore deposits (subsidary rifting facies).There is an unconformable contact between Saghand formation and overlying units which is characterized by reworked fragments of metasomatic rocks derived from Saghand formation.
ft is concluded that the oldest sialic crust of Iran was formed during upper Precambrian (about 1000
m.y.) which was covered by green schist facies metamorphosed continental slope-type submature sediments before 874-750 m.y. Two different facies of rock units were deposited over the old basement:
1-The riftogenic sequence of Saghand formation in central Iran, and probably Hormoz Series in Zagros
domain that resulted from tectonomagmatic reactivation and development of taphrogenic structures.
2- Epi-continental platform- type sediments of Bayandor and Soltanieh formations which may have a non-conformable contact with underlying basement.
Subject List : Riftogenic facies