Writers : Samani, Bahram; Grasso, Tao; Koan, Tao
Refference : Geosciences Scientific Quarterly Journal,No.:10,P.:40
Publishing Year : 1372

Abstract :

from the remote sensing (TM, MSS), and geophysical data interpretation, isotopic geology and field surveying in Central Iran it is proved that the formerly named Tashk formation, Chapedony and Boneh Shurow complexes originally were a unique miogeosynclinal- type clastic sequence. This rock unit is named Natk formation which is 874- 750 m.y. old (Sm- Nd isotopic age calculated from REE depleted- type basalt model).
The oldest sialic crust (continental margin- type) is 1300- 1000 my. in Iran, where the Natk formation with continental slope facies deposited over it.
The geological evolution of Upper Precambrian is defined as the following:
a)- The Natk formation was formed on Upper Precambrian (> 874- 750 my.). This unit is a mature clastic rocks that metamorphesed in greenschist facies grade. K- type granites were intruded to this formation. The measured isotopic ages (Zircon, Uraninite) are 850- 800 my. which is characteristics of a mature sialic crust consolidation. Granodioritic magmatism occurred in 749! 97 m.y. (Pb 207/Pb 206) and followed by 624± 5 m.y. (isochrons of Pb 207/U 235 and Pb 206/U 238) granitization.
b)- Development of Saghand formation with riftogenic facies, consists of five lithologic members. This rock Unit is unconformably underlained by Naik formation. This formation has peculiarities and features of intracontinental rift, where Fe/UI Th, FlEE, and apatite deposits were formed. This major rifting started not later than 750 m.y. but ended by 583 my. It is characterized by enriched- mantle magmatism, extensive linear metasomatism, and related ore formation.
c)- Alkaline metasomatism, and granitization interrupted after 583 my. by uplifting and development of erosional surface. The subsidiary rifting caused high K-volcanism (561 ± 73 m,y.) which was derived from an enriched- mantle, grading into normal- type. This stage of rifting is characterized by volcanics, evaporites, shales, clastics, and carbonates with Vendian- early Cambrian age. This sequence is named Desu Series and Soltarrieh Dalomite after stratigraphic correlation.
d)- The only recorded metamorphism of Upper Precambrian age is older than Saghand formation. The main components of conglomeratic beds at the base of Saghand formation show the greenschist fades metamorphism, like the Natk formation.
e)- The Natk, Saghand and Desu rock Units were metamorphosed on the western part of Bafq- posht-e Badam Block on Mesozoic and early Cenozoic time. The former one is high pressure- type and could be related to the detachment faulting of late Triassic- Early Jurassic time. The later dynamothermal high grade metamorphism occurred at 52 my. (K/Ar, Zircon minerals, Rb/Sr), as a result of Neotethys oceanic crust subduction beneath the central continental crust. Granitization, migmatization and high grade metamorphism ol Khoshumi, Neibaz and Chapedony is related to this stage of deformation. Finally, it is concluded that, two types of uppermost Precambrian rock units crop out in the region: 1) riftogenic series, and , 2) platform- type cover namely Saghand formation and Desu Series for first type and Barut- Soltanieh Formations as platform cover.

Subject List : Stratigraphy