The Kabul Block (Afghanistan), a segment of the Columbia Supercontinent, with a Neoproterozoic metamorphic overprint
Abstrakt
We report field relationships, petrography and isotopic ages from two superposed basement units of the Kabul Block, the so called Lower Sherdarwaza and Upper Welayati formations. The Sherdarwaza Formation is represented mostly by migmatites and gneisses that are derived from pelitic and psammitic lithologies with lenses and layers of mafic and carbonate rocks.
Several bodies of orthogneisses are also exposed in the Sherdarwaza Formation. The Upper Welayati Formation is characterized by micaschist, quartzite and amphibolites.
SHRIMP U-Pb data on zircon from the orthogneiss in the Sherdarwaza Formation indicates a Neoarchean age of ca 25-2.8 Ga for their magmatic crystallization. The rocks exhibit granulite facies conditions of 5-7 kbar and 800 degrees C that are documented by the presence of orthopyroxene and Ti-rich biotite in the orthogneiss and by olivine and phlogopite in some talc-silicate rocks at contact with marble.
A Paleoproterozoic age of ca. 1.85-1.80 Ga for this metamorphism was obtained using U-Pb SHRIMP dating on zircon and U-Th dating on monazite. Mineral textural relations also show a younger amphibolite facies metamorphism that is documented in both the Sherdarwaza and Welayati formations.
This metamorphism occurred at relatively higher pressure conditions of up to 9 kbar at ca. 650 degrees C, compared to the granulite facies event. A Neoproterozoic age of ca 0.85-0.9 Ga, for this metamorphism is confirmed by Ar-Ar data on biotite and white mica as well as by U-Th data on monazite.