A moderate exhumation rate for the high‐pressure Maksyutov Complex, southern Urals, Russia

Abstract

AbstractThe high‐pressure Maksyutov Complex in the southern Urals contains a regionally coherent crustal unit that experienced peak metamorphic conditions of ∼2 GPa (∼60 km) and ∼550°C at 378 ± 6 Ma. Exhumation of the high‐pressure unit occurred under retrograde blueschist and middle/lower greenschist facies conditions, indicating a pronounced cooling during decompression. The rise of the high‐pressure unit to a mid‐crustal depth (∼20 km) was accommodated along retrograde ductile shear zones that formed in quartz‐rich lithologies. Rb‐Sr white mica ages from these quartzitic mylonites cluster at ∼360 Ma, while Ar‐Ar ages are more variable, between 370 ± 7 and 344 ± 7 Ma. We interpret the Rb‐Sr ages to date the mylonitic deformation in the retrograde shear zones and infer that by 360 ± 8 Ma the high‐pressure Maksyutov Complex has reached mid‐crustal depth. The pressure–temperature–time data suggest an average exhumation rate of 2 (+2/−0.5) mm a−1 for the first exhumation stage from 60 to 20 km. Using our Rb‐Sr data and published fission‐track data, a slower exhumation rate of 0.3 ( ± 0.1) mm a−1 is inferred for the rise of the high‐pressure rocks from the middle to the upper crust. The moderate exhumation rate of 2 mm a−1 is in accordance with published thermal models, which demonstrate that a relatively slow exhumation is required for high‐pressure rocks to experience syn‐decompressional cooling. Such syn‐decompressional cooling, as inferred for the Maksyutov Complex, contrasts with isothermal decompression observed in many other regional high‐pressure terrains that experienced a relatively fast exhumation. The two main factors that controlled the unusual thermal evolution of the southern Urals may be the low radiogenic heat production of the crust and the stable, ∼200 km thick lithospheric mantle beneath the orogen. Copyright © 2000 John Wiley & Sons, Ltd.