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

Abstract

The 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 AE 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 AE 7 and 344 AE 7 Ma. We interpret the Rb-Sr ages to date the mylonitic deformation in the retrograde shear zones and infer that by 360 AE 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 / À 0.5 ) mm a À1 for the ®rst exhumation stage from 60 to 20 km. Using our Rb-Sr data and published ®ssion-track data, a slower exhumation rate of 0.3 ( AE 0.1) mm a À 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 À 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.

Protolith Signatures and Element Mobility of the Maksyutov Complex Subducted Slab, Southern Ural Mountains, Russia

“…The URSEIS project reflection seismic profile shows that that the east-dipping MUF separates the East European platform on the west from the accreted Siberian-Kazakhian terrane assemblage to the east (Fig. 1B;Brown et al, 1998;Hetzel and Romer, 2000;Hamilton, 1970). The footwall beneath the Main Uralian fault in the south Uralide orogen consists of a generally west-vergent thrust stack (Brown et al, 1998;Brown and Spadea, 1999).…”

“…The blueschist-facies Yumaguzino unit consists of white quartzite with a structural thickness of 600 m . A ductile shear zone (360 ± 8 Ma; Hetzel and Romer, 2000;Beane and Connelly, 2000) is well developed in this unit, and also juxtaposed the Kairakli and Karamola units during exhumation (Hetzel et al, 1998).…”

“…The timing of retrograde shear zone development ranges from 370 to 344 Ma, based on 40 Ar/ 39 Ar and Rb-Sr ages (Beane and Connelly, 2000;Hetzel and Romer, 2000). A weighted age estimate for retrograde shear is 360 ± 8 Ma (Hetzel and Romer, 2000). Apatite fission-track data indicate that exhumation and late-stage cooling occurred at 300 ± 25 Ma (Leech and Stockli, 2000).…”

“…White mica ages from Yumaguzino samples and from a retrograde sample of the Kairakli unit range from 355 to 365 Ma (Beane and Connelly, 2000). These ages are interpreted to reflect development of a shear zone during lower blueschist-facies conditions (Beane and Connelly, 2000;Hetzel and Romer, 2000;Leech and Ernst, 2000). The many documented fractures in this shear zone (Hetzel et al, 1998) would have facilitated fluid flow between the three units of the Maksyutov Complex.…”

“…The timing of retrograde shear zone development ranges from 370 to 344 Ma, based on 40 Ar/ 39 Ar and Rb-Sr ages (Beane and Connelly, 2000;Hetzel and Romer, 2000). A weighted age estimate for retrograde shear is 360 ± 8 Ma (Hetzel and Romer, 2000).…”