Surface signals of an arc-continent collision: The detritus of the Upper Devonian Zilair Formation in the Southern Urals, Russia

Laser 40Ar/39Ar ages of single detrital white mica grains related to the exhumation of Neoproterozoic and Late Devonian high pressure rocks in the Southern Urals (Russia)

“…The resulting age/composition pattern can corroborate and refine previous provenance studies (Willner et al , 2002a(Willner et al , 2003 and yield more detailed information about the surface composition during the already well-known Neoproterozoic and Early Devonian exhumation histories of the two highpressure metamorphic complexes representing part of the source areas. The scope of this paper is to obtain single grain Ar-Ar ages from detrital white mica grains of known composition, including phengite and muscovite, from these two siliciclastic formations.…”

“…The spread of ages from the four analysed samples collected from different stratigraphic and regional positions within the Zilair Formation is similar, suggesting a similar composition and source of the Zilair detritus in space and time (Willner et al 2002a). Although our ages have large errors, most of the phengites of the younger age range of 342-421 Ma seem to be older than the estimated time of peak metamorphism at 375 ± 2 Ma (Glodny et al 2002) of the high-pressure rocks of the current lower Unit 1 in the Maksyutovo Complex.…”

“…1; see Willner et al 2001 for petrography) were selected. In both formations the detrital white mica populations contain muscovite and phengite (Willner et al , 2002a. Samples Z15 and Z19 are from the Middle Zilair Formation, west of the Main Uralian Fault, and samples Z8 and Z28 are from the Lower Zilair Formation to the east of the Main Uralian Fault ( Fig.…”

“…1). Willner et al (2002a) suggested that ophiolites of the Main Uralian Fault zone and metamorphic complexes to the west are the prime source, as well as the volcanosedimentary series of the Magnitogorsk arc. The metamorphic source contained low-to medium-grade rocks including high-pressure rocks as indicated by detrital phengite crystals with Sicontents up to 3.45 per fomula unit (p.f.u.)…”

“…The detritus is composed of (1) mainly metamorphic lithoclasts and heavy minerals of metamorphic origin (epidote, garnet, tourmaline, Ca-amphibole, glaucophane, chloritoid, titanite and rutile) and to a minor extent by (2) volcanic lithoclasts of a calc-alkaline source, (3) a few serpentinite and chert lithoclasts as well as abundant Cr-spinel from an ophiolithic source, and (4) intraformational sedimentary lithoclasts. This time period (∼ 376-354 Ma, maximum absolute age interval for the Famennian: Tucker et al 1998) represents surface uplift of a narrow ridge or axial rise in the suture zone between the collisional accretionary prism in the west and the forearc basin in the east during exhumation of highpressure rocks (Willner et al 2002a). and rare detrital glaucophane.…”