Bimodal metavolcanic rocks

Unit symbol: Das
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Metamorphic
Group name: Bimodal metavolcanic rocks
Interlayered white- to medium-gray-weathering metarhyolite and dark-green-weathering metabasite, exposed primarily within the Schist belt of Till and others (2008a) in the Brooks Range. Unit also includes minor pale-gray-weathering marble and brown- to dark-gray-weathering calcareous, pelitic, and carbonaceous schist. Unit occurs as large lenses interfolded within unit D<aqm and D<acs. Generally known as the Ambler sequence of Hitzman and others (1982) or Ambler metavolcanic rocks, it is best studied near the boundary of the Ambler River and Survey Pass quadrangles, largely because of the presence of the world-class Arctic mineral deposit (Hitzman and others, 1986; Schmidt, 1986), but a much larger area of exposure occurs in the Wiseman quadrangle (Dillon and others, 1986). The felsic part of the unit is characterized by porphyritic metarhyolite that has megacrysts of feldspar and quartz eyes, as well as aphanitic metarhyolite showing rare flow banding, breccia textures, and possible welded shard textures (Hitzman and others, 1986). Hitzman and others (1986) report that metabasite occurs as pods and lenses where exposures in the Ambler River quadrangle retain remnant pillow structures. Metarhyolite, including at the Arctic deposit, has yielded U/Pb zircon ages that range from 386 to 378 Ma with one outlier at 405 Ma (see Till and others, 2008a). A conodont collection from marble in the Wiseman quadrangle yielded a Devonian age (Till and others, 2008a). Till and others (2008a) discuss megafossil collections that “have been reported but are not now considered definitive. In an abstract, Smith and others (1978) reported that poorly preserved favositid corals, crinoid columnals, bryozoans, and ichnofossils were found at a locality in the Ambler district, and assigned the rocks a tentative Middle Devonian to Early Mississippian age. The faunal assemblage was examined by two researchers. G.D. Webster (written commun. to I.L. Tailleur, 1977) indicated a Devonian age for the corals, and considered crinoid columnals to be of probable Middle Devonian and possible Devonian or Mississippian age. William A. Oliver, Jr. (written commun. to I.L. Tailleur, 1977) suggested that the solitary corals indicate a post-Middle Ordovician age and that possible thamnoporoid corals suggested a Silurian-Devonian age. Both paleontologists have reconsidered their findings (written commun. to A. Till, 1992), and neither now suggest a Middle Devonian to Early Mississippian age for the assemblage. Therefore, the age reported in Smith and others (1978) and derivative publications (for example, Hitzman and others, 1982; 1986) can not be supported by megafossil collections.” However, Till and others (2008a) report Devonian conodonts and interpret, given this and available radiometric ages, that the unit is at least in part Devonian

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Df
Unit name Felsic schist
Description Schistose felsic volcanic rocks, including flows, tuffs, and porphyritic hypabyssal? rocks, interlayered with metasedimentary rocks belonging to units PzPxsa and Pzca. An important component of this unit is the widely recognized "button schist" -a porphyritic rhyolite composed of quartz megacrysts and albite porphyroblasts in a groundmass of quartz, potassium feldspar, and muscovite. Occurs as thin layers in pelitic schist, calc schist, quartzite, and carbonate rocks in Wiseman and Survey Pass quadrangles
Lithology Metamorphic

Correlated geologic units

Label Davf
Description Very light gray to brownish red weathering resistant schistose felsic volcanic rocks, Ambler sequence?
Geologic age Devonian
Geologic setting Extrusive
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Major
Schist < Metamorphic Minor