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 Till, A.B., Dumoulin, J.A., Harris, A.G., Moore, T.E., Bleick, Heather, and Siwiec, Benjamin, 2008, Bedrock geologic map of the southern Brooks Range, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2008-1149, 88 p., 2 sheets, scale 1:500,000.
Symbol Da
Unit name Ambler sequence
Description Interlayered white to medium gray weathering metarhyolite, dark green weathering metabasite, pale gray weathering marble, and brown to dark gray weathering calcareous, pelitic and carbonaceous schist exposed in two areas in the Schist belt. "Metarhyolite porphyries" with megacrysts of feldspar up to 5 cm across and quartz quartz eyes up to 1 cm across are typical of the unit. "Aphanitic metarhyolite" layers and lenses are characteristic and show rare flow banding, breccia textures, and possible welded shard textures. Metabasites occur as pods and lenses; exposures in the Ambler River quadrangle retain remnant pillow structures. Near the Ambler River Survey Pass quadrangle boundary, where it has been studied in the most detail, the unit is thought to be 700-1,850 meters thick ; there, massive sulfide deposits are associated with the metarhyolites, including the world-class Arctic deposit. This unit is part of the Schist belt.
Lithology Metamorphic

Correlated geologic units

Label Dav
Description Ambler metavolcanic and lesser metasedimentary rocks
Geologic age Devonian
Geologic setting Extrusive
Lithology Form Importance
Metavolcanic < Metaigneous < Metamorphic Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Quartz-feldspar-schist < Schist < Metamorphic Indeterminate, major