High-grade metamorphic rocks of the Seward Peninsula

Unit symbol: PzPxkg
Age range earliest Paleozoic to Proterozoic (1600 to 485.4 Ma)
Lithology: Metamorphic
Group name: High-grade metamorphic rocks of the Seward Peninsula
Consists of brown, light-brown, reddish-brown, black, and gray-weathering schist and gneiss, in part derived from blueschist-facies rocks of the Nome Complex (PzPxnc) that have undergone another metamorphic episode (Till and others, 2011). Exposed in the Kigluaik, Bendeleben, and Darby Mountains on the Seward Peninsula, assemblages in all three ranges record multiple metamorphic events; the highest event reached upper amphibolite to granulite grade. Metamorphic foliation is gneissic to schistose, though locally no foliation is apparent (Till and others, 2011). The unit is lithologically variable on a scale of centimeters and meters; and lithology also varies between the three mountain ranges, but generally includes schist and gneiss of pelitic, quartzose, calcareous, mafic, graphitic, and other compositions (Till and others, 2011). “Till and others (1986) recognized sequences of rocks lithologically similar to those mapped within the Nome Complex in western Bendeleben and northern Darby Mountains. * * * Metamorphic foliations and lithologic layering in the Kigluaik Mountains define a dome, with highest-grade rocks in its core. Metamorphic grade decreases towards the flanks of the dome, where biotite-grade metamorphic assemblages overprint low-grade metamorphic assemblages of the Nome Complex (Thurston, 1985; Hannula and others, 1995; Amato and Miller, 2004). * * * Metamorphic foliations in the Bendeleben Mountains define a dome that spans the area between the large, ovoid pluton in the eastern part of the range and the smaller, irregularly shaped pluton in the west part of the range; the dome coincides with sillimanite-bearing peak thermal assemblages (Gottlieb and Amato, 2008). * * * Metamorphic rocks in the Darby Mountains lack any domal structure; instead, map-scale folds of lithologic sequences with near-vertical axial planes are present where rocks exhibit higher-grade assemblages (Till and others, 1986). * * * In all three mountain ranges, decompression post-dated the thermal peak, and is recorded in aluminum- and iron-rich metasedimentary rocks. In these volumetrically minor but significant rocks, assemblages containing sillimanite or kyanite (± hercynite spinel) and orthoamphibole were overprinted by assemblages containing cordierite and staurolite or garnet. * * * While decompression assemblages apparently formed at about 82 Ma in the Bendeleben range (Gottlieb and Amato, 2008), similar assemblages in the Darby range are probably older: the 100-Ma Darby pluton cross-cuts the metamorphic gradient in the Darby range * * *” (Till and others, 2011). Associated with the metamorphic rocks of the Bendeleben Mountains are foliated lenses and sill-shaped bodies, thought to be metavolcanic rocks, that are exposed in outcrop and rubble fields in the southwest part of the mountains. According Amato and others (2009), contacts and internal foliation are conformable to the foliation of the enclosing metamorphic rocks. Till and others (2011) report that these are the oldest dated rocks on Seward Peninsula: a U/Pb date from zircon gave a protolith age of 870 Ma (Gottlieb and Amato, 2007, 2008; Amato and others, 2009)

Source map information

Source map Till, A.B., Dumoulin, J.A., Gamble, B.M., Kaufman, D.S., and Carroll, P.I., 1986, Preliminary geologic map and fossil data, Solomon, Bendeleben, and southern Kotzebue quadrangles, Seward Peninsula, Alaska: U.S. Geological Survey Open-File Report 86-276, scale 1:250,000.
Symbol PzpCh
Unit name High-grade schist, undivided
Description Metasedimentary and metaigneous schist and gneiss above biotite grade; contains rocks which may be equivalent to units described above
Lithology Metamorphic

Correlated geologic units

Label PzZh
Description Kigluaik Group high-grade schist
Geologic age Neoproterozoic to Cambrian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Schist < Metamorphic Amphibolite Major