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., Werdon, M.B., and Bleick, H.A., 2011, Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Scientific Investigations Map 3131, 2 sheets, scale 1:500,000, 1 pamphlet, 75 p., and database, available at http://pubs.usgs.gov/sim/3131/.
Symbol PzZh
Unit name High-grade metasedimentary and metaigneous rocks, High-grade Metamorphic and associated igneous rocks
Description Brown, light brown, reddish brown, black, and gray-weathering schist and gneiss exposed in the Kigluaik, Bendeleben, and Darby Mountains. Highest-grade metamorphic assemblages are upper amphibolite to granulite grade; assemblages in all three ranges record multiple metamorphic events. Metamorphic foliations range from gneissic to schistose; locally, foliation is lacking and crystallization of metamorphic minerals appears to be static (Till and others, 1986; Lieberman, 1988; Calvert and others, 1999). Lithologically variable on a scale of centimeters and meters, the unit includes pelitic, semi-pelitic, quartzose, calcareous, aluminous, mafic, ultramafic, and graphitic schist and gneiss. Dominant lithologies vary within each mountain range. Till and others (1986) recognized packages of rocks lithologically similar to those mapped within the Nome Complex in the western Bendeleben and northern Darby Mountains. Thurston (1985), Patrick and Lieberman (1988), Hannula and others (1995) and Calvert and others (1999) showed that the high-grade metamorphic event in the Kigluaik Mountains overprinted blueschist-facies assemblages in the Nome Complex. At least part of |<h, perhaps a significant part, was originally blueschist-facies rocks of the Nome Complex. Metamorphic foliations and lithologic layering in the Kigluaik Mountains define a dome, with highest-grade rocks in its core. The earliest-formed metamorphic assemblages known in the range are found at the base of the section, below Mt. Osborn. There, garnets in pelitic rocks locally contain kyanite inclusions (Lieberman, 1988); garnet lherzolite occurs as large, meter-scale xenoliths in pegmatite and is present in abundance in glacial moraine (Till, 1980, 1981; Lieberman and Till, 1987). Garnet lherzolite is stable at eclogite facies (Evans, 1977; Spear, 1993). The dominant metamorphic fabric in the Kigluaiks post-dates this high-pressure event and contains peak granulite-facies assemblages. The garnet lherzolite is partially overprinted by spinel-bearing assemblages stable at granulite facies. Two-pyroxene semipelitic and mafic gneiss, diagnostic of the facies, are also found in the core of the range (Till and Dumoulin, 1994). Most of the rocks in the range crystallized at temperatures above the second sillimanite isograd (above the temperature stability of muscovite; Sheet 2; Amato and Miller, 2004). 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 isograds along the flanks of the dome are closely spaced (Till, 1980; Patrick and Lieberman, 1988; Miller and others, 1992; 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). West of the dome, southwest of Mt. Bendeleben, kyanite-bearing assemblages predate sillimanite-bearing assemblages in pelitic rocks (Sheet 2; Till and Dumoulin, 1994; Till, unpublished data). The kyanite-bearing assemblages are apparently the oldest in the range. Metamorphic rocks in the Darby Mountains lack any domal structure; instead, map-scale folds of lithologic packages with near-vertical axial planes are present where rocks exhibit higher-grade assemblages (Till and others, 1986). In the northern Darby Mountains, units of the low-grade Nome Complex are shallowly-dipping and partially overprinted by biotite-grade metamorphic assemblages (Till and others, 1986). Southward along the crest of the range, kyanite- and staurolite-bearing assemblages occur. The kyanite is texturally older and may be relict of an earlier metamorphic event. Further south, metamorphic grade increases with the appearance of sillimanite and locally sillimanite plus K-feldspar in pelitic schists. Metamorphic grade culminates near Mt. Arathlatuluk. There, rare two pyroxene schists are diagnostic of granulite facies, and small granitic bodies appear to be anatectic melts (Sheet 2). The anatectic granites were formed around 102 Ma (Till and Dumoulin, 1994). A fault-bounded block of high-grade rocks on the western flank of the Darby range contains similar high-grade metamorphic assemblages (formed above the second-sillimanite isograd) (Sheet 2). 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 (Sheet 2). Corona or symplectite textures are common in these rocks. The aluminosilicate plus orthoamphibole assemblage is stable at moderate to high pressures, above about 5 kb, and the cordierite-bearing assemblage is stable at low pressures, below about 5 kb (Spear, 1993). 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 100-106-Ma Darby pluton (Amato, written commun., 2008) cross-cuts the metamorphic gradient in the Darby range; andalusite formed in its contact aureole (Sheet 2). Andalusite forms at pressures lower than 4 kb. These high-grade metamorphic rocks are equivalent to generalized units "PzpCh", "PzpCg", "PzpCg" and units representing upgraded Nome Complex "Oimh", "Ocsh", "OC-xh" and "CpCsh" of Till and others (1986)
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