Schist of the Telsitna River

Unit symbol: Pxqm
Age range Proterozoic (1600 to 541 Ma)
Lithology: Metamorphic
Group name: Basement of the White Mountain sequence
Chiefly pelitic and quartzose metasedimentary rocks of greenschist facies exposed in west-central Alaska in the northeastern Medfra, southeastern Ruby and southwestern Kantishna River quadrangles. Micaceous quartzite and quartz-chlorite schist grade into quartz-muscovite-biotite-garnet schist; calc-schist and marble are subordinate. Unit locally includes small bodies of greenstone and greenschist-facies metabasite, granitic gneiss, and metamorphosed quartz porphyry. The porphyry is fine-grained, foliated, and banded felsic volcanic rock composed of large phenocrysts of embayed quartz and plagioclase in a very fine-grained quartzo-feldspathic groundmass that has a distinct micaceous overprint. Zircon fractions from two suites of samples of metamorphosed quartz porphyry yielded upper intercept ages of 850±30 Ma and 1,265±50 Ma, interpreted to be crystallization ages (Patton and others, 1980, 2009). Metamorphic minerals from the schist yield K/Ar isotopic cooling ages that range from 921 to 284.2 Ma (Patton and others, 2009), which indicate that this terrane of metamorphic rocks likely did not experience the mid-Cretaceous metamorphism commonly seen in other metamorphic rocks in Alaska

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 Pxs
Unit name Pelitic schist, quartzite, quartzo-feldspathic gneiss, and greenstone
Description Chiefly pelitic and quartzose metasedimentary rocks of greenschist metamorphic facies. Micaeous quartzite and quartz-chlorite schist grade into quartz-muscovite-biotite-garnet schist. Subordinate calc schist and marble. Unit locally includes small bodies of greenstone and greenschist metabasite, granitic gneiss, and metamorphosed quartz porphyry. North-central Medfra quadrangle and southeast Ruby quadrangle
Lithology Metamorphic

Correlated geologic units

Label YZns
Description Pelitic schist of Nixon Fork basement
Geologic age Mesoproterozoic to Neoproterozoic
Geologic setting Metamorphic, undivided
Lithology Form Importance
Metasedimentary < Metamorphic Greenschist Major
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Major
Quartz-feldspar-schist < Schist < Metamorphic Greenschist Major