Mafic schist and amphibolite

Unit symbol: Pzyms
Age range Devonian or older Paleozoic (541 to 358.9 Ma)
Lithology: Metamorphic
Group name: Gneiss, amphibolite, schist, quartzite, and marble (Yukon-Tanana crystalline complex)
“Green, quartz-chlorite-carbonate schist, commonly having abundant plagioclase porphyroblasts. Associated with amphibolitic schist and minor marble, quartzite, and pelitic schist. Thought to represent metamorphosed mafic pyroclastic rocks interbedded with schists of unit |<yqs” in the Circle quadrangle (Wilson and others, 1998; see also Foster and others, 1983). In the south-central Circle and adjacent Big Delta quadrangle unit varies from greenish-gray actinolitic greenschist to dark green, fine- to coarse-grained, schistose amphibolite of unit |<yqm, which is interlayered or interlaminated with muscovite-feldspar-quartz felsite (Smith and others, 1994). In the southeast Big Delta quadrangle, unit is similarly composed of dark-green, fine- to medium-grained, strongly foliated, hornblende-biotite amphibolite gneiss associated with augen gneiss of unit MDag or interlayered with foliated, medium-grained, equigranular calc-silicate schist and, locally, quartzite of unit |ymi (Day and others, 2007). In the southeastern Eagle quadrangle, unit consists of moderately- to nonfoliated, fine- to medium-grained hornblende metagabbro and metadiabase (Werdon and others, 2001).Unit also contains a green and white, medium- to coarse-grained, slightly- to nonfoliated metaigneous rock that has pseudomorphs after clinopyroxene and plagioclase of fine-grained actinolite, chlorite, epidote, clinozoisite, albite, and (or) sphene (Szumigala and others, 2002)

Source map information

Source map Werdon, M.B., Newberry, R.J., Szumigala, D.J., and Pinney, D.S., 2001, Geologic map of the Eagle A-2 quadrangle, Fortymile mining district, Alaska: Alaska Division of Geological and Geophysical Surveys Preliminary Interpretive Report 2001-3A, 1 sheet, scale 1:63,360.
Symbol uPzmg
Description Moderately- to non-foliated, fine- to medium-grained, hornblende metagabbro and metadiabase. The bulk of this unit is interpreted to represent a volcanic arc upon which more extrusive volcanic-dominated (uPzv) and sediment-dominated (uPzst, uPzl) units were deposited. Possibly a sill- or dike-swarm, as suggested by fine-grained intra-unit contacts and common occurrence of this unit as sills or dikes in unit uPzst. These rocks are nearly indistinguishable in major- and trace-element composition from the metabasalt of unit uPzst. In thin section, these rocks display a variably strong greenschist facies overprint, with primary igneous (?) hornblende and clinopyroxene replaced by fine-grained actinolite + chlorite + calcite and plagioclase replaced by albite + carbonate + sericite. Magnetic susceptibility is usually high, 5–20 x 10 SI, but considerably lower where sheared. Abundant dikes of Taylor Mountain batholith-type quartz diorite are locally present in this unit on both sides of the South Fork of the Fortymile River east of Chicken; however, abrupt, sheared contacts suggest that original intrusive contacts have been displaced by high-angle faults. Similarly, although dikes and sills of metagabbro/metadiabase are present in unit uPzst, the contact between uPzmg and uPzst is mostly faulted. Amphibole from uPzmg and from a dike of this unit in greenstone yielded disturbed Ar/ Ar spectra, with highest-temperature fraction ages of 227 and 225 Ma and pseudo-plateau ages of 213 and 201 Ma (Cushing, 1984). The spectra indicate a pre-mid Triassic magmatic age with a thermal reset caused by intrusion of the Taylor Mountain Batholith and younger plutons
Lithology Metamorphic

Correlated geologic units

Label PzZms
Description Mafic schist
Geologic age Paleozoic to Devonian
Geologic setting Metamorphic, undivided
Lithology Form Importance
Calc-silicate-schist < Schist < Metamorphic Greenschist Major
Amphibole-schist < Schist < Metamorphic Greenschist Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Greenschist Minor
Quartzite < Metaclastic < Metasedimentary < Metamorphic Greenschist Minor
Quartz-feldspar-schist < Schist < Metamorphic Greenschist Minor