Gabbro and orthogneiss

Unit symbol: Pzgb
Age range late Paleozoic (419.2 to 252.2 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Medium- to coarse-grained, massive to foliated, altered gabbro, leucogabbro, and minor diorite in sills, dikes, and discordant plutons (Richter, 1976; MacKevett, 1978; Winkler and others, 1981) in the Nabesna, McCarthy and Valdez quadrangles. These rocks are similar to the more mafic part of the diorite complex of unit P*gi, described below, and yield similar radiometric ages; K/Ar on biotite of 271±11 Ma (MacKevett, 1978) and U/Pb on zircon of 307±2 Ma (Plaker and others, 1989). Gabbroic parts of the unit in the McCarthy quadrangle were interpreted by MacKevett (1978) to be part of the oceanic crust that underlies the Skolai arc. In the Valdez quadrangle, however, as the unit is emplaced at a stratigraphic level near the base of rocks equivalent to the lower Permian Hasen Creek Formation (Winkler and others, 1981, unit Peh here), the Devonian zircon age, if interpreted as an emplacement age, is problematic. Unit also includes a mass that is predominantly medium-grained leucocratic gabbro and hornblende gabbro intimately mixed with altered anorthositic rocks in the Nabesna quadrangle north of the Denali Fault System and in the adjacent Yukon. This gabbro has a hypidiomorphic texture and consists of calcic plagioclase, clinopyroxene, and hornblende; anorthositic rocks lack clinopyroxene, but locally contain abundant segregations, lenses, and wavy bands of fibrous tremolite-actinolite and minor clinozoisite (Richter, 1976)

Source map information

Source map MacKevett, Jr., E.M., 1978, Geologic map of the McCarthy quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1032, 1 sheet, scale 1:250,000.
Symbol IPgo
Unit name Gabbro and Orthogneiss
Description Interlayered quartz monzonite gneiss, monzonite gneiss, and associated altered and foliated gabbro, with minor dioritic phases. Gabbro and orthogneiss extend into Valdez Quad.
Lithology Igneous

Correlated geologic units

Label IPgb
Description Gabbro and leucogabbro, Late Paleozoic, Permian to Pennsylvanian
Geologic age Pennsylvanian to Permian
Geologic setting Intrusive
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Gabbroic < Plutonic < Igneous Dike or sill Indeterminate, major
Gabbroic < Plutonic < Igneous Pluton Indeterminate, major
Orthogneiss < Metaigneous < Metamorphic Indeterminate, major