Gabbronorite and other mafic and ultramafic rocks of Hidden terrane

Unit symbol: Jum
Age range Early Jurassic (201.3 to 163.5 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Complexly intermixed series of ultramafic and mafic to intermediate plutonic rocks. “Plutons consist of gabbronorite, hornblende gabbro, diorite, and tonalite. * * * Xenoliths of gabbro show ductile deformation as though they still were warm when intruded by silicic magmas and migmatite textures are common at contacts between lithologies” (Winkler, 1992). Plutons are cut by steeply dipping faults that form the northern part of the Border Ranges Fault System. Fault-bounded cumulate ultramafic and mafic rocks of the Wolverine and Eklutna complexes of Winkler (1992) are also included in unit and have an inferred age of Middle and Early Jurassic based on correlation with Tonsina complex of adjacent Valdez quadrangle and intrusion by Middle and Early Jurassic dikes (Winkler, 1992). These rocks have typically been considered part of the root of the Talkeetna arc (Burns, 1985), which includes rocks of three units: Jtk, the Talkeetna Formation; Jtr, trondhjemite; and Jgr, the Jurassic phase of the Alaska-Aleutian Range batholith. An alternative explanation associates these rocks with the slightly older magmatic arc of the Hidden terrane. Unit also includes rocks of the Uyak Complex, in the Kodiak Island archipelago, which consist of gabbroic and ultramafic rocks including layered gabbro, clinopyroxenite, dunite, and plagioclase peridotite. The mafic and ultramafic rocks of the Uyak Complex occur as kilometer-sized fault-bounded slabs in the westernmost exposures of the Uyak Complex (Connelly, 1978) within the Border Ranges Fault Zone. The margins of these slabs have pronounced serpentinization

Source map information

Source map Winkler, G.R., compiler, 1992, Geologic map and summary geochronology of the Anchorage 1° x 3° quadrangle, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2283, scale 1:250,000.
Symbol Jum
Unit name Ultramafic and mafic rocks
Description Cumulate ultramafic and mafic rocks forming two large fault-bounded sequences in the northern Chugach Mountains; the Eklutna and Wolverine complexes. The Eklutna is 2-3 km thick and 11 km long. From base to top consists of chromite-bearing dunite, wehrlite, olivine clinopyroxenite, clinopyroxenite, and websterite. Associated with both positive gravity and magnetic anomalies. The Wolverine is 1-2 km thick and consists of chromite-bearing dunite grading up into clinopyroxenite. These rocks are correlative to the Tonsina sequence in the Valdez quad.
Lithology Igneous

Correlated geologic units

Label Jmu
Description Gabbronorite and other mafic and ultramafic rocks, Middle and Early Jurassic of Hidden terrane (Chugach Mountains)
Geologic age Early-Jurassic to Middle-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Gabbronorite < Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Pluton Minor
Pyroxenite < Ultramafic < Plutonic < Igneous Pluton Minor
Serpentinite < Metaigneous < Metamorphic Minor