Plutonic rocks of Hidden terrane

Unit symbol: Jhg
Age range Early Jurassic (201.3 to 174.1 Ma)
Lithology: Igneous - Plutonic
Group name: Intermediate to mafic plutonic rocks
Relatively homogeneous, fine- to medium-grained quartz diorite and tonalite that have extensive areas that have been sheared and altered (Winkler, 1992). Unit is found within the Border Ranges Fault Zone in the Anchorage and Valdez quadrangles. U/Pb zircon ages range between 193.9 and 183 Ma (Rioux and others, 2007), which are similar to, but marginally older than plutons of Alaska-Aleutian Range batholith that have similar composition. The plutons of this unit are often considered part of the Alaska-Aleutian Range batholith and considered part of the Talkeetna magmatic arc, but we interpret these rocks, in companion with mafic rocks of unit Jum, to represent a slightly older but distinct magmatic arc. As analyzed by Rioux and others (2007), unit also includes trondhjemite, which yields significantly older U/Pb ages—between 193.3 and 184.1 Ma—than the trondhjemite of map unit Jtr, which is part of Alaska-Aleutian Range batholith. K/Ar ages generally range from 194 to 161 Ma, but most ages are older than 172 Ma (Winkler, 1992; Hacker and others, 2011)

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 TKc
Unit name Calaclasite
Description Chlorite-rich, fine-grained granular rocks formed by cataclasis and retrograde alteration of mafic and ultramafic plutonic rocks and mafic volcanic rocks; May be equivalent lithologically to sheared gabbronorite (Jgs). Lithologies and fabrics of these rocks may resemble parts of the Haley Creek metamorphic assemblage. Deformation in the Border Ranges fault is at least as old as Early Cretaceous; however, much of the fabric may be an Eocene overprint from reactivation of the old thrust as a strike-slip boundary.
Lithology Metamorphic

Correlated geologic units

Label Jemi
Description Early Jurassic mafic and intermediate plutonic rocks of Hidden terrane (Chugach Mountains)
Geologic age Early-Jurassic to Pliensbachian
Geologic setting Intrusive, gabbro
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Indeterminate, major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Indeterminate, major
Gabbronorite < Gabbro < Gabbroic < Plutonic < Igneous Pluton Indeterminate, major
Tonalite < Granitic < Plutonic < Igneous Pluton Indeterminate, major