Diorite and gabbro of southeast Alaska

Unit symbol: KJdg
Age range Cretaceous? and Jurassic (201.3 to 132.9 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks in southern Alaska
Extensively altered and sheared, green to gray-green, medium-grained, hornblende ± biotite diorite, gabbro, and minor quartz diorite that forms extensive sills and dikes within the Whitestripe Marble and along the contacts of the Whitestripe Marble and Goon Dip Greenstone southwest of the Peril Strait Fault. Mafic minerals are generally altered to chlorite and epidote or clinozoisite. Rocks are generally foliated and locally have extensive shear and cataclastic textures. Similar sheared dioritic rock intrudes marble and greenstone and also occurs as large blocks in the mélange facies of the Yakutat Group (unit KJyg here), which is thought to be correlative with the Kelp Bay Group (Johnson and Karl, 1985) (unit K^m, here). Unit spatially associated with rocks of unit KJse in southeast Alaska. On Halleck Island, unit has yielded several ages: a U/Pb zircon age of 192.8 ±2.4 Ma; 40Ar /39Ar hornblende ages of 191.9±1.2 and 185.4±3.8 Ma; and an 40Ar /39Ar white mica cooling age of 157.2±2.6 Ma (S.M. Karl, unpub. data). In the Juneau and Ketchikan quadrangles, unit also includes altered and (or) metamorphosed and undated diorite and quartz diorite that are exposed in small areas associated with rocks of the Gastineau Group (unit }|sv) and the Gravina-Nutzotin unit (unit KJgn) (Gehrels and Berg, 1992; Karl, 1992; S.M. Karl, unpub. data)

Source map information

Source map Johnson, B.R., and Karl, S.M., 1985, Geologic map of western Chichagof and Yakobi Islands, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1506, pamphlet, 15 p., 1 sheet, scale 1:125,000.
Symbol KJqg
Unit name Quartz gabbro and gabbro
Description Medium to dark gray (C.I. 25-35), medium grained, pyroxene + hornblende + or - biotite quartz gabbro and gabbro crops out west of Ushk Bay. Contain as much as 2 percent magnetite and are principally hypidiomorphic, equigranular, and weakly foliated. Locally banded, granitic veins are common.
Lithology Igneous

Correlated geologic units

Label KJgg
Description Gabbro and diorite
Geologic age Jurassic to Valanginian
Geologic setting Intrusive
Lithology Form Importance
Diorite < Dioritic < Plutonic < Igneous Pluton Major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major