Metamorphic rocks of Baranof Island

Unit symbol: TrPzbi
Age range Triassic or older (358.9 to 208.5 Ma)
Lithology: Metamorphic
Group name: Metamorphic rocks of Baranof Island
High-grade metamorphic rocks composed chiefly of amphibolite, gneiss, and schist, locally intercalated with thin units of marble and calc-silicate granofels, as described by Johnson and Karl (1985). These rocks are faulted against a lower grade heterogeneous unit of metasedimentary and metavolcanic rocks to the west, which are of higher metamorphic grade than the adjacent Goon Dip Greenstone (unit ^n here), and grade through a migmatitic phase to dioritic rocks (unit KJse) to the east. The contact with the Kelp Bay Group to the southeast is likely a fault. The most common lithologies in the higher grade part of the unit are quartz-andesine-biotite-hornblende schist and andesine-hornblende amphibolite (Loney and others, 1975). The lower grade part consists of a lower unit of sandstone and siltstone, a middle unit of limestone, and thin-bedded chert at the highest stratigraphic level (Loney and others, 1965). Correlation of this unit is equivocal; Johnson and Karl (1985) suggested that, as it appears to occupy a stratigraphic position similar to that of the Skolai Group (unit PIPms) relative to the Nikolai Greenstone (Trn), these rocks might be equivalent to the Skolai Group; however, they acknowledge that there is very little lithologic correspondence between these units and the Skolai Group. Another correlation, based on age and stratigraphic position (S.M. Karl, unpub. data), could be with the Sicker Group of Vancouver Island (Muller, 1980; Massey and Friday, 1986), but again, the lithologic correspondence is questionable, as these rocks have significantly more carbonate than the Sicker Group, and the Sicker Group has significant thicknesses of chert in its upper part (Massey and Friday, 1986) that are not present in this unit. Johnson and Karl (1985) thought that the protoliths of this unit, on the basis of bulk composition, were probably mafic volcanic rocks and marine sedimentary rocks. Chert nodules and contorted ribbon chert are abundant at some localities (D.A. Brew, 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 MzPzs
Unit name Siliceous metasedimentary and metavolcanic rocks
Description A heterogeneous unit of metasedimentary and metavolcanic rock of slightly higher metamorphic grade than the overlying Goon Dip Greenstone. Because the Goon Dip Greenstone has been correlated with the Nikolai Greenstone (Jones and others, 1977), which overlies the Skolai Group of Southern Alaska, the rocks of this unit may be equivalent to the Skolai rocks. However, there is very little lithologic correspondence between these two units. Thus, this unit is lithologically distinct from the Skolai Group, but apparently occupies a similar stratigraphic position. NOTE: More extensive description in pamphlet.
Lithology Metamorphic

Correlated geologic units

Label TrPzbi
Description Metamorphic rocks of Baranof Island
Geologic age Carboniferous to Norian
Geologic setting Metaigneous and metaseds, high-grade
Lithology Form Importance
Amphibolite < Metamorphic Indeterminate, major
Gneiss < Metamorphic Indeterminate, major
Amphibole-schist < Schist < Metamorphic Indeterminate, major