Bokan Mountain peralkaline granite and syenite

Unit symbol: Jag
Age range Middle Jurassic (182.7 to 170.3 Ma)
Lithology: Igneous - Plutonic
Group name: Bokan Mountain peralkaline granite and syenite
Alkalic granitic rocks, including light-gray peralkaline granite and syenite (MacKevett, 1963). The Bokan Mountain granite on Prince of Wales Island is characterized by diverse grain sizes and textures, high quartz content, and sodium-bearing mafic minerals. It ranges from fine- to coarse-grained, and its textures, dominantly porphyritic and cataclastic, include porphyritic, seriate porphyritic, protoclastic, cataclastic-hypidiomorphic granular, and xenomorphic granular. The porphyritic textures are characterized by euhedral quartz phenocrysts, which range from 5 to 20 mm in length and are embedded in a fine- or medium-grained groundmass. The cataclastic textures are characterized by granulated quartz or fractured minerals—chiefly quartz and feldspar (MacKevett, 1963). The Bokan Mountain peralkaline granite porphyry has a core of arfvedsonite granite and an outer ring of predominantly aegirine granite, which forms a nearly complete ring about 180 m in thickness around the core of arfvedsonite granite. The granite is characterized by 2–10 volume percent subhedral arfvedsonite and aegirine (Dostal and others, 2013). Aegirine syenite forms cylindrical to tabular masses that have sharp contacts in the southeastern part of the Bokan Mountain complex (Thompson and others, 1982) and the granite is intruded by late phase quartz-albite-K-feldspar pegmatite enriched in Th and rare earth elements (REE) relative to the main phase as well as late stage, light-colored aplite porphyry dikes. Similar alkaline granite porphyry at Dora Bay has a rim of migmatitic nepheline-eudialyte-bearing syenite, locally ~100 m thick (Eberlein and others, 1983). Lanphere and others (1964) reported K/Ar ages on riebeckite of 190±8 and 185±8 Ma for the Bokan Mountain granite, and de Saint-Ander and others (1983) reported a U/Pb age on discordant zircon fractions of 171±5 Ma; de Saint-Andre and Lancelot (1986) revised that age to 167 +7/-5 Ma. A 87Sr/86Sr—87Rb/86Sr isochron age of 151±5 Ma was interpreted by Armstrong (1985) to be a minimum age. A concordant U/Pb zircon age of 177.2±0.2 Ma and two 40Ar /39Ar plateau ages of 176.3±0.8 and 175.5±06 Ma on amphibole suggest rapid cooling of the granite at Bokan Mountain (Dostal and others, 2013). Hornblende in the peralkaline granite at Dora Bay yielded a K/Ar age of 175.4±6.6 Ma (Nora Shew, written commun., 1992). Mined for uranium in the past (Thompson and others, 1982; Thompson, 1988), Bokan Mountain is rich in rare earth elements (Philpotts and others, 1996)

Source map information

Source map Gehrels, G.E., 1992, Geologic map of southern Prince of Wales Island, southeastern Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2169, 23 p., 1 sheet, scale 1:63,360.
Symbol Jbr
Unit name Riebeckite granite porphyry of Bokan Mountain
Description Granite porphyry containing quartz phenocrysts to 5 mm in diameter, subhedral riebeckite phenocrysts to 4 mm, albite, and K-feldspar. Grades outward to aegirine granite porphyry.
Lithology Igneous

Correlated geologic units

Label Jpg
Description Bokan Mountain peralkaline granite and syenite
Geologic age Toarcian to Aalenian
Geologic setting Intrusive
Lithology Form Importance
Granite < Granitic < Plutonic < Igneous Pluton Major
Syenite < Syenitic < Plutonic < Igneous Pluton Major
Melilitic < Plutonic < Igneous Pluton Indeterminate, major
Monzonite < Syenitic < Plutonic < Igneous Pluton Indeterminate, major