Granodiorite, syenite, and other granitic rocks

Unit symbol: PIPgi
Age range early Permian and Pennsylvanian (323.2 to 272.3 Ma)
Lithology: Igneous - Plutonic
Group name: Granodiorite, syenite, and other granitic rocks
Granodiorite, syenite, granite, monzonite, and other granitic rocks that form a discontinuous belt from southeast Alaska to south-central Alaska and typically yield Permian radiometric ages. The rocks are fine- to coarse-grained, may be massive or schistose, and are locally porphyritic. Unit includes shoshonitic Ahtell pluton (Richter, 1966; W.J. Nokleberg, written commun., 1997), also called the Ahtell complex by Beard and Barker (1989) in the Gulkana quadrangle, which has been dated by K/Ar to between 291 and 288 Ma. Map unit also includes the granodiorite of Rainbow Mountain in the Mount Hayes quadrangle (unit grrm, Nokleberg and others, 1992a), which yielded a K/Ar age of 325.94±9.78 Ma and a U/Pb zircon age of 309±2 Ma (Nokleberg and others, 1992b). Richter and others (1975) and Richter (1976) defined a diorite complex to the east of the Ahtell pluton and considered it to be of Jurassic and Triassic age on the basis of K/Ar dates that range from 204 to 167 Ma (recalculated using constants of Steiger and Jager, 1977); subsequent U/Pb dating reported by Beard and Barker (1989) indicated that the diorite complex was mostly of Pennsylvanian age, between 311 and 290 Ma, and possibly older than the Ahtell pluton. The diorite complex includes a range of lithologies, from quartz diorite to gabbro, including minor anorthosite and gabbro cumulates and a small area of pink biotite-hornblende syenite-monzonite gneiss, gray hornblende diorite gneiss, minor dark biotite schist, and small syenite pegmatite dikes. In the McCarthy and Bering Glacier quadrangles, MacKevett (1978) and George Plafker (written commun., 2006) mapped a monzonitic to granitic complex that consists of “medium-grained, equigranular granitic rocks with fine- to coarse-grained variants. Abundant granite and quartz monzonite and some quartz syenite, syenite, and monzonite, with border zones of quartz monzodiorite, monzodiorite, and gabbro” (MacKevett, 1978). Radiometric ages for this complex range from 312 to 279 Ma, and most K/Ar ages are only slightly younger than the U/Pb age, unlike the dates on the diorite complex, which are highly discordant. Nokleberg and others (1992a), mapping in the Mount Hayes quadrangle, reported sparse to locally abundant andesite and lesser dacite and rhyolite stocks, sills, and dikes that intrude the Permian to Pennsylvanian Slana Spur Formation and Tetelna Volcanics but not the Permian Eagle Creek Formation. These igneous rocks have been metamorphosed to lower greenschist facies and have granoblastic overprint texture and local weak schistosity; their age is inferred from intrusive relations. The rocks of this unit, in the Bering Glacier, Gulkana, McCarthy, and Mount Haues quadrangles, are inferred to represent the plutonic root of the Skolai magmatic arc (Nokleberg and others, 1994). Small, undated, exposures of medium- to medium dark-gray, fine- to coarse-grained diorite appear to intrude the Totatlanika Schist (unit MDts) in the Big Delta quadrangle (Weber and others, 1978). Distally associated with the rocks of this map unit are felsic to mafic rocks that intrude the Retreat Group (unit |<rg) in the Juneau and Sitka quadrangles, orthogneiss in the Sumdum and Petersburg quadrangles, and syenitic rocks in the Craig quadrangle of southeast Alaska. In the Craig quadrangle, syenite and leucosyenite of the map unit locally grade to sodic diorite (Eberlein and others, 1983) and are compositionally similar to the other parts of this unit; they yield K/Ar and 40Ar/39Ar ages between 293 and 276 Ma, comparable to the K/Ar ages in south-central Alaska. Though included in this map unit on the basis of common age and lithology, these rocks may not all be genetically related

Source map information

Source map Brew, D.A., compiler, 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
Symbol PIPsy
Unit name Leucosyenite of Klawock and Sukkwan Island
Description Biotite- and hornblende-bearing syenite near Klawock. Sphene-apatite-amphibole-aegirine/augite-bearing biotite leucosyenite on Sukkwan Island is surrounded by extensive hornfels aureole.
Lithology Igneous

Correlated geologic units

Label PIPsy
Description Syenite, southeast Alaska
Geologic age Pennsylvanian to Cisuralian
Geologic setting Intrusive
Lithology Form Importance
Syenite < Syenitic < Plutonic < Igneous Pluton Major