Quartz diorite and granodiorite

Unit symbol: Trqd
Age range Triassic (242 to 201.3 Ma)
Lithology: Igneous - Plutonic
Group name: Quartz diorite and granodiorite
Granodiorite and similar rocks of Triassic age occur primarily in three parts of Alaska. In southwest Alaska, the Afognak pluton is the largest exposure, but small exposures of rocks of similar age and composition are found in the Barren Islands of the Afognak quadrangle and in the Seldovia quadrangle of south-central Alaska. The Afognak pluton is exposed on the west side of the Kodiak Island archipelago in the Karluk, Kodiak, and Afognak quadrangles. It is a large, multiphase hornblende diorite, quartz diorite, and tonalite pluton that has a well-developed contact-metamorphic aureole in the Shuyak Formation; its boundary with the schist of Kodiak Island (unit Jsch) is apparently a fault (Roeske and others, 1989). In the Seldovia quadrangle, unit includes the diorite of Point Bede of Bradley and others (1999), which is a fine- to medium-grained quartz diorite, and the tonalite of Dogfish Bay of Bradley and others (1999), a medium-grained tonalite that shows chloritic alteration similar to that found in diorite of Point Bede and, hence, was assigned a similar age. Bradley and others (1999) originally assigned plutons in the Seldovia quadrangle a Jurassic age on the basis of correlation with the pluton in the Barren Islands that had yielded a K/Ar hornblende age of 191±1.3 Ma (Cowan and Boss, 1978). A Triassic age of 227.7±0.6 Ma was determined on zircon from the diorite of Point Bede (Bradley and Miller, 2006). Unit also includes diabasic hypabyssal intrusions in Shuyak Formation (Connelly and Moore, 1979). A fission-track age determination on zircon yielded 153±10 Ma (Clendenen, 1991). K/Ar ages on hornblende from the Afognak pluton and associated migmatite range from 197±5.8 to 187.5±5.5 Ma (Roeske and others, 1989), which we interpret as a cooling age; a U/Pb age of 217±10 Ma is interpreted as the emplacement age (Roeske and others, 1989). We infer a similar history for the plutons of the Barren Islands. The second major area of exposure of Triassic plutonic rocks is the Taylor Mountain batholith in the Eagle and Tanacross quadrangles of east-central Alaska (Foster, 1970, 1976). The batholith is medium- to coarse-grained, subequigranular biotite–hornblende quartz monzodiorite, tonalite, granodiorite, and quartz diorite that ranges from slightly foliated in its interior to strongly foliated at its margins (Werdon and others, 2001). Contacts with surrounding rocks are complex; zones of intimately foliated quartz dioritic dikes and sills are locally present in the country rocks near the batholith, but sheared contacts apparently predominate: Jurassic deformation, and later high-angle faulting, has disrupted most of the original contacts (Werdon and others, 2001). Rocks of the Taylor Mountain batholith have yielded a U/Pb (sphene) age of 214 Ma and zircon ages between 215.7±3.1 and 196±4 Ma (Aleinikoff and others, 1981; Dusel-Bacon and others, 2009; Day and others, 2014); K/Ar ages of 183 to 177 Ma (Wilson and others, 1985); and 40Ar/39Ar plateau ages of 209±3 Ma on hornblende and 204±9 Ma on biotite (Cushing and others, 1984a, b, written commun., 1992), and 210 Ma on biotite and 211 Ma on hornblende (P.W. Layer, University of Alaska-Fairbanks, unpub. data; see Werdon and others, 2001). The most likely magmatic age is about 214 Ma, with younger apparent ages caused by heating and deformation during the Early Jurassic (Werdon and others, 2001). In southeast Alaska, the primary exposure of Triassic plutonic rocks is the Texas Creek granodiorite of Berg and others (1988) in the Ketchikan and Bradfield Canal quadrangles and adjacent British Columbia. The pluton consists mainly of recrystallized, locally cataclastically deformed granodiorite and minor quartz diorite. In general, the unit is relatively massive and lacks pronounced primary or metamorphic foliation. Where present, the intensity of the cataclastic texture generally is low, but locally the granodiorite is converted to mylonite (Berg and others, 1988). The Texas Creek granodiorite yields latest Triassic apparent ages, but discordant K/Ar dates on hornblende, (Smith, 1977). U/Pb zircon age determinations by Alldrick and others (1987) on the pluton in adjacent British Columbia yield Early Jurassic apparent ages, which they interpreted to be metamorphic ages. Other small plutons, exposed on northern Admiralty Island, described as very light- to medium-gray, nonmagnetic, locally foliated, lineated, garnet-bearing granodiorite have reported U/Pb zircon ages of 227.3±2.0 and 221.8±3.0 Ma and an 40Ar/39Ar age of 236.3±3.8 Ma on hornblende; the plutons are locally migmatitic and some contacts are mylonitic (S.M. Karl, unpub. data). In the Skagway quadrangle, a number of dioritic dikes and small stocks mapped by Redman and others (1985) and Gilbert and others (1987) are of presumed Triassic age

Source map information

Correlated geologic units

Label Jmgd
Description Texas Creek granodiorite (metamorphosed)
Geologic age Norian to Sinemurian
Geologic setting Intrusive
Lithology Form Importance
Orthogneiss < Metaigneous < Metamorphic Major