Calcareous and metabasaltic schist and phyllite

Unit symbol: JPs
Age range Early Jurassic to Permian (298.9 to 174.1 Ma)
Lithology: Metamorphic
Group name: Calcareous and metabasaltic schist and phyllite (Togiak-Tikchik Complex)
Consists of a number of interrelated map units defined by Hoare and Coonrad (1978) and Box (1985) in the Goodnews Bay and Hagemeister Island region of southwest Alaska. Consists of three main subunits; the rocks of the first subunit include recrystallized schistose and phyllitic calcareous sandstone, shale, limestone, limestone conglomerate, greenish tuffaceous rocks, mafic volcanic rocks, and volcanic conglomerate; the second subunit consists of metamorphosed conglomerate, sandstone, and shale; and the third subunit is partially to completely recrystallized glaucophane-hornblende epidote schistose rocks derived from mafic igneous rocks that include pillow basalt, angular volcanic breccia, diabase, and gabbro and associated sedimentary rocks, that include pebbly mudstone, fine-grained tuffaceous sedimentary rocks, and rare chert (Box, 1985). Turbidite characteristics are locally preserved in clastic metasedimentary rocks. Mineral assemblages indicate greenschist to transitional greenschist-blueschist facies metamorphism (Box, 1985; Dusel-Bacon and others, 1996); structurally the unit shows evidence of multiple metamorphic events (Box, 1985). Metamorphic fabric is similar in all three subunits, which suggests that all three units were deformed during the same metamorphic event (Box, 1985). Locally, unit is structurally overlain by Devonian to Ordovician limestone and the Kanektok metamorphic complex (Wilson and others, 2013). West of Goodnews Bay, the unit structurally overlies Permian volcaniclastic rocks and underlies Permian limestone. Intruded by nonfoliated Middle Jurassic gabbroic rocks (Jgb). Schist has yielded K/Ar mica ages of 155±8 Ma and 150±8 Ma (Wilson and others, 2013), possibly related to a Late Jurassic thermal event suggested by Box (1985). The ages are suspect, however, because the K2O content of the mica (1.71 and 2.94 percent, respectively) is much lower than typical for mica (8 to 10 percent), suggesting alteration. A K/Ar cooling age on amphibole of 231.2±6.9 Ma (Wilson and others, 2013) from the metasedimentary rocks suggests Middle Triassic metamorphism; age is constrained to be older than the intrusion of Middle Jurassic gabbroic rocks. Age of protolith may be Permian or older (Hoare and Coonrad, 1978; Box, 1985). Main part of unit is spatially associated with a Triassic ophiolite complex

Source map information

Source map Box, S.E., 1985, Mesozoic tectonic evolution of the northern Bristol Bay region, southwestern Alaska: Santa Cruz, University of California, Ph.D. dissertation, 163 p., 7 tables, 21 figures, 2 plates.
Symbol JTrmb
Unit name Upper Triassic-Lower Jurassic (?) metabasaltic schists
Description Partially to completely recrystallized schistose rocks with mafic igneous protoliths, including pillow basalts, angular volcanic breccia, pebbly mudstone, fine tuffaceous sedimentary rocks, diabase, gabbro, and rare chert. Greenschist to transitional greenschist-blueschist metamorphic mineral assemblages. Strong fabric development by flattening and/or synmetamorphic mylonitization (D2). Foliation (S2) moderately to shallowly dipping. Locally developed mineral lineation (plagioclase, amphibole) and stretching lineation (L2). Sporadically developed crenulation cleavage (S3), often in two sets at 60-90 degrees apart and with opposing microfold asymmetry, suggesting west- to northwest-directed compression. Both D2 and D3 features folded around northeast-trending open folds (D4). Schists cut by unfoliated hornblende gabbro bodies (Jmg) of Middle Jurassic age. Similarity of protoliths to those structurally overlying rocks (Trub, Trud, Trug) suggests Upper Triassic protolith ages. Two mica separates yield Late Jurassic K-Ar ages, probably resulting from Late Jurassic thermal event. Unit bounded by low-angle faults from overlying unfoliated mafic rocks (Trub, Trud, Trug) and from underlying schistose clastic rocks (JTrmvss). Lower fault marked by sporadic serpentinite (su) with gabbro (Trug) and Trondhjemite (Trut) blocks.
Lithology Metamorphic

Correlated geologic units

Label TrPrmb
Description Metabasaltic schist
Geologic age Late-Triassic to Early-Jurassic
Geologic setting Extrusive
Lithology Form Importance
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Major
Schist < Metamorphic Indeterminate, major