Newenham ophiolite complex

Unit symbol: JTros
Age range Jurassic and Triassic (235 to 163.5 Ma)
Lithology: Tectonite
Group name: Newenham ophiolite complex
Considered a dismembered ophiolite (Box, 1985; Decker and others, 1994), the ophiolite assemblage, as defined, also includes the serpentinized ultramafic rocks of Cape Newenham, which are included in map unit Jmu. Included here is aphanitic to porphyritic pillow basalt that contains interbedded pillow breccia, aquagene tuff, and inter-pillow and interbedded red and white radiolarian chert as described by Box (1985). Basalt is commonly amygdaloidal; abundant vesicles are filled with chlorite, clinozoisite, prehnite, pumpellyite, and (or) calcite. Box (1985) reported a number of radiolarian collections, but only two were age diagnostic, yielding Middle Triassic to Early Jurassic and Middle to Late Triassic ages. Box (1985) considered the pillow basalt to be Late Triassic. Also part of the ophiolite is light-gray, medium-grained trondhjemite (Hoare and Coonrad, 1978; Box, 1985) in small bodies in the Hagemeister Island and Goodnews Bay quadrangles. Box (1985) indicated that, in addition to the mapped bodies, other small trondhjemite bodies occur within altered diabase near Cape Newenham. Locally, Box (1985) described the trondhjemite as “a northeast-dipping slab faulted above and below against schistose rocks * * *.” Box (1985) tentatively assigned a Late Triassic age based on probable co-genesis with the Late Triassic pillow basalt (his unit Trub; unit J^os here) and other components of the ophiolite, whereas Hoare and Coonrad (1978) assigned a Jurassic age based on the association of the trondhjemite with Jurassic gabbro and ultramafic rocks (unit Jmu). Additional components of the ophiolite include subophitic, holocrystalline plagioclase-clinopyroxene diabase and altered clinopyroxene gabbro that locally contains up to 5 percent orthopyroxene (Box, 1985), which locally grades into the diabase. Plagioclase is partially to completely replaced by a fine aggregate of albite, epidote, chlorite, calcite, and iron oxides. Box (1985) mapped the gabbro as a thrust sheet, and locally mylonite is found along contacts; similar mylonite is found sporadically along the contact between gabbro and structurally underlying diabase near Cape Newenham. Serpentinite, medium-grained serpentinized dunite, wehrlite, clinopyroxenite, and other ultramafic rocks form a number of intrusive bodies and tectonic blocks(?) within fault zones that separare the pillow basalt from the pyroxene gabbro (Hoare and Coonrad, 1978; Box, 1985). The margins of these bodies are cut by numerous coarsely pegmatitic hornblendite dikes that have contact metamorphic zones as much as 800 m wide (Box, 1985). According to Box (1985), the northeast flank of one ultramafic body appears to grade into hornblende gabbro of map unit Jgb, which yielded a K/Ar age of 162.4±4.9 Ma—significantly younger than the dated dikes that cut another one of the ultramafic bodies, whose K/Ar ages were 186.9±5.6 Ma and 176.4±5.3 Ma on amphibole from the cross-cutting hornblendite dikes

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 Jmu
Unit name Middle Jurassic ultramafic plutons
Description Occur only at Red and Susie Mtns. at west end of map area. Medium-grained dunite, wehrlite, and clinopyroxenite, partially (less than 25 percent) replaced by serpentine minerals. Around margins cut by numerous coarsely pegmatitic hornblendite dikes. Northeast flank of Susie Mtn. appears to grade into adjacent hornblende gabbro body. Intrusive into surrounding schistose country rock, with moderately wide (1/2 mile) zone of hornblende- and biotite hornfels. Hornblendite dikes yield Middle Jurassic K-Ar amphibole ages. Apparent source of platinum placers between Red Mtn. and Susie Mtn. (Bird and Clark, 1976).
Lithology Igneous

Correlated geologic units

Label JTrosu
Description Serpentinized ultramafic rocks
Geologic age Late-Triassic to Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Serpentinite < Metaigneous < Metamorphic Major
Ultramafic < Plutonic < Igneous Indeterminate, major