Ophiolite of the Brooks Range

Unit symbol: JTrob
Age range Jurassic to Triassic? (252.2 to 145 Ma)
Lithology: Igneous - Volcanic
Group name: Mafic and ultramafic rocks in central, western, and northern Alaska
Predominantly mafic and ultramafic rocks considered by most workers to represent an essentially complete ophiolite sequence (Mayfield and others, 1988; Moore and others, 1994; Saltus and others, 2001; Dover and others, 2004). Unit grades upward from tectonized and serpentinized mantle peridotite, dunite, harzburgite, and lherzolite at the base through a crustal sequence of cumulate ultramafic rocks and layered gabbro, massive gabbro, high-level felsic igneous differentiates, and sheeted diabase dikes, and is capped by basalt tuffs (Dover and others, 2004). Located in the western Brooks Range and on western Saint Lawrence Island, this ophiolite is best studied at Siniktanneyak Mountain and Memorial Creek in the Howard Pass quadrangle and is commonly referred to as the Misheguk igneous sequence. Dover and others (2004) recognized six subunits in the sequence. At its base, an intrusive phase (1) consists of predominantly orange-weathering dunite, but harzburgite is common locally, as are lesser amounts of lherzolite, serpentinized peridotite, and olivine pyroxenite; most lithologies are typically tectonized and foliated. The next-higher intrusive phase in the complex is (2) a gray-green cumulate layered gabbro as thick as 4 km that includes interlayered ultramafic rocks in its lower part. The third level of the intrusive sequence is (3) predominantly grayish-weathering, medium- to coarse-grained, hypersthene-bearing hornblende-pyroxene gabbro that has a generally directionless texture but has locally well developed mineral banding. Diabase occurs in localized swarms of subparallel dikes as thick as 2 m and has chilled margins. Structurally overlying the gabbro and diabase is a subunit (4) that typically ranges from diorite to hornblende-plagiogranite; alaskite dikes are also common. Dover and others (2004) interpreted that this fourth subunit formed above, and by differentiation from, massive gabbro (5) that is the second-highest intrusive phase in the complex. Locally as thick as 2 km, it is intruded by late-stage diabase dikes. At the top of the section is (6) a predominantly brown to greenish-gray, vesicular and amygdaloidal, locally pillowed basalt and minor volcanic breccia, tuff, and volcaniclastic rocks, as well as lenses of interpillow radiolarian chert and fossiliferous limestone. Fossils in the chert and limestone are varied, and, depending on location, are pre-Permian (Mississippian?), Permian(?),Middle and Late Triassic, and Early(?) Jurassic radiolarians in chert; the limestone lenses have early Late Devonian to early Early Mississippian conodonts and Permian brachiopods. Mayfield and others (1987) inferred a Triassic age for the section in the Noatak quadrangle on the basis of lithologic correlation with similar rocks in Misheguk Mountain quadrangle. Mayfield and others (1987) also inferred a Jurassic age based on the possibility that gabbroic dikes and sills may have been feeders for some of the basalt. These dikes and sills are similar to those in Misheguk igneous sequence, which has yielded K/Ar dates that range from 164.0±7.0 on hornblende (Ellersieck and others, 1982) to 153.0±7.6 on biotite (Nelson and Nelson, 1982) in the Misheguk Mountain and Howard Pass quadrangles, respectively. Wirth and others (1993) reported widely ranging 40Ar/39Ar ages, between 196.6±12.6 and 134.3±5.8 Ma; very few samples yielded plateau ages, but those that did ranged between168.8±4.2 and 163.1±4.0 Ma. The Misheguk igneous sequence is generally considered to be far-traveled oceanic crust exposed as thin klippen at the highest structural level in the Brooks Range (see for example Patton and others, 1977; Roeder and Mull, 1978). Modeling of magnetic and gravity data by Saltus and others (2001), however, suggests that the ophiolite is at least 8 km thick, which they postulate is inconsistent with the interpretation that the sequence is a thin klippen. Saltus and others (2001) suggest that the ophiolite may have formed in an extensional basin on a broad continental shelf. Harris and others (2003) disagreed with that interpretation, arguing that field relations and petrochemical data support an allochthonous origin for the Misheguk igneous sequence and that Saltus and others’ (2001) geophysical data had inadequate resolution. An interpretation that reconciles both points of view is yet to be published

Source map information

Source map Dover, J.H., Tailleur, I.L., and Dumoulin, J.A., 2004, Geologic and fossil locality maps of the west-central part of the Howard Pass and part of the adjacent Misheguk Mountain quadrangles, western Brooks Range, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2413, pamphlet, 75 p., 3 map sheets, scale 1:100,000.
Symbol Jslg
Unit name Layered gabbro
Description Predominantly gray-green cumulate gabbro that includes interlayered ultramafic rocks in the lower part; banding typically expressed by melanocratic and leucocratic layers. As much as 4 km thick. This is one of six map units that is distinguished within the Jsu unit. It represents the second lowest intrusive setting in the complex.
Lithology Igneous

Correlated geologic units

Label Jbod
Description Gabbro, diabase, basalt, microgabbro, and minor diorite; dark-grayish-green to dark-gray. Brooks Range
Geologic age Jurassic
Geologic setting Intrusive, gabbro
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Hypabyssal-basalt < Mafic-hypabyssal < Hypabyssal < Igneous Indeterminate, major
Diorite < Dioritic < Plutonic < Igneous Pluton Minor