Igneous rocks

Unit symbol: JDoc
Age range Jurassic to Devonian (419.2 to 174.1 Ma)
Lithology: Sedimentary
Group name: Igneous rocks (Angayucham)
Dominantly basalt, greenstone, gabbro, diabase, and chert, and lesser ultramafic rocks. Minor basaltic tuff, volcanic breccia, and carbonate rocks. Basalt and greenstone include pillow basalt and metamorphosed spilitic basalt. Unit consists of discontinuous and large unsheared blocks and lenses of incipiently recrystallized mafic rocks in a low metamorphic grade, blastomylonitic, metasedimentary matrix. Matrix is bedded chert of Triassic age; argillite, slate, and limestone of Mississippian age; and andesitic and basaltic tuff. (Note: much of the description of this unit, in particular the Angayucham part, is derived from Till and others, 2008a.) Hitzman and others (1982) and Pallister and Carlson (1988) recognized several distinct subunits within the package, although the subunits are lithologically similar and each is internally imbricated. Primary igneous and sedimentary textures in the metabasalt and metagabbro are partially overprinted by metamorphic minerals, which indicate prehnite-pumpellyite- to greenschist-facies metamorphism. Some metabasalt is foliated and lineated in the western Ambler River quadrangle (A.B. Till, unpub. data) and in the Angayucham Mountains in the Survey Pass quadrangle. Barker and others (1988) also reported albite-epidote-amphibolite-facies assemblages in metabasalt in the Bettles quadrangle. A sliver of mafic schist in the Angayucham Mountains retains relict hornblende and garnet from an amphibolite-facies metamorphic assemblage, contains a foliation-forming greenschist-facies assemblage, and is cut by prehnite-bearing veins (Pallister and Carlson, 1988). Locally, glaucophane is present, indicating high-pressure metamorphism (Till and others, 2008a). Devonian, Mississippian, Triassic, and Jurassic radiolarians, conodonts, and megafossils have been collected from chert, cherty tuff, metalimestone layers, interpillow sedimentary rocks, and fault slivers of carbonate rocks and chert (Pallister and Carlson, 1988; Jones and others, 1988; Till and others, 2008a). Conodonts of late Silurian to Early Devonian age are the oldest fossils collected from the unit, and Early Jurassic radiolarians are the youngest (Pallister and Carlson, 1988; Jones and others, 1988). Pennsylvanian-Permian, Permian, and early Permian radiolarians were collected near the southeast corner of the Wiseman quadrangle (Jones and others, 1988). Contrasting ages within the unit could reflect structural juxtaposition of rocks of different ages or reworking of older fossils into younger strata (Till and others, 2008a). The Tozitna and Rampart assemblages are characterized by variably altered and metamorphosed flows and shallow intrusive rocks of basalt, diabase, and gabbro interbedded with varying proportions of chert, argillite, slate, phyllite, volcaniclastic rocks, graywacke, and carbonate rocks. Sparse megafossils from these carbonate rocks range in age from Devonian to Permian (Patton and others, 2009). The basalt, diabase, and gabbro are weakly metamorphosed to prehnite-pumpellyite facies and generally increase in metamorphic grade structurally downward. Greenschist facies metamorphism and locally high-pressure blueschist metamorphism, as indicated by the presence of glaucophane and lawsonite, occur near the base assemblage where it structurally overlies the Ruby terrane (Till and others, 2008a). The chert includes both interpillow and bedded varieties and ranges from pure radiolarian and spiculitic chert to cherty tuff. In the southeastern part of the Nulato quadrangle and adjoining parts of the Ruby quadrangle, the unit is characterized by sill-like bodies of diabase and gabbro, argillaceous rocks, fine-grained to conglomeritic graywacke, and chert. Associated with these extrusive and shallow intrusive rocks are ultramafic complexes that consist of serpentinized peridotite, dunite, and harzburgite, associated layered gabbro and anorthosite, and, locally, garnet-amphibolite tectonite, possibly derived from eclogite. K/Ar hornblende ages on hornblende gabbro and hornblende-bearing dikes range from 172 to 138 Ma and dates on garnet-amphibolite range from 172 to 155 Ma (Patton and others, 1977, 1994a). These are considered cooling ages related to tectonic emplacement of the ultramafic and mafic rocks. Gabbro in the Rampart Group yielded a K/Ar age of 210±6 Ma (Brosgé and others, 1969; age recalculated using constants of Steiger and Jager, 1977). In the Angayucham Mountains, where the unit has been mapped in detail, the basalts are tholeiitic and fall into “within-plate” fields on trace element discrimination diagrams; the light rare earth elements (LREE) are enriched relative to chondrite in some basalts and gabbros, but show little to no enrichment in other rocks (Barker and others, 1988; Pallister and others, 1989, Till and others, 2008a). Based on these characteristics, the mafic rocks are thought to have been parts of oceanic plateaus or islands. The Angayucham assemblage, along with the other related assemblages here, may compose a part of a collapsed ocean basin (Till and others, 2008a). It was emplaced in a high structural position during the Brooks Range orogeny. Angayucham metabasalt along the southern flank of the Brooks Range, the “Narvak panel” of Patton and Box (1989) is typically correlated with basalt of the “Copter Peak allochthon” which is exposed at the crest of the range (Moore and others, 1994). One interpretation considers these rocks as part of a dismembered ophiolite derived from the root of a volcanic arc, rather than the more typical mid-ocean ridge setting for an ophiolite (Loney and Himmelberg, 1985; Patton and others, 1994a; see also Patton and others, 1994b)

Source map information

Source map Reifenstuhl, R.R., Dover, J.H., Pinney, D.S., Newberry, R.J., Clautice, K.H., Liss, S.A., Blodgett, R.B., Bundtzen, T.K., and Weber, F.R., 1997, Geologic map of the Tanana B-1 quadrangle, central Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-15a, 17 p., 1 sheet, scale 1:63,360.
Symbol TrMrg
Unit name Gabbro
Description Very dark gray and dark greenish dark gray, fine-grained hornblende-pyroxene gabbro and diabase which constitute the most abundant igneous rock in the Rampart Group. Hornblende occurs both as reaction rims surrounding clinopyroxene, and as isolated, interstitial (late magmatic?) grains. The characteristic texture is diabasic, with subhedral-altered pyroxenes and interstitial hornblende intergrown with subhedral to euhedral former plagioclase grains. No primary quartz has been noted, but 1-3 percent normative quartz is common. Greenschist alteration is pervasive. Clinopyroxene is commonly altered to a very fine-grained mixture of chlorite, actinolite, calcite, and quartz; plagioclase is altered to a very fine-grained mixture of epidote, calcite, sericite, and quartz; and ilmenite is altered to rutile. Rampart Group gabbro tends to have higher Ti02 than Rampart Group basalts, but are otherwise indistinguishable in major and minor element composition from the basalts (Liss and others, 1997; Newberry and Haug, 1997
Lithology Igneous

Correlated geologic units

Label JPztm
Description Mafic and ultramafic rocks, undivided, Tozitna (Jurassic? and/or Paleozoic?)
Geologic age Devonian to Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Quartz-diorite < Dioritic < Plutonic < Igneous Pluton Minor
Quartz-monzonite < Syenitic < Plutonic < Igneous Pluton Minor
Pyroxenite < Ultramafic < Plutonic < Igneous Pluton Incidental