Unit symbol: MzPzmb
Age range Mesozoic and Paleozoic (541 to 145 Ma)
Lithology: Metamorphic
Group name: Metabasite
Consists of a variety of metabasite bodies exposed around the Yukon-Koyukuk Basin. Typically small bodies, these are found associated with the Schist belt of Till and others (2008a) in the Brooks Range (units D<aqm and D<acs, here), similar schist of the Ruby geanticline in the Nulato and Tanana quadrangles (included in unit |<rqm, here) and also includes metamorphosed mafic rocks and serpentinite associated with the Kugruk Fault Zone of Till and others (2010). Varies from thinly layered greenschist to more massively layered greenstone bodies derived from altered mafic and intermediate volcanic and shallow intrusive rocks. Also includes small bodies of serpentinite, peridotite, dunite, gabbro, diorite, metabasite, and minor talc schist and roddingite. The rocks of this unit exhibit a broad range of intrusive and (or) metamorphic textures and fabrics. Age control is generally poor and inferred ages range from late Proterozoic to Mesozoic. Unit likely includes rocks of several different ages. Unit may include some rocks in the Brooks Range that might more appropriately be assigned to the Ambler sequence of Hitzman and others (1982; unit Das, here). Some of the bodies are interlayered with the Devonian felsic schist of unit Df of Nelson and Grybeck (1980) and Dillon and others (1986) and some of the bodies are part of a bimodal volcanic assemblage. Other bodies are interlayered with carbonate rocks that contain fossils of probable Devonian and Ordovician age (unit D<acs, here). Still other bodies may represent tectonically emplaced slices of unit JDv of the Angayucham-Tozitna terrane. The rocks associated with the Kugruk Fault Zone are “a tectonic assemblage of metagabbro, metabasalt, amphibolite, serpentinite, and minor chert, exposed in rubble fields and poor outcrops along the trend of the Kugruk Fault Zone. Mafic rocks within the fault zone include minor nonmetamorphosed (but altered) rocks, rocks with relict igneous textures and a single metamorphic overprint, and rocks that have experienced more than one metamorphic event. Pumpellyite- and prehnite-bearing veins that cross foliation are common in many lithologies” (Till and others, 2011). The “mafic rocks in the northern part of the fault zone (Bendeleben D-1 and Kotzebue A-1 quadrangles) display evidence of an albite-epidote amphibolite facies event overprinted by a lower grade event. Blue-green amphibole, epidote, and albite, with and without garnet, occur in equilibrium metamorphic textures. These albite-epidote-amphibolite-facies assemblages are slightly to significantly overprinted by epidote-blueschist or greenschist assemblages, largely on mineral rims or cracks. The later metamorphic event is likely the same event that affected mafic rocks in the unit further to the south” (Till and others, 2011)

Source map information

Correlated geologic units

Label MzZum
Description Undivided dunite and serpentinite, Mesozoic or Paleozoic
Geologic age Paleozoic to Early-Jurassic
Geologic setting Intrusive
Lithology Form Importance
Serpentinite < Metaigneous < Metamorphic Greenschist Major
Gabbro < Gabbroic < Plutonic < Igneous Pluton Indeterminate, major
Dunite < Peridotite < Ultramafic < Plutonic < Igneous Indeterminate, major
Schist < Metamorphic Greenschist Minor