Metamorphism and exhumation of mid-crustal gneiss domes in the Arctic Alaska terrane

Metamorphism and exhumation of mid-crustal gneiss domes in the Arctic Alaska terrane
Calvert, A. T.
University of California, Santa Barbara
198 p., Illus.
Geophysical Institute Library: THESIS NOT UAF
The Kigluaik Mountains of the Seward Peninsula, Alaska represent one of several domal uplifts of high-grade Barrovian-series metasedimentary gneissic rocks that lie structurally below, or rise through, the blueschist- and greenschist-facies rocks which underlie most of the Seward Peninsula. Along the southern front of the mountain range, including the study area in the eastern Kigluaik Mountains, an unusual, but consistent relation exists between the two metamorphic rocks types. Although early mapping suggested a fault contact between the two, later work has documented structural continuity across the zone, raising questions about the exact relation of these two groups of rocks formed under different geothermal gradients. The entire structural package in the map area is metamorphosed and the absolute ages of metasedimentary rocks and their original stratigraphic relations are difficult to constrain. Deepest structural levels are primarily pelitic and quartzose lithologies (Kigluaik Group) and underlie the rugged Kigluaik Mountains. The lowest part of the sequence is cut off by the east- west trending, range-bounding Kigluaik Fault which has primarily normal displacement. Kigluaik Group rocks retain minerals and structures related to the highest grade of metamorphism of the package with metamorphic minerals beyond the sillimanite+K-feldspar isograd at the base of the section. Metamorphic grade increases very rapidly from the first appearance of biotite in the highest unit of the Kigluaik Group through staurolite-in, sillimanite-in and sillimanite+K-feldspar isograds in less than 5 km of section. The Kigluaik Group is divided into five units, the highest of which is a graphitic quartzite and schist unit with a mylonitic texture and which separates the Kigluaik Group from the overlying, lower-grade Nome Group. Nome Group rocks typically contain several deformations. The first preserves fabrics related to the blueschist-facies metamorphism and is completely transposed by isoclinal folding from the younger deformational event apparently associated with the Kigluaik Group metamorphism. The age of this high pressure event is somewhat in question. K-Ar geochronology by Armstrong et al (1986) suggested a 160 Ma age, but new data suggests that metamorphism took place at 207 Ma or earlier based on (super 40) Ar/ (super 39) Ar phengite dating (Hannula et al., in press). Only a small interval of Nome Group rocks is exposed in the map area and it is divided into four units. None of these rocks contain relict blueschist-facies assemblages, but similar units just south of the map area contain blueschist-facies minerals and pseudomorphs. Two types of intrusive rocks crop out in the map area. Both are found in the Kigluaik Group and are biotite-bearing granites in composition, but the first is foliated and involved in the deformation, while the second is cross-cutting. The end of high-grade metamorphism of the Kigluaik Group can be bracketed between the 105 Ma U-Pb zircon age of the pre- to syn-metamorphic granite and the 83 Ma U-Pb zircon age of the post-metamorphic granite. Several episodes of deformation accompanying these metamorphic events appear to have caused the unusual juxtaposition of the blueschist-facies Nome Group and the amphibolite-facies Kigluaik Group rocks on the Seward Peninsula, but the details of this history are not clear from previously published work in the area. This study focused primarily on strain involved with the Cretaceous thermal event responsible for high-grade metamorphism of the Kigluaik Group. Structural thinning by ductile flow late in the event may have caused the unusual juxtaposition of the Nome and Kigluaik Group rocks. Evidence to support this includes (1) continuous metamorphic fabric orientations across the transition, (2) evidence for high amounts of strain late in the thermal event which could allow contact of hotter and cooler rocks without complete thermal overprinting, and (3) an unusually high field metamorphic gradient which is difficult to explain under static metamorphic condition s. Three metamorphic and deformational events are apparent in the post-depositional geologic history of the rocks in the map area: (1) the blueschist-facies metamorphism with fabrics apparent in Nome Group rocks, (2) the high-grade thermal event associated with the amphibolite- to granulite-facies rocks in the Kigluaik Group and a greenschist-facies overprint in the Nome Group rocks, and (3) structures and fabrics associated with the late-stage juxtaposition of two types of rocks. The relation of the Seward Peninsula blueschists and higher-grade rocks to the rest of Alaska is unknown. It has been described as a terrane with geology unrelated to that of the rest of Alaska (Jones et al., 1987), as an extension of the Brooks Range because carbonates northwest of the map area on the Seward Peninsula correlate with sequences in the western Brooks Range (Dumoulin and Harris, 1987), and as possibly related to rocks with blueschist-facies fabrics on the southern flank of the Brooks Range (Dusel-Bacon, et al., 1989).
Ph.D. dissertation
Minerals Data and Information Rescue in Alaska (MDIRA)