Petrology of Eclogitic Rocks from the Fairbanks District, Alaska

Abstract

P e tr o lo g y o f E c lo g itic R o c k s f r o m th e F a ir b a n k s D is t r ic t , A la s k a ABSTRACTEclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystal line schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies.No basic igneous analogs were found for the eclogitic rocks, and their bulk com positions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and thecalcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks.The eclogitic rocks and associated amphibolites are characterized by the follow ing essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants.The garnets in these eclogites are compositionally similar to those from "group C" eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane.The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White's (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by data on garnet-clinopyroxene pairs.Based on experimental data from various authors, and thermodynamic consid erations, these eclogitic mineral assemblages were probably crystallized at tempera tures of 540° to 590°C at 5.5 to 7.5 kb.K40/Ar40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.Eclogite occurrences were associated with two belts of calc-magnesian rocks in a terrane otherwise dominated by pelitic schists. The belts are defined by discon tinuous masses of calc-magnesian schists that are elongate parallel to the northeast strike of compositional layering and crystallization foliation in the surrounding pelitic schists. The calc-magnesian masses contain various metamorphic rock types including amphibolite, impure marble, and eclogitic rocks.Prindle (1913) distinguished three varieties of ec...

Phase petrology of eclogitic rocks in the Fairbanks district, Alaska

“…Similar eclogitic rocks have been discovered recently northeast of the Chatanika, in a structural setting similar to that discussed in this paper (Laird and others in press), and several other blocks of exotic eclogitic rocks have been found along the cratonal margin in the Yukon near the Tintina Fault (Tempelman-Kluit, 1970;Erdmer and Helmstaedt 1983). This suite of eclogitic fragments is Swainbank and Forbes (1975). Inset map modified from Erdmer and Helmstaedt (1983) and Jones and others (1982).…”

“…5x13 km) about 20 km north of Fairbanks, Alaska ( Figure 1). As previously described by Swainbank and Forbes (1975), the eclogite-bearing terrane is surrounded by rocks of different lithology and metamorphic grade (lower P and T), and the terrane is probably a tectonic window or klippe. The terrane appears to be truncated on the northwest by the Chatanika Lineament, a possible splay of the Tintina Fault that extends hundreds of kilometers to the southeast through Alaska and the Canadian Yukon ( Figure 1).…”

“…The purpose of the present study is to establish the P-T conditions of metamorphism, to evaluate relevant metamorphic geothermometers, and to compare and contrast the phase associations with other eclogites and amphibolites in order to determine reaction relations and phase stability fields among eclogitic assemblages. Swainbank and Forbes (1975) previously attempted to establish P-T parameters for the crystallization of the eclogitebearing terrane based on wet chemically analyzed minerals and whole rocks. They estimated that the eclogites had crystallized at temperatures between 540°-620°C, at pressures exceeding 5 kb.…”

Geologic map of Alaska

“…Other estimates of pressure-temperature conditions range from 590 °C and 5.5 to 7.5 kbar (Swainbank and Forbes, 1975) to 600±50 °C and 13-15 kbar (Brown and Forbes, 1986) on the basis of mineral-pair geothermometers and geobarometers. Newberry and others (1996) Ar, K/Ar, and Pb-Pb studies; Forbes (1982) reported a questionable K/Ar protolith age of 487±35 Ma on low-potassium amphibole and K/Ar ages on mica that range from 170.1±2.3 to 108.7±3.5 Ma, which he attributed to Mesozoic metamorphism.…”

“…On this map, assignment of rocks in the eastern Eagle quadrangle to the Klondike Schist or the Keevy Peak Formation (unit |kp) must be considered tentative; Permian (~255 Ma) U/Pb zircon ages, as well as Mississippian and Devonian zircon ages are reported by Dusel- Bacon and others (2006) for the Nasina assemblage, here assigned to the Keevy Peak map unit (unit |kp). On generalized map, included as part of unit |kn |e Eclogite and associated rocks (Paleozoic)-Garnet-and omphacite-bearing biotite-muscovite schist, micaceous marble, black quartzite, and amphibolite in thin interlayers within a metamorphic sequence, described as allochthonous, garnet-bearing, quartz-biotite-muscovite schist and quartzite (Swainbank and Forbes, 1975;Brown and Forbes, 1986;Weber and others, 1992;Foster and others, 1994) and designated as the Chatanika assemblage by Robinson and others (1990). The Chatanika assemblage is primarily epidote-amphibolite facies rocks, but also includes eclogitic rocks and has some lithologic similarities with the structurally underlying quartz and pelitic schist (unit |<yqs).…”