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We report 76 previously unpublished K-Ar mineral ages from 47 metamorphic and igneous rocks in the southwestern Brooks Range. The pattern of radiometric ages is complex, reflecting the complex geologic history of this area. Local and regional radiometric evidence suggests that the southern Brooks Range schist belt has, at least in part, undergone a late Precambrian metamorphism and that the parent sedimentary and igneous rocks for the metamorphic rocks dated as late Precambrian are at least this old (Precambrian Z). This schist terrane experienced a major thermal event in midCretaceous time, causing widespread resetting of nearly all K-Ar mica ages. A series of apparent ages intermediate between late Precambrian and mid-Cretaceous are interpreted as indicating varying amounts of partial argon loss from older rocks during the Cretaceous event. The schist belt is characterized by dominant metasediments and subordinate metabasites and metafelsites. Blueschists occur within the schist belt from the Chandalar quadrangle westward to the Baird Mountains quadrangle, but geologic evidence does not support the existence of a fossil subduction zone. INTRODUCTION This project was begun in 1973 as a cooperative effort involving personnel from the Alaska Division of Geological and Geophysical Surveys, U.S. Geological Survey, and the Geophysical Institute of the University of Alaska. During that summer, a N-S mapping and sampling traverse across the Southern Brooks Range schist belt was completed by Forbes, Turner, Wyatt Gilbert, and John Garden with D.G.G.S. helicopter support. In 1974 Forbes and Garden completed additional field work in the Baird Mountains quadrangle with D.G.G.S. helicopter support. Laboratory work for K-Ar dating was supported by the D.G.G.S. and the National Science Foundation (NSF Grant GA 43004). Additional samples for dating were provided by the D.G.G.S. and U.S.G.S. from the collections of J. Fritts, R. Garland, G. Eakins, G. Pessel, I. Tailleur, W. Brosge, C. F. Mayfield and M. Wiltse. The final preparation of this report was supported by the U.S.G.S. The report is divided into three principal parts, a section on the petrology and structural setting of the metamorphic rocks by R. B. Forbes; a section on radiometric dating by D. L. Turner; and a section on evidence for the age of metamorphism from the stratigraphic record by C. F. Mayfield. A total of 76 new K-Ar mineral ages from 47 different metamorphic and igneous rock samples in the southwestern Brooks Range are reported. Radiometric dating was done at the Geochronology Laboratory of the Geophysical Institute, University of Alaska, Fairbanks. Analytical techniques have been reported previously (Turner and others, 1973). Analytical data for age determinations are given in Table 2. Data for previously published U.S.G.S. K-Ar ages in the area are given in Table 3. Sample locations, along with ages and mineral symbols, are shown on Plates 1-3. All of the dated samples come from the schist belt and nearby granitic plutons which crop out over a distance of more than 600 km along the southern flank of the Brooks Range (Beikman and Lathram, 1976). Data presented here come from the western and central parts of this schist belt in the Baird Mountains, Ambler River and Survey Pass quadrangles. PETROLOGY AND STRUCTURAL SETTING Geologic Characteristics of the Schist Belt: The central and western segments of the southwestern Brooks Range schist belt are characterized by dominant metasediments and subordinate metabasites and metafelsites. The parent terrane was apparently composed of argillaceous and quartzose sedimentary rocks, with intercalated carbonates, basalts and felsic volcanic rocks. The belt was recrystallized under conditions of the blueschist, greenschist and lower amphibolite facies. In some areas the terrane appears to be a two-sided metamorphic belt, with highest metamorphic grade displayed by rocks along the axial zone. In other areas, however, metamorphic grade increases to the north where the effects of synkinematic recrystallization are overprinted by thermal aureoles associated with the emplacement of granitic plutons. Along some segments of the belt, including the Walker Lake and Kiana areas, the north flank of the schist terrane is concealed by allochthonous carbonates. Blueschist and Greenschist Metamorphism Blueschists have been found within the schist terrane from the Chandalar quadrangle westward to the Ambler River and Baird Mountains quadrangles. As presently mapped, most of the blueschists occur as discontinuous zones or large boudin-like masses derived from parental mafic volcanic and intrusive masses. However, a few examples of chloritoidand jadeite-bearing blueschists have been discovered, which are clearly of metasedimentary origin. Whole-rock chemical analyses show that there is no significant difference in the sodium content of analyzed blueschists, compared to greenshists in other parts of the schist terrane. In some cases the compositionally-analogous greenschists are retrograded blueschists. However, some of these greenschists are transitional to epidote amphibolites in prograde sections within the terrane. Additionally, sodic amphiboles occur in schists with a sodium content as low as 0.97 wt. %. The evidence clearly indicates that the presence of sodic amphibole in' the metabasites is due to the high pressure/low temperature regime of blueschist facies metamorphism rather than high sodium content. Whole-rock sodium concentrations of Brooks Range blueschists and / greenschists are similar to those in metabasites from other Alaskan metamorphic terranes, with the exception of the highly sodic crossiteepidote schists in the Seldovia-Kodiak terranes which were derived from parental spillites (J. Garden, University of Alaska Ph.D. dissertation, in preparation). Petrofabric studies of the metabasites (blueschists, greenschists and epidote amphibolites) indicate a polymetamorphic history involving a later greenschist facies thermal event which overprinted the earlier blueschists, amphibolites and eclogites (?) with imperfectly developed greenschist mineral assemblages. This event has had a pervasive effect 40 40 on the apparent K/ Ar mineral ages, as discussed in the following sections. Metasediments derived from argillaceous and quartzose sediments show little evidence of the later greenschist facies thermal event because the constituent mineral assemblages tend to be stable under both 40 40 blueschist and greenschist facies P/T conditions. K/ Ar dating, however, shows that the micas and amphiboles in these rocks have frequently been partially or totally reset by a later thermal pulse. The earlier metamorphic event appears to have been under conditions of the low temperature sub-division of the blueschist facies (Taylor and Coleman, 1968), based on the occurrence of coarse relict jadeite in a metawacke collected from the cirque 1/4 mile east of VABM Ruby, and as metacrysts in a metabasite which crops out on the southern tip of the ridge which forms the divide between the Kogoluktuk and Mauneluk Rivers. Significantly, the jadeitic metacrysts are rimmed by secondary sodic amphibole which probably formed during the waning stages of blueschist facies metamorphism. Additional support for a pervasive blueschist facies metamorphic event is given by the general absence of biotite in pelitic schists and metabasites in this segment of the schist belt. Notable exceptions are the biotite-bearing assemblages of the felsic schists ("button schists") which are highly potassic and have whole-rock compositions analogous to rhyolites or potassic granites, and biotite which is in contact thermal aureoles around granitic plutons north of the Walker Lake Fault. The origin of the constituent biotite and the genesis of the "button schists" is controversial. Biotite and muscovite from these schists give Cretaceous 40 40 K/ Ar ages, apparently produced by the pervasive Cretaceous age overprint discussed in the following section. The absence of lawsonite and aragonite in the blueschists is no doubt due to the retrograde effects of the late greenschist facies thermal event, which has inverted the former aragonite to calcite, and promoted reactions between lawsonite and other minerals to produce clinozoisite and epidote. Petrorraphic evidence for these reactions includes biaxial calcite with relict optic angles up to 40°, and discordant clinozoisite-paragonite fabrics which are believed to be related to retrograded lawsonite. 18 16 Initial results of oxygen isotope (0 /O ) analyses by James O'Neil (personal communication) indicate that quartz-glaucophane mineral pairs were not equilibrated, whereas quartz-mica pairs from the same rocks give crystallization temperatures of 500°-600°C which would correlate with conditions of the upper greenschist fades. In addition to documenting the validity of subsequent greenschist facies metamorphism, these data are compatible with the survival of coarse-grained glaucophane in the polymetamorphic fabrics; and with the inherited argon problem associated with the dating of some of the coarse-grained glaucophanes discussed in a following section. Structure The most pronounced and easily recognized structure in this part of the schist belt is an anticlinally folded nappe-like structure (Figure 1) which was first called the Kalurivik Arch (Pessel and others, 1973). The axis of this structure is 5 km north of VABM Ruby. The arch is defined by the deformation of S.. surfaces, but S 9 surfaces were apparently synchronous with the development of the antiform. The arching may be related to the emplacement of the Redstone, Shishsakshinovik and Mt. Igipak-Arrigetch Peaks plutons, because these plutons are elongate parallel to the axis of the arch and have gneissic margins (Plate 2). The Walker Lake Fault (Figure 1) was first described by Fritts (1972). Fritts believed that he could trace the fault for 100 km along the strike of th

K-Ar geochronology of the Survey Pass, Ambler River and Eastern Baird Mountains quadrangles, southwestern Brooks Range, Alaska