Mafic
Dikes of the
Stephen S. Harlan
2005
Department
of Environmental Science and Policy, George Mason University, Fairfax, VA
22030-4444; tel: 703-330-5714; e-mail: sharlan(AT)gmu.edu
Introduction
Swarms of mafic dikes are a common feature of the basement-cored uplifts of the Laramide foreland province of Wyoming and Montana (Fig. 1.). Regionally, the dikes are of diverse strikes, but NW-, NE-, and N-trending dike swarms are the most commonly observed orientations. The dikes exposed within these uplifts have been studied by numerous workers and this work has been summarized by Snyder et al. (1988). The geologic record provided by these dikes is significant because they provide virtually the only evidence for magmatic and tectonic events that have affected the Wyoming craton since the “stabilization” of the Archean crust at about 2.7 Ga, and the uplift and deposition of Paleozoic sediments during the early Cambrian.
Early Isotopic dating of the mafic dikes, conducted
mainly by the whole-rock Rb-Sr and whole-rock K-Ar techniques, yielded evidence
for at least seven apparent age groups at 2550 Ma; 2200-2100 Ma; 1800-1700 Ma;
1450 Ma; 1330 Ma; 1150 Ma, and 750 Ma (Rb-Sr and K-Ar ages from pre-1977
sources cited in this report have not been corrected for the modern decay
constants of Steiger and Jaeger [1977]). (Baadsgaard and Mueller, 1973; Larson
and Reynolds, 1973; Wooden et al., 1978; Snyder et al., 1988). Given the
problems associated with the dating of mafic dikes by either the Rb-Sr and
conventional K-Ar dating methods (Hanes, 1988), the extent to which any of
these age groupings represent discrete magmatic events is somewhat problematic.
Although the dikes in individual uplifits have been studied by numerous
workers, the geochemical or geochronologic correlation of individual mafic dike
swarms between various uplifts has been tenuous at best and it is unclear if
the dikes of the Wyoming craton can be related to those from other Precambrian
cratons.
Recent advances in isotopic dating techniques, especially U-Pb and 40Ar/39Ar techniques have provided evidence for magmatic events at 2160 Ma in the southern Wind River Range (Harlan et al., 2003), a 2010 ± 10 Ma magmatic event in northeast-striking dikes along the southeastern margin of the Wyoming craton (Premo and Van Schmus, 1989; Cox et al., 1995), 1467 Ma mafic magmatism in the Wind River and Granite Ranges (Chamberlain and Frost, 1995), and about 780 Ma in the Beartooth Mountains and Tobacco Root and Ruby Ranges of southwestern Montana (Harlan et al., 1987). The 2010 Ma event is thought to record continental rifting from whatever Precambrian continental block may have existed adjacent to the southeast margin of the craton prior to that time. The 1467 Ma mafic magmatic event, based on U-Pb dating of W- and NW-trending dikes in the Granite Mountains and Wind River Range, may be related to continental extension associated with development of the Mesoproterozoic Belt Basin along the northwestern margin of the Wyoming craton. The U-Pb dates obtained from these dikes are identical to those obtained from mafic sills that intrude the Belt Supergroup. The 780 Ma event, based on U-Pb dating and 40Ar/39Ar dating of northwest-trending dikes in the Beartooth Mountains and the southern Tobacco Root Ranges, appears to be related to a widespread and synchronous magmatic event (Gunbarrel mafic magmatic event) along the western margin of North America that extends from the Teton Range in Wyoming north to the Mackenzie Mountains of Canada. These dikes may be part of a giant radiating dike swarm related to an ancient mantle plume and may record initial breakup, rifting, and demise of the supercontinent Rodinia (Park et al., 1995; Harlan et al., 1997; Harlan et al., 2003). At present, there appears to be no compelling evidence for magmatic events at 1300 Ma or 1150 Ma.
The purpose of this paper is to briefly review the geologic record of mafic dikes and intrusions exposed in the Beartooth Mountains (exclusive of the Stillwater Complex), especially with respect to those exposed along the Beartooth Highway (U.S. Highway 212). (Fig. 2)
Mafic Dikes of the
Mafic dikes in the Beartooth Mountains (Fig. 1; Fig. 2) probably represent the most extensively studied group of dikes in the uplifts of the Wyoming craton. Early petrographic descriptions and geochemical analyses of the dikes include Eckleman and Poldevaart, (1957); Spencer (1959); Prinz (1964); and Casella (1969). Probably the most complete geologic map of the dikes is that of Prinz (1964), but other geologic maps that include references to mafic dikes include those of Fraser et al. (1969); Reid et al. (1969); Wedow et al. (1975); Simons et al. (1979); Casella et al. (1982); Segerstrom and Carlson, 1982; Timm, 1982; and Elliott et al. (983). More recent geochemical studies include those of Mueller and Rogers (1973) and Simons et al. (1979). Geochronologic studies of the dikes have been conducted by Condie et al. (1969); Mueller (1970); Rowan and Mueller (1971); Baadsgaard and Mueller (1973), Larson et al. (1973); Wooden and Mueller (1979); and Harlan et al. (1997). Paleomagnetic studies of the Beartooth dikes have been conducted by Larson et al. (1973) and Harlan et al. (1997).
Early workers divided the dikes of the Beartooth Mountains into four main groups: 1) Pre-folding and pre-metamorphic orthoamphibolite or metagabbro dikes and sills; 2) emplacement of metadolerite dikes that that cut pegmatites in the Archean basement rocks; 3) emplacement of unmetamorphosed Proterozoic quartz dolerite dikes; and 4) intrusion of olivine dolerite dikes. This latter group of dikes was originally considered to be Tertiary in age and perhaps related to volcanic rocks of the Eocene Absaroka Supergroup (Prinz, 1964), but they are now known to be Proterozoic in age. The metadolerites are perhaps the most lithologically diverse group and include rocks described as metadolerite, metaharzburgite, metabronzite, and intrusion breccia. Also included within this group are the distinctive “leopard rock” dikes, essentially a porphyritic diabase dike in which the interior zones of the dikes contain concentrations of large (up to 10 cm diameter) white subhedral plagioclase phenocrysts (Prinz, 1964) (Fig. 3). The concentration of the phenocrysts in the interiors of these dikes is thought to be due to flow differentiation in which the large plagioclase phenocrysts are entrained by faster moving magma in the center of the dike during emplacement. Dike essentially identical in appearance are found in the northern Bighorn Mountains, approximately 150 km to the southeast (Osterwald, 1978).
More recently, Mueller et al. (1982) presented evidence for six distinct mafic intrusive events in the Beartooth Mountains. The first event consisted of emplacement of dioritic dikes and sills (now amphibolite) at about 2.9 to 3.0 Ga. The second event is represented by the emplacement of the Stillwater Complex at about 2.7 Ga (Premo et al., 1990). The third event is represented by the emplacement of early clinopyroxene-bearing, hypersthene-normative dikes at about 2.5 to 2.8 Ga. The fourth event consisted of emplacement of north-striking dikes at about 2.1 to 2.2 Ga. The fifth magmatic episode is represented a swarm of olivine-normative mafic dikes with an apparent age of 1.3 Ga, which because of its trend and age are thought to be related to extension accompanying development of the Mesoproterozoic Belt Basin. This 1300 Ma event is somewhat problematic because the apparent age does not correlate with known mafic magmatic events in the Belt Basin, which have now been reliably dated by the U-Pb method as occurring between 1430 Ma and 1470 Ma (Höy, 1989; Hunt, 1962; Zartman et al., 1982; Anderson and Davis, 1995; Sears et al., 1998). The last event is represented by emplacement of quartz tholeiitic diabase dikes which yielded a K-Ar whole-rock isochron age of 740 Ma (Baadsgaard and Mueller, 1973).
Geochemical
data presented by Mueller and Rogers (1973) indicate that the ca. 2550 Ma to
740 Ma dikes of the Beartooth Mountains are characterized primarily as
continental tholeiites with low Al2O3 contents. The dikes
can be subdivided into four groups on the basis of TiO2 content. The
four groups correlate with respect to apparent isotopic ages. Group I dikes
consist of metadolerites of Prinz (1964), but also includes ultramafic,
diabasic, noritic, and anorthositic rocks. The rocks are composed primarily of
clinopyroxene (augite), plagioclase and minor magnetite. Rb-Sr and K-Ar age
determinations indicate that these dikes are about 2550 Ma (Baadsgaard and
Mueller, 1973). Group II dikes contain hornblende (with relic pyroxene) and
plagioclase and commonly show evidence of a metamorphic texture and/or evidence
of shearing. The dikes of this group are geochronologically indistinguishable
from the group I dikes, but thought to represent a compositionally distinctive
magma. Group III dikes are glassy fresh-looking dikes that consist of olivine,
clinopyroxene (titanaugite), plagioclase, and titanomagnetite. Chemically,
these rocks are transitional between tholeiites and alkali basalts (Mueller and
Rodgers, 1970). These dikes largely correspond to the olivine dolerite swarm of
Prinz (1964) which originally was thought to be Tertiary in age. Rb-Sr and K-Ar
dates from these rocks yield an apparent age of about 1300 Ma (Baadsgaard and
Mueller, 1973). Group IIIA dikes consist of plagioclase, clinopyroxene, minor hornblende,
and magnetite and ilmenite. Whole rock K-Ar isochron analyses from the Group
IIIA dikes yielded an apparent age of 740 ± 32 Ma (2s) (Baadsgaard and Mueller,
1973). A Neoproterozoic age for one these dikes, the Christmas Lake dike (Fig.
2), has been confirmed by new U-Pb and 40Ar/39Ar age
determinations. A single 207Pb/206Pb age from a
baddeleyite fraction from this dike gave an age of 779 ± 4 Ma (2s),
whereas hornblende from this dike an identical 40Ar/39Ar
hornblende plateau date of 779 ± 4 Ma (2s) (Harlan et al., 1997;
Harlan et al., 2003). These dates are essentially identical to a U-Pb
baddeleyite age of 782.4 ± 4.9 Ma from northwest-trending dikes in the southern
Overall, the relationship between the different geochemical groups and dike orientations is complex. However, the ca. 2100 Ma dikes were found to have largely north strikes, whereas dikes of the ca. 1300 Ma event have strikes approximately N30°W (Wooden and Mueller, 1979). The youngest group of dikes (group IIIA), which includes the Christmas Lake dike, have a consistent trend of N75°W (Wooden and Mueller, 1979). Simons et al. (1979) obtained essentially similar results, although they reported a slightly more westerly dike strike maxima of N55-60°W for the alkalic olivine diabase dikes.
Given problems associated with the Rb-Sr and K-Ar dating of mafic dikes observed in many mafic dike studies, modern geochronologic methods, especially the high-precision dating of dikes by the U-Pb method, combined with geochemistry, petrology and paleomagnetism are needed to better understand the age and distribution of dike swarms in the Beartooth Mountains and surrounding uplifts of the Wyoming craton. At present, only the Neoproterozoic Christmas Lake dike can be credibly related to mafic magmatic events elsewhere in the Cordillera.
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