- UW Department of Geology and Geophysics
Proterozoic plutonism in Colorado and Wyoming was initiated ∼1.8 Ga with scattered tholeiitic mafic complexes coeval with widespread synorogenic bimodal volcanism. Limited Nd and Sr isotopic data for the metavolcanic rocks show derivation from depleted mantle. Major Paleoproterozoic granitic plutonism followed at 1.67–1.77 Ga. Most of the earliest plutons are distinctly calc-alkaline; they range largely from quartz diorite to granodiorite to trondhjemite in composition, and have trace-element signatures similar to plutons within magmatic arcs related to subduction zones. Later Paleoproterozoic plutons at 1.71 Ga include increased volumes of felsic rock types and, independent of silica, are shifted to more peraluminous and iron-rich compositions. The earliest appearance of anorthosite occurs with the 1.76-Ga Horse Creek anorthosite complex of Wyoming, and the earliest occurrence of A-type granite includes the late-kinematic, 1.66-Ga Garell Peak batholith of southern Colorado. Elemental and isotopic compositions of the younger Early Paleoproterozoic granitic plutons are consistent with a systematically increasing crustal component as a function of age in waning orogenic stages of crust formation in the region.
After a 200 m.y. hiatus, renewed granitic plutonism occurred at 1.36–1.44 Ga. Plutonism was associated with emplacement of over a dozen Mesoproterozoic A-type granite batholiths and many smaller intrusions as part of a global “anorogenic” mid-Proterozoic event that commonly includes associated intrusions of anorthosite and charnockite. Across the former Laurentia supercontinent, three geographic and petrologic subprovinces merge in Colorado and Wyoming. An ilmenite-series granitic province, which includes the Sherman Granite and associated Laramie anorthosite complex of Wyoming, extends northeastward through Wisconsin to Labrador and the classic rapakivi granite-anorthosite intrusions of the Baltic region. A magnetite-series granite subprovince ranges across the southern mid-continent to California and includes the San Isabel and Eolus batholiths of southern Colorado. The third subprovince is peraluminous, comprised of two-mica granite, and geographically extends from central Colorado to Arizona. Granites of this suite are the most common in Colorado and include the Silver Plume batholith. Granites defining the three Mesoproterozoic provinces have distinctly different elemental and oxygen isotopic compositions, which presumably reflect fundamental shifts in composition of the lower continental Laurentian crust. The mid-Proterozoic intrusions of Colorado and Wyoming coincided in time with emplacement of regional, north-trending mafic dike swarms, implying widespread extension during this period.
After another magmatic hiatus, one of ∼300 m.y., intrusion of the A-type, 1.08-Ga Pikes Peak batholith formed the last Proterozoic magmatic episode of the Colorado-Wyoming Front Range.
Over a century ago, Sederholm (1891) offered one of the earliest characterizations utilizing the changing character of plutonism within Proterozoic orogens as a function of timing of intrusion relative to peak deformation. Eskola (1932) and Marmo (1971) made similar contributions, leading to a widely accepted three-part classification of syn-, late-, and post-kinematic or syn-, late-, and anorogenic categories for batholiths within orogenic belts. Although details of their respective classifications differed, all noted the increasing abundance of potassium and other incompatible elements with decreasing age of granitic batholiths within orogenic belts. The classic rapakivi granites of the Baltic region figured prominently in the post-kinematic or anorogenic suite, along with nearly coeval emplacement of anorthosite and charnockite. The chemical features of rapakivi and other anorogenic granites subsequently led to the definition of A-type granite (Loiselle and Wones, 1979; Collins et al., 1982; Whalen et al., 1987), which includes as essential attributes high abundances of alkalies and other incompatible elements (including Rb, Ba, REE, U, Th, Zr, Nb, Y, Hf, and Ta), high Fe/Mg ratios, high abundance of K-feldspar, and accessory fluorite.
Although Sederholm based his work on observations in the Baltic shield, similar age-dependent chemical changes occur for Proterozoic plutonism of North America, including that of Colorado and Wyoming (Reed et al., 1993). The oldest Proterozoic units of this region include metasedimentary and bimodal metavolcanic sequences (Condie and Nuter, 1981; Bickford and Boardman, 1984; Boardman and Condie, 1986; Houston, 1993) that correlate to similar units throughout the western and southwestern U.S.A. In Colorado, the mafic metavolcanic rocks are tholeiitic in composition as are scattered mafic, tholeiitic intrusions such as the Elkhorn Mountain (Snyder et al., 1988) and Mullen Creek (Loucks et al., 1988) complexes. Ages of the volcanic successions and mafic complexes range from 1.77 to 1.79 Ga (see summary by Reed et al., 1993) and mark the initial phase of orogenic Proterozoic igneous activity and Proterozoic crust formation in the basement of Colorado and Wyoming.
Between 1.63 and 1.75 Ga, this region of North America underwent a profound era of batholith emplacement similar to that found throughout Laurentia. Now variably metamorphosed and foliated, numerous batholiths of diorite, quartz diorite, and more felsic rocks invaded the older supracrustal sections (Fig. 1). Reed et al. (1993) recognized that the earlier “synorogenic” Proterozoic plutons of Colorado and Wyoming had trace element “volcanic arc” features, whereas later plutons had increasing aspects of “within-plate” chemical attributes.
Later plutonism in Colorado and Wyoming was Mesoproterozoic, ranging in age largely from 1.36–1.44 Ga, with a repeated phase at ∼1.1 Ga. From major batholiths to small plutons, these intrusions of Colorado and Wyoming are part of a profound Proterozoic episode that occurred on a global scale. Reddish-colored, alkali feldspar-rich, A-type granite is the dominant rock type, and along with associated intrusions of charnockite and anorthosite, comprise the “anorogenic trinity” of Anderson (1983). In Colorado and Wyoming, charnockite and anorthosite occur in the Laramie anorthosite complex (Frost et al., 1993; Mitchell et al., 1995, 1996; Scoates and Frost, 1996; O'Connor and Morrison, 1999), but are rare elsewhere.
This paper is focused on the Paleo- and Mesoproterozoic granitic plutonism of Colorado and Wyoming with emphasis on plutons emplaced at ∼1.7 and ∼1.4 Ga. Figures 2 and 3 are presented to show the essential chemical attributes of plutons in both events. Later Mesoproterozoic plutonism includes the 1.08-Ga Pikes Peak batholith (Barker et al., 1975, 1976b) which is described by Smith et al. in this issue.
The Archean province of Wyoming represents the oldest crust-forming event in this region and includes a number of plutons as young as 2.55 Ga (Reed et al., 1993; Houston, 1993; Frost et al., 1998). Both here and elsewhere on a global scale, a remarkable 800 m.y. hiatus in granitic plutonism occurred after 2.6 Ga. The scattered ∼1.8-Ga mafic plutons mentioned above represent the onset of renewed igneous activity in Wyoming and Colorado. Voluminous Proterozoic granitic plutonism began shortly thereafter, including the 1.75-Ga Sierra Madre intrusion (Premo and Van Schmus, 1989) of Wyoming, the 1.73-Ga Twilight gneiss (Barker, 1969, 1976a; Bickford et al., 1969), and 1.71-Ga Boulder Creek batholith (Gable, 1980; Premo and Fanning, 2000 in review) of Colorado. The Sierra Madre and Boulder Creek plutons are comprised of magnetiteseries, calc-alkaline trondhjemite and tonalite to granodiorite. Fundamentally metaluminous, the rocks typically are hornblende-, sphene-, and biotite-bearing. As shown in Figures 2 and 3, the Boulder Creek batholith has arc-related chemical attributes including low alkalies and calc-alkalic alkali-lime indexes. Independent of silica level, the Boulder Creek has the lowest iron numbers [ratio of FeO*/(FeO* + MgO)] relative to other 1.7-Ga plutons. In contrast, the Twilight gneiss intrusion is peraluminous and has a more evolved composition similar to younger 1.7 Ga plutons. Although trace element data are not available for the Twilight gneiss and the Sierra Madre plutons, Nb and Rb abundances for the Boulder Creek batholith (Anderson, unpublished data) fall in the “volcanic arc” (VAG) field of Pearce et al. (1984).
Later Paleoproterozoic plutons include the 1.71-Ga Crampton Mountain and Twin Mountain batholiths, the 1.69-Ga Pitts Meadow pluton, and the 1.67-Ga Kroenke granodiorite (Wobus et al., 1985; Bickford et al., 1969; Hallett, 1990; Reed et al., 1993). Although less silicic members include diorite and quartz diorite, significant parts are more felsic and include trondhjemite to monzogranite. The rocks contain hornblende, biotite, and sphene in more mafic metaluminous members and biotite ± sphene in peraluminous, more siliceous members; all are magnetite bearing. The plutons share some chemical attributes with the older intrusions in being calcalkaline, but clearly they are more iron-enriched (at the same level of silica) and more potassic (Fig. 2). Nb and Rb abundances range from VAG to the “within plate granite” (WPG) field, based on criteria of Pearce et al. (1984). Similarly aged plutonism occurred in the Black Hills of South Dakota, including the 1.71-Ga, post-tectonic Harney Peak granite. In contrast to ∼ 1.7-Ga plutons of Wyoming and Colorado, the Harney Peak is strongly peraluminous and, based on Nd and Pb isotopic data (Nabelek et al., this issue), was derived from crustal sources having a significant Archean component.
The oldest Proterozoic anorthosite includes the 1.76-Ga Horse Creek anorthosite complex of the Laramie Mountains of Wyoming (Scoates and Chamberlain, 1997; Frost et al., 2000 in press). As one of the younger Paleoproterozoic plutons of Colorado, the late-orogenic, 1.66-Ga Garell Peak batholith (Wobus et al. 1979; Bickford et al., 1989) represents the earliest intrusion of A-type granite. This is a characterization based on high alkalies (4 to > 6 wt. % K2O), an alkali-calcic alkali-lime index, and a range of elevated trace-element attributes. Composition of the Garell Peak is similar to numerous other Paleoproterozoic, A-type batholiths, including the 1.76-Ga Montello batholith of Wisconsin (Anderson et al., 1980) and a large number of 1.70–1.72-Ga batholiths in southern California and adjacent regions of western Arizona and southern Nevada (Wooden and Miller, 1990; Bender et al., 1993). The Garell Peak ranges in rock-type from metaluminous hornblende-biotite granodiorite to peraluminous biotite syeno-granite. Sphene and magnetite are ubiquitous accessory mineral phases. Based on total K2O + Na2O, less silicic parts fall into the “alkaline” field of Irvine and Baragar (1971). The batholith also is strongly iron-enriched, independent of silica level, with ratios of FeO*/(FeO*+MgO) ranging above 0.9 (Fig. 2). In terms of Nb and Rb, most samples fall in the WPG field of Pearce et al. (1984; Fig. 3). REE abundances also are higher relative to older plutons, such as that of the Crampton and Phantom Canyon batholiths (Cullers, unpublished data). All of these plutons, however, exhibit similar LREE/HREE ratios and negative Eu anomalies (Fig. 4). By all chemical criteria, the Garell Peak batholith is indistinguishable from 1.4-Ga biotite ± hornblende granites described below.
Following a 200 m.y. hiatus in igneous activity, a renewed period of plutonism at ∼1.4 Ga was part of a continuous belt of 1.3–1.6-Ga granitic intrusions that extends across Laurentia from the Baltic region to California. Almost all of these granites have textural, chemical, and mineralogic similarities to the classic rapakivi granites of Finland. Vorma (1971) succinctly described the attributes of rapakivi granites. Although rapakivi texture is notable only for the wiborgitic rapakivi granites, other textural varieties include pyterlitic (seriate with ovoidal, unmantled K-feldspars), porphyritic, even-grained, and tirilitic (green-colored and containing pyroxene). The last is considered part of the charnockite suite that is intimately associated with anorthositic intrusions worldwide. Other features include the presence of accessory fluorite, the abundance of K-feldspar, and a range of chemical attributes subsequently used as part of the definition of A-type granite (Whalen et al., 1987).
Anderson and Morrison (1992) presented data to show that this transcontinental A-type magmatic province is divisible into three major subprovinces, as follows: (1) low fO2, ilmenite-series granitic intrusions from the Baltic region to Wyoming; (2) high fO2, magnetite-series granitic intrusions of the central to southwestern U.S.A.; and (3) peraluminous, two-mica granitic intrusions from Colorado to central Arizona.
Anderson and Morrison (1998) also found that these mineralogic divisions are mirrored by substantial elemental and oxygen isotopic differences. The ilmenite-series granites, which often contain classic rapakivi textures and have the highest Fe/Mg ratios, also have the lowest whole rock δ18O values at 5.8–7.9‰. The magnetite-series granites were found to have lower Fe/Mg ratios and have higher whole rock δ18O values, ranging 7.8–10.1‰. Although they retain A-type characteristics, the peraluminous granites have the lowest Fe/Mg ratios, higher abundances of compatible elements including Mg and Sr, and the highest whole-rock δ18O values, ranging from 9.8 to 12.5‰.
Notably, all three of these subprovinces are represented in Colorado and Wyoming (Fig. 5). The ilmenite-series group includes the Sherman batholith, parts of the Virginia Dale ring complex, and the Log Cabin batholith of Wyoming and northern Colorado. Likewise, the magnetite-series granites include the San Isabel, Vernal Mesa, and Eolus batholiths of southern and western Colorado, and the peraluminous suite includes the Silver Plume, St. Vrain, Mt. Evans, St. Kevin, Cripple Creek, Curecanti, West McCoy, Oak Creek, and other two-mica granites of central and southern Colorado. Notably, Silver Plume-type granites are hosted largely in a regionally extensive Paleoproterozoic section of metasedimentary schists and gneisses, an observation made by Tweto (1987). In contrast, the other granites, which are fundamentally metaluminous, are hosted in an older supracrustal unit, in which the dominant lithology is metavolcanic.
Edwards (1993), Frost and Frost (1997), and Frost et al. (1999) described the Sherman Granite, which borders and intrudes the Laramie anorthosite. A sizeable portion is composed of red, coarse-grained, biotite ± hornblende seriate granite that, in hand sample, is indistinguishable from the Wolf River granite of Wisconsin (Anderson and Cullers, 1978) and pyterlitic phases of the Wiborg massif of Finland (Vorma, 1971). Notably, all three are within the ilmenite-series granite subprovince. Although numerous authors have drawn attention to the spatial and age relation between rapakivi granites and anorthosite (Anderson, 1983; Anderson and Morrison, 1992; Emslie, 1991, Windley, 1993), it is only the ilmenite-series rapakivi granites that occur with anorthosite and/or charnockitic members of the “anorogenic trinity.”
Parts of the Sherman Granite are inferred to extend southward across the Wyoming-Colorado border as part of the Virginia Dale ring complex, where they are intruded by two-mica granite of the Silver Plume suite (Eggler, 1968; Vasek, 1995; Vasek and Kolker, this issue). The Virginia Dale intrusion was interpreted by Eggler (1968) to be a subvolcanic, caldera structure. Vasek and Kolker (this issue) present macroscopic and supporting chemical evidence for the effects of mingling and mixing of mafic ferrodiorite with granite, correlated with that of the Sherman batholith in the Virginia Dale intrusion. Vasek and Kolker also offer an interpretation of the Virginia Dale structure, in which intrusion of mafic magma occurred into and along the floor of a part of the Sherman Granite batholith. The Log Cabin batholith of northern Colorado is similar to the Sherman Granite, but it largely lacks hornblende. Granites of the Sherman, Virginia Dale, and Log Cabin exhibit common chemical attributes, including high iron-enrichment. In contrast to most 1.7-Ga plutons, all have alkali-calcic alkali-lime indexes (Fig. 2). Nb, Y, and Rb abundances largely, but not exclusively, fall in the WPG field (Fig. 3).
The most abundant 1.4-Ga granites in Colorado are peraluminous and contain variable amounts of biotite + muscovite. Anderson and Morrison (1992) characterized these as “Silver Plume-type” and further noted correlative plutons in the southwestern U.S.A., including parts of New Mexico and central Arizona. The rocks are notably less iron-rich than other granites of this age. Although they are more potassic than most of the other 1.4-Ga granites, they also have greater abundances of some compatible elements, including Mg and Sr. However, their Nb, Y, and Rb trace-element compositions fall well within the WPG field of Pearce et al. (1984). Parts of the Silver Plume and Oak Creek batholiths (Anderson and Thomas, 1985; Cullers et al., 1993) have strikingly high Th. They also are mineralogically distinctive. In addition to magmatic muscovite, monazite is a common accessory phase. The Silver Plume and St. Vrain batholiths of central Colorado also contain magmatic sillimanite. Magnetite is the dominant oxide, and sphene commonly is absent.
The peraluminous 1.4-Ga granites have similarities to S-type granites of orogenic belts (Thompson and Barnes, 1999), including their aluminous mineralogy, high δ18O, and low Fe/Mg ratios. However, they are also very different. As depicted in Figure 6, these granites have much higher abundances of K, Y, La, Th, and Zr relative to the classic S-type granites of Australia (Chappell and White, (1992) and other peraluminous granites of orogenic belts. Fundamentally, these attributes reinforce the interpretation that they are simply a subset of A-type granites, like the other 1.4-Ga anorogenic granites of this region, and North America in general.
Although several of these granites in Colorado are fairly homogeneous, Cullers et al. (1992, 1993) have shown two batholiths, the 1.36-Ga San Isabel and 1.44-Ga Oak Creek, to be unusual in having significant parts with high mineralogic concentrations of feldspar, biotite, and accessory minerals. Associated high concentrations of Fe, Mg, Ti, Sc, Ba, and Sr fall on linear arrays explained by variable degrees of unmixing of melt and cumulate solid phases. Similar mineralogical and chemical attributes have been described for parts of the 1.49-Ga Wolf River batholith of Wisconsin (Anderson and Cullers, 1978).
The San Isabel, Vernal Mesa, and Eolus batholiths of southern Colorado (Hansen and Peterman, 1968; Bickford and Cudzilo, 1975; Thomas et al., 1984; Collier, 1989; Cullers et al., 1992) are magnetite-series, metaluminous granite batholiths that have counterparts throughout the midcontinent and into the southwestern U.S.A. (Anderson and Morrison, 1992). Both the San Isabel and the Vernal Mesa have sufficient magnetite to generate positive magnetic anomalies (Case, 1966; Zietz and Kirby, 1972). The abundance of magnetite potentially signifies high fO2 during magmatic crystallization, and this inference is supported by lower Fe/Mg ratios in mafic silicates (Cullers et al., 1992) relative to that observed for other 1.4-Ga granites. The San Isabel batholith is distinctive in being the most deeply emplaced batholith of all Mesoproterozoic A-type batholiths known worldwide. Based on hornblende barometry, Cullers et al. (1992) estimated emplacement conditions on the order of 6 kbar, consistent with presence of magmatic epidote. As such, the San Isabel appears to be the only known granite batholith of rapakivi affinity to be so deeply emplaced and to have magmatic epidote. The San Isabel granite, and the nearby Oak Creek batholith, have very high REE abundances. Notably, the 1.4-Ga granites have higher REE than those of 1.7-Ga age (Fig. 4).
The 1.08-Ga Pikes Peak batholith (Barker et al., 1975, 1976b) of the Colorado Front Range comprises one of several 1.03–1.10-Ga anorogenic intrusions in the central and southern U.S.A. The others include the Enchanted Rock, Lone Grove, and related intrusions of the Llano region of central Texas (Smith et al., 1997) along with the Red Bluff batholith of west Texas (Shannon et al., 1997).
Smith et al. (this issue) offer a current review of work on the Pikes Peak batholith, and thus only salient points will be noted here. The Pikes Peak is comprised of a potassic and a sodic series. The former is volumetrically dominant and is largely comprised of biotite and biotite-hornblende granite. The principal oxide phase is ilmenite. This sizeable part of the batholith, therefore, is chemically similar to reduced, ilmenite-bearing 1.4-Ga granites, such as the Sherman Granite. Rock units within the Sherman and Pikes Peak intrusions not only share similar rock and iron-rich mineral chemistries, but also are similar in appearance. The most common rock of the Pikes Peak is a reddish, coarse-grained, seriate syenogranite like much of the Sherman batholith. Thus, it should be of no surprise that early workers considered the Pikes Peak and Sherman granites to be correlative (see review by Tweto, 1977).
The sodic series of the batholith includes a number of satellite intrusions, such as the Sugarloaf pluton (Beane and Wobus, this issue). These plutons are principally syenitic and contain sodic amphiboles as accessory phases.
Each of the two series of the Pikes Peak batholith are A-type in conventional granite terminology, but clearly they have much different origins. Barker et al. (1975) argued that the sodic rocks of the Pikes Peak batholith were derived by fractionation from an alkali basaltic parent melt, whereas granites of the potassic series were considered to result from crustal melting.
ISOTOPIC CONSTRAINTS ON ORIGIN
Based on data provided by DePaolo (1981), Bennett and DePaolo (1987), and Frost et al. (1999), the Nd isotopic composition of Proterozoic igneous rocks of Colorado and Wyoming provides substantial insight with respect to the origins of the major batholiths of the region. DePaolo's Nd isotopic data for Paleoproterozoic rocks came principally from volcanic successions augmented by samples of granulite xenoliths. Values of ϵNdt in these units range from +3.2 to +4.2 at ∼2.0 Ga, and are consistent with a depleted mantle origin for these oldest Proterozoic rocks (Fig. 7). In contrast, data for 1.7–1.8 batholiths have ϵNdt values of + 1.4 to +3.6 at ∼1.8 Ga and are displaced below DePaolo's trend for depleted mantle. Although considerable more work needs to be done, the existing Nd data are compatible with a range of mantle and crustal components in the origin of these magma systems.
The Nd isotopic composition of the Sherman batholith (Frost et al., 1999) yields ϵNdt values of + 1.8 to −1.2 and the ϵNdt data for the Silver Plume, St. Vrain, and other peraluminous granites of DePaolo (1981) range from + 0.2 to −2.3. The data are consistent with derivation from melting of older Protero-zoic crust of differing Sm/Nd ratios. Minor incorporation of an Archean component is possible for the Sherman batholith only in its northern-most region (Frost et al., 1999).
Based on their work on the ilmenite-series, Wolf River batholith of Wisconsin, Anderson and Cullers (1978) proposed that such rapakivi granites were produced from limited melting of lower crustal rocks of tonalitic composition. Creaser et al. (1991) presented support for this model. Anderson and Morrison (1998) found that the 1.4–1.5-Ga plutons have significant whole rock oxygen isotopic variations, including δ18O at 5.8–7.9‰ for ilmenite-series plutons such as the Sherman, 7.0–10.1‰ for magnetite-series granites, and 9.8–12.5‰ for peraluminous granites, including the Silver Plume and others of Colorado. Frost and Frost (1997) concluded that the low fO2, elemental, and isotopic composition of the Sherman batholith are best explained by melting of a reduced lower crustal source comprised of mafic and related differentiated units derived from the Laramie anorthosite complex. This model is consistent with the low oxygen isotopic composition of these rocks and thus may represent a viable model for ilmenite-series rapakivi granites in general.
Anderson and Morrison (1998) concluded that a more oxidized crustal source is required for magnetite-series metaluminous granites, such as the San Isabel and Vernal Mesa granites. They also suggested that a significant metasedimentary component is required in the crustal source for the two-mica granites, such as the Silver Plume and similar granite intrusions.
TECTONIC SETTING OF 1.4-Ga PLUTONISM
Numerous investigators have concluded that the 1.4-Ga granite batholiths of Colorado and Wyoming are anorogenic in origin and are part of a transcontinental event that spans Laurentia (Anderson, 1983; Anderson and Thomas, 1985; Tweto, 1987; Cullers et al., 1992, 1993). However, recent work in Colorado (Graubard, 1991, Shaw et al., 1999) and elsewhere in the western U.S.A. (Nyman et al., 1994; Nyman and Karlstrom, 1994; Kirby et al., 1995; Duebendorfer and Christensen, 1995) has questioned this view. The questions are based on observations that: (1) several granitic batholiths of ∼1.4-Ga age have foliations indicative of emplacement synchronous with regional tectonic deformation; and (2) many Paleoproterozoic metamorphic rocks yield cooling ages close to 1.4 Ga.
As elsewhere in North America, the ∼1.4 Ga-granite batholiths of Colorado and Wyoming typically are foliated on a local scale, particularly near outer margins and along outer contacts of major blocks of older units. Our interpretation is that much of this foliation is magmatic in origin. Reed et al. (1993) summarized a range of data for Proterozoic plutons in Colorado, noting that almost all of these plutons contain areas of foliation. Reed et al. (1993) further attribute most of these foliations to a magmatic origin.
However, the fact that these Mesoproterozoic plutons contain structural elements of tectonic deformation is not new. In regard to the Silver Plume and similar batholiths of Colorado, Tweto (1987, p. 31) acknowledged their anorogenic setting and wrote: “Although they were emplaced long after the major deformation … they were accompanied by deformation in some areas. This deformation was principally by fracture, and it led to development or reactivation of major shear zones and faults, mainly of northeast and north-northwest trends.” Tweto (1987, p. 46) further noted that these zones of deformation originated prior to 1.7 Ga and thus suggested that these “preexistent orogenic features” served as a guide to intrusion 300 m.y. later.
Metamorphic and/or cooling ages in Proterozoic metasedimentary rocks throughout much of North America commonly yield dates at ∼1.4 Ga. The 1.4-Ga plutons crosscut these supracrustal rocks and are not metamorphosed. A vast amount of 1.4-Ga granite magma has intruded into the older crust now exposed in Colorado and Wyoming, including ∼35 percent currently exposed. Unknown parts of the Colorado Proterozoic section had 1.4-Ga plutons that either passed through or lie emplaced below current exposure levels. The 1.4-Ga granite-forming event was profound in its regional extent, and it should not be surprising that considerable re-setting of isotopic systems occurred at 1.4 Ga.
OROGENY AND 1.4-Ga DIKE SWARMS
Many 1.4-Ga plutons of the western U.S.A. contain areas of foliation. Although some of the foliation was tectonic in origin, we do not interpret these foliations as evidence of orogenic events. There are many definitions of “orogeny.” We embrace Hatcher's (1995, p. 490) definition of orogeny as “… a process of mountain building, accompanied by metamorphism, plutonism, and associated deformation, resulting from subduction, terrane accretion, and/or continental collision.” In particular, the period of 1.4–1.5 Ga in this region lacks the basic attributes of orogeny, including terrane accretion and production of vast volumes of juvenile magmas typical either of modern or ancient subduction zones. This is in contrast to 1.4–1.5-Ga plutons in Brazil, for example, which appears to be one of a few regions in the world that was undergoing orogenic processes at that time, including emplacement of mantle-derived intermediate magmas (Geraldes et al., 1998). Elsewhere, the age of 1.4 to 1.5 Ga coincides with emplacement of discordant masses of rapakivi granite, charnockite, and anorthosite, including the Baltic region of northern Europe (Bridgwater and Windley, 1973; Ramo, 1991; Ramo and Haapala, 1995), the Ural Mountains (Aberg, 1988), Ukraine (Herz, 1969), peninsular India (Leelanandam and Reddy, 1988), southern Africa (Conradie and Schoch, 1986), and central China (Jianhua et al., 1991).
The fact that some 1.4-Ga plutons were intruded synchronously with faulting is important, but the existence of such regional tectonism does not necessarily preclude that the plutons were emplaced during an anorogenic period. We suggest that at 1.4 Ga the regional stress-field, in what is now Wyoming and Colorado, was broadly extensional. We base that conclusion on the widespread distribution of immense mafic dike swarms that intrude the Sherman Granite and correlative plutons but predate emplacement of the Silver Plume and similar two-mica granites (Tweto, 1987). Existence of the swarms has been noted for some time (Eggler, 1968; Braddock et al., 1970; Abbott, 1976) and existing hornblende K-Ar dates of 1.35 ± 0.04 Ga (Peterman et al., 1968) and 1.42 ± 0.07 Ga (Ferris and Krueger, 1964) are consistent with the interpretation that the dikes are coeval with intrusion of 1.4 Ga plutons. Comprised of tholeiitic diabase and tholeiitic andesite (Snyder, 1978, 1980), the dikes have a persistent N-NW trend throughout the Front, Park, Sawatch, and Laramie Ranges. Similar mafic intrusions occur in the Virginia Dale intrusion (Vasek and Kolker, this issue). A later mafic dike-swarm postdates timing of the Silver Plume intrusions, which includes the “Iron Dike” of Wahlstrom (1956). The “Iron Dike” is sonamed because of its high titaniferous magnetite content. The swarm is exposed from near Boulder, Colorado to the eastern Medicine Bow Mountains of Wyoming. Rocks of the post-Silver Plume swarm are undated, but they are believed to be Mesoproterozoic in age based on style of structural emplacement (Tweto, 1987).
Anderson (1987) and Anderson and Bender (1989) offered a “crustal overturn” model for origin of the Mesoproterozoic anorogenic granites. Characteristically, Paleoproterozoic rapid crust production above undepleted mantle led to rise of mantle-derived, anorthositic magmas and subsequent melting of the lower crust. Hoffman (1989) presented essentially the same model, coining the term “superswells” for unstable parts of the mantle beneath crust formed during the Proterozoic orogenies. A key observation leading to these models is the significant Proterozoic (and minimal Archean) crustal component in the source of these granites (Anderson, 1983; Anderson and Morrison, 1992).
Paleo-to Mesoproterozoic granitic plutonism of Colorado and Wyoming exhibits classic features of variable styles of igneous activity that occurred during and after orogeny. South of the Wyoming Archean craton, crustal development of areas that encompassed southern Wyoming, all of Colorado, and the remainder of the western and southwestern U.S.A. originated from orogenic processes of tectonic accretion and magmatism at 1.7–1.8 Ga. The earliest volcanic and plutonic suites were tholeiitic in composition, which soon were succeeded by large calc-alkaline batholiths, such as the 1.7-Ga Boulder Creek batholith. Younger (1.6–1.7 Ga) granitic batholiths exhibit a shift in composition toward higher levels of incompatible elements, and one of the youngest plutons (the 1.67-Ga Garell Peak batholith) is an A-type granite.
After a 200–300 m.y. hiatus in igneous activity, renewed plutonism in Colorado and Wyoming at ∼1.4 Ga was fundamentally A-type. This was part of an “anorogenic” magmatic event that occurred throughout Laurentia and elsewhere in the world. The typically reddish-colored, alkali feldspar-rich granites have considerable mineralogic, elemental, and isotopic diversity, consistent with variations in composition of the lower crust from which they were derived. In particular, their isotopic, elemental, and mineralogic variation is attributed to the state of oxidation and extent of metasedimentary component in lower crustal levels. Much of their crustal source was Proterozoic in age, and any Archean component was minimal. We also see no evidence for origin by fractionation from more mafic magmas, unlike the sodic, riebeckite-bearing parts of the Pikes Peak batholith. Although fractionation and accumulation of mineral phases was a common and active process in these magma systems (Cullers et al., 1992, 1993), the most primitive magmas were fundamentally granitic in composition.
Demonstration that the ultimate control on magma origin did not lie with tectonic setting comes from the facts that: (1) anorthosite originated as early as the 1.76-Ga Horse Creek complex; and (2) A-type granitic magmas appeared as early as the 1.67-Ga Garell Peak batholith. In regard to origin of the diverse range of granitoid batholiths in Colorado and Wyoming, it seems clear that the principal control lay with elemental and isotopic composition of crustal sources and the conditions of P-T-fH2O-fO2 attending magma formation.
We suggest that the principal source of the 1.4-Ga granites was a tonalitic or similar crustal rocktype of Proterozoic age. At 1.4 Ga, mantle-derived intrusions led to partial melting, the extent of which was limited to stability of hydrous phases in this crust under vapor-absent conditions. At low fO2, such as with fractionated phases of the anorthosite suite (Frost and Frost, 1997), the hydrous phases remained stable until fairly high temperatures (> 900°C). Subsequent low fractions of melting led to extreme A-type compositions of the reduced, ilmenite-series plutons (e.g., 1.4-Ga Sherman and 1.1-Ga Pikes Peak batholiths). Lower-crustal sections comprised of magnetite-series calc-alkaline plutons would have melted at lower temperatures, and resulting higher fractions of melting would have lowered the abundance of incompatible elements as well as the Fe/Mg ratio of derived melts. Noting that Silver Plume-type granites are nearly exclusively hosted in country-rock terranes dominated by metasedimentary gneisses, and assuming that such materials exist at depth, partial melting would have proceeded at even lower temperatures, and the extent of melting would have been even greater. The abundance of K and Th could be expected to have remained high due to effects of source composition and preferential involvement of micas in melting reactions. The higher degree of melting, of course, would have further lowered abundances of most incompatible elements and increased the abundances of Ca, Mg, Sr, and other compatible elements, as observed in these granites.
Although an “anorogenic” setting for the Mesoproterozoic has been questioned, we draw attention to the: (1) abundance of extension-related dike swarms emplaced during and after this magmatic episode; (2) lack of magmatism typical of subduction zones; and (3) lack of processes of terrane accretion at this time.
This manuscript benefited immensely from reviews by Calvin Barnes, Ron Frost, and Jean Morrison. The authors also very much appreciate the input and patience exhibited by our editor-in-chief, Carol Frost.
- Received January 28, 1999.
- Revision received April 6, 1999.
- Accepted April 12, 1999.