In the Wet Mountains of central Colorado, we document evidence for increasing metamorphic grade and associated higher amounts of partial melting along a transect from northwest to southeast. Field observations of structural orientation and style, qualitative assessment of strain intensity, analysis of metamorphic mineral assemblages, and macroscopic identification of leucosomes and migmatites are complemented by the use of melt microstructures to carefully document the presence and locations of former partial melt and to identify melt-producing reactions. In the northwest Wet Mountains, migmatitic foliation is moderately well developed, and partial melting occurred via granite wet melting and muscovite-dehydration melting, with rare melt pseudomorphs remaining. At Dawson Mountain in the central part of the range, inferred former melt channels are preserved along grain and subgrain boundaries, deformation appears more intense, and anatexis occurred through biotite-dehydration melting. Farthest to the south, the highest intensity of strain is inferred, with very closely spaced foliations, abundant dynamic recrystallization, and local mylonitization occurring in rocks of granitic composition, and partial melting occurring via granite wet melting. Metapelitic rocks experienced biotite-dehydration melting and contain garnet with Mn-rich rims and Mn-poor cores mantled by plagioclase, decussate biotite, and quartz, textures indicating back-reaction between melt and garnet. These textures indicate there was abundant melt within these highest-grade rocks and extensive melt-rock interaction. Throughout the Wet Mountains, deformation is concentrated in areas where melt-producing reactions occurred, and melt appears to be localized along deformation-related features, suggesting a correlation between partial melting and deformation. The northern Wet Mountains contain few plutons, whereas the central and southern portions of the Wet Mountains contain more pervasive dikes and sills and may contain more former melt as a result of both higher metamorphic grade and widespread thermal insulation. The Wet Mountains represent an exhumed section of formerly molten middle crust located at the transition between upper and lower crust and provide insight into processes ongoing at depth in modern orogenic belts. The microstructures indicative of former partial melt, textures associated with melt-rock interaction, and melting reactions we have identified in the Wet Mountains will greatly facilitate the recognition of other such exhumed sections.
Geophysical imaging and recognition of high heat flow in modern orogenic belts provide evidence for a partially molten middle and lower crust. Bright seismic reflectors, high conductivity, and low-velocity zones have been identified at depths of 15–20 km below the Tibetan Plateau (Nelson et al., 1996; Brown et al., 1996) and are interpreted as a midcrustal magma layer. High electrical conductivity and an inferred partial melt layer have also been identified beneath the Bolivian Altiplano (Schilling et al., 1997; Brasse et al., 2002), the Pyrenees (Pous et al., 1995), and eastern Anatolia (Türkoğlu et al., 2008), suggesting the phenomenon of a midcrustal partially molten layer may be a characteristic of many orogens. The presence of a molten layer in the middle crust will strongly affect how the crust deforms, and this layer has been called upon to explain orogenic collapse and subsequent channel flow (Clark and Royden, 2000; Beaumont et al., 2001; Rey et al., 2001), and strain partitioning into the rheologically weak layer (Rosenberg and Handy, 2005). Rosenberg and Handy (2005) demonstrated that there is a dramatic decrease in strength when the melt fraction is quite small (even less than 0.05), and they proposed that at the onset of syntectonic melting, deformation is immediately localized into the melt. Melt segregation in migmatites is driven primarily by deformation of partially molten areas (McLellan, 1988; Sawyer, 1994; Brown, 1994; Brown and Rushmer, 1997), but deformation can also promote partial melting reactions, with deformation contributing to melting in migmatites through enhanced diffusion rates (Etheridge et al., 1984). As a result, deformation and partial melting are likely to each enhance the other concurrent process.
Exhumed pieces of once partially molten middle and lower crust should be preserved within the cores of many Precambrian orogenic belts, and detailed investigation of the granitic plutons and networks of dikes, migmatites, and residual granulites that comprise these ancient orogens can shed light on the processes that were operating at depth in these terrains and are currently operating in modern orogens. The Wet Mountains of south-central Colorado provide an example of one such exhumed section of formerly molten Proterozoic middle crust and provide insight into modern orogenic processes (Shaw et al., 2005; Jones et al., 2010). Rocks in the Wet Mountains are characterized by lower metamorphic grade in the northwestern portion of the range that increases in grade toward the southeast, with abundant migmatites present in the central and southern areas (Siddoway et al., 2000; Jones et al., 2010). Abundant granitic bodies are found throughout the Wet Mountains, but their character changes from coherent, elliptical plutonic bodies in the north to a more sheet-like body in the central Wet Mountains, interpreted as a midcrustal magma layer (Shaw et al., 2005), and numerous small plutonic bodies, arranged in more of a dike and sill network in the southern part of the range (Jones et al., 2010).
In this paper, we analyze melting reactions, microstructures associated with partial melting, and the strain history of the rocks in the Wet Mountains, paying careful attention to the influence of an increasing metamorphic gradient and change in style of magmatism toward the southeast. Petrographic and microstructural analysis are used to provide better constraints on metamorphic pressure and temperature conditions, and clarify the relationships among deformation, metamorphism, and partial melting, particularly in the central and southern Wet Mountains. We provide careful documentation of the types of microstructures that are characteristic of partial melting in the middle crust along a transect from northwest to southeast in the Wet Mountains. Our detailed analysis and documentation of these features will help in identifying other ancient examples of partially molten middle and lower crust and in understanding the types of processes that may be operating in modern orogens.
MELTING REACTIONS AND ASSOCIATED MICROSTRUCTURES
In migmatite terrains worldwide, two melting reactions are common: granitic wet melting reactions and dehydration melting reactions. Fluid-present granite eutectic melting (quartz + plagioclase + K-feldspar + H2O = melt) is generally the first melting isograd to be crossed in migmatites with a granitic protolith at temperatures of ∼640 °C (Wyllie, 1977). Although this assemblage is ubiquitous in migmatite terrains, and the reaction is fairly simple, it can be difficult to determine where melting occurred within a given sample, because of the lack of recognizable peritectic reaction products.
Dehydration reactions have the advantage of clearly indicating the site of melt generation, because of obvious peritectic products of melting, but they occur at higher temperatures than fluid-present melting of granitic protoliths. The muscovite-dehydration reaction (quartz + muscovite = melt + sillimanite + K-feldspar) typically occurs at 675–800 °C, depending on pressure (Thompson and Algor, 1977; Le Breton and Thompson, 1988; Spear, 1995; Spear et al., 1999). Biotite-dehydration melting (biotite + quartz + plagioclase + sillimanite = melt + garnet + K-feldspar) occurs at slightly higher temperatures than muscovite-dehydration melting, generally 760–800 °C, increasing at higher pressures and depending on the presence of other minerals involved in the reaction (Le Breton and Thompson, 1988).
Microstructures indicative of former partial melt or textures recording melting reactions have been described recently by many authors (Sawyer, 1999, 2001; Waters, 2001; Barbey, 2007; Holness and Sawyer, 2008; Kriegsman and Álvarez-Valero, 2010). In rocks of a metapelitic composition where peritectic products of melting formed, melt textures are comparatively easy to identify. They include: solid products of melting reactions with euhedral crystal faces against the melt (Sawyer, 1999, 2001), reactant phases with rounded or corroded boundaries surrounded by melt films (Mehnert et al., 1973; Büsch et al., 1974), small cuspate-shaped melt pools similar to those formed in experimental studies (Harte et al., 1991; Sawyer, 1999; Rosenberg and Riller, 2000; Holness and Sawyer, 2008), and also intergrowths between quartz and solid products of melting (Waters, 2001; Barbey, 2007). More specifically, biotite-dehydration melting may lead to garnet growth, characterized by garnet containing abundant quartz inclusions and surrounded by a halo of coarse-grained quartz, absent of biotite (Waters, 2001; Barbey, 2007).
Microstructures are more difficult to identify in rocks of a granitic composition because the crystallized melt is made up of the same components as the reactants (Holness and Sawyer, 2008; Sawyer, 2010). Some textures that can be used to infer the former presence of melt specifically in granitic rocks include: cuspate or serrate grain boundaries with low dihedral angles, pseudomorphs of melt along grain boundaries of unlike phases (i.e., K-feldspar between two plagioclase grains), melt pseudomorphs present at grain boundary triple junctions and multiple grain junctions, and grains with straight crystal faces that indicate crystallization from a melt (Vernon and Collins, 1988; Harte et al., 1991; Rosenberg and Riller, 2000; Sawyer, 2001; Holness and Sawyer, 2008).
Throughout this paper, the melt pseudomorphs are described as being composed of the minerals that now occupy the location of the former melt. In granitic eutectic melting, the melt is composed of quartz, plagioclase, and K-feldspar components. When the melt begins to crystallize, each component of the melt tends to nucleate and crystallize onto any preexisting mineral grains of the same composition (Harte et al., 1991; Rosenberg and Riller, 2000; Holness and Sawyer, 2008). For example, if melt is present between plagioclase and K-feldspar, the K-feldspar component of the melt will crystallize onto the preexisting K-feldspar grain, and the plagioclase will crystallize onto preexisting plagioclase grains. If quartz is the limiting phase of the reaction, the quartz will not have any preexisting grains of quartz on which to nucleate and crystallize, so the quartz will be preserved on the plagioclase–K-feldspar grain boundary as a melt pseudomorph.
GEOLOGICAL SETTING AND PREVIOUS WORK
The Wet Mountains are a northwest-trending mountain range, composed of Proterozoic gneisses, schists, amphibolites, and granites, located in the Rocky Mountains of south-central Colorado (Fig. 1). From north-northwest to south-southeast, the metamorphic grade increases, from greenschist-amphibolite to granulite facies, as a result of exhumation of progressively deeper crust toward the south (Siddoway et al., 2000). Associated with this increasing depth of crustal exposure, there is a change in structural style from upright open folds and vertical foliation in the north to shallowly to moderately dipping foliation and more widespread and pervasive plutonism in the central and southern parts of the range (Jones et al., 2010). The oldest metamorphic rocks are interpreted to have been a volcanic and sedimentary sequence formed in a convergent margin between 1780 and 1717 Ma (Bickford et al., 1989; Reed et al., 1987). Rocks in the northern Wet Mountains include garnet, sillimanite, and cordierite gneisses or schists, amphibolites, felsic gneisses, and siliceous schists, and they are the lowest-grade rocks in the area, with temperatures and pressures of ∼700 °C and 0.5 GPa (Givot and Siddoway, 1998; Siddoway et al., 2000). Higher-grade rocks, including migmatitic gneisses and amphibolites, are found throughout the central and southern Wet Mountains and have experienced uppermost amphibolite- to granulite-facies metamorphism (Lanzirotti and Condie, 1988; Brock and Singewald, 1968).
Three episodes of granitic plutonism are recognized from field relationships and U-Pb geochronology (Bickford et al., 1989; Jones et al., 2010). The oldest plutons are found in the northern Wet Mountains and are Paleoproterozoic in age, ranging from 1705 to 1615 Ma (Bickford et al., 1989). Additionally, some smaller dikes and sheets of granitoid intrusions can be linked with these Paleoproterozoic intrusive bodies (Bickford et al., 1989; Siddoway et al., 2000). All Paleoproterozoic magmatism is described as the G1 magmatic event (Jones et al., 2010). Mesoproterozoic plutons are divided into those older than 1400 Ma, and a smaller group of ca. 1360 Ma plutons (Bickford et al., 1989; Jones et al., 2010). In addition to larger plutons, smaller dikes and sills also yield similar Mesoproterozoic ages.
New U-Pb geochronologic data from Jones et al. (2010) have served to better distinguish the two episodes of Mesoproterozoic magmatism. In the northern Wet Mountains, the Mesoproterozoic magmatism appears to have been part of a single protracted episode. A syntectonic pegmatite within the Five Points deformation zone gives a U-Pb zircon age of 1430 +5/–3 Ma. The West McCoy Gulch pluton, yields an age of 1474 ± 7 Ma (Bickford et al., 1989) and is located several kilometers southwest of the Five Points deformation zone. The Oak Creek pluton located 10 km south to southwest of Cañon City, gives a U-Pb zircon age of 1442 ± 7 Ma (Bickford et al., 1989; Cullers et al., 1993). In the southern Wet Mountains, Jones et al. (2010) have distinguished foliated coarse-grained granites within 1–5-m-thick sills (G2) concordant with host gneiss foliation from finer-grained foliated granites within 0.5–5-m-thick sills (G3) that crosscut both host-rock foliation and the foliation in G2 granites. The coarse-grained G2 granite gives a U-Pb zircon age of 1435 ± 4 Ma, whereas the fine-grained G3 granite gives a younger U-Pb zircon age of 1390 ± 10 Ma (Jones et al., 2010).
Rocks in the Wet Mountains have undergone three phases of deformation, but differences in crustal exposure from north to south have led to preservation of primary bedding and earliest deformation fabrics only in low-strain locations in the northern Wet Mountains (Siddoway et al., 2000). Cordierite poikiloblasts, up to 20 cm in length, within cordierite schists exposed in the Five Points Gulch, near the Arkansas River Canyon of the northern Wet Mountains, preserve evidence of relict bedding, which is defined by patterns of quartzofeldspathic and opaque mineral inclusions (Siddoway et al., 2000). In this vicinity, D1 is characterized by the crenulation of S0 bedding, forming F1 folds, and associated development of an S1 crenulation cleavage, which were both synchronous with growth of the cordierite poikiloblasts (Siddoway et al., 2000). Exposures in the central and southern Wet Mountains preserve little evidence of D1 due to overprinting by higher-grade migmatitic fabrics (Siddoway et al., 2000).
The S2 foliation is the dominant foliation throughout the Wet Mountains and is typically found with an associated mineral lineation, L2. In the northern Wet Mountains, this foliation strikes east-west to northwest-southeast and dips moderately to steeply, with lineations plunging moderately to the north-northeast, whereas in the central and southern Wet Mountains, foliations strike east-west to northeast-southwest and dip moderately to steeply, with lineations plunging moderately to the northeast, north, and northwest (Siddoway et al., 2000; Jones et al., 2010). F2 folds are tight to isoclinal in style and are more easily recognized in the compositionally varied gneisses of the northern Wet Mountains than in more compositionally uniform migmatites of the central and southern Wet Mountains (Siddoway et al., 2000).
D1 is constrained to have occurred synchronously with 1.67 Ga metamorphism based on U-Pb zircon ages from granulites in the central portion of the range, and granodioritic sills containing foliation that is concordant with host-rock foliation (1.67 Ga; Bickford et al., 1989). The subsequent D2 deformation had been thought to coincide with Paleoproterozoic magmatism; however, new U-Pb data from Jones et al. (2010) show that D2 was associated with the Mesoproterozoic G2 magmatism. The timing of D2 has been determined on the basis of U-Pb ages for metamorphic zircon in amphibolite gneisses that are 1436 ± 2 Ma and for igneous zircons from G2 granitic sills that give an average age of 1435 ± 4 Ma.
Throughout the Wet Mountains, high-strain zones formed during D3 exhibit an S3 foliation that is distinct in orientation from S2 (Siddoway et al., 2000). One of the zones, the Five Points deformation zone, is a 2–5-km-wide shear zone with north-south–striking foliation that truncates a kilometer-scale F2 fold on its western boundary and bounds domains of contrasting metamorphic grade (Siddoway et al., 2000; Jones et al., 2010). High-strain zones in the central and southern Wet Mountains have west-southwesterly striking foliations with shallower dips than lower-strain fabrics outside of these zones. The boundaries of the high-strain zones are poorly defined and probably are gradational.
This study relied on previous detailed structural mapping throughout the Wet Mountains and the recognition of an increase in metamorphic grade from northwest to south-southeast (Brock and Singewald, 1968; Lanzirotti, 1988; Siddoway et al., 2000, and references therein; Jones, 2005; Jones et al., 2010) to identify optimal study locations within high-strain domains developed within migmatites. Five locations were studied, with one in the north, one within the central Wet Mountains, and three in the southern part of the range (Fig. 1). Although each location contains migmatites, the abundance of former melt, used as a qualitative indication of the amount of melt formerly in these rocks, is variable between locations, and the degree of strain is not always uniform across a location. The term former melt is used more than leucosome throughout the paper, because, in many locations, the former melt is not macroscopically visible. Qualitative determination of the amount of melt formerly contained within these rocks was based on the abundance of melt microstructures, degree of preservation of original textures, corrosion of reactant minerals, and inferred back-reaction between melt and peritectic minerals. An actual estimate of the amount of melt produced during anatexis was beyond the scope of this study. Qualitative determination of strain magnitude came from analyzing foliation spacing, aspect ratios of quartz and feldspar, and degree of dynamic recrystallization. These rocks do not provide any strain indicators that can be used to quantitatively assess the degree of strain across the Wet Mountains. However, several recent studies have used foliation intensity, given by foliation spacing and aspect ratio of planar elements as a qualitative measure of strain magnitude (Goscombe et al., 2006; Waters-Tormey and Tikoff, 2007). Foliation spacing is also dependent on the original composition of the rocks, however, so it is only valid to use foliation spacing when comparing the same rock types. The rocks described in detail in this study are either granitic gneisses or metapelites, and comparisons within each of these rock types provide a qualitative estimate of the degree of strain across the Wet Mountains. Additionally, dynamic recrystallization is driven by internal plastic strain; hence, more dynamically recrystallized rocks record more strain. Data collected at each of the five field localities (Fig. 1) include measurements of foliations and lineations, as well as fold hinges when available, descriptions of mineral assemblages, and types and textures of migmatites, and oriented samples.
FIVE POINTS DEFORMATION ZONE
The Five Points deformation zone extends for 2–5 km along the Arkansas River and is well exposed in spectacular road cuts along Highway 50 (Fig. 1). West of the Five Points deformation zone, the Texas Creek domain (Fig. 1) is dominated by a kilometer-scale F2 anticline that is truncated by the Five Points deformation zone (Siddoway et al., 2000). Rocks within the Texas Creek domain are compositionally similar to those in the Five Points deformation zone. East of the deformation zone, there is the Sheep Basin domain (Fig. 1), which is composed mainly of tonalites, quartz diorite, and granodiorites of the Crampton Mountain pluton (Bickford et al., 1989).
Rocks found within the deformation, or high-strain, zone are dominated by muscovite-biotite-quartzofeldspathic gneisses (Fig. 2A). Many rocks contain lineation-parallel “pods” of sillimanite and muscovite, up to 5 cm in length. Garnet occurs in the centers of some of these pods of muscovite and sillimanite (Fig. 2A). Magnetite porphyroblasts are present within elliptical pods of quartzofeldspathic material and resemble peritectic products of partial melting, but instead formed as a result of solid-state biotite- or amphibole-dehydration reactions and are known as flecked gneisses (Trumbull, 1988). Other rocks found in this area include amphibolites, biotite-rich granitic gneisses, and rarer sillimanite-biotite schists.
Structurally, these rocks are characterized by the east-west– to northwest-southeast–striking, steeply north- to northeast-dipping S2 foliation (Fig. 3A). L2 lineations are usually defined by aligned “pods” of sillimanite and muscovite, or biotite, and they plunge moderately to steeply to the north-northeast (Fig. 3A). Abundant granitic and pegmatitic veins are present, and most of them are deformed; most veins are isoclinally folded and foliated, concordant with foliation in the host rock, but rarely boudinaged. However, a few pegmatite dikes, generally 20–50 cm wide, cut across the host-rock foliation.
Mineral Assemblages and Associated Deformation
These rocks are composed of quartz, plagioclase, K-feldspar, biotite and muscovite ± sillimanite, amphibole, porphyroblasts of garnet or magnetite, and accessory minerals, including apatite, zircon, epidote, and titanite. The muscovite-biotite-quartzofeldspathic gneisses have a weak to moderately well-developed foliation defined by aligned biotite ± muscovite alternating with layers of elongate quartz and feldspar, parallel to foliation (Fig. 4A). Foliation spacing ranges from 1 to 5 mm, and aspect ratios in quartz and feldspar are typically 1:1 or 2:1, with rare grains that have ratios as high as 3.5:1. Additionally, there are small pockets of coarser quartz and feldspar, up to several millimeters in diameter; these pockets are aligned parallel to foliation. Plagioclase is variably sericitized in these rocks, and subgrains are locally poorly developed. K-feldspar displays microcline twinning and is typically perthitic. Quartz has well-developed subgrains present and rarely displays chessboard extinction. Chessboard extinction, so-called because there are two sets of subgrain boundaries perpendicular to each other, occurs when basal and prism slip occur together, and it is indicative of the transition from low to high quartz (Kruhl, 1996).
A few of the samples from the Five Points deformation zone contain garnet, but in all but one sample, the garnet is highly disaggregated. These disaggregated garnets are separated into round or square, small, 50–200-μm-diameter crystals, surrounded by quartz, and they appear to have once been a single larger garnet crystal. Garnet is surrounded by quartz that has well-developed subgrains and is relatively inclusion free (Fig. 4B). However, the disaggregated grains are commonly joined together by coarse single grains of muscovite or biotite, or fine-grained chlorite (Fig. 4B). Rocks from the Texas Creek domain to the west of Five Points have similar textures, but garnet is more highly disaggregated and is joined by coarse calcite, chlorite, and muscovite. Within the Five Points area, one sample containing garnet exhibits a xenoblastic texture, but it is not disaggregated. The garnet is also surrounded by a leucocratic segregation composed mostly of deformed quartz, but there is a discontinuous envelope of slightly sericitized plagioclase immediately surrounding the garnet.
Interpreted Melt Microstructures and Metamorphic Reactions
Microstructures indicative of former melt in rocks at Five Points are either found adjacent to minerals that are interpreted to have broken down in a melt-producing reaction, such as muscovite, as melt pseudomorphs along grain boundaries, or as crystals with serrate or cuspate grain boundaries adjacent to melt pseudomorphs (Figs. 4C and 4D). Samples containing abundant muscovite provide evidence for the greatest number of microstructures interpreted to indicate the former presence of melt, including former melt adjacent to embayed and corroded muscovite grains and along grain boundaries, typically between quartz and plagioclase. This space formerly occupied by melt is typically preserved as K-feldspar or quartz; quartz is more prevalent adjacent to corroded muscovite.
Reaction 1 occurs at temperatures of 675–800 °C (Thompson and Algor, 1977; Le Breton and Thompson, 1988; Spear, 1995; Spear et al., 1999), depending on pressure, and reaction 2 occurs at temperatures up to 725 °C (Thompson and Algor, 1977). The presence of chessboard extinction in quartz in some muscovite-rich samples may suggest that reaction 1 was the primary reaction in these rocks, as it can occur at higher temperatures than reaction 2. Based on previous pressure estimates in this area of 0.5 GPa (Givot and Siddoway, 1998), reaction 1 could occur at temperatures of around 750 °C, whereas reaction 2 would occur at temperatures around or below 700 °C.
Very few samples lack muscovite, and those that do lack muscovite preserve only rare examples of microstructures attributed to partial melting. However, the presence of a few examples of melt microstructures, particularly thin films of K-feldspar between quartz and plagioclase, suggests that these rocks did undergo wet melting of a granitic protolith. Fluid-present granitic eutectic melting occurs at temperatures lower than muscovite-dehydration melting, but the lack of abundant microstructures attributed to partial melting may indicate that K-feldspar was the limiting phase in this reaction. This interpretation is further supported by the presence of K-feldspar now occupying most of the sites of former melt-filled pores. At a pressure of 0.5 GPa, granitic fluid-present melting would start to occur at temperatures of ∼650 °C (Wyllie, 1977; Spear, 1995). Samples lacking muscovite also lack chessboard extinction within quartz grains, which also suggests that these rocks could have melted at lower temperatures than muscovite-rich rocks.
The presence of abundant muscovite suggests that all of the muscovite in these rocks was not used up during muscovite-dehydration melting and/or that some of the muscovite is retrograde. Although much of the muscovite in these rocks appears skeletal and corroded, there is some fresh-looking muscovite, which may have been formed during retrograde metamorphism. In summary, it is likely that some fluid-present granitic melting occurred in these rocks, but samples that preserve the best melt microstructures are rich in muscovite. Consequently, muscovite-dehydration melting was likely the dominant melting reaction.
Dawson Mountain is adjacent to the northern part of the Oak Creek pluton (Fig. 1), a compositionally variable pluton that ranges from diorite to granodiorite to monzogranite (Cullers et al., 1993; Dean et al., 2002). The Oak Creek pluton has a U-Pb zircon age of 1442 ± 7 Ma (Bickford et al., 1989) and on its margins has a well-developed foliation that is concordant with the foliation in its host rocks. Previous workers (Cullers et al., 1993; Dean et al., 2002) proposed that the Oak Creek pluton was emplaced during high-temperature, 600–800 °C, and low-pressure, 0.2–0.5 GPa, metamorphic conditions.
Gneisses at Dawson Mountain have a closely spaced S2 foliation and are compositionally diverse, ranging from cordierite-garnet-biotite-sillimanite gneisses (Fig. 2B) to biotite-rich granitic gneisses, to granitic gneisses containing ellipsoidal quartzofeldspathic patches with magnetite porphyroblasts. These rocks all contain a pervasive dominantly northeast-striking and southeast-dipping S2 foliation, and some L2 lineations are present, typically defined by aligned biotite, plunging moderately to the southwest (Fig. 3B). Some of the cordierite-garnet-sillimanite–rich, metapelite layers are isoclinally folded, with hinge lines parallel to the biotite lineation. These rocks have undergone partial melting, with clear evidence for in situ partial melting indicated by garnet surrounded by leucosomes and small patches of leucosome parallel to foliation (Fig. 2B). In addition to evidence for in situ partial melting, there are abundant granitic and pegmatitic veins, some concordant to foliation and others crosscutting host gneiss foliation, throughout the field area, at least some of which appear to be externally derived, possibly from the adjacent Oak Creek pluton.
In the field, biotite-rich granitic gneisses are the most abundant rock type, but in scattered locations, no more than 10 m2 in size, garnet-rich metapelites show abundant evidence for localized partial melting. Field evidence for partial melting within these metapelites includes abundant small foliation-parallel leucosomes and rounded to elliptical leucosomes immediately surrounding the garnet. Some metapelites contain more continuous veins or veinlets of leucocratic material enveloping multiple garnet crystals, slightly oblique to the main foliation, spaced 2–3 cm apart. A few contain foliation-parallel, laterally continuous leucocratic veins. These rocks would be best characterized as metatexite migmatites, as they are heterogeneous on the outcrop scale and appear to form from low degrees of partial melting (Sawyer, 2008). The metapelitic rocks in this vicinity contain leucosome patches, and all of the garnet is located within leucosomes. These metapelites are the focus of most of the microstructural and chemical analyses.
Mineral Assemblages and Associated Deformation
Metapelitic rocks at Dawson Mountain have fairly uniform mineral assemblages, including: quartz, plagioclase, K-feldspar, biotite, garnet, sillimanite, and cordierite, plus accessory minerals, including epidote, zircon, and hematite. These rocks are very well foliated, with foliation planes spaced at 1–2 mm. The S2 foliation is defined by aligned biotite and sillimanite, in some cases within coarse grains of cordierite or K-feldspar, alternating with layers of elongate quartzofeldspathic material, dominantly quartz, which has typical aspect ratios of 2:1–4:1 (Fig. 5A). The presence of coarse grains of K-feldspar primarily within the more iron and aluminous layers, and the lack of plagioclase in these samples will be discussed in the next section. The S2 foliation defined by sillimanite has a somewhat wispy character (Fig. 5A), and sillimanite defines the rarely seen S1 foliation visible only in the hinges of F2 isoclinal folds.
Garnet crystals are up to 2 cm in diameter and have a rounded to moderately embayed shape. Within embayed areas, garnet tends to be disaggregated, and strings of disaggregated material generally parallel the foliation (Fig. 5A). Some of the garnet contains inclusions of sillimanite that are needlelike and in some places look somewhat ragged. Within several of the garnet crystals containing sillimanite, some sillimanite outlines the shape of an isoclinal fold. It is unclear whether the sillimanite has actually been folded or whether the sillimanite is mimicking minerals it has replaced that were once folded, possibly crenulated biotite. In either case, it is clear some deformation preceded the growth of garnet.
Microprobe analyses of garnet from the metapelites indicate they are rich in almandine (Table 1). On a transect from rim to rim across the garnet, no major element zoning was observed, suggesting they are equilibrated; they have compositions in the range of XFe = 0.75–0.79 (where XFe = Fegar/[Fe + Mg + Mn + Ca]gar), XMg = 0.18–0.19, XMn = 0.01–0.06, and XCa = 0.01 (Table 1). Most garnet crystals are rimmed by a very thin, <1-mm-thick rim of plagioclase and then surrounded by a lens of leucocratic material that is composed entirely of quartz (Figs. 5A and 5B). In many samples, a single, thin, optically continuous film of plagioclase rimming garnet also can be found rimming coarse grains of cordierite or K-feldspar, which themselves contain biotite and sillimanite inclusions. The quartz is quite coarse grained and commonly displays chessboard extinction (Fig. 5C). Plagioclase rims around garnet are optically continuous and commonly surround more than one garnet or isolated portions of the same garnet. The leucocratic envelopes surrounding garnet contain no biotite, sillimanite, K-feldspar, or cordierite (Figs. 5A and 5B).
K-feldspar or cordierite grains are found parallel to foliation, along grain boundaries and a few subgrain boundaries, with inclusions of sillimanite, rare biotites, and some rounded quartz grains (Figs. 5D–5F). The relationship between the K-feldspar and sillimanite is the same as that between cordierite and sillimanite, with optically continuous K-feldspar or cordierite extending along the entire length, 4 cm, of the thin section parallel to the foliation. These bands are pervasive in most thin sections; they have a wispy character with variable thickness, but they generally range from 50 to 200 μm thick, with rare areas up to 1–2 mm thick. K-feldspar is typically perthitic, and cordierite is commonly pinitized with yellowish, brownish, or greenish, isotropic, somewhat ragged material along the edges, and locally containing fine needles of sericite (Figs. 5A and 5E). Throughout some thin sections, large swaths of pinite are present, with almost no original cordierite remaining. The pinite is compositionally identical to cordierite, except that it is more hydrous. Plagioclase rims are found on some of the pinite, cordierite, or K-feldspar that contain sillimanite and biotite (Fig. 5E). Along some grain and subgrain boundaries, particularly between quartz grains, there are also optically continuous, up to 0.1-mm-thick films of plagioclase surrounding slightly thicker, up to 0.3-mm-thick films of cuspate-shaped K-feldspar (Fig. 5D). Some of the sillimanite within K-feldspar, cordierite, and pinite is idioblastic, with a square to slightly rectangular cross section (an end section); other sillimanite is needlelike and may have more tapered ends (Figs. 5D–5G). Biotite grains within the K-feldspar/cordierite/pinite do not contain idioblastic crystal faces; instead, they are jagged or appear somewhat skeletal (Figs. 5D and 5E). Within some of the coarse grains of K-feldspar/cordierite/pinite, these skeletal grains of biotite are in contact with idioblastic sillimanite crystals, which have grown parallel to or at a slight angle to the cleavage in biotite.
In summary, the key minerals present in these metapelites are garnet, K-feldspar, cordierite/pinite, plagioclase, biotite, sillimanite, and quartz. Many biotite grains appear a bit ragged and skeletal; sillimanite grains are commonly idioblastic and are locally in contact with biotite, parallel to biotite cleavage planes. Both biotite and sillimanite, and some quartz grains, are inclusions within coarse-grained K-feldspar/cordierite/pinite, which are foliation parallel and extend along much of the length of the thin section. Garnet crystals are commonly surrounded by a thin film of plagioclase, which may continue in optical continuity around biotite and sillimanite grains, or the coarse-grained K-feldspar/cordierite/pinite. Thin films of plagioclase, surrounding a thicker film of K-feldspar are found along grain boundaries and some subgrain boundaries, particularly in areas of abundant small quartz grains. Quartz is present parallel to the foliation, but it also forms an elliptical patch surrounding the plagioclase rimming garnet.
Interpreted Melt Microstructures and Metamorphic Reactions
Garnet-rich metapelites at Dawson Mountain contain distinctive melt microstructures that appear to be former “channels” filled with melt (Figs. 5D and 5G) and envelopes of former melt surrounding peritectic products of melting reactions (Figs. 5B and 5D). Both of these textures were described briefly in the previous section, but here, we discuss their association with melting. The interpretation of the microstructures as former “channels” filled with melt is based on (1) volumetrically small, optically and laterally continuous grains of K-feldspar/pinite/cordierite that extend, parallel to foliation, along the length of the thin section, (2) the presence of prismatic sillimanite, rare skeletal or jagged biotite, and rounded quartz grains within these laterally continuous grains (Fig. 5G), (3) the presence of single optically continuous grains of plagioclase locally rimming garnet (Fig. 5B), and K-feldspar/cordierite/pinite grains, which themselves contain inclusions of biotite, quartz, and sillimanite, and (4) the presence of the thin films of plagioclase, sometimes encompassing slightly thicker films of K-feldspar along grain boundaries and subgrain boundaries, particularly around small quartz grains (Figs. 5D). It is well established that melt migrates through rocks along grain boundaries, particularly those that are foliation parallel (Mehnert et al., 1973; Sawyer, 2000, 2001; Marchildon and Brown, 2003), and the microstructures described herein that have the appearance of channels are interpreted as channels along which melt migrated. The melt that may have flowed through these channelways would likely be connected with larger conduits that moved melt to higher crustal levels. However, no larger conduits are preserved within the samples analyzed in this study.
Biotite grains that are slightly jagged, embayed, or skeletal in appearance and found either within or adjacent to the K-feldspar/cordierite/pinite regions have a morphology suggesting that they broke down as a result of biotite-dehydration melting. The association of sillimanite adjacent to biotite and in some cases continuing parallel to the cleavage in biotite suggests that sillimanite is replacing biotite. It is unclear whether biotite broke down and was replaced by sillimanite in a dehydration-melting reaction, or whether it was a premelting dehydration reaction. The presence of small grains of well-rounded quartz, biotite that is somewhat jagged and skeletal, and abundant sillimanite within the coarse-grained K-feldspar/cordierite/pinite suggests that the K-feldspar/cordierite/pinite grains may be peritectic products of reaction. If so, the shape and lateral extent of the grains parallel to foliation (Figs. 5D and 5G) suggest open channelways in which these peritectic minerals formed. It is possible that some of the very prismatic sillimanite within and along the edges of K-feldspar, adjacent to rounded quartz grains (Fig. 5G), may actually represent sillimanite that formed as a result of minor back-reaction with melt. Garnet present in these samples also contains inclusions of quartz and sillimanite, but rarely biotite. The sillimanite grains within the garnet tend to be less idioblastic and prismatic than the sillimanite within K-feldspar/cordierite/pinite grains. The other distinctive melt microstructure within the Dawson Mountain metapelites is the thin rim of plagioclase ± K-feldspar surrounding many of the garnet, K-feldspar/cordierite/pinite grains, and along the grain boundaries and subgrain boundaries of quartz grains (Figs. 5B, 5D, and 5F).
Garnet crystals and K-feldspar/cordierite/pinite appear to have similar relationships with sillimanite and quartz; both minerals are inclusions within the much larger crystals. Additionally, thin films of plagioclase are found bounding garnet and continuing in optical continuity around K-feldspar/cordierite/pinite crystals. Thus, the garnet and K-feldspar/cordierite/pinite could be peritectic minerals of a melting reaction. In this case, when the garnet was produced as a peritectic product of melting, the plagioclase and any K-feldspar components of the melt crystallized as a thin rim surrounding the garnet. The quartz component of the melt crystallized as the coarse quartz grains that commonly display chessboard extinction (Fig. 5C), indicating deformation after crystallization of melt. However, there is far more abundant coarse-grained quartz surrounding the garnet, which is not found surrounding the K-feldspar/cordierite/pinite grains. Thus, the garnet may have formed earlier than the K-feldspar/cordierite/pinite during a subsolidus reaction and is surrounded by a quartz segregation, which is unrelated to a melting reaction.
The limiting phase for reaction 4 is likely plagioclase, as the only plagioclase seen in these samples is the thin film along grain boundaries, and rimming the peritectic minerals. In areas where K-feldspar is a peritectic mineral, the K-feldspar component of the melt could have nucleated on these peritectic K-feldspar grains, and the quartz component of the melt could have nucleated onto preexisting quartz grains. Thus, a thin film of plagioclase would be left behind indicating the former location of melt. In quartz-rich domains, there is a thin film of plagioclase and a slightly thicker film of K-feldspar, perhaps suggesting that both of these minerals crystallized from the melt, whereas the quartz component of the melt crystallized onto preexisting quartz grains.
In some samples, the garnet is much smaller than the grains shown in Figures 5A and 5B. These garnet crystals are surrounded by thicker and more continuous films of plagioclase, with smaller amounts of quartz. The small garnet crystals may provide evidence for some garnet produced as a peritectic product of melting via reaction 5. Biotite-dehydration melting is likely to occur at temperatures of 760–800 °C, depending on pressure (Le Breton and Thompson, 1988; Spear et al., 1999). As the melt crystallized, the biotite-dehydration reaction was not entirely reversed, as evidenced by a lack of resorption zoning of Mn within garnets. If garnet had been extensively involved in reaction with melt, it would likely have Mn-rich rims, with low-Mn cores (Nyström and Kriegsman, 2003). A minimal amount of reaction between garnet and melt may have occurred, but the volume of melt was probably small enough that zoning did not develop. The lack of clearly residual rocks at Dawson Mountain indicates that although melt channelways appear to be present, most of the melt solidified in place, and large volumes of melt did not pass through these inferred channelways.
WILLIAMS CREEK ROAD
The Williams Creek Road is in the southern Wet Mountains (Fig. 1), within the San Isabel National Forest. Most of the best exposures are located along Forest Service Road 402, which parallels Williams Creek. These rocks are dominantly granitic gneisses and are adjacent to and include Mesoproterozoic plutonic rocks. Rocks along the Williams Creek Road are highly strained, with local development of mylonites.
Rocks along the Williams Creek Road are biotite-hornblende-granitic gneisses that appear to be made up of interlayered G2 and G3 granites. G2 granitic gneisses are more abundant and are identified by their coarse grain size, K-feldspar augen, well-developed foliation, and streaky biotite lineation, which locally weathers into rods (Fig. 2C). The degree of strain appears to be variable in these rocks, but some rocks have a protomylonitic fabric. G3 granitic gneisses are rarer but can be identified by their fine grain size, less abundant mafic phases, and locally mylonitic fabric. S2 foliation within both the G2 and G3 rocks strikes northeast-southwest and dips moderately toward the northwest. Lineations plunge shallowly to moderately toward the north-northwest (Fig. 3C). Shear-sense indicators are generally rare in these rocks, but when visible, they suggest a top-to-the-southwest motion. In outcrops along Williams Creek Road, from north to south, the degree of strain increases, with protomylonites and mylonites in only the southernmost outcrops.
Mineral Assemblages and Associated Deformation
These rocks are dominantly composed of quartz, K-feldspar, plagioclase, biotite, and hornblende; they contain accessory phases, including zircon, apatite, epidote, chlorite, titanite, and opaques such as hematite. Rocks along the northern, lower-strain part of the road have a moderately developed foliation, with a spacing of 2–3 mm, and the southern rocks with higher degrees of strain have a more closely spaced foliation, in some areas spaced at less than 0.5 mm. As mentioned in the methodology section, foliation spacing can represent original differences in lithology, but as these rocks are all granitic gneisses, the change in foliation spacing is more likely to indicate a change in the intensity of strain. A dextral, top-to-the-southwest shear sense is visible in one sample (Fig. 6A).
Equant quartz grains have well-developed chessboard extinction; quartz grains, elongate parallel to foliation, form ribbons (up to 2 mm in length) with aspect ratios of 6–10:1, and, in rare cases, the quartz ribbons also display chessboard extinction (Figs. 6B and 6C). Plagioclase grains tend to be equant and have either undulatory extinction or moderately developed subgrains. Plagioclase grains that are adjacent to K-feldspar typically have edges that are serrate and cuspate (Fig. 6D) and are marked by a change in extinction that represents a more albitic rim. The degree of sericitization is variable in plagioclase, and myrmekite is locally developed (Fig. 6D). K-feldspar in these rocks has undergone pervasive recrystallization and tends to have an equant or slightly elongate shape. In monomineralic domains, the K-feldspar grains appear statically recrystallized with 120° grain intersections (Figs. 6B and 6C). K-feldspar is also found as small (less than 0.2 mm in length) blebs of material within quartz ribbons that formed via high-temperature grain boundary migration recrystallization or along grain boundaries, usually between quartz and plagioclase. Microcline twinning is displayed in all K-feldspar grains, and they have a perthitic texture. Along grain boundaries between plagioclase and K-feldspar, abundant new grains of both minerals, ranging in size from 5 to 40 μm in diameter, have formed as a result of primarily high-temperature grain boundary migration, with some subgrain rotation recrystallization (Fig. 6D). The degree of recrystallization is variable in these rocks, but it typically ranges from 10%–20% newly formed grains up to nearly 70% in the mylonitic sample.
Interpreted Melt Microstructures and Metamorphic Reactions
Melt microstructures in these rocks include former melt found along many grain boundaries, triple junctions, and serrate/cuspate grain boundaries adjacent to melt pseudomorphs (Figs. 6B–6D). In this area, the melt pseudomorphs are generally preserved as K-feldspar between grains of quartz and plagioclase, which appears to be common for rocks throughout the Wet Mountains. Recrystallization appears to have preferentially occurred along grain boundaries, locations of former melt, between plagioclase and K-feldspar. Thin films of optically continuous K-feldspar, interpreted to represent former melt, commonly grade into new grains formed as a result of recrystallization, along the length of a plagioclase grain with serrate/cuspate edges (Fig. 6D).
Melt pseudomorphs are more easily identified in less-strained rocks, largely because the former melt is found on grain boundaries, and these locations are marked by dynamic recrystallization in more highly strained rocks. In areas of low amounts of dynamic recrystallization, along some grain boundaries in these rocks, there are cuspate-shaped films of K-feldspar between quartz and feldspar grains. These resemble pseudomorphs of former melt-filled pores that have been documented in experiments (Harte et al., 1991; Sawyer, 1999; Rosenberg and Riller, 2000; Holness and Sawyer, 2008). Although this rock is quite rich in K-feldspar, along grain boundaries between quartz and plagioclase, there is no K-feldspar on which the melt can nucleate. Thus, it is likely that more highly strained rocks had at least as many melt pseudomorphs as lower-strain samples, but the melt pseudomorphs were recrystallized. In some locations, the serrate/cuspate grain boundaries are visible next to areas of pervasive recrystallization (Fig. 6D). The recrystallization of the pseudomorphs of former melt-filled pores indicates that deformation occurred in the rocks after the crystallization of the melt.
Former melt is also interpreted to be present along the edges of the quartz ribbons (Figs. 6B and 6C) and in some cases on the boundary between two ribbons. Several of the quartz ribbons contain melt pseudomorphs, now K-feldspar, along subgrain boundaries that either parallel or are oblique to the long direction of the quartz grains (Figs. 6B and 6C). All of the melt microstructures are located along grain or subgrain boundaries between quartzofeldspathic phases, indicating that the dominant melting reaction in these rocks is reaction 3, the granitic wet melting reaction previously described from the Five Points deformation zone.
The ability to clearly identify melt pseudomorphs in granitic rocks that do not have peritectic minerals produced during melting is quite difficult (Holness and Sawyer, 2008; Sawyer, 2010). It is further complicated by deformation after crystallization of the melt, which seems to have overprinted many of the melt microstructures by dynamic recrystallization. These rocks are cryptic to interpret, but based on the presence of some melt microstructures in the lower-strain portions of samples, it is likely that they did undergo partial melting, and the amount of partial melting may be underestimated due to later recrystallization of melt pseudomorphs. Rocks found along Williams Creek Road are dominantly granitic gneisses, and based on dating of rocks nearby, it appears that D2 deformation was synchronous with intrusion of the G2 granites into the country rock (Jones et al., 2010). The deformation in these rocks was likely partitioned into the melt-rich portions of the rock because it was rheologically weak (Rosenberg and Handy, 2005), and deformation continued after the crystallization of the melt.
SOUTHERN GREENHORN PEAKS
The Salt Road, also known as Forest Service Road 409, and the Cisneros Trail are within the San Isabel National Forest and are the southernmost exposures investigated in the Wet Mountains (Fig. 1). Rocks in these locations are not adjacent to any mapped granitic plutons, but there are abundant G2 and G3 intrusions present throughout the area.
Rocks in this area are composed dominantly of migmatitic biotite ± hornblende granitic gneisses and migmatitic garnet-biotite ± sillimanite gneisses. Both rock types contain foliation-parallel stromatic migmatites that are locally isoclinally folded. The dominant S2 foliation in the area strikes west-northwest and dips moderately to steeply to the north-northeast, and no S1 foliation is visible in these rocks (Fig. 3C). This foliation is steeper and dips more north-northeasterly than the foliation found in the Williams Creek area. Granitic gneisses are the most abundant rock type in this area, and their foliation is defined by alternating quartzofeldspathic layers and biotite ± hornblende-rich layers. They are quite pink in color, indicating the abundance of K-feldspar, but unlike rocks from Williams Creek, they do not contain abundant K-feldspar augen.
Garnet-rich metapelitic gneisses contain coarse garnet crystals, up to 3 cm in diameter, which are commonly surrounded by leucocratic material, or are within 2 cm of foliation-parallel leucocratic layers, although some garnet is located within the restitic material (Fig. 2D). All of the rocks in this vicinity, regardless of rock type, provide evidence for abundant former melt in the system despite the absence of larger plutons. Many foliation-parallel pegmatites and granitic dikes and several crosscutting granitic dikes are present and likely represent some of the G2 and G3 granitic dikes that have been intruded into the rocks. In metapelitic areas, garnet is surrounded by leucosome material, and these rocks can be characterized as stromatic migmatites, with evidence for formation via partial melting presented next. These rocks appear to contain more former melt of both anatectic and intruded sources than any of the other localities in the Wet Mountains.
Mineral Assemblages and Associated Deformation
Granitic gneisses are composed of quartz, K-feldspar, plagioclase, biotite ± hornblende and accessory phases, including apatite, titanite, magnetite, epidote, and zircon. S2 foliation in these rocks is defined by alternating layers of elongate quartzofeldspathic material with a spacing of 1–3 mm, and in some cases quartz ribbons and biotite grains are aligned parallel to foliation. Quartz grains are typically equant with chessboard extinction, or they are more elongate, with aspect ratios ranging from 1:1–6:1 (with only rare ribbons with high aspect ratios like the rocks from Williams Creek), and they have sharp subgrain boundaries. Plagioclase is far less abundant than K-feldspar and contains variable amounts of sericite. Plagioclase grains typically display either continuous undulatory extinction or have poorly developed subgrains. Myrmekite appears to be located along subgrain boundaries within plagioclase grains, but it is also found in grains that do not have subgrains. K-feldspar has well-developed microcline twinning and contains abundant perthitic texture. It is generally equant and coarse grained, commonly containing inclusions of plagioclase, or it is elongate parallel to the foliation. Locally, it is found as thin films on grain boundaries, particularly between plagioclase and quartz.
Metapelitic samples contain quartz, plagioclase, garnet, and biotite, ± sillimanite, cordierite, K-feldspar, and accessory phases, including apatite, zircon, and magnetite. K-feldspar and cordierite are not present within the same thin sections, perhaps due to variation in bulk composition across the area. Foliation in these rocks is defined by alternating layers of aligned biotite ± sillimanite and elongate quartzofeldspathic material, locally containing garnet, with a spacing of 1–2 mm (Figs. 7A and 7B). In most garnet-rich samples, the biotite-rich layers appear to wrap around garnet, as well as the pocket of leucosome surrounding the garnet (Fig. 7B). Quartz in these samples either displays chessboard extinction or has well-developed subgrains; locally, quartz ribbons with aspect ratios as high as 5–6:1 are present. Plagioclase and cordierite both display continuous undulatory extinction or rare subgrains.
Garnet crystals are generally poikiloblastic with rounded to square inclusions of quartz, biotite, and plagioclase (Figs. 7B and 7C). They are generally surrounded by an envelope between 100 and 800 μm thick of plagioclase and decussate biotite, or vermicular cordierite and biotite, or plagioclase, vermicular cordierite, and biotite (Figs. 7C–7H). Some of these envelopes, particularly the plagioclase-rich ones, have undulatory extinction. This envelope immediately surrounding the garnet is either surrounded by the typical foliation in the rock or an envelope of quartz, commonly displaying chessboard extinction.
Microprobe analyses were conducted on five garnet-rich samples from this vicinity, and two of the samples display resorption zoning of Mn in garnet (Figs. 7D and 7G); the others have flat zoning profiles. All of the garnet crystals are Fe rich, but there is variation in the Mn and Mg content in zoned samples (Table 2). Rims of zoned garnet crystals have compositions of XFe = 0.67–0.72, XMg = 0.05–0.08, XMn = 0.17–0.25, and XCa = 0.03–0.04 (Table 2). The cores have compositions of XFe = 0.73–0.76, XMg = 0.09–0.13, XMn = 0.09–0.15, and XCa = 0.03–0.04 (Table 2). Samples that have garnet with fairly flat zoning profiles have XFe = 0.75–0.76, XMg = 0.16–0.17, XMn = 0.04–0.05, and XCa = 0.02 (Table 2).
Biotite in these rocks generally has two different morphologies and compositions. Biotite defining the foliation has light-brown to brown pleochroism and a composition of: XFe = 0.57–0.58 or 0.42, XMg = 0.36–0.4 or 0.54, and XTi = 0.03–0.06 (Table 2). The brown biotite has different Fe and Mg contents, depending on the sample, but all brown biotite has high Ti content relative to the green biotite. Green biotite crystals are found surrounding and within garnet, and they are commonly corroded, or very narrow, and have apleochroism from pale green to light brownish-yellow. Their composition is XFe = 0.57–0.58, XMg = 0.40–0.42, and XTi = 0.003–0.005 (Table 2).
Interpreted Melt Microstructures and Metamorphic Reactions
The rocks of the southernmost Wet Mountains contain melt microstructures in the form of melt pseudomorphs adjacent to corroded and embayed biotite grains and envelopes of plagioclase or cordierite surrounding garnet (Figs. 7C–7H). These envelopes surrounding the garnet are not typically classified as a melt microstructure, but in these rocks, their formation is interpreted to be related to the melting process; they appear to represent a back-reaction between garnet and melt. Additionally, there are very rare films of K-feldspar between quartz and plagioclase.
Melt pseudomorphs adjacent to biotite are typically quartz, between biotite and feldspar, or biotite and cordierite; in samples that have an appropriate bulk composition, the films may also be composed of K-feldspar. In sillimanite-rich rocks, the films of quartz adjacent to biotite commonly contain idioblastic, prismatic sillimanite, similar in appearance to the sillimanite at Dawson Mountain that may have crystallized from a melt. These biotite-quartz ± sillimanite intergrowths have been interpreted to record reaction between melt and biotite during crystallization (Waters, 2001; Kriegsman and Álvarez-Valero, 2010).
Sample WMG123–2 has well-developed zoning in all of the garnet crystals, with Mn-rich rims and Mn-poor cores (Fig. 7D). These garnet crystals are surrounded by an envelope of plagioclase with decussate biotite within the envelope (Figs. 7C and 7E). Plagioclase is also found rimming quartz inclusions within the garnet (Figs. 7C and 7E). Inclusions in garnet containing quartz rimmed by plagioclase tend to be square in shape, possibly indicating the phase in contact with the garnet was melt, which is now preserved as plagioclase. The quartz component of the melt is interpreted to have crystallized onto adjacent quartz, and thus the melt pseudomorphs are preserved as plagioclase, the remaining component of the melt. Undulatory extinction within some of the plagioclase enveloping the garnet indicates deformation continued after the growth of the plagioclase. Biotite within the envelopes is green in color, but biotite defining the foliation is much browner in color, indicating higher Ti contents. This difference in biotite composition suggests that the foliation-parallel biotite grew at a different time than the decussate biotite within the plagioclase envelope. Low-Ti green biotite that is texturally late has been interpreted to represent a melt-consuming reaction (Kriegsman and Álvarez-Valero, 2010).
At such high-temperatures, garnet will typically exhibit flat zoning profiles because they have been homogenized (Spear, 1991), but the presence of Mn-rich rims is attributed to garnet resorption via back-reaction with melt (Nyström and Kriegsman, 2003; Kriegsman and Álvarez-Valero, 2010). The Mn present within the garnet will diffuse toward the center of the garnet as the garnet breaks down, because the other mineral phases surrounding the garnet will take in minimal Mn. Consequently, the Mn present in the edge of the garnet has diffused inward, but if a large portion of the garnet volume is not lost, the interior of the garnet will retain the homogenized core composition.
Although sillimanite and K-feldspar may have been involved in the original melt-producing reaction, sillimanite is no longer present in these rocks, and K-feldspar is only found in the matrix, as an elongate, foliation-defining mineral.
Cordierite is generally only present in these rocks surrounding garnet or as single grains with a vermicular texture. These isolated grains of cordierite are interpreted to be adjacent to garnet that is not exposed in thin section. Evidence for this comes from several thin sections that were remade and consequently had a new surface exposed at least 50 μm below the original sample. In the original thin section, cordierite was visible, and in the new one, garnet was present in the same location, instead of the cordierite. The proximity of cordierite grains to garnet suggests cordierite growth is associated with garnet breakdown.
The presence of resorption zoning of Mn within garnet from two different locations, surrounded by an envelope of plagioclase and biotite, likely biotite-dehydration melting reactants, suggests that abundant melt was in the system and may have pooled within rocks of the southern Wet Mountains. The presence of far more abundant stromatic migmatites in this location also provides evidence for far more abundant melt in this vicinity.
Exposures in the Wet Mountains provide evidence for an increase in metamorphic grade and abundance of melt toward the south-southeast. We used the presence of melt microstructures along grain boundaries, and relationships between peritectic products of melting and their surrounding minerals to identify anatectic melt in metapelitic rocks and granitic gneisses. We found very little evidence of partial melting in the northwest portion of the Wet Mountains; the central part of the range is characterized by fossil melt channels with little interaction between peritectic minerals and melt; and the southern part of the Wet Mountains had much more abundant partial melting and melt pooled in the system, causing extensive back-reaction between garnet and melt. Deformation was concurrent with, and in some locations outlasted or preceded partial melting, and strain intensity qualitatively appears to correlate with the amount of partial melt present in the rocks. The central and southern portions of the Wet Mountains may be partially insulated by a midcrustal magma layer, as proposed by Shaw et al. (2005), which would lead to enhanced partial melting.
In the Five Points deformation zone, evidence for partial melting comes from melt pseudomorphs along grain boundaries as well as the embayed, somewhat corroded muscovite grains, in association with sillimanite and K-feldspar. However, the presence of some muscovite within these rocks indicates that muscovite breakdown was not complete or that some muscovite regrew during minor back-reaction between melt and peritectic products of melting.
Foliation is moderately well developed with a spacing of 1–5 mm, and quartz and feldspar are elongate parallel to foliation, with aspect ratios ranging from 1:1 to 3.5:1, but there is little evidence for recrystallization in these rocks. The best evidence for high-temperature deformation in these rocks comes from coarse-grained quartz, with well-developed subgrains and, in rare cases, chessboard extinction. Chessboard extinction occurs at the transition from low to high quartz, and this typically occurs at temperatures at or higher than 800 °C for pressures of 0.5 GPa (Kruhl, 1996). Rare examples of chessboard extinction and more typical examples of only well-developed subgrains suggest that most deformation occurred at temperatures below 800 °C. Rare melt pseudomorphs in conjunction with moderate deformation indicate that the Five Points deformation zone had low amounts of melting and relatively low amounts of deformation in comparison to other portions of the Wet Mountains.
Farther to the south and east at Dawson Mountain, both the degree of melting and intensity of strain have increased. The most striking feature at Dawson Mountain is the preservation of former melt channels in metapelites, containing former melt composed of plagioclase and K-feldspar, which rim peritectic minerals including: cordierite, pinite, K-feldspar, or garnet, and the likely reactants of biotite-dehydration melting: biotite, sillimanite, and quartz. The laterally continuous K-feldspar/cordierite/pinite grains parallel to the foliation are interpreted as peritectic products grown in channel-shaped zones. The former melt channels suggest that melt may have flowed through these rocks, primarily along grain boundaries and subgrain boundaries, parallel to foliation. Alternatively, the melt may have been forming along grain and subgrain boundaries but was crystallized in place before through-going flow occurred. In these rocks, unlike the ones in the very southern Wet Mountains, no zoning is observed in garnet, even when garnet is in contact with these former melt channels. The lack of zoning in garnet indicates that garnet was only minimally or not at all involved in reaction prior to the crystallization of melt, and/or that some of the garnet, particularly those coarse grains surrounded by large quartz grains, may have formed earlier during a subsolidus reaction. The volume of melt present in these rocks is far smaller than that in the southern Greenhorn Peaks area. Shaw et al. (2005) proposed that the central and southern Wet Mountains are adjacent to a midcrustal magma layer, and the geotherms were consequently steeper in this area as a result of magmatism. Hence, rocks were hotter than those in the Five Points region. The location at Dawson Mountain is proximal to the Oak Creek pluton, and some of this melt may have been associated with magmatism related to the pluton.
The foliation at Dawson Mountain has a closer spacing, and grains have higher aspect ratios in these rocks than in the Five Points deformation zone, with typical spacing of 1–2 mm and aspect ratios of foliation-parallel quartz grains that range from 2:1 to 4:1. Inferred former melt channels at Dawson Mountain are found along grain boundaries and subgrain boundaries; the subgrain boundaries where partial melting occurred are found within the quartz grains that display chessboard extinction. Here, melting preferentially occurred along the same apparent high-strain locations where deformation had previously been localized. Thus, deformation appears to have started before melting and outlasted crystallization of melt.
Rocks of mostly granitic composition are found in the Williams Creek exposures. Former melt is typically found on grain boundaries, triple junctions, and adjacent to serrate-cuspate grain boundaries with low dihedral angles in these rocks, which is typical of melt microstructures attributed to granitic wet melting and also dehydration melting (Harte et al., 1991; Rosenberg and Riller, 2000; Sawyer, 2001, 2010). Thus, these features are important for identifying partial melting in rocks that do not have peritectic products of reaction. Deformation in these rocks is characterized by extensively developed quartz ribbons, with aspect ratios up to 10:1, sometimes displaying chessboard extinction, a strong lattice preferred orientation, up to 60%–70% feldspar recrystallization, and development of an S-C fabric indicating local mylonitization. These observations suggest the Williams Creek rocks have the highest degrees of strain in the field area. Because deformation occurred synchronously with melting, it appears that many of the melt microstructures have been overprinted by recrystallization. All of these rocks have evidence for former melt, but in many locations, well-preserved melt pseudomorphs along a grain boundary transition into an area that is pervasively recrystallized with abundant formation of new grains (see Fig. 6D). This association indicates that areas once containing melt were preferentially recrystallized, perhaps because they were rheologically weaker.
The southernmost Wet Mountains (southern Greenhorn Peaks area) provide evidence for abundant melt in the system, as indicated both by stromatic migmatites exposed in all rock types, as well as inferred widespread back-reaction between garnet and melt. Garnet in some of these rocks has an increase in Mn content at its rims and is surrounded by an envelope of plagioclase/cordierite and low-Ti biotite. This pattern of Mn zoning, with Mn-rich rims, indicates that garnet has been resorbed, and the envelope surrounding garnet is composed of the likely products of the garnet resorption reaction. These textures are quite different than the interpreted former melt channels from Dawson Mountain, despite rocks in both areas providing evidence for biotite-dehydration melting. The lack of zoning in Dawson Mountain garnet indicates that there was minimal reaction between garnet and melt, likely as a result of small volumes of melt moving through the rock or because the garnet formed earlier as a result of subsolidus reaction.
Along a transect from northwest to southeast in the Wet Mountains, the amount of former melt that formed via partial melting increases. The cause of the increase in partial melting and increased back-reaction between melt and peritectic minerals to the southeast could be related to increased metamorphic grade, but it may also be related to increased insulation of the crust via a midcrustal magma layer, discussed further next. The products of partial melting (i.e., leucosomes, former melt films, melt pseudomorphs) are deformed, and the strain intensity appears to increase toward the southeast along with increased evidence for partial melting. The increase in strain intensity likely reflects strain partitioning into the rheologically weaker partially molten rocks (Brown, 1994; Brown and Rushmer, 1997; Brown and Solar, 1998; Rosenberg and Handy, 2005). However, because deformation was synchronous with partial melting, it is possible that deformation may have aided partial melting reactions via enhanced diffusion and reaction rates (Kerrich et al., 1977; Brodie and Rutter, 1985; Yund and Tullis, 1991). Deformation acts to decrease grain size through dynamic recrystallization, creating more or shorter intergranular pathways for enhanced diffusion and increased surface area along which reactions can take place (Kerrich et al., 1977; Brodie and Rutter, 1985; Yund and Tullis, 1991). Foliations formed during deformation provide easy pathways for diffusion and flow of melt. Stress gradients also enhance fluid flow and diffusion rates (Etheridge et al., 1984).
The association of increased partial melting coupled with increased strain intensity suggests a possible positive feedback loop where deformation is preferentially concentrated in melt-rich regions that are rheologically weaker and where increased deformation enhances the melting reaction. The former is generally accepted (Brown, 1994; Brown and Rushmer, 1997; Brown and Solar, 1998), and the latter is consistent with previous work indicating that grain-size reduction as a result of recrystallization will enhance reaction rates (Kerrich et al., 1977; Brodie and Rutter, 1985; Yund and Tullis, 1991). Previous workers have suggested that on the scale of an orogen, contemporaneous deformation and melting shouldlead to enhancement of both processes (Hollister and Crawford, 1986; Brown and Solar, 1998; Lissenberg and van Staal, 2006). Our results from the Wet Mountains support this suggestion of positive feedback between the two processes.
CORRELATION WITH TECTONIC MODELS
Previous work on the Wet Mountains has focused on the larger tectonic picture and has provided excellent timing constraints on magmatism in the area. Shaw et al. (2005) and Jones et al. (2010) have proposed a tectonic model where the northern Wet Mountains are interpreted as upper-crustal exposures and the southern Wet Mountains are interpreted as lower-crustal exposures (Fig. 8). Jones et al. (2010) documented partitioned deformation, steep fabrics, and discrete plutonism in the northern Wet Mountains and correlated these with other upper-crustal shear zones in Colorado. Shaw et al. (2005) proposed a midcrustal magma layer, which is coincident with the Oak Creek pluton in the central Wet Mountains, and numerous plutonic bodies and widespread lower-crustal flow in the southern Wet Mountains.
These observations and the proposed tectonic model correlate well with observations based primarily on melt microstructures and detailed petrography of these rocks. Rocks with the least apparent strain are documented in the northern Wet Mountains, with rare melt pseudomorphs, and the lowest-grade rocks of the area. The lack of melt pseudomorphs and abundant dikes and veins is consistent with a small number of discrete Mesoproterozoic-aged plutons in the northern Wet Mountains. These fairly small discrete plutons would have provided only a small degree of thermal insulation during deformation and melting. Here, fabrics are steep, and the rocks within the Five Points deformation zone are consistent with several other near-vertical shear zones, such as the Fish Creek–Soda Creek shear zone and the Homestake shear zone (Shaw et al., 2005; Lee et al., 2012). These rocks have a very different structural style from the rocks present farther to the south, as well as less plutonism; this correlates well with a potential midcrustal magma layer between the Five Points deformation zone and rocks exposed at Dawson Mountain.
Shaw et al. (2005) proposed that rocks in southern Colorado and northern New Mexico have an elevated geotherm because they are below a midcrustal layer of plutons that marks a transition between advective and conductive heat flow below the plutons to only conductive heat flow above the plutons. These plutons may have been emplaced along a rheological and thermal barrier and provide insulation to the rocks present below this layer (Shaw et al., 2005; Jones et al., 2010). Less than 1 km north of the Oak Creek pluton at Dawson Mountain (Fig. 1), we see inferred former melt channels in the rocks, consistent with a midcrustal magma layer. If these rocks are located adjacent to a midcrustal magma layer, they would experience more melting than the rocks farther north. The inferred melt channels indicate that anatectic melt was wetting foliation-parallel grain boundaries, consistent with the proximal location near the proposed midcrustal magma layer. The presence of a midcrustal magma layer is consistent with the sheet-like aspect of the Oak Creek pluton adjacent to the granitic rocks at Dawson Mountain (Dean et al., 2002; Jones et al., 2010).
The southern Wet Mountains contain the San Isabel granite, a large plutonic body, as well as abundant dikes and sills, all of which indicate high temperatures consistent with the grade of metamorphism in the area. Granitic sills within the southern Wet Mountains have ages that range from 1435 ± 4 Ma for G2 granites to 1390 ± 10 Ma for G3 granites (Jones et al., 2010). These granites are both deformed, and the G2 granites have foliation concordant with host-rock gneisses. Metapelites from the southern Wet Mountains provide abundant evidence for melt in the system as evidenced by back-reaction between melt and garnet and abundant stromatic migmatites, consistent with widespread lower-crustal magmatism. These rocks were not directly adjacent to granitic plutons and are more consistent with a hot middle crust and a pervasive dike and sill network. It is unclear whether granite plutonism or partial melting actually began first, but they were broadly synchronous and provide additional evidence for 1.4 Ga magmatism, in the form of dike and sill networks associated with regional deformation. Rocks in the southern Wet Mountains underwent a high degree of partial melting, and the intrusion of granitic veins in this region concurrent with deformation and anatexis may indicate that partial melting was aided by additional heat input from a pervasive dike and sill network. It is possible that the intrusion of G2 and G3 granites led to increased partial melting and provided additional influx of melt onto already wet grain boundaries.
The identification of midcrustal magma layers from bright seismic reflectors and high-conductivity, low-velocity zones in modern orogens suggests that we should find fossil midcrustal magma layers in older, now-cold orogens. The Wet Mountains provide an excellent example, and the results of this study should help in recognition of others. Many exposures of the middle crust have been studied, and detailed deformational histories have been constructed, but a renewed evaluation of evidence for partial melting in other Proterozoic orogens could provide more examples of thermally insulated crust and possibly midcrustal magma layers. In addition, careful study of melting reactions, melt microstructures, and patterns of granitic magmatism in these older, preserved sections of formerly molten middle crust will provide better constraints on the nature of midcrustal melting that is occurring in modern orogens.
Samples collected from the Wet Mountains, from northwest to southeast, show an increase in metamorphic grade, an increase in the amount of former partial melt present, and an apparent increase in strain. In the northern Wet Mountains, there is little evidence of partial melting; in the central Wet Mountains, anatectic melt is preserved on foliation-parallel grain boundaries of metapelites; and in the southern Wet Mountains, there is far more abundant former melt, with evidence for back-reaction between garnet and melt, indicating melt may have pooled in these rocks. The southern Wet Mountains contain abundant syntectonic granite dikes and sills, which appear to have insulated this crust and led to more widespread partial melting. The degree of strain determined qualitatively from foliation spacing, aspect ratios in quartz and feldspar and degree of dynamic recrystallization, appears to increase with the amount of melt present in rocks of the Wet Mountains suggesting the processes of deformation and partial melting enhance each other. Analysis of detailed petrography and melt microstructures correlates well with larger-scale plutonism and tectonic models for the Wet Mountains. The higher amount of partial melt in the central and southern Wet Mountains suggests that there may be a midcrustal magma layer that is providing insulation and promoting partial melting, similar to midcrustal magma layers beneath modern orogens inferred from geophysical imaging. Finally, we suggest that the identification of melt microstructures and melting reactions is critical to understanding the rheology and tectonic history of metamorphic terranes, and study of examples such as the Wet Mountains will provide critical insights into modern tectonic processes.
This research was supported by a Geological Society of America student research grant and the Geology Foundation at the Jackson School for Geosciences, University of Texas at Austin. We would like to thank Bill Carlson, Mark Cloos, and Nathan Daczko for comments on an earlier version of this manuscript. We would also like to thank Mike Williams, Christopher Gerbi, and Ed Sawyer for their constructive reviews that greatly improved this manuscript.