Rocky Mountain Geology 35.1

Reflections on Proterozoic magmatism of the Rocky Mountains and environs: Past and present

Carol D. Frost

Keywords: History of geology, western geological surveys, Hayden, King.

2.01-Ga Kennedy dike swarm, southeastern Wyoming: Record of a rifted margin along the southern Wyoming Province

David M. Cox, Carol D. Frost, and Kevin R. Chamberlain

Keywords: Mafic dikes, Paleoproterozoic, Wyoming, petrology, geochemistry, Nd isotopes, Sr isotopes, Pb isotopes, U-Pb Geochronology

The Kennedy dike swarm of the central and northern Laramie Mountains is the most extensive Precambrian dike swarm of the Wyoming province. The dikes occupy from 15 to 20% of a 100-km-wide exposure of Archean basement, and include diabasic, plagioclase-phyric, and peridotitic roc types. U-Pb zircon and baddeleyite data indicate that dykes were intruded at 2010 Ma. The dikes were deformed locally and metamorphic zircon replaced igneous baddeleyite to varying degrees during the collisional Medicine Bow orogeny that affected the southern margin of the Wyoming province between 1780 and 1760 Ma.

The geochemical compositions of the dike swarm exhibit a continuum with considerable overlap between diabase, plagioclase porphyry, and peridotite. Medium-grained rocks along the margins of peridotite are the best estimate of parental, mantle-derived magma compositions. The variations in major element compositions are the result of magmatic differentiations and crystal accumulation. Minor amounts of assimilation of felsic Archean crust, identified from Nd, Sr, and Pb isotopic compositions, have significantly affected the abundances of some trace elements including the rare earth elements.

The Kennedy dike swarm records paleoproterozoic rifting fo the southern Wyoming province. Identification of the rifted continental block rests in part on the recognition of a coeval dole swarm. The only well-dated dikes of similar age are the Lacde Gras dike swarm of the Slave craton. A Tectonic reconstruction juxtaposing the southeastern Wyoming province with the northern Slave province is proposed.

SHRIMP U-Pb zircon ages for Big Creek geneiss, Wyoming and Boulder Creek batholith, Colorado: Implications for timing of Paleoproterozoic accretion of the northern Colorado province

Wayne R. Premo and C. Mark Fanning

Keywords: Accretion, SHRIMP, zircon, U-Pb ages, Proterozoic, Big Creek gneiss, Boulder Creek batholith, Colorado province.

Sensitive, high-resolution, ion microprobe (SHRIMP) U-Pb zircon ages from a sample of the high-grade, hornblende-feldspathic big Creek gneiss of the southeastern Sierra Madre, along with samples of a quartz monzonitic phase of the Boulder Creek batholith, help define timing of three major Paleoproterozoic thermo-tectonic events within the northern Colorado province at approximately 1810, 1710, and 1610 Ma. Previous ages determined for these key rock units were problematic; they hindered regional tectonic interpretations of the Paleoproterozoic crustal accretion history of the Colorado province that extends from the Cheyenne belt of southern Wyoming to north-central New Mexico. The Colorado province has been popularly modeled as a series of accreted oceanic volcano-plutonic arc systems and associated sediments, although alternative interpretations suggest that the series represents continental-margin arc rocks.

The Big Creek gneiss has been interpreted as a high-grade basement equivalent of the oldest arc volcanic rocks exposed within the Green Mountain arc terrane, but it also has been suspected of being either an older block of pre-arc or basement or perhaps an allochthonous piece of crust from slightly older orogents to the east and north. Previous ID-TIMS work on mg-size zircon fractions yielded U-Pb Concordia upper-intercept ages of 1618 ± 22 and 1684 ± 5 Ma as well as negative lower-intercept ages, indicating complex U-Pb isotopic systematics involving at least two ages of zircon growth overprinted by at least one episode of Pb-loss. Zircons from this gneiss were analyzed using the SHRIMP, and a total of 32 spot analyses on both centers and rims produced a range of different 207Pb/206Pb ages between ~1840 and ~1560 Ma. The weighted mean of the oldest 207Pb/206Pb ages is 1812 ± 12 Ma and is interpreted to estimate the age of the protolith that appears to be slightly older than lower-grade metabasalts and associated plutons at ~1790-1775 Ma. This protolith age of 1812 Ma further implies that significantly older crust ( >1820 Ma; e.g., Penokean orogeny) is not found in the Green Mountain magmatic arc. The youngest 207Pb/206Pb ages of ~1610 Ma are interpreted to represent a time of new zircon growth during highly localized high-grade metamorphism—an event that also produced local granitic magmatism at ~1625 Ma.
The Boulder Creek batholith had been dated previously using the ID-TIMS, U-Pb zircon technique that yielded ages at ~1670 and ~1714 Ma, a 45-m.y. discrepancy that left the true age of the batholith in doubt. Zircons from two samples, previously dated using the ID-TIMS method, were analyzed using SHRIMP, and yielded Concordia upper-intercept ages of 1714 ± 10 and 1721 ± 15 Ma. These results, combined with two earlier U-Pb zircon determinations, help to establish the age of the Boulder Creek batholith at 1714 ± 4.6 Ma (weighted mean), an age more compatible with those for the other large, tonalitic to quartz monzonitic, syntectonic plutons within the northern Colorado province. The new Boulder Creek age helps to establish a discrete periodof plutonism (~1735-1705 Ma) that is syn-to post-tectonic with respect to major regional structures of deformation and metamorphism in the northern Colorado province. Assuming the multiple oceanic arc accretion model, the new age for the mid-crustal emplacement of this batholith into a deforming composite back-arc basin may date the closure of that basin during crustal shortening.

Age and Pb-Sr-Nd isotopic systematics of plutonic rocks from the Green Mountain magmatic arc, southeastern Wyoming: Isotopic characterization of a paleoproterozoic island arc system

Wayne R. Premo and R. R. Loucks

Keywords: Isotopic ages, isotopic characteristics, paleoproterozoic, plutonic rocks, island arcs, Green Mountain, colorado province, zircon ages, magmatic arc

Three new U-Pb zircon ages and the Pb-Sr-Nd isotopic systematics of 24 whole-rock samples from mainly plutonic rocks of the Sierra Madre and Medicine Bow Mountains near the Colorado-Wyoming border help establish the Green Mountain magmatic arc as a Paleoproterozoic variably eroded, island arc terrane. The Green Mountain magmatic arc, a terrane composed of variably metamorphosed volcanic and volcaniclastic rocks, minor metasedimentary rocks, high grade gneisses, and plutons ranging from gabbro to granodiorite, was formed between ca. 1792 and 1744 Ma. It is the northernmost and oldest part of the Colorado province and is separated from Archean rocks to the north by the east-west-trending Cheyenne belt.

New U-Pb zircon ages were determined for two dioritic samples of the Mullen Creek complex (1778 ± 2 and 1778 ± 17 Ma; an ultramafic/mafic layered intrusion) and for a sample of the Rambler granite (1771 ± 3.4 Ma); both units are exposed in the Medicine Bow Mountains. A Sm-Nd whole-rock isochron age of (1750 ± 24 Ma (E_Ndi = +3.8) was determined that is within error of the Sm=Nd whole-rock isochron age for the entire Lake Owen Sample database (1775 ± 45 Ma). Initial Nd signatures ( +3.3 to 4.8) indicate that the bulk of the arc rocks was derived from a depleted mantle source at 1.78 Ga. Although the Rb-Sr systematics appear disturbed, data from extremely low Rb/sr, non-hydrous, ultramafic layered units indicate an initial 87Sr/86Sr value of 0.7024. The range of initial Sr isotopic values for these rocks is elevated relative to depleted mantle sources at 1.78 Ga, and isotopic distinction of modern primitive oceanic island arc systems. The U-Pb data on the same mafic rock samples are consistent with the other isotopic results. The values define average initial Pb values of 206Pb/204Pb = 15.7 and 206Pb/204Pb = 15.5, indicative of a depleted mantle source at 1.78 Ga.

Felsic plutonic arc rocks exhibit disturbed Pb and Sr isotopic behavior. They are characterized by the same depleted mantle signature with initial E_Nc values of ~2.9-4.4, however, indicating little crustal contamination of source magmas for granites and precluding their derivation by subduction of Archean crustal components during collisional accretion of the arc.

The 1.76-Ga Horse Creek anorthosite complex, Wyoming: A massif anorthosite emplaced late in the Medicine Bow orogeny

Carol D. Frost, Kevin R. Chamberlain, B. Ronald Frost, and James S. Scoates

Keywords: Anorthosite, monzonite, granite, Proterozoic, isotopes, Wyoming

The 100 km2 Horse Creek anorthosite complex, which was emplaced between 1770 ± 3 and 1755 ± 2 Ma, is the older of two anorthosite complexes located in the southern Laramie Mountains, southeastern Wyoming. The complex is composed of four lithologic units, which are in order of decreasing age the Ragged Top granodiorite, Horse Creek granite, Horse Creek anorthosite, and Horse Creek monzonite. Nd, Sr, and Pb isotopic compositions of all units indicate that they are dominated by sources such as contemporary depleted mantle and crustal rocks with short crustal residence ages. However, the involvement of a small amount of Archean material is required by the Pb isotopic data.

Both the Horse Creek anorthosite complex and the 1.43-Ga Laramie anorthosite complex were emplaced along the Cheyenne belt, which marks the suture between the Archean Wyoming province and Proterozoic Colorado province. The time of emplacement of the Horse Creek anorthosite complex coincides with late stages of the Medicine Bow orogeny, in which Proterozoic island arcs collided with the Wyoming province. The Horse Creek anorthosite complex was emplace in a zone of local transtension formed as a result of oblique collision of the Colorado province along the southern margin of the Wyoming craton.

Dynamic versus anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado: Does the interpretation depend on level of exposure?

Christine S. Siddoway, Rima M. Givot, Christopher D. Bodle, and Matt T. Heizler

Key Words: Mesoproterozoic • Colorado • Wet Mountains • Proterozoic shear zones • ⁴⁰Ar-³⁹Ar hornblende • 1.4-Ga magmatism

New field investigations in the Wet Mountains of Colorado reveal informative structural-plutonic relationships surrounding Mesoproterozoic intrusions. Gneisses and schists of the Wet Mountains host syntectonic 1.7-Ga and 1.4-Ga plutons plus two to three generations of sills and stocks. Comparison of two study areas reveals a variation in metamorphic grade, crustal position, and structural rigidity of gneisses hosting the 1.4-Ga intrusions, with implications for the interpretation of dynamic versus anorogenic intrusive settings. An episode of post-1.4-Ga mineral growth was recorded in the Wet Mountains by overprinting mineral textures and ⁴⁰Ar/³⁹Ar hornblende ages of 1369 ± 4 to 1342 ± 6 Ma.

Mineral textures and rock fabrics provide evidence of three significant Proterozoic deformational events in the Wet Mountains. Predominant northwest- to west-striking foliation is a second-phase fabric, S₂, developed during regional plutonism at 1.66–1.7 Ga. S₀, relict sedimentary layering, and S₁, an earlier penetrative foliation, are preserved within cordierite.

Fabric development and metamorphism during 1.4-Ga magmatism varied across the range. Middle amphibolite-grade gneisses of the Arkansas River Canyon in the north give way to stromatic migmatites in the central Wet Mountains. S₂ was completely transposed in two discrete shear zones. The Five Points Gulch shear zone strikes approximately north-south and records sinistral-oblique displacement along a sillimanite mineral lineation. The Newlin Creek shear zone strikes northwest, with top-southwest transport.

Fabric within 1.4-Ga intrusions varies from locally developed foliation on discordant margins (northern Wet Mountains) to strong concordant foliation in extensive sills (central Wet Mountains). Later sills are less well-foliated and slightly discordant, indicating syntectonic granitic emplacement. Blocks of host gneiss were assimilated along some sill margins, attesting to a similarity in temperature between intruded material and country rock.

The variation in degree of metamorphic recrystallization, degree of transposition, and style of intrusion from north to south in the Wet Mountains is attributed to southward increase in temperature and structural depth. This study suggests that pluton emplacement depth influenced structural development and thus bears on the interpretation of dynamic versus anorogenic context for 1.4-Ga magmatism.

An overview of the petrology and geochemistry of the shermand batholith, southeastern Wyoming: Identifying multiple sources of Mesoproterozoic magmatism

Ben R. Edwards and Carol D. Frost

Key Words: Proterozoic • granite • anorogenic • isotopes • A-type • Wyoming • Colorado

The 1.43-Ga Sherman batholith of southeastern Wyoming and northeastern Colorado is a texturally and geochemically heterogeneous intrusion that comprises rocks derived from at least four different sources. The coarse-grained, metaluminous, biotite-hornblende Sherman Granite is volumetrically the most significant unit in the batholith. It has geochemical characteristics at the extreme end of A-type suites with high Fe^# > 88, high K₂O wt. % (generally > 5%), molar Na/K generally < 1, and high abundances of incompatible elements. The source for the Sherman Granite is constrained to be a Fe-rich, low oxygen fugacity source mafic material by initial isotopic ratios ( = –0.8–1.1 and initial ⁸⁷Sr/⁸⁶Sr = 0.7024–0.7126), low oxygen fugacity (–0.1 to –0.5 log units below the fayalite-magnetite-quartz buffer reaction) and water activity (0.7), comparisons to experimental melt compositions, and mineralogical and geochemical similarities to monzonitic intrusions from the Laramie anorthosite complex (LAC). The medium-grained, peraluminous Lincoln granite, which is volumetrically subordinate to the Sherman Granite, has less extreme A-type geochemical characteristics. Initial isotopic ratios (_Nd = –1 and initial ⁸⁷Sr/⁸⁶Sr = 0.7189 and 0.7238), comparisons to experimental melt compositions, and geochemical similarities to intrusions from the peraluminous Silver Plume suite favor a parental magma for the Lincoln granite derived from intermediate to felsic crustal rocks from the Colorado province, which forms the basement to the Sherman batholith. A suite of mineralogically heterogeneous biotite-hornblende porphyritic quartz monzonites and granites has geochemical characteristics that also fit the A-type classification but are less extreme than that for the Sherman Granite, with distinctly lower Fe^# (< 88) and molar Na/K (generally > 1). Field and geochemical observations are consistent with many of the porphyritic granites having formed by mixing between granitic and monzodioritic magmas and highlight the importance of magma mixing in the formation of the batholith. Mafic rocks are present in the batholith in minor quantities and include gabbro, ferrodiorite, and monzonite in addition to monzodiorite. All of the mafic rocks are geochemically and mineralogically similar to rocks from the adjacent LAC, and the gabbro presumably is derived from a mantle source similar to that for gabbroic rocks from the LAC. Geochemically distinct sodic granodiorite, which occurs in minor quantities in the batholith, represents a distinct unit likely derived from partially melted metabasalt.

The rocks of the Sherman batholith record the evolution of Mesoproterozoic lithosphere in five ways: 1) gabbro records asthenospheric input of heat and mass(?) into the juvenile, Paleoproterozoic lithosphere; 2) peraluminous granite records partial melting of the Paleoproterozoic lithosphere in response to the influx of asthenospheric melts; 3) hybrid porphyritic rocks record the direct interaction of mantle-derived magma and crustal materials to produce lithosphere with intermediate characteristics; 4) Na-rich rocks record melting of metabasalt emplaced prior to or during widespread magmagenesis; and 5) the Sherman Granite records direct partial melting of juvenile lithosphere and subsidiary fractional crystallization locally accompanied by crustal assimilation.