Laramie Anorthosite Research Articles

Pseudomorphic 9-line silician ferrihydrite and Fe-rich serpentine-group minerals in FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, U.S.A.

AbstractLow-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the replacement of an igneous sulfide such as pyrrhotite. Electron diffraction shows the ferrihydrite to be a 9-line variety. Si-rich cronstedtite formed thin rims around the ferrihydrite, and talc grew patchily around the cronstedtite. Greenalite formed in ~10 μm cracks through all the above minerals and olivine, and hisingerite microveinlets partially replaced olivine. Igneous minerals remaining include olivine Fa46, magnetite, ilmenite, hornblende, biotite, and trace clinopyroxene. Correlations among the constituents of ferrihydrite determined by electron microprobe, including anhydrous totals, indicate progress during the growth of two charge-balanced exchanges involving silica enrichment: an inverse cronstedtite substitution (MgFe2+,Si) (Fe3+Mn3+)–2 and an inverse hydrogarnet substitution SiH–4. The cronstedtite exchange requires charge and size balance across nearest-neighbor T and O crystal sites, suggesting crystal-interior rather than crystal-surface control. Ferrihydrite’s composition reflects time- and space-related variations in the chemical potentials of components in the hydrous fluid at the site of alteration. An upper limit for SiO2 of 14–15 wt%, or ≈1.0 Si per 5-cation formula unit, would seem to correspond to the limit of availability in ferrihydrite of tetrahedral sites open to the entry of Si. Our EPMA data, projected to zero SiO2, indicate an anhydrous total of ≈83 wt% for end-member ferrihydrite, a number that matches the formula: Fe10O15·9H2O. The geochemical properties of Laramie ferrihydrite are shared by some samples of altered chondritic and Martian meteorites. Ferrihydrite on Earth commonly occurs as a surface deposit; unlike the Laramie occurrence, these lack the microspatial coherence of replacements/pseudomorphs to show systematic, structure-related element variations. The superior crystal quality of the Laramie ferrihydrite likely contributed to its unique compositional variability.

CHEMICAL AND ISOTOPIC EVOLUTION OF THE ANORTHOSITIC PLUTONS OF THE LARAMIE ANORTHOSITE COMPLEX: EXPLANATIONS FOR VARIATIONS IN SILICA ACTIVITY AND OXYGEN FUGACITY OF MASSIF ANORTHOSITES

The Laramie anorthosite complex includes three anorthositic plutons: the Poe Mountain, Chugwater, and Snow Creek intrusions. The Poe Mountain and Chugwater bodies exhibit mappable magmatic stratigraphy that records fractionation processes in a magma chamber at or near the present level of exposure followed by doming and expulsion of residual liquid. The Snow Creek pluton is more poorly exposed, although layering is locally observed, and there is geochemical evidence for removal of interstitial liquid in the eastern portion of the body. The three plutons were emplaced in close succession. The U–Pb zircon and baddeleyite geochronology of anorthosite from the Chugwater and Snow Creek plutons, and a monzodiorite dike, indicates that the Chugwater is the oldest of the bodies, with a weighted average 207 Pb/ 206 Pb age of five samples of 1435.5 ± 0.3 Ma. The Snow Creek anorthosite intrudes the previously dated 1434 Ma Poe Mountain anorthosite and is, in turn, cut by the 1432.8 ± 2.4 Ma monzodiorite dike. The LAC and associated granites of the Sherman batholith suite were emplaced over a period no greater than 12 million years, and possibly in as little as three million years. Despite their similar ages, each of the LAC anorthosite plutons displays a distinctive assemblage of minerals. The Poe Mountain anorthosite is characterized by olivine, augite, low Ca-pyroxene, ilmenite and magnetite, and plagioclase in the range An 43–53 . The Chugwater anorthosite generally contains no olivine but is composed of augite, low Ca-pyroxene, ilmenite and magnetite, and iridescent plagioclase in the range An 50–56 . The Snow Creek anorthosite is characterized by a lack of olivine and magnetite and the presence of iridescent plagioclase of composition An 47 to An 56 ; it is commonly quartz-bearing. These assemblages record a range of silica activities and oxygen fugacities. Higher activities of silica and fugacities of oxygen are correlated with greater amounts of crustal assimilation, as indicated by Nd and Sr isotopic compositions and the presence of inherited components in zircon. We demonstrate that crustal assimilation can produce a range of mineral assemblages and Sr–Nd isotopic compositions in LAC anorthosites crystallized from common mantle-derived parental magmas, and that assimilation of crust is likely an important control on the compositional variations documented in other Proterozoic massif anorthosites.

MAGMATIC AND STRUCTURAL EVOLUTION OF AN ANORTHOSITIC MAGMA CHAMBER: THE POE MOUNTAIN INTRUSION, LARAMIE ANORTHOSITE COMPLEX, WYOMING

The 200 km 2 Poe Mountain intrusion, part of the 1.43 Ga Laramie anorthosite complex in southeast Wyoming, USA, preserves an exceptional range of primary magmatic and secondary deformation-induced structures that document the sequence of events involved in the emplacement, crystallization, and high-temperature deformation of a Proterozoic anorthosite. The intrusion consists of a steeply dipping (40–90°) olivine leucogabbroic to anorthositic margin of layered cumulates (minimum stratigraphic thickness of 5 km) that contains structures typical of mafic layered intrusions. The layered margin grades inward to a core of granoblastic (recrystallized) anorthosite characteristic of Proterozoic anorthosite plutonic suites worldwide. From the lowest to the highest stratigraphic levels, the layered series is divided into the anorthositic layered zone (ALZ) and leucogabbroic layered zone (LLZ). Igneous layering occurs at all scales from laterally discontinuous cm-thick horizons to distinct mappable packages of layered rocks up to several hundred meters thick. Contacts between the layered zones and internal subsections can be traced ~20 km along strike. The layered rocks are strongly laminated, defined by the orientation of tabular plagioclase; the abundance of layers in the ALZ containing blocky megacrysts of plagioclase increases down-section toward the core. Most of the tabular plagioclase appears to have grown within the upper crustal magma reservoir; plagioclase megacrysts were likely entrained from depth. Isolated examples of layer disruption record periodic re-organization of the semi-consolidated cumulate pile in response to slumping. Structures related to the impact of settled blocks, combined with scours and a mixed magma horizon, indicate that layering and lamination formed directly at or near the magma – pile interface, and that a dynamic magma chamber was present throughout the crystallization history of the Poe Mountain intrusion. Because the rejected interstitial liquid in anorthositic cumulates is denser than intermediate-composition plagioclase, the intrusion floor must have been inclined to allow for drainage. A sloping floor formed in the Poe Mountain intrusion owing to the contemporaneous diapiric rise of the relatively buoyant plagioclase-megacryst-rich rocks in the core; diapirism was the driving force for pervasive recrystallization of the core anorthosite and much of the ALZ. The Poe Mountain intrusion demonstrates that the assembly of Proterozoic anorthosites required active, periodically replenished magma chambers where the efficient segregation of plagioclase and removal of dense rejected melt, aided by syn- to post-crystallization deformation, led to the formation of vast expanses of nearly monomineralic cumulates.

PETROLOGY, GEOCHEMISTRY, AND STRUCTURE OF THE CHUGWATER ANORTHOSITE, LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

The Chugwater Anorthosite is one of several 1.43 Ga intrusions that make up the Laramie Anorthosite Complex in the Laramie Mountains, Wyoming, USA. The southwestern portion of the Chugwater Anorthosite has a mappable magmatic stratigraphy totaling at least 8000 meters, and probably 10,000 m or more. It consists of three major units, each of which comprises a lower, dominantly anorthosite portion (>90 vol.% plagioclase) and an upper, dominantly gabbroic anorthosite section (80–90 vol.% plagioclase). Each anorthosite portion contains layers of gabbroic anorthosite and vice versa , on a variety of scales ranging down to centimeters. Plagioclase is dominantly An 50–55 , although higher and lower values are also found. This “main series” of the Chugwater Anorthosite mainly lacks modal and normative olivine. Oxygen fugacity ranged from FMQ to FMQ + 0.5. We interpret that the “main series” was produced by at least three major injections of mildly hyperfeldspathic magma containing approximately 40% entrained plagioclase megacrysts (tabular crystals at least 5 cm across). The Ti contents of the megacrysts (0.3–0.4 wt% TiO 2 ) suggest that these had crystallized at pressures near 10 kbar, consistent with initial formation in a magma chamber at or near the base of the crust. Emplacement pressure is poorly constrained, but appears to have been near 3.5–4 kbar. However, before the main series had completely solidified, it was repeatedly intruded by at least two (and probably more) leucotroctolitic magmas. Contacts of leucotroctolite are sharp against anorthosite, but commonly are diffuse against gabbroic anorthosite, suggesting that sufficient residual melt remained in the latter to permit local mixing with leucotroctolite. We estimate that 80–85% of the Chugwater Anorthosite is main-series, 10–15% is mixed rock, and 5% is leucotroctolite. Prior to final solidification, the entire Chugwater Anorthosite was domed, probably as a result of gravitational instability of the relatively buoyant plagioclase-rich material. The emplacement of the Chugwater Anorthosite as a series of crystal-rich magmas, followed by continued fractionation in a magma chamber at a mid-crustal level and subsequent doming, are characteristics that make the Chugwater Anorthosite intermediate between the Poe Mountain Anorthosite to the north ( in situ fractionation in a magma chamber) and the classic diapiric emplacement of a crystal-rich mush.

Geochemical and Nd–Hf isotopic constraints on the origin of the ~ 1.74-Ga Damiao anorthosite complex, North China Craton

The ~1.74-Ga Damiao complex in the North China Craton consists of anorthosite (85%), norite (10%), mangerite (4%) and minor troctolite (<1%), all of which are cut by gabbroic and ferrodioritic dikes. The complex hosts abundant Fe–Ti–P oxide ores and has an emplacement sequence from anorthosite and norite to Fe–Ti–P oxide ores to mangerite. All of the different lithologies in the complex have similar light rare earth element (LREE)-enriched patterns and Nd isotopic compositions with εNd(t) values mostly within the range of −5.0 to −4.0. Average εHf(t) values of zircon are −4.7 for mangerite and −5.9 for norite. Plagioclase and clinopyroxene show a continuous range of composition from anorthosite and norite to Fe–Ti–P oxide ores to mangerite, suggesting that these lithologies formed by differentiation from a common parental magma. Gabbroic dikes in the Damiao complex are characterized by high Al2O3 (14.5–17.1 wt.%), Sr (~1000 ppm) and Mg#s [100 Mg/(Mg+Fe2+)]=56–73), similar to the high-Al gabbros parental to the Harp Lake and Laramie anorthosite complexes in North America. The high-Al gabbroic dikes of the Damiao complex may reflect the parental melt from which the complex formed, whereas the ferrodioritic dikes may represent residual melts after anorthosite crystallization from the high-Al gabbroic magma. The high-Al gabbros have incompatible element ratios (Zr/Nb=14–24; La/Nb=3.6–4.8), which differ significantly from those of nearby coeval, mantle-derived, mafic dike swarms and volcanic rocks but are similar to those of the lower crust. Their zircon εHf(t) and whole-rock εNd(t) compositions plot along the evolution lines of the ~2.5 Ga Archean rocks in the North China Craton, suggesting that the Damiao complex was derived from an ancient lower crust. Exceptionally high temperatures and high-degrees of melting (>75%) would have been required to form the parental high-Al gabbroic magma from the lower crust. We propose that the crustal rocks were dragged into the upper mantle during amalgamation of the North China Craton, where the ambient temperature was high enough (>1271 °C at 12 kbar) to cause extensive melting, producing a deep-seated magma chamber. The melt then evolved through polybaric crystallization, producing anorthositic magmas with abundant suspended plagioclase that were finally injected at mid-crustal depth to form the Damiao complex.

ORIGIN OF ZIRCON RIMS AROUND ILMENITE IN MAFIC PLUTONIC ROCKS OF PROTEROZOIC ANORTHOSITE SUITES

Zircon rims around ilmenite occur in a wide variety of mafic plutonic rocks (Fe–Ti oxide ores, anorthosite, and gabbro) from unmetamorphosed Proterozoic intrusions in the Grenville Province of eastern Canada (Havre-Saint-Pierre, Mattawa and Saint- Urbain anorthosites, Quebec; Twin Lakes intrusive complex, Ontario) and the western U.S. (Laramie anorthosite, Wyoming). The rims range from a few μm to 100 μm across and occur along grain boundaries between Ti-based oxide minerals (mainly ilmenite and exsolved ferrian ilmenite, but also rutile) or between silicate minerals (plagioclase and biotite) and Ti-based oxide minerals. The zircon rims contain very low concentrations of P (<447 ppm), U (0.9–1.1 ppm) and Th (15–19 ppm), which, coupled with the restriction of the rims to ilmenite grain boundaries, rules out formation from high-temperature, zircon-saturated interstitial liquid and from low-temperature hydrothermal fluids. There is no textural evidence that zircon replaced pre-existing baddeleyite that may have formed by an oxidation–exsolution mechanism from ilmenite. The textural and compositional characteristics of the zircon rims are consistent with an origin by diffusion of Zr from ilmenite to grain boundaries during slow cooling and subsequent reaction with available Si from adjacent silicate minerals (or from ilmenite itself) to form zircon. A positive correlation between whole-rock TiO2 and Zr contents in samples of Fe–Ti oxide ores from Saint-Urbain and Big Island (Havre-Saint-Pierre) (up to 500 ppm in massive ilmenite ore) confirms that Zr is compatible in ilmenite crystallized from ferrobasaltic magmas. Mass-balance constraints demonstrate that the observed quantities of zircon can be derived from Zr-bearing ilmenite crystallized from basaltic magmas with typical Zr concentrations (161–252 ppm) after fractionation of Zr-free minerals (150–247 ppm). Zircon rims are not expected to occur in ilmenite-bearing basaltic lavas and high-level intrusions owing to rapid cooling, but zircon rims may be a common feature of slowly cooled plutonic rocks that crystallize from ferrobasaltic magmas. The U–Pb dating of these rims may provide additional constraints on the thermal evolution of a given plutonic rock.

Can crystallization of olivine tholeiite give rise to potassic rhyolites?—an experimental investigation

Experiments were conducted to determine whether the rhyolites and basalts of the intraplate silica-saturated potassic suites could be genetically related through crystallization. Extreme crystallization (96–97%) of a high-MgO (10.62 wt%) olivine tholeiite from the Snake River Plain with an initial bulk water content of 0.4 wt% at a mid-crustal pressure of 4.3 kbar generated potassic rhyolitic liquids similar in major element chemistry to those found in the Quaternary rhyolite domes of the Snake River Plain and their plutonic equivalents in the Proterozoic Laramie Anorthosite Complex. Residual liquids comparable in composition to the bulk rock compositions of intermediate rocks found at the Craters of the Moon and Cedar Butte eruptive centers in the Snake River Plain are also generated along this crystallization path.

Petrogenesis of the Red Mountain pluton, Laramie anorthosite complex, Wyoming: implications for the origin of A-type granite

Abstract The Red Mountain pluton, the youngest intrusion in the Laramie anorthosite complex, is one of five monzonitic to granitic A-type plutons intruded at 1.43 Ga in the Laramie Mountains of southeastern Wyoming. The Red Mountain pluton ranges in composition from fayalite monzonite, clinopyroxene quartz monzonite, and biotite–hornblende quartz syenite to granite. The Red Mountain pluton is petrologically and geochemically distinct from the northern and southern Sherman batholith, the Sybille intrusion, and the Maloin Ranch pluton. It has higher FeO t /(FeO t +MgO) and higher K 2 O, is more strongly enriched in REEs, and at any given silica content it has lower abundances of TiO 2 , FeO, MgO, CaO, and P 2 O 5 . The fayalite monzonite in the Red Mountain pluton crystallized at temperatures around 1000 °C and oxygen fugacities 1–2 log units below that of the FMQ buffer, similar to the crystallization conditions of the Sybille and Maloin Ranch plutons. The biotite–hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of FMQ. Most of the units in the pluton carry mantle-like Nd, Sr, and Pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the LAC, indicating that the pluton evolved mainly via differentiation. Substantial crustal assimilation occurred only in the late dikes and the granite. We suggest that all of the 1.43 Ga A-type intrusions in southeastern Wyoming are ultimately derived from tholeiites. Extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust. The differences between the Red Mountain pluton and other A-type granitoids in southeastern Wyoming are related to the extent of differentiation prior to crustal assimilation. The compositional ranges of A-type granitoids in southeastern Wyoming suggest that the geochemical spectrum of A-type granites worldwide similarly are produced from evolved mantle-derived melt that has assimilated varying proportions of crustal material.

Geochronologic, geochemical and isotopic constraints on the origin of monzonitic and related rocks in the Laramie anorthosite complex, Wyoming, USA

We present new U-Pb zircon ages for monzonitic rocks of the 1.43 Ga Laramie anorthosite complex, a major Proterozoic anorthosite plutonic suite in southeastern Wyoming, USA, associated with contemporaneous reduced rapakivi-type anorogenic granites of the Sherman batholith. Our results constrain (1) the geochemical evolution of Fe-enriched residual liquids to anorthosite, (2) the duration and cooling history of mid-Proterozoic magmatism in the region, and (3) the composition and structure of the crust below the exposed plutonic rocks. Monzonitic intrusions and dikes, ranging in composition from monzodiorite to quartz monzonite, account for nearly one-quarter of the Laramie anorthosite complex, forming a discontinuous rim around the anorthositic intrusions. All of the monzonitic rocks crystallized at high temperatures (>950°C) and under relatively reducing conditions (fO2 1000°C was relatively rapid (20-25°C per million years) during the initial 30 million years following emplacement and slowed considerably once ambient temperatures of about 350°C were reached. Each of the monzonitic intrusions and all of the monzodioritic dikes within the Laramie anorthosite complex are characterized by a wide range of initial 87Sr/86Sr and limited variation of initial eNd, consistent with simple binary mixing between anorthosite residual liquids and Archean crust. The ages and isotopic compositions of most of the monzonitic and related rocks indicate that their ferrodioritic parents were extracted from anorthosite at depth and that they were contaminated during ascent through Archean crust prior to emplacement within and along the margins of the currently exposed anorthosites in the Laramie Mountains. The combined geochronological and geochemical results of this study demonstrate that Proterozoic anorthosite plutonic suites are characterized by extensive, relatively long-lived (∼10 million years) conduit systems, with intrusions distributed at different depths throughout the crust, which are linked and cut by dikes derived from deeper crustal levels. © 2003 Elsevier Science B.V. All rights reserved.

The relationship between A-type granites and residual magmas from anorthosite: evidence from the northern Sherman batholith, Laramie Mountains, Wyoming, USA

Anorthosite complexes commonly are spatially and temporally associated with A-type granite batholiths, but their genetic relationship, if any, has been difficult to determine. This study focuses on the northern portion of the 1.43–1.44 Ga Sherman batholith, which is located adjacent to the 1.44 Ga Laramie anorthosite complex, Laramie Mountains, Wyoming, USA. The northern Sherman batholith is an ideal place to resolve the relationship of A-type granite with anorthosite because of the age of the crust into which it is emplaced. Most A-type granites are intruded into crust only slightly older than the granites themselves, and there is little isotopic contrast between potential mantle and crustal source rocks. The northern Sherman batholith intrudes Archean crust that by 1.44 Ga had developed Nd, Sr and Pb isotopic compositions that differed greatly from contemporaneous mantle. The northern Sherman batholith is an iron-enriched, metaluminous, alkalic to alkali-calcic batholith. Most of the batholith is composed of fayalite granite, with minor volumes of monzodiorite and olivine gabbro. Hypabyssal dikes of these compositions crop out in the highest structural levels exposed in the area. The northern Sherman batholith is less differentiated and has assimilated less felsic crust than the southern portions, which intrude more fertile Proterozoic crust. Northern Sherman batholith rocks are compositionally and isotopically very similar to monzonitic and dioritic rocks of the Laramie anorthosite complex. None of these rocks have isotopic compositions similar to their Archean host rocks, precluding derivation solely from such sources. Instead, northern Sherman batholith granites, like Laramie anorthosite complex rocks, are related to mantle-derived tholeiitic magmas emplaced at the base of the crust. The tectonic environment and magmatic processes that lead to the formation of massif anorthosite are also conducive to the formation of voluminous A-type granite, of which the northern Sherman granite is a prime example.

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

The 100 km 2 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 emplaced 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.

Paleo- and Mesoproterozoic granite plutonism of Colorado and Wyoming

Proterozoic plutonism in Colorado and Wyoming was initiated ∼1.8 Ga with scattered tholeiitic mafic complexes coeval with widespread synorogenic bimodal volcanism. Limited Nd and Sr isotopic data for the metavolcanic rocks show derivation from depleted mantle. Major Paleoproterozoic granitic plutonism followed at 1.67–1.77 Ga. Most of the earliest plutons are distinctly calc-alkaline; they range largely from quartz diorite to granodiorite to trondhjemite in composition, and have trace-element signatures similar to plutons within magmatic arcs related to subduction zones. Later Paleoproterozoic plutons at 1.71 Ga include increased volumes of felsic rock types and, independent of silica, are shifted to more peraluminous and iron-rich compositions. The earliest appearance of anorthosite occurs with the 1.76-Ga Horse Creek anorthosite complex of Wyoming, and the earliest occurrence of A-type granite includes the late-kinematic, 1.66-Ga Garell Peak batholith of southern Colorado. Elemental and isotopic compositions of the younger Early Paleoproterozoic granitic plutons are consistent with a systematically increasing crustal component as a function of age in waning orogenic stages of crust formation in the region. After a 200 m.y. hiatus, renewed granitic plutonism occurred at 1.36–1.44 Ga. Plutonism was associated with emplacement of over a dozen Mesoproterozoic A-type granite batholiths and many smaller intrusions as part of a global “anorogenic” mid-Proterozoic event that commonly includes associated intrusions of anorthosite and charnockite. Across the former Laurentia supercontinent, three geographic and petrologic subprovinces merge in Colorado and Wyoming. An ilmenite-series granitic province, which includes the Sherman Granite and associated Laramie anorthosite complex of Wyoming, extends northeastward through Wisconsin to Labrador and the classic rapakivi granite-anorthosite intrusions of the Baltic region. A magnetite-series granite subprovince ranges across the southern mid-continent to California and includes the San Isabel and Eolus batholiths of southern Colorado. The third subprovince is peraluminous, comprised of two-mica granite, and geographically extends from central Colorado to Arizona. Granites of this suite are the most common in Colorado and include the Silver Plume batholith. Granites defining the three Mesoproterozoic provinces have distinctly different elemental and oxygen isotopic compositions, which presumably reflect fundamental shifts in composition of the lower continental Laurentian crust. The mid-Proterozoic intrusions of Colorado and Wyoming coincided in time with emplacement of regional, north-trending mafic dike swarms, implying widespread extension during this period. After another magmatic hiatus, one of ∼300 m.y., intrusion of the A-type, 1.08-Ga Pikes Peak batholith formed the last Proterozoic magmatic episode of the Colorado-Wyoming Front Range.

Oxygen isotope constraints on the petrogenesis of the Sybille intrusion of the Proterozoic Laramie Anorthosite Complex

The origin of monzonitic intrusions that are associated with Proterozoic massif-type anorthosite complexes is controversial. A detailed oxygen isotope study of the Sybille intrusion, a monzonitic intrusion of the Laramie Anorthosite Complex (Wyoming), indicates that either derivation from a basaltic magma of mantle origin with a metasedimentary component (∼20%) incorporated early in its magmatic history, or a partial melt of lower crustal rocks is consistent with the data. The oxygen isotope compositions of plagioclase, pyroxene and zircon from the Sybille monzosyenite, the dominant rock type in the Sybille intrusion, were analyzed in order to establish the isotopic composition of the source of the magma. Plagioclase δ18O values range from 6.77 to 9.17‰. We interpret the higher plagioclase δ18O values (average 8.69 ± 0.30‰, n = 19) to be magmatic in origin, lower plagioclase δ18O values (average 7.51 ± 0.44‰, n = 22) to be the result of variable subsolidus alteration, and pyroxene δ18O values (average 6.34 ± 0.38‰, n = 19) to be the result of closed-system diffusional exchange during cooling. Low magnetic zircons, which have been shown to retain magmatic oxygen isotope values despite high grade metamorphism and extensive subsolidus hydrothermal alteration, have δ18O values (7.40 ± 0.24‰, n = 11) which are consistent with our interpretation of the plagioclase and pyroxene results. Oxygen isotope data from all three minerals indicate that the magmatic oxygen isotope composition of the Sybille intrusion is enriched in 18O relative to the composition of average or “normal” mantle-derived magmas. This enrichment is approximately twice the oxygen isotope enrichment that could result from closed-system fractionation, rendering a closed-system, comag- matic petrogenetic model between the Sybille intrusion and the mantle-derived anorthositic lithologies of the Laramie Anorthosite Complex improbable.

Contrasting responses of Rb–Sr systematics to regional and contact metamorphism, Laramie Mountains, Wyoming, USA

Archean supracrustal sequences of pelitic, quartzitic, calcareous and mafic compositions in the central Laramie Mountains, Wyoming, have been affected by two metamorphic events: a 1.78 Ga amphibolite‐grade regional metamorphism, and a 1.43 Ga contact metamorphism resulting from the intrusion of the Laramie Anorthosite Complex (LAC). Rb–Sr whole‐rock isotopic data from both outside and within the LAC contact aureole define a linear array that lies along a 1.78 Ga isochron. This date has been independently established as the time of amphibolite facies regional metamorphism associated with collision of the Archean Wyoming province and the Proterozoic Colorado province along the Cheyenne belt. The Rb–Sr isotopic data require that Sr was redistributed during regional metamorphism on a scale of at least tens of metres. Although within the 2 km‐wide aureole of LAC the pelitic rocks were thermally metamorphosed at temperatures greater than 800 °C, none of the whole‐rock Rb–Sr data from samples within the LAC aureole show evidence of resetting at 1.43 Ga. It is interpreted that the regional metamorphism involved fluid transport which facilitated Sr isotopic resetting, whereas the contact metamorphism occurred in a relatively dry environment in which isotopic mobility was restricted to centimetre‐scale or less. Rb–Sr data for biotite, feldspar and whole rock from a regional metamorphosed pelitic schist give an isochron age of 1450±40 Ma, which is interpreted as a cooling age resulting from crustal uplift. Rb–Sr data for biotite, quartz+feldspar and whole rock from a pelitic schist affected by contact metamorphism give an isochron age of 1420±43 Ma, the time of isotopic re‐equilibration in response either to crustal uplift or to both contact metamorphism and crustal uplift. This study demonstrates that although the response of isotopic systems to metamorphism is complex, isotopic data provide insight into metamorphic processes that is difficult to obtain by other means.

Medicine Bow orogeny: Timing of deformation and model of crustal structure produced during continent-arc collision, ca. 1.78 Ga, southeastern Wyoming

A four-dimensional model for the evolution of the late-Paleoproterozoic Cheyenne belt arc-continent suture is presented based on available geologic mapping, structural analysis, geophysical constraints, geobarometry, geochronology, and isotopic data. All of the data are consistent with a southeast-dipping suture and deformation that lasted from 1.78 Ga to at least 1.76 Ga, and possibly as late as 1.74 Ga. Archean crustal components and/or detritus were subducted at least 30 to 70 km south of the trace of the suture. There is considerable variation in crustal structure and tectonic evolution along strike of the Cheyenne belt. Thick-skinned, intracratonic uplift along the Laramie Peak shear zone and synorogenic emplacement of the Horse Creek anorthosite complex occurred in the east, whereas low-grade metamorphism and late cataclastic thrust faulting occurred in the west. Some of this lateral variation may reflect the influence of preexisting crustal features such as high-angle normal faults and crustal heterogeneities related to ca. 2.0-Ga rifting, whereas other differences may reflect variations in collision geometry. The lithospheric architecture of this arc-continent suture created compositional and structural anisotropies that influenced later deformation and magmatism, such as the generation and emplacement of the 1.43-Ga Laramie anorthosite complex and location of Paleozoic diamond-bearing diatremes. In this paper, the term “Medicine Bow orogeny” is proposed to describe the ca. 1.78-Ga arc-continent collision that formed the Cheyenne belt suture, and the subsequent structural evolution of the orogenic zone, which may have continued to ca. 1.74 Ga. The orogenic belt trends from southeastern Wyoming to northeastern Nevada, a distance of ∼1900 km. This term is invoked to differentiate the tectonic history along the Cheyenne belt from both the Yavapai orogeny to the south, and Central Plains orogeny to the east, because these orogenies include younger rocks and younger deformation. The arc terrane involved in the Medicine Bow orogeny is probably 50 to 100 km wide, and it makes up the basement of northern Colorado and continues at least as far south as the Soda Creek-Fish Creek shear zone in north-central Colorado. Accretion during the Medicine Bow orogeny represents a substantial addition to the North American continent.

Orogenic to Post‐Orogenic Origin For the 1.76 Ga Horse Creek Anorthosite Complex, Wyoming, Usa

The age and inferred tectonic setting of the 1.76 Ga Horse Creek anorthosite complex (HCAC) in the Laramie Mountains of southeastern Wyoming place important constraints on the origin of middle Proterozoic anorthosite complexes. The 100 km2 HCAC consists of strongly recrystallized anorthosite and two small intrusions of monzonite and granite. U‐Pb crystallization ages from euhedral zircons in anorthosite and monzonite are 1761.5 ± 2 Ma and 1754.5 ± 2.2 Ma, respectively. An additional period of zircon growth in the anorthosite occurred at 1753.4 ± 2 Ma, as represented by a small population of anhedral zircon. We attribute the origin of this second morphological variety of zircon in the anorthosite to the loss of Zr from ilmenite during reaction with plagioclase to form sphene. This reaction took place in response to heat and fluid influx during intrusion of the adjacent monzonite. The HCAC and the younger 1.43 Ga Laramie anorthosite complex to the north were intruded along or near a Paleoproterozoic suture zone, the Cheyenne belt, marking the boundary between Archean rocks of the Wyoming Province to the north and Proterozoic island arc terranes to the south. We propose that the the HCAC was emplaced into young crust during or several million years after collision along the suture in an environment of late‐to post‐orogenic transtension. The presence of pre‐existing crustal structures, especially Archean/Proterozoic boundaries, strongly influences the generation and emplacement of many middle Proterozoic anorthosite complexes.

U-Pb sphene dating of metamorphism: the importance of sphene growth in the contact aureole of the Red Mountain pluton, Laramie Mountains, Wyoming

The relative importance of thermal diffusion versus new growth or recrystallization on U-Pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the Red Mountain pluton, Laramie anorthosite complex, Wyoming. Samples were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0.13 to 2.65 km from the contact. The 207Pb/206Pb ages of sphene from amphibolite samples range between 1.43 Ga, the intrusive age of the Red Mountain pluton, to 1.78 Ga, the age of regional metamorphism. The 207Pb/206Pb ages of sphene in rocks metamorphosed above 700° C are within error of the intrusive age of the pluton, and appear to have resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1.43 Ga. At greater distance from the contact the 207Pb/206Pb ages range from 1.45 to 1.72 Ga. This 300 million year spread in ages is interpreted to result from two periods of sphene growth, one produced during regional metamorphism at 1.78 Ga and another generation of newly grown or recrystallized sphene that formed during contact metamorphism at 1.43 Ga. These two age populations may be identified on the basis of petrographic textures, the morphologies and color differences of grain separates as well as by the U-Pb systematics. In rocks metamorphosed to temperatures less than 700° C, sphene growth was the dominant process controlling the response of the U-Pb isotope system to contact metamorphism. Sphene grew well outside the zone of obvious contact metamorphism. The U-Pb sphene ages were reset by diffusion only at high temperatures, supporting the experimentally determined closure temperature estimates for the U-Pb system in sphene of around 650° C (Cherniak 1993). This study demonstrates that U-Pb ages of sphene can be used to date metamorphism not only in areas with a simple geologic history, such as igneous intrusion or single metamorphic or deformational events, but also to date multiple events so long as different generations of sphene can be identified and separated.

A strontium and neodymium isotopic investigation of the Laramie anorthosites, Wyoming, USA: Implications for magma chamber processes and the evolution of magma conduits in Proterozoic anorthosites

A strontium and neodymium isotopic investigation of plagioclase-rich cumulate rocks in the 1.43 Ga Laramie anorthosite complex (LAC) provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of Proterozoic anorthosites. The 725 km 2 LAC intruded the boundary between Archean rocks of the Wyoming Province in the north and Proterozoic island arc terranes in the south, the Cheyenne belt, at pressures of 3–4 kilobars. Initial strontium and ε ND isotopic ratios from the three large composite anorthositic intrusions (Poe Mountain, Chugwater, and Snow Creek) and late troctolitic intrusions range from 0.7034–0.7055 and +0.9 to −4.9, respectively. The isotopic ratios in the cumulates vary as a result of (1) the isotopic composition of the parental mantle-derived high-Al gabbroic magmas, (2) the amount of contamination by Archean rocks during ascent through the crust, and (3) mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement. Strontium isotopic and normative An variations across the 5–7 km thick stratigraphic section of layered plagioclase-rich cumulates in the 200 km 2 Poe Mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in Proterozoic anorthosites. The earliest and stratigraphically lowest intrusion was emplaced as a plagioclase-rich magma that crystallized large volumes of compositionally homogeneous anorthosite. At least three subsequent chamber-wide episodes of magma replenishment, followed by mixing, are indicated by abrupt shifts in I Sr and/or normative An in the stratigraphically higher levels of the Poe Mountain anorthosite. Strontium and neodymium isotopic disequilibrium between a high-Al clinopyroxene megacryst and the layered cumulates is consistent with a high-pressure origin for Al-rich pyroxene megacrysts in Proterozoic anorthosites. The megacryst crystallized in a magma chamber at pressures of 10–12 kilobars from relatively uncontaminated basaltic parent magma (high-Al gabbro) and was transported through the crust in progressively contaminated anorthositic magma. The range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high-Al gabbroic dikes in the LAC, supporting the proposition that high-Al gabbros are parental to anorthosite (Mitchell et al., 1995). The isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by Archean orthogneisses and/or metapelitic rocks. Decreasing I Sr and increasing ε Nd with a relative decrease in age of the intrusions (Snow Creek → Poe Mountain/Chugwater → troctolites) indicates that the magma conduit became increasingly insulated from crustal contamination over time. This study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal/upper mantle pressures, during the ascent of magma diapirs, and within individual magma chambers in Proterozoic anorthosites, and underscores the need for stratigraphic control when addressing the origin of plagioclase-rich cumulates.

The Geochemical Evolution of Anorthosite Residual Magmas in the Laramie Anorthosite Complex, Wyoming

Olivine- and pyroxene-bearing Fe-enriched dioritic rocks in the 1434 Ma Laramie anorthosite complex are interpreted to represent variably fractionated and contaminated magmas residual after the crystallization of anorthosite. Geochemical characteristics of this suite include the following: high contents of TiO2, Fe2O3T⁠, and P2O5; high incompatible trace element contents; rare earth element patterns with a large range of Eu anomalies; and isotopic compositions that reflect the geographic location of individual samples, with ISr increasing and ∈Nd decreasing from south to north. After extraction from anorthosite, fractionation of ferrodioritic residual magmas resulted in secondary residual monzodioritic melts and complementary oxide-rich ferrodiorite cumulates. Geographic trends in isotopic composition reflect an increasing Archean crustal component from south to north. Dioritic dikes and cumulates with isotopic compositions similar to associated anorthosites were derived locally. Large isotopic discrepancies between some diorites and their hosting anorthosites reflect preferential contamination of residual magma during ascent and emplacement of mantle-derived plagioclase-rich diapirs, followed by subsequent extraction and isolation of Fe-enriched interstitial melt. Strong isotopic contrasts between anorthosite and associated Fe-enriched rocks in anorthosite complexes do not preclude a direct relationship between them and reflect the diversity and complexity of processes during their petrogenesis.

Residual-liquid origin for a monzonitic intrusion in a mid-Proterozoic anorthosite complex: The Sybille intrusion, Laramie anorthosite complex, Wyoming

The Sybille intrusion (≈100 km 2) is one of three large monzonitic intrusions in the 1.43 Ga Laramie anorthosite complex of southeastern Wyoming. The petrographic, geochemical, isotopic, and geophysical characteristics of Sybille monzonitic rocks are consistent with an origin by extensive crystallization of liquids residual to nearby anorthositic cumulates (ferrodiorites) and contamination by Archean wall rocks. The exposed part of the intrusion is composed mainly of coarse-grained monzosyenites with abundant alkali feldspar phenocrysts. The monzosyenites preserve mineralogical evidence for high crystallization temperatures (>1000 °C), mid-crustal emplacement pressures (≈3 kbar), relatively reduced crystallization conditions (2 log units below the fayalite + magnetite + quartz [FMQ] oxygen buffer), and they crystallized in the presence of a CO2-rich fluid phase (Fuhrman et al., 1988; Frost and Touret, 1989). The eastern monzosyenites, those adjacent to contemporaneous anorthosite, are distinguished by an anhydrous mineral assemblage (Fo16-Fo8 olivine, high-Ca pyroxene) lacking modal quartz, silica contents of 60 wt%, and smaller Eu anomalies (Eu/Eu* = 1.2 to 1.3). Abundant xenoliths of Archean wall rocks and anorthosite from the adjacent intrusions in all monzosyenites attest to a stoping emplacement mechanism near the roof of the chamber. We propose that the monzosyenites represent a relatively thin, 0.5-1.0-km-thick, roof to a magma chamber dominated by dense ferrodioritic cumulates at depth. Extensive, open-system fractionation of a ferrodioritic parent magma, residual after crystallization of anorthosite, produced Fe-enriched monzodioritic and/or monzonitic magma in the upper part of the chamber and complementary Fe- and Ti-rich cumulates in the lower levels. We have corroborated the production of monzonitic liquids from crystallization of ferrodiorite through a series of reconnaissance equilibrium-crystallization experiments. The presence of dense ferrodioritic cumulates at depth is consistent with the prominent positive gravity anomaly associated with the Sybille intrusion (Hodge et al., 1973). In the upper parts of the chamber, the fractionated monzodioritic and/or monzonitic magmas eventually became saturated in alkali feldspar. Owing to density contrasts, the alkali feldspar phenocrysts floated to the roof of the chamber, thus producing the exposed porphyritic monzosyenites. In addition, the roof of the chamber was the site of significant melting of Archean gneiss and, locally, metapelite. The Sr and Nd isotopic compositions of the monzosyenites, with Sr isotopic ratios becoming increasingly radiogenic from east ( I Sr = 0.7059 and initial ϵNd = −2.5) to west ( I Sr = 0.7092 and initial ϵNd = −2.6), are consistent with a 5% to 15% addition of Archean orthogneiss to a ferrodioritic parent magma that had isotopic characteristics similar to adjacent anorthositic rocks. The stratigraphic and compositional similarity of the Sybille monzosyenites to mangerites in the Bjerkreim-Sokndal intrusion of the Rogaland anorthosite complex, southern Norway, indicates that similar open-system magmatic processes are capable of having produced high-temperature, K-rich monzonitic rocks in other Proterozoic anorthosite complexes.