Kevin R. Chamberlain

Sphene (titanite): phase relations and role as a geochronometer

Abstract Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO 5 ) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock. Sphene from igneous rocks and orthogneisses has initial U contents ranging from 10 to over 100 ppm and ratios of initial U to common Pb ranging from 10 to 1000, ratios that may potentially yield high precision U–Pb ages. Sphene in marbles, calc-silicates, and metagraywackes has a similar range in composition to that from igneous rocks, but sphene from metabasites may have initial U contents of less than 1 ppm and ratios of initial U to common Pb lower than 1, making them unsuitable for geochronology. These low-U sphenes are most commonly found in weakly metamorphosed metabasites. Strategies to extract age information from sphene with moderate initial U/common Pb ratios include estimation of common Pb isotopic composition of sphene from coexisting low-U phases, use of U–Pb and Pb–Pb isochron plots, and step-wise leaching methods to improve 206 Pb/ 204 Pb spread. By correlating sphene compositions to metamorphic or hydrothermal reactions, age determinations on sphene can be used to directly date metamorphism, deformation, and hydrothermal alteration.

The Late Archean history of the Wyoming province as recorded by granitic magmatism in the Wind River Range, Wyoming

Abstract The Wyoming province, a small, ca. 500 000 km 2 Archean craton, is the most southwestern of the Archean provinces in North America. It is composed primarily of Late Archean potassium-rich granitic rocks. In contrast to many other Archean provinces, rocks of tonalite-trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2.8 Ga. Field, petrologic, geochemical and isotopic study of the Late Archean granites exposed in the Wind River Range have allowed us to identify at least four periods of potassic calc-alkalic magmatism at ∼2.8, 2.67, 2.63 and 2.55 Ga. Granitic rocks of these ages appear to be widespread across the Wyoming province. The oldest calc-alkalic granites of the Wind River Range, emplaced at ca. 2.8 Ga, appear to be derived predominantly from pre-existing crust. However, Nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses. During at least two other periods of plutonism, at 2.67 and 2.63 Ga, generation of the Wind River Range batholiths involved the incorporation of substantial amounts of isotopically juvenile material, either from depleted mantle or young continental crust. The information presented below, as well as data available from elsewhere in the Wyoming province, is interpreted to suggest that the Wyoming province, unlike other Archean cratons, is not composed of a tectonic amalgamation of smaller, exotic terranes. Although the Wyoming province did experience crustal addition in Archean time, it was not by lateral accretion, but by incorporation of mantle-derived melts into large granitic batholiths.

Apatite–feldspar U–Pb thermochronometer: a reliable, mid-range (∼450°C), diffusion-controlled system

The U–Pb systematics of apatite are commonly dominated by simple volume diffusion, without secondary growth and recrystallization that can affect U–Pb systematics of sphene, monazite and zircon. Although apatite is a relatively low-U mineral, both U and Pb are lattice-bound and thus the U–Pb systematics are not controlled by micro-inclusions as appears to be the case for other low-U minerals such as garnet and staurolite. Closure temperature estimates from both experimental and empirical studies range from 425°C to as high as 500°C for typical diffusion radii (25 μm) and cooling rates (2–100°C/Ma, respectively). This range can be exploited to constrain cooling and exhumation histories of both igneous and metamorphic rocks. In addition, the closure temperature for Pb diffusion in apatite fills a gap between that for Pb diffusion in sphene (650°C) and diffusion of Ar in hornblende (500°C), muscovite (350°C) and biotite (300–325°C). Apatite is a common accessory phase in many rock types, and U–Pb apatite dating can be a reliable and powerful thermochronometer as it combines mid-range cooling ages with the strength of the U–Pb system in testing for isotopic closure. Precisions of U–Pb apatite dates can range from ±1.2% to as low as ±0.3% for Proterozoic or Archean dates. The main limitation to accuracy of U–Pb apatite dates is a moderate to low ratio of U to Pb, and corresponding dependence of concordia coordinates on the choice of initial Pb isotopic composition. For Proterozoic-aged samples with measured 206Pb/204Pb of 100, the range of geologically reasonable initial Pb isotopic compositions can shift the calculated U–Pb age by 60 Ma. Fortunately, even though the diffusion rate of Pb in alkali feldspar is less than in apatite, it appears that the isotopic composition of Pb in coexisting feldspar is a good approximation of the initial isotopic composition in apatite for many geologic settings. Examples are given from an Archean terrane in SE Wyoming that was unroofed and cooled during Proterozoic uplift and from Proterozoic terranes in SW US that have contrasting thermal histories.

Evidence for extensive Proterozoic remobilization of the Aldan Shield and implications for Proterozoic plate tectonic reconstructions of Siberia and Laurentia

Abstract A geological traverse across the Aldan shield along the Aldan River shows that the area is underlain by two distinct rock associations. The Middle Aldan association consists of metasedimentary rocks, mainly quartzite, that have been intruded by a potassic biotite granite. Downstream, north of the Middle Aldan association, lies the Lower Aldan association, which consists mostly of charnockite with rafts of older granulite gneiss that contains abundant metasedimentary layers. UPb dating of zircons from the Middle Aldan association granite (1900 Ma) and Lower Aldan association charnockite (maximum age 1918 Ma) and granitic gneiss (maximum age 2230 Ma) shows that the majority of the rocks exposed along the Aldan River are Proterozoic in age. Although they have Proterozic crystallization ages, the granite, granite gneiss and charnockite yield Archean Nd model ages, suggesting that they formed by remobilization—that is, partial or complete remelting—of earlier Archean crust. In contrast, pelitic gneisses included within the charnockites of the Lower Aldan association give Proterozoic Nd model ages, indicating that a substantial amount of Proterozoic rock is incorporated within the Lower Aldan association. The present results show that the rocks along the Aldan River, which are part of the Aldan block, display a significantly different history from those in the Olekma block to the west. The Olekma block contains ca 3.0 Ga greenstone belts Late Archean amphibolite-grade granitic gneisses; the Proterozoic remobilization that typifies the Aldan terrane is absent. The Olekma block was thrust under the Aldan block during the ca 1.9 Ga orogeny contemporaneous with, or slightly after the 1.9 Ga charnockitic event in the Aldan. The 1.9 Ga magmatic and granulite event seen in the Aldan is similar in age and character to the Thelon magmatic zone of northern Canada. This correlation allows the development of a preferred construction for the Precambrian Laurentia-Siberia connection in which the present day southern portion of the Siberia platform was connected to the northern margin of Laurentia.

Crustal growth by magmatic underplating: Isotopic evidence from the northern Sherman batholith

It is accepted that continental growth takes place both in intraplate and convergent margin settings, but the relative importance of crustal growth in these two tectonic environments is the subject of ongoing debate. In this study we suggest that magmatic underplating in continental interiors results in significant increases in continental volume. We maintain that A-type, or anorogenic, granites are derived from young, underplated mafic crust. Magma sources for A-type granites typically are difficult to identify due to the lack of isotopic contrast between mantle and crustal sources. However, the northern portion of the Mesoproterozoic Sherman batholith, southeastern Wyoming, intrudes Archean gneiss. Isotopic data from these granites preclude derivation from felsic crust, and instead require the involvement of a mantle or mantle-like isotopic reservoir. The data are analogous to those for eruptive equivalents of A-type granites, the fayalite rhyolites of Yellowstone, which also ascended through Archean felsic crust but carry little Archean isotopic signature. Anorogenic granites thus may represent a middle to upper crustal record of magmatic underplating at depth.

Baddeleyite (ZrO2) and zircon (ZrSiO4) from anorthositic rocks of the Laramie anorthosite complex, Wyoming: Petrologic consequences and U-Pb ages

Two types of chromite deposits occur in the Hongguleleng ophiolite in Xinjiang, northwest China. One is located in the mantle sequence, the other occurs in the transition zone between the mantle sequence and layered cumulates. Abundant primary silicate inclusions such as phlogopite, pargasite, clinopyroxene, orthopyroxene, and olivine are found in the segregated chromite, but silicate inclusions occur only rarely in accessory chromite of the ultramafic rocks from the transition zone and cumulates. These silicate phases are considered to have been entrapped as discrete and rare composite inclusions during magmatic precipitation of chromite rather than formed by postmagmatic entrapment. Phlogopite is the most abundant mineral found as inclusions in chromite in the Hongguleleng ophiolite. There are two types of substitutions for K in the phlogopite inclusions: (l) Na substitutes for K, and phlogopite shows a continuous range from almost pure sodium phlogopite to phlogopite; (2) Ca partially substitutes for I! resulting in the formation of Ca-bearing phlogopite. It is proposed that the alkalic aqueous liquid (melt) responsible for the formation of the phlogopite inclusions was derived from the mixture of the K-rich aqueous liquid related to the subduction of the oceanic slab and the Na-rich aqueous liquid from the primary magma of the ophiolite. Two types of phlogopite hydrates, hydrate I and possibly a new hydrate (hydrate H), occur as inclusions in the chromite and result from later hydrothermal processes. The fractures from brittle deformation provided passage for meteoric water to enter and react with the phlogopite inclusions. Compared with those in the transition zone, the inclusions of phlogopite and phlogopite hydrates in the mantle sequence are characterizedby (l) smaller grain size and greater abundance, (2) undulatory extinction, (3) higher Si, Cr, Ni, and Ca, and (4) lower Ti and Al. These differences are possibly due to (l) P, T, and composition of chromite-precipitating magma, (2) subsolidus reequilibration with the host chromite, and (3) postmagrnatic deformation and hydrothermal processes.

Eocene clocks agree: Coeval 40Ar/39Ar, U-Pb, and astronomical ages from the Green River Formation

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from 40 Ar/ 39 Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238 U- 206 Pb age of 51.66 ± 0.20 Ma (full external 2σ uncertainty), indistinguishable from the recalibrated 40 Ar/ 39 Ar age of 51.40 ± 0.25 Ma (full external 2σ uncertainty), which is adjusted to eliminate samples with low radiogenic 40 Ar*. Significant 238 U- 206 Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean 238 U- 206 Pb age of 49.23 ± 0.13 Ma (full external 2σ uncertainty), whereas the sanidine 40 Ar/ 39 Ar age is 49.24 ± 0.18 Ma (full external 2σ uncertainty). Calibrating Green River Formation 40 Ar/ 39 Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of specific Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima.

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.

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

Abstract 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.

Mesozoic tectonics and metamorphism in the Pequop Mountains and Wood Hills region, northeast Nevada: Implications for the architecture and evolution of the Sevier orogen: Discussion and reply

The Pequop Mountains–Wood Hills–East Humboldt Range region, northeast Nevada, exposes a nearly continuous cross section of Precambrian to Mesozoic strata representing middle to upper crustal levels of the Mesozoic hinterland of the Sevier orogen. These rocks preserve the transition from unmetamorphosed Mesozoic upper crust to partially melted middle crust. Integration of new structural, metamorphic, and U-Pb thermochronologic data from the Wood Hills and Pequop Mountains, coupled with a regional tectonic reconstruction, reveals substantial Cretaceous metamorphism, contraction, and extension in the Sevier hinterland in northeast Nevada. We report two phases of contraction not previously recognized that are accommodated by top-to-the-southeast thrust faults, the Windermere and Independence thrusts. Contraction was succeeded by two phases of extension along west-rooted normal faults, the Late Cretaceous Pequop fault and Tertiary Mary9s River fault system. The earliest phase of thrust faulting resulted in as much as 30 km of crustal thickening and an estimated minimum of 69 km of shortening along an inferred fault called the Windermere thrust. The timing of this thrusting event is bracketed between Late Jurassic (ca. 153 Ma) and Late Cretaceous (84 Ma). Relaxation of crustal isotherms following and perhaps during thrusting resulted in Barrovian-style metamorphism of footwall rocks, and partial melting of metapelite at deep levels. Peak metamorphism was attained ca. 84 Ma, and by this time hinterland crustal thickening had reached a maximum. During 84–75 Ma another minor pulse of shortening and thickening along the Independence thrust was followed by partial exhumation of the metamorphic rocks and as much as 10 km of crustal thinning along the Pequop fault. Thus the interval from 84 to 75 Ma in northeast Nevada marks a fundamental, and apparently permanent, change from horizontal contraction to extension in the upper to middle crust in the hinterland. Final exhumation of the metamorphic rocks was accomplished by the Tertiary Mary9s River fault system. Our data indicate that much of the metamorphism and some of the contraction in the Sevier hinterland in northeast Nevada, which was previously thought to be largely Late Jurassic, is actually Cretaceous in age. Furthermore, the data indicate that widespread metamorphism of the middle crust is a byproduct of tectonic burial, and that hinterland and foreland thrust faulting were coeval, suggesting that thrust faults in the Sevier orogen do not form a simple foreland younging sequence.