Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. U/Pb dating is accomplished by laser-ablation ICP-MS in a small pit on the exterior of the crystal, and He dates are then determined on the bulk grain by conventional laser-heating and dissolution techniques. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. We present a method for estimating the extent of resetting of each grain and the thermal history of the sample, based on coupled (U-Th)/(He-Pb) age patterns among all the grains.

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(U-Th)/(He-Pb) double dating of detrital zircons

The 2.95–2.82 Ga quartzofeldspathic gneisses and granitoids in the Bighorn, western Owl Creek, and northeastern Wind River uplifts in the central Wyoming Province include low-K tonalite–trondhjemit...

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The tonalite–trondhjemite–granodiorite (TTG) to granodiorite–granite (GG) transition in the late Archean plutonic rocks of the central Wyoming Province

Long-term tectonothermal history of Laramide basement from zircon–He age-eU correlations

The Bighorn Mountains of the central Wyoming Province expose a large tract of Archean crust that has been tectonically inactive and at relatively high crustal levels since ~2.7 Ga. Seven sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon and titanite age determinations on samples of the main lithologic units provide a geochronological framework for the evolution of this area. The oldest, precisely dated magmatic event occurred at 2950 ± 5 Ma, when diorite to granite dykes and sills intruded an older gneiss complex exposed in the central and southern Bighorn Mountains. Rocks as old as 3.25 Ga may be present in this gneissic basement, as indicated by the oldest dates obtained on areas of zircon grains that are interpreted as inherited cores. A tonalitic gneiss was intruded into the gneiss complex at 2886 ± 5 Ma. Deformation of the central and southern gneisses preceded the intrusion of the Bighorn batholith, a tonalitic to granitic intrusion that occupies the northern portion of the uplift. This ...

Archean geochronological framework of the Bighorn Mountains, Wyoming

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 rock types. U-Pb zircon and baddeleyite data indicate that dikes 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 differentiation 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 of the southern Wyoming province. Identification of the rifted continental block rests in part on the recognition of a coeval dike swarm. The only well-dated dikes of similar age are the Lac de Gras dike swarm of the Slave craton. A tectonic reconstruction juxtaposing the southeastern Wyoming province with the northern Slave province is proposed.

R.A. Heimlich

Papers

Variance of Precambrian K-Ar biotite dates, Bighorn Mountains, Wyoming

Flow differentiation within leopard rock dikes, Bighorn Mountains, Wyoming

Interpretation of Precambrian K-Ar biotite dates in the bighorn mountains, wyoming