Search for: All records

Abstract Detailed geochronology from two compositionally distinct generations of dikes and sills intruded into the Alta metamorphic aureole, north‐central Utah, complement previous geochronologic studies from the Alta stock, providing information on the timing of magmatism and the nature of emplacement. Uranium/thorium‐lead dates and chemistry were measured in zircon and monazite from these intrusions and associated reaction selvages in hornfels by split‐stream laser ablation techniques. Concordant zircon U‐Pb dates (n = 532) define a dispersed population of dates that range from ∼38 to 32 Ma. Monazite Th‐Pb dates (n = 888) from granodioritic compositions range from ∼40 to 32 Ma. Evaluation of²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates with respect to common Pb, U and Th/U values allows rigorous evaluation of the effects of excess²⁰⁶Pb in these young monazites, yielding concordant²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates in monazites from the granodiorite, consistent with zircon dates from the same thin sections. Leucogranite sills and dikes, which cross‐cut the older granodiorite, have younger monazite dates from ∼33 to 28 Ma. Elevated heavy rare earth element concentrations and trends of larger negative Eu anomalies in the youngest monazites suggest crystallization from an evolved melt. Integration of these new geochronology results and field relationships with prior results from the Alta stock indicate the granodiorite represents the oldest material emplaced in the Alta system. Leucogranite aplite/pegmatite dikes and sills in the inner Alta aureole were emplaced during the final stage of Alta stock construction by injection of evolved water‐rich magmas. 

Paleoenvironmental reconstructions of the Last Glacial Period of eastern Africa paint a picture of a landscape dominated by grasslands and herds of diverse grazing herbivores unlike anywhere in modern Africa. However, the scale of such reconstructions is often at the site level and greatly time-averaged. To elucidate the impact of glacial conditions on tropical grassland animal behavior, a more direct proxy is needed. Using stable strontium, carbon, and oxygen isotopes, we reconstruct paleoenvironmental conditions and behavior of 18 bovid and equid species from four sites dating to the Last Glacial Period in Kenya (Karungu, Rusinga, Kibogo, and Lukenya Hill). In doing so, we address i) migration patterns, ii) seasonality of precipitation and diet, and iii) the role that seasonal responses played in niche separation of closely related species. We find that migration played a similar role in Last Glacial Period grasslands to what it does today but with a notably different set of species; that animals had relatively stable, grass-dominated diets year-round, peaking in C4 grass abundance during the Last Glacial Maximum; that precipitation and seasonality fell within the range of modern eastern African ecosystems; and that a diverse guild of ungulate grazers was able to coexist due to niche separation detectable as isotopic differences. These results combine to extend the theory that eastern African grasslands were greatly expanded and resource-rich year-round during the Last Glacial Period, creating highly favorable conditions for grazing ungulates. Additionally, they demonstrate the geologic recency of the modern guild of migratory species in eastern Africa, which replaced a set of now-extinct migratory species once common in grasslands during the Last Glacial Period, most notably the enigmatic bovid Rusingoryx. Our results illustrate the ecosystem dynamics of Late Pleistocene Kenya on a scale not attainable with most other paleoenvironmental proxies: the scale of individual animals’ lifetimes. This is nearly as close as possible to an actualistic ecological survey of ungulate behavior during the Last Glacial Period in a setting not analogous to any ecosystem on Earth today.