Showing papers by "Paul R. Renne published in 2000"

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

Abstract: from variable amounts of mixing between melts derived from an The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line, Equatorial West Africa, ranges in from an amphibole-bearing peridotite source (lithospheric HSr age from Oligocene to Recent. It is characterized by basanitic, alkali component). New Ar/Ar ages for Mts Oku and Bambouto basaltic and transitional basaltic series. Mineral chemistry, major basalts, combined with previous Ar/Ar and K/Ar ages of and trace element bulk-rock compositions, and geochemical modelling basaltic and silicic volcanics, and with volcanic stratigraphy, suggest suggest that the magmatic series evolved mainly at low pressure a NE–SW younging of the peak magmatic activity in the Western (2–4 kbar) through fractional crystallization of clinopyroxene and Highlands. This SW younging trend, extending from the Oligocene olivine±magnetite, at moderately hydrated (H2O= 0·5–1 wt %) volcanism in northern Cameroon (e.g. Mt Oku) to the still active and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conMt Cameroon, suggests that the African plate is moving above a ditions. Basalts from Ngaoundere (Miocene to Quaternary) and deep-seated mantle thermal anomaly. However, the age and location from the early activity (31–14 Ma) of the Western Highlands have of the Ngaoundere volcanism does not conform to the NE–SW incompatible trace element and Sr–Nd isotopic compositions similar younging trend, implying that the continental sector of the Cameroon to those of oceanic Cameroon Line basalts, pointing to a similar Volcanic Line cannot be easily interpreted as the surface expression asthenospheric mantle source. By contrast, the late (15–4 Ma) of a single hotspot system. Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric

New evidence for geologically instantaneous emplacement of earliest Jurassic Central Atlantic magmatic province basalts on the North American margin

Abstract: Dikes in the southeastern United States represent a major component of the Central Atlantic magmatic province and record kinematics of Pangean breakup near the critical, predrift junction of three major continental masses. Until now, the age of these dikes had not been determined with the same precision as those of Central Atlantic magmatic province basalts on other parts of the circum-Atlantic margin. Our new results for three dike samples from the South Carolina Piedmont yield plateau ages of 198.8 ± 2.2, 199.5 ± 1.8, and 199.7 ± 1.5 Ma. For comparison, we present new age determinations of the benchmark Watchung flows I and III of the Newark basin: 201.0 ± 2.1 and 198.8 ± 2.0 Ma, respectively. Collectively, these data suggest that basaltic volcanism responsible for the dikes, flows, and sills of eastern North America occurred within ∼1 m.y. of 200 Ma. The timing, brief duration, and extent of the Central Atlantic magmatism imply that it may have been causally related to Triassic-Jurassic mass extinctions. The distribution and timing of this magmatism and the absence of regional uplift or an identifiable hotspot track lead us to favor strong lithospheric control on the origin of the Central Atlantic magmatic province, consistent with the modern generation of plume incubation or edge-driven convection models.

Lunar Impact History from 40Ar/39Ar Dating of Glass Spherules

Abstract: Lunar spherules are small glass beads that are formed mainly as a result of small impacts on the lunar surface; the ages of these impacts can be determined by the 40 Ar/ 39 Ar isochron technique. Here, 155 spherules separated from 1 gram of Apollo 14 soil were analyzed using this technique. The data show that over the last ∼3.5 billion years, the cratering rate decreased by a factor of 2 to 3 to a low about 500 to 600 million years ago, then increased by a factor of 3.7 ± 1.2 in the last 400 million years. This latter period coincided with rapid biotic evolutionary radiation on Earth.

Volcanism, tectonism, sedimentation, and the paleoanthropological record in the Ethiopian Rift system

Abstract: The Ethiopian Rift System consists of basins that are in different stages of evolution. Some of the rift-related basins in southwestern Ethiopia are half-grabens that have not evolved to symmetrical rifts since the initiation of rifting here in the middle Miocene. These basins contain fossiliferous Pliocene–Pleistocene volcaniclastic sediments and volcanic rocks and have been occupied by early hominid populations. The Afar and the Main Ethiopian Rifts are symmetrical, with both margins fully developed. Several paleoanthropological localities, ranging in age from the Quaternary to the Pliocene, were discovered within these rift basins. The discovery of Australopithecus afarensis (the “Lucy” species) at Hadar and Ardipithecus ramidus and Australopithecus garhi in the Middle Awash makes the region the most prolific early hominid area in the world. Many of the known Pliocene–Pleistocene paleoanthropological localities that have given us information about our ancestors are concentrated in the East African Rift System. This is not a coincidence, because the volcanic and tectonic activities that were responsible for the formation of the rift basins and coeval sedimentation created ideal environments for the proliferation of life and the preservation of faunal and floral remains. Volcanic and tectonic activities created plateaus and mountains; most of the sediments in the basins were derived from these topographic highs located within and outside the rift valleys. Volcaniclastic sediments and volcanic ash were responsible for the quick burial and preservation of fossils during diagenesis. Diagenetic processes involving silicification, calcification, zeolitization, feldspathization, clay formation, and pedogenesis all played roles in fossil preservation in the volcaniclastic sediments. Volcanic rocks interbedded with the fossiliferous sediments also provide temporal information about geologic processes, faunal evolution, paleoenvironment, and early hominid behavior and lithic technology.

Development of Cretaceous transpressional cleavage synchronous with batholith emplacement, central Sierra Nevada, California

Abstract: Metamorphosed strata of the Ritter Range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels (<8 km) in the Late Cretaceous Sierra Nevada magmatic arc Paleozoic to Middle Jurassic strata of the pendant have been penetratively shortened 30%–50%, forming slates, phyllites, and schists with bedding-inclined cleavage and associated linear structures The cleavage, secondary cleavages, and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the Farallon and North American plates Microstructural relations show that cleavage development accompanied greenschist to amphibolite facies metamorphism Metamorphic hornblende, muscovite, and biotite in the wall rocks show weak to strong preferred orientation and have 40Ar/39Ar plateau ages ranging from about 85 to 80 Ma Hornblende, and possibly muscovite, are expected to retain 40Ar at or near estimated peak metamorphic temperatures of ∼500 °C, thus their 40Ar/39Ar ages closely correspond to the time of cleavage formation Granitic plutons adjacent to the Ritter Range pendant have yielded U-Pb and other ages that cluster from 91 to 82 Ma We conclude that bedding-inclined cleavage and metamorphism in the Ritter Range wall rocks developed in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons

Progress and challenges in geochronology

Abstract: Rapidly improving technologies have provided increasingly precise and widely applicable means for measuring geologic time. Whilst the ability to resolve age continues to improve, the absolute accuracy will remain limited by sources of systematic error that loom ever larger in comparison. The decay constants that characterize radioactive decay rates, and govern the relationship between isotopic data and their corresponding radioisotopic ages, are inexactly known. In consequence, the accuracy of some of the most precise dating methods, such as the 40Ar/39Ar technique, may be an order of magnitude or more worse than their precision. Scientific inquiries requiring comparison of ages determined by different radioisotopic systems, such as thermochronology or early Solar System evolution, are most impacted by these accuracy-limiting effects. Amelioration of the condition is straightforward (though not necessarily easy), but requires shifting research priorities and more interdisciplinary interaction.