Showing papers by "Paul R. Renne published in 1993"

Early and Late Alkali Igneous Pulses and a High-3He Plume Origin for the Deccan Flood Basalts

Abstract: Several alkalic igneous complexes of nephelinite-carbonatite affinities occur in extensional zones around a region of high heat flow and positive gravity anomaly within the continental flood basalt (CFB) province of Deccan, India. Biotites from two of the complexes yield (40)Ar/(39)Ar dates of 68.53 +/- 0.16 and 68.57 +/- 0.08 million years. Biotite from a third complex, which intrudes the flood basalts, yields an (40)Ar/(39)Ar date of 64.96 +/- 0.1 1 million years. The complexes thus represent early and late magmatism with respect to the main pulse of CFB volcanism 65 million years ago. Rocks from the older complexes show a (3)He/(4)He ratio of 14.0 times the air ratio, an initial (87)Sr/(86)Sr ratio of 0.70483, and other geochemical characteristics similar to ocean island basalts; the later alkalic pulse shows isotopic evidence of crustal contamination. The data document 3.5 million years of incubation of a primitive, high-(3)He mantle plume before the rapid eruption of the Deccan CFB.

New data from Hadar (Ethiopia) support orbitally tuned time scale to 3.3 MA

Abstract: Support for the astronomically calibrated geomagnetic polarity time scale is provided by new 40Ar/39Ar dating and magnetostratigraphic data from the Hadar Formation, Ethiopia. Three tuffs and a basalt flow at Hadar span the Mammoth subchron of the Gauss chron, and range in age from 3.18±0.01 to 3.40 ±0.03 Ma. Interpolation of sediment accumulation rates in two magnetostratigraphic sections yields an estimated age range of 3.30±0.02 to 3.21±0.01 Ma for the Mammoth subchron, closely coinciding with orbitally based predictions of 3.33 to 3.22 Ma rather than with previous estimates of 3.15 to 3.05 Ma.

Thermochronologic record of pluton emplacement, deformation, and exhumation at Courtright shear zone, central Sierra Nevada, California

Abstract: Thermochronology and thermobarometry of two plutons and a ductile shear zone at their contact in the central Sierra Nevada, California, as defined over the range 700 to 300 °C by ^(40)Ar/^(39)Ar dating of hornblende and biotite and U-Pb dating of zircon, indicate that ductile shear coincided closely with dramatically increased cooling and possible decompression of both plutons. Deformation occurred as the younger (∼90 Ma) Mount Givens pluton was emplaced against the older (∼102 Ma) Dinkey Creek pluton, which was softened by conductive heating, during extension. These events coincided with significant regional exhumation, possibly a result of crustal inflation, recorded by a major pulse of rapid sediment accumulation in contemporaneous fore-arc strata of the Great Valley sequence.

Potassic volcanism near Mono basin, California: Evidence for high water and oxygen fugacities inherited from subduction

Abstract: Exposed around the margins of Mono basin and the periphery of Long Valley caldera are ∼100 km 3 of potassic, basic to intermediate lavas that record high water and oxygen fugacities. Most eruptions occurred between 4 and 2 Ma, although sporadic potassic volcanism continued into the Quaternary with a pulse between 0.1 and 0.5 Ma, ∼15 km northeast of Mono basin. The lava types include absarokite, minette, hornblende lamprophyre, trachybasalt, and trachyandesite. Estimated water contents for lavas without hydrous phenocrysts range between 2 and 3 wt% considerably more than mid-ocean ridge, oceanic island, or back-arc magmas. Calculated f O 2 , values fall between -0.4 and +1.2 log units of the Ni-NiO buffer. Although this potassic suite was erupted in an extensional tectonic setting at the western margin of the Basin and Range province, its high K 2 O/TiO 2 and low Zr/Ba ratios suggest a subduction-modified mantle source. If subduction was the process that enriched the lithospheric mantle in large-ion lithophile elements (K, Ba, etc.), it was also a mechanism for mantle oxidation and hydration.

Mid‐Paleozoic olistoliths in eastern Hayfork Terrane Mélange, Klamath Mountains: Implications for Late Paleozoic‐Early Mesozoic Cordilleran forearc development

Abstract: The western Paleozoic and Triassic belt in the southern Klamath Mountains contains several convergent-margin tectonic assemblages that show great variation in lithology and structural patterns. Many preserve evidence of intermediate- to high-pressure facies-series metamorphism consistent with a subduction and/or accretionary origin prior to regional mid-Jurassic magmatism and shortening. Retention of a pre-Middle Jurassic record is revealed by new 40Ar/39Ar data from the eastern Hayfork terrane chert-argillite melange, which document unexpectedly old cooling ages of 346–360 Ma for micas and amphiboles in a variety of blocks within melange matrix. These Early Mississippian cooling ages and petrologic criteria indicate that the blocks probably originated as olistoliths derived from an exhumed Central Metamorphic belt in the late Paleozoic. Minor low-temperature discordance of the block ages suggests low-grade reheating between about 240 and 280 Ma, coincident with an age on eastern Hayfork matrix of 263 Ma that reflects Early Permian or younger melange formation. The mineral spectra indicate regional temperatures have not significantly exceeded ∼300°C since the Mississippian. This Early Permian accretionary history may only be preserved in the southern Klamath Mountains where mid-Jurassic granitoids are sparse. We suggest that a late Paleozoic eastern Hayfork accretionary complex was constructed proximally to denuded Eastern Klamath forearc basement in an environment analogous to the modern Sumatran convergent margin.