The Paraná-Etendeka flood volcanic event produced approximately 1.5 x 10(6) cubic kilometers of volcanic rocks, ranging from basalts to rhyolites, before the separation of South America and Africa during the Cretaceous period. New (40)Ar/(39)Ar data combined with earlier paleomagnetic results indicate that Paraná flood volcanism in southern Brazil began at 133 +/- 1 million years ago and lasted less than 1 million years. The implied mean eruption rate on the order of 1.5 cubic kilometers per year is consistent with a mantle plume origin for the event and is comparable to eruption rates determined for other well-documented continental flood volcanic events. Paraná flood volcanism occurred before the initiation of sea floor spreading in the South Atlantic and was probably precipitated by uplift and weakening of the lithosphere by the Tristan da Cunha plume. The Parana event postdates most current estimates for the age of the faunal mass extinction associated with the Jurassic-Cretaceous boundary.

No abstract

[1] The multielemental composition of 31 lavas sampled in the extrusive section of the Oman ophiolite was determined by inductively coupled plasma-source mass spectrometry (ICP-MS). This study allowed us to define clear geochemical criteria to characterize the different lavas types in Oman. Most of the Oman ophiolite extrusive sequence is composed of lavas of composition similar to present-day MORB with the exception of slight Nb-Ta negative anomalies (V1 magmatism). V1 lavas display REE patterns and Zr/Hf ratios in the range of MORB (Zr/Hf = 33.15-38.7). In contrast, the overlaying V2 lavas, which outcrop only in the northern part of the ophiolite, are REE depleted relative to V1 and display low Zr/Hf ratios (23.6-30.5). V2 lavas may be further classified into two sub-groups. V2 type I lavas display a continuous decrease from HREE to LREE. The upper V2 type II lavas are more depleted in HREE and MREE but they are enriched in LREE. They are also distinguished by their low Nb/Ta ratios (10.53-11.65) relative to other Oman basalts (V1: 12.5-14; V2 Type I: 12.35-14.4). As for N-MORB, melting of an upwelling mantle beneath an oceanic spreading centre formed V1 lavas. V2 resulted from fluid-enhanced melting of previously depleted mantle residual after V1 extraction. This process probably occurred at different melt/rock ratios thus resulting in the two V2 sub-groups, and was enhanced in the northern part of the ophiolite. V3 lavas overlie the V1-V2 sequence in the Salahi area. They display incompatible element rich patterns (LREE > MREE > HREE) and a high Nb/Ta ratio typical of within-plate basalts. In the different studied areas, the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data.

No abstract

Despite the limited number of paleointensity data available for the last 400 Ma, some general features of the magnetic field in the past can be drawn from their analysis. A mild selection applied to the set drastically reduced the number of determinations, underscoring the unequal quality of the paleointensity estimates at hand and the clear need for many more new reliable studies. However, with or without selection, the record is characterized by a succession of periods with alternatively low and high fields, but data available are yet insufficient to propose any model of transition between both regimes. For the last 400 Ma, the dipole nature of the main field is preserved. This is also true when only data from the Mesozoic Dipole Low (120-260 Ma) are considered. Moreover it is shown that the Mesozoic data are very unlikely to represent an insufficient sampling of a Neogene-type field. These last observations strengthen the reality of this long period where the intensity of the main field was roughly only one third of the present-day value. A possible relation between field strength and secular variation (approximated by standard deviation) appears to exist, although this remark is compromised by the existence of a similar relation between standard deviation and number of determinations. The distribution of all Virtual Dipole Moments is log-normal, as shown before, but when only the selected data set is considered the distribution is clearly bi-modal. An oscillatory or bimodal paleointensity behaviour rather than a monotone variation is not at all unexpected given the highly non-linear geodynamo equations.