Abstract

This study reports paleomagnetic results from nine localities (22 sites) of the Uairen Formation, the basal and oldest unit of the Roraima sedimentary sequence (Roraima Group) in the Precambrian Guayana shield (southeastern Venezuela). Three distinct natural remanent magnetizations (NRM's), U1N, U1R, and U2, were isolated by alternating-field and thermal demagnetization. U1N and U1R NRM's have mean directions D = 342°, I = 6° (k = 14, α95 = 10°, N = 14 sites) and D = 155°, I = 10° (k = 8, α95 = 17°, N = 8 sites), respectively. Because of their high coercivities (HC) and high (>580 °C) laboratory unblocking temperatures (TUB), it is believed that these NRM's are carried mainly by fine-grained magnetite and (or) hematite. The U2 NRM has D = 338°, I = −7° (k = 5, α95 = 25°, N = 6 sites). U2 is a low-HC (20 mT) and low-TUB (<300 °C) NRM carried by coarse-grained magnetite. Despite its low Fisher's k value, the U2 grand mean closely resembles that for U1N. Directional similarities between U1N, U1R, and U2 to groups I and II previously reported for diabase dikes and sills that intruded the Roraima sedimentary sequence (Roraima Intrusive Suite) suggest that primary NRM's of these sediments have been completely overprinted by thermal and hydrothermal effects associated with the intrusive suite. The apparent absence of diabase intrusions at the sampled localities, but mainly the low metamorphic grade of the Uairen samples revealed by thin-section analyses, lead to the alternative conclusion that U2 is a primary detrital remanent magnetization (DRM) dating from the time of deposition of the Uairen sediments, and U1N and U1R are later thermochemical overprints (TCRM's) acquired shortly after deposition by oxidation of fine-grained magnetite. Directional scatter of U2 could be explained as the consequence of misaligning effects of mechanic over magnetic torques acting on coarse-grained magnetite. An age between 1580 and 1960 Ma, derived from radiometric constraints, is suggested for the Uairen components. Thus U2 would be one of the oldest DRM's ever reported.

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