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A paleomagnetic, geochronologic and petrographic study was undertaken on the flat-lying gabbros and basalts of the Nova Floresta Formation of Rondo “ nia state, western Brazil in order to constrain the Mesoproterozoic paleogeography of the Amazon craton. Measurement of the anisotropy of magnetic susceptibility on the gabbroic samples reveals a flat-lying foliation with a radiating pattern of lineations, supporting the field evidence that the gabbros are part of a large, undeformed sill. Petrographic observations of oxides in the gabbros reveals two populations of magnetite grains produced during the original cooling of the sill: large, oxyexsolved titanomagnetite grains and fine-grained magnetite in igneous reaction rims. New 40 Ar/ 39 Ar age dating of biotite and plagioclase yield ages of V1.2 Ga, which represent the rapid cooling following emplacement of the mafic magma. Whole rock dating of basalt samples yields total gas ages of 1062 ˛ 3 Ma, similar to the V1.0 Ga K/Ar ages reported by previous workers. However, the strong compositional dependence of the age spectrum renders this younger whole rock age unreliable except as a minimum constraint. A single magnetic component is found in the basalts, indistinguishable from the characteristic remanence found in the gabbros that is oriented WNW and steeply upward. This magnetization is considered to be primary and was acquired during the cooling of the sill and associated lavas. A paleomagnetic pole calculated from the Nova Floresta Formation (n = 16 sites, Plat: = 24.6‡N, Plong: = 164.6‡E, A95 = 5.5‡, Q = 5), the first reported pole for the Amazon craton for the 1200^600 Ma Rodinia time period, constrains the paleogeographic position of Amazonia at V1.2 Ga. Juxtaposition of the western Amazon craton with the Llano segment of the Laurentia’s Grenville margin causes the NF pole to lie on the 1.2 Ga portion of the combined APWP for Laurentia and Greenland, which indicates that a collision with the Amazon craton could have caused the Llano deformation in early Grenvillian times. fl 2002 Elsevier Science B.V. All rights reserved. |
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