Summary
Until recently all basic rocks cropping out in the Amazonian Platform were regarded either as dikes, sills or as flows. Stocks and volcanic pipes were recognized in the Roraima region by RADAM Project geologists, only as late as 1974. Subsequent work by CPRM in the Negro river region established that the Tapuruquara structures named II and III are made up by high alumina gabbros and ultra-basites (olivine-websterite and lherzolite). The gabbros are poor in silica, very poor in Na2O+K,O, and very rich in A12O3, with a strong subalkaline and a strong subsaturated character. The normative anorthite content is high, ranging from 77% up to 93% (a bytownitic composition), while the differentiation index (Q+Ab + Or) is very low, ranging from 5,99 to 12,60, because of the low content in alkalies. The Tapuruquara gabbros are richer in A12O3 than KUNO's high alumina basalt (17,90% A12O3). and they also have a high content of Cr, V, Cu, Ni and Co. particularly the first two, with over than 5,000 ppm in both soil and rock. The complex has a pre-Uatumã age (Early Proterozoic to Archean), while two K/Ar datings performed on whole rock samples gave 2,250 m.y. and 2,910 m.y. ages. Several tectonic parameters such as tectonic setting, stratigraphic position, shape and size of the pipes, geographic distribution, very high A12O3 content, and ultrabasic association, were used to correlate the Tapuruquara Suite with both the Parima river and the Mucajaí river basic/ultrabasic assemblages.