U–Pb and Sm–Nd evidence for 1.76–1.77 Ga magmatism in the Moriru region, Mato Grosso, Brazil: implications for province boundaries in the SW Amazon Craton

The Moriru region, situated in the SW Amazon Craton, Mato Grosso, Brazil, is composed of two main domains: (1) an area of undeformed bimodal volcanic rocks and (2) an area of deformed granitic–gneissic–migmatite rocks that also include a small occurrence of slightly deformed rhyolite. The undeformed domain is composed of felsic to mafic flows and volcanoclastic sediments; the felsic flows are essentially explosive deposits classified as ignimbrites, however flows lava occurs interlayed. The compositions of ignimbrites and associated felsic lava are rhyodacite to dacite. The mafic volcanics are phaneritic, fine-grained, and are often strongly hydrothermally altered. Unaltered samples have tholeiitic to transitional compositions and have within-plate petrologic and geochemical characteristics. The felsic volcanics are enriched in light REE, show a negative Eu anomaly, and are depleted in heavy REE. The trace elements indicate a post-collisional environment. U–Pb data on single zircon grains from the volcanic sequence yield ages between 1770±6 Ma and 1778±6 Ma. εNd(t) values range from −1.4 to +1.5 (t=1.75 Ga), and the TDM model ages range from 2.02 to 2.16 Ga. from the deformed domain are calc-alkaline to transitional and have metaluminous to peraluminous compositions. The metaluminous granites are hornblende-monzogranites and the peraluminous rocks are biotite-granites and granodiorite. Both groups plot in post-tectonic fields of trace element diagrams. U–Pb data on single zircon grains from the deformed granite–gneiss–migmatite domain range from 1662±13 Ma to 1803±6 Ma, but most cluster about a mean of 1763 Ma (1759±3 Ma to 1767±2 Ma). εNd(t) values (t=1.75 Ga) vary from −3.4 to +3.0, and the TDM model ages range from 1.94 to 2.28 Ga. proximately 1775 Ma volcano-plutonic rocks in the area probably resulted from partial melting of slightly older continental lithosphere and may have formed during an extensional event about 1.78–1.76 Ga. The plutonic rocks seem to have a similar source and may be intrusive into the volcanic sequence, despite their more deformed nature. These ages are similar to those proposed for older parts of the 1.80–1.55 Ga Rio Negro-Juruena province and younger than 1.9–1.8 Ga, as proposed for the Ventuari-Tapajós province. Sm–Nd isotopic data are consistent with formation of these rocks from ca. 1.9–2.2 Ga continental lithosphere. It is not yet necessary to relocate the proposed boundary between the Rio Negro-Juruena and Ventuari-Tapajós provinces, since provinces should be defined by the ages of the main accretionary assemblages and not the ages of younger intrusive suites. More studies are needed to refine these province boundaries.