Provenance of neoproterozoic ophiolitic mélange sediments in the brasilia belt, central Brazil

Abstract

Mélanges are a chaotic mixture of different rock assemblages that can be associated with orogenic belts and suture zones. The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, which contains a poorly understood ophiolitic mélange. In order to better understand the sedimentation that was associated with this mélange, isotopic U–Pb and Lu–Hf analyses on single zircon grains, and Sm–Nd on whole rock samples was conducted on mélange metasedimentary rock (garnet-mica-quartz schists and quartzites) and basement (mylonitic granites) assemblages. The quartzite samples have a maximum age of deposition of about 930 Ma, while their negative ƐNd (T) values and TDM, −7.78 to −11.57 and 1.70 to 2.07 Ga, respectively, are similar to other metasedimentary rock units interpreted to have formed in a passive margin environment. ƐHf and Hf TDM model values from the quartzite samples range from −12.72 to 8.99 and 2.56 to 1.36 Ga, respectively. The mylonitic granite has an age of ~2.1 Ga, ƐHf values ranging from −0.97 to 2.18, and TDM model values ranging from 2.38 to 2.53 Ga, indicating that it was derived from juvenile Paleoproterozoic basement. Results of the garnet-mica schists reveal that they represent two different rock units, defined by two different periods of sedimentation. The first group of garnet-mica schist has a maximum age of deposition of 672 Ma and ƐNd (T) values and TDM -7.90 to −7.62 and 1.82 to 1.88 Ga, respectively. ƐHf and TDM model values range from −23.50 to 11.69 and 3.23 to 1.45 Ga, respectively, and indicate the presence of similar zircon populations as the passive margin sequence and basement zircon population; there is also a contribution of a younger zircon population derived from older reworked crustal material. The second group of garnet-mica schists is marked by a maximum age of deposition of 616 Ma and ƐNd (T) values and TDM ranging from 0.61 to 0.22 and 1.16 to 1.25 Ga, respectively. ƐHf and TDM model values range from −8.24 to 16.04 and 2.97 to 0.79 Ga, respectively. These results also indicate the presence of passive margin and basement zircon populations, but the younger zircon populations are derived from juvenile sources. The depositional periods of the garnet-mica schists occurred between roughly 800 to 650 Ma and 650 to 600 Ma, and indicate that the rocks from the Brasília Belt register the presence of different depositional environments and basins, which opened, closed and were later dismembered. The quartzites, mylonitic granites and ophiolites most likely represent blocks that were introduced into the mélange by sedimentation or tectonic processes.