Mesoproterozoic rapakivi granites of the Rondônia Tin Province, southwestern border of the Amazonian craton, Brazil — I. Reconnaissance U–Pb geochronology and regional implications

Abstract

Rapakivi granites and associated mafic and ultramafic rocks in the Rondônia Tin Province, southwestern Amazonian craton, Brazil were emplaced during six discrete episodes of magmatism between ca 1600 and 970 Ma. The seven rapakivi granite suites emplaced at this time were the Serra da Providência Intrusive Suite (U–Pb ages between 1606 and 1532 Ma); Santo Antônio Intrusive Suite (U–Pb age 1406 Ma); Teotônio Intrusive Suite (U–Pb age 1387 Ma); Alto Candeias Intrusive Suite (U–Pb ages between 1346 and 1338 Ma); São Lourenço–Caripunas Intrusive Suite (U–Pb ages between 1314 and 1309 Ma); Santa Clara Intrusive Suite (U–Pb ages between 1082 and 1074 Ma); and Younger Granites of Rondônia (U–Pb ages between 998 and 974 Ma). The Serra da Providência Intrusive Suite intruded the Paleoproterozoic (1.80 to 1.70 Ga) Rio Negro–Juruena crust whereas the other suites were emplaced into the 1.50 to 1.30 Ga Rondônia–San Ignacio crust. Their intrusion was contemporaneous with orogenic activity in other parts of the southwestern Amazonian craton, except for the oldest, Serra da Providência Intrusive Suite. Orogenic events coeval with emplacement of the Serra da Providência Intrusive Suite are not clearly recognized in the region. The Santo Antônio, Teotônio, Alto Candeias and São Lourenço–Caripunas Intrusive Suites are interpreted to represent extensional anorogenic magmatism associated with the terminal stages of the Rondônian–San Ignacio orogeny. At least the São Lourenço–Caripunas rapakivi granites and coeval intra-continental rift sedimentary rocks may, in contrast, represent the products of extensional tectonics and rifting preceding the Sunsás/Aguapeı́ orogeny (1.25 to 1.0 Ga). The two youngest rapakivi suites, the Santa Clara Intrusive Suite and Younger Granites of Rondônia, seemingly represent inboard magmatism in the Rondônian–San Ignacio Province during a younger episode of reworking in the Rio Negro–Juruena Province during the waning stages of the collisional 1.1 to 1.0 Ga Sunsás/Aguapeı́ orogeny. The six intra-plate rapakivi granite episodes in the southwestern part of the Amazonian craton form three broad periods of anorogenic magmatism that have age-correlative events composed of similar rocks and geologic environments in eastern Laurentia and Baltica, although the exact timing of magmatism appears slightly different. Recognition of lithologic and chronological correlations between various cratons provide important constraints to models explaining the interplay between rapakivi granite magmatism and deep crustal evolution of an early Mesoproterozoic supercontinent. They are, furthermore, important to plate tectonic models for the assembly, dispersal and reassembly of Amazonia, Laurentia and Baltica in the Mesoproterozoic and Neoproterozoic.