Zircon U-Pb Geochronology of a Manganese-rich Supracrustal Succession in Northeast Brazil: Adding a New Piece to the Paleoproterozoic Manganese Mineralization Puzzle

A vast accumulation of manganese occurred in the Paleoproterozoic and it is closely related to the Paleoproterozoic Great Oxidation Event. Although the largest Mn deposits are located in the African continent, relevant deposits that potentially correlate with the African ones are found in South America, specifically in the Brazilian territory. The Borborema Province in Northeast Brazil hosts several Mn-rich meta-sedimentary sequences, containing up to 40 wt.% MnO. These sequences are composed of oxidized and manganese silicate lenses that alternate with spessartite-quartzite layers. This succession is hosted by graphite-bearing pelitic gneisses, is metamorphosed to the amphibolite facies, and is intruded by granitic bodies. Although some preliminary studies report Palaeoproterozoic ages for these meta-sedimentary sequences, the age at which the Mn-rich sequences were deposited has not yet been determined. We investigate in this study the Lagoa do Riacho Manganese Deposit, a representative member of the Northern Borborema Province Mn-rich sequences. Not only was the age of the Mn-rich protolith deposition determined by LA-SF-ICP-MS zircon U-Pb geochronology, but also the timing of the metamorphic reworking of the Mn-rich succession was inferred. The youngest detrital zircon population from a meter-sized bed of spessartite-quartzite interlayered with an oxidized manganese ore from drillhole Ocr-1 yielded a maximum depositional age close to 2130 Ma. Similarly, a gneiss hosting the manganese mineralization at the base of drillhole Ocr-1 also yielded a maximum depositional age in the Rhyacian, close to 2156 Ma. We infer from the U-Pb dating of metamorphic overgrowths on zircon from the Mn-bearing sequences and host rocks and the U-Pb crystallization age of an intrusive leucocratic granite that the minimum age of the Mn-rich succession to be 2023-2106 Ma. The metamorphic reworking of the Mn-rich succession is time-equivalent to the Transamazonian/Eburnean orogeny. Thus, these constraints point to a Rhyacian age (ca. 2100-2200 Ma) for the deposition of the Mn-rich protolith, chrono-correlated with other world-class manganese deposits, such as those from the Franceville Basin in Gabon.

Lithostratigraphy of the Mesoproterozoic Windpoort Granite (Spektakel Suite), western Namaqualand, South Africa

The Windpoort Granite is a porphyritic, leucocratic granite belonging to the Spektakel Suite, a group of late- to post-tectonic granites intruded into the orthogneisses and supracrustal metamorphic rocks in western Namaqualand. Like other granites of this type, it is devoid of penetrative tectonic foliation, at most displaying a magmatic foliation parallel to the boundaries of the intrusion. Its main characteristic setting it apart from other Spektakel Suite granites in western Namaqualand is its tightly packed arrangement of small stubby alkali feldspar phenocrysts. Its U-Pb LA-ICPMS age of 1087 ± 11 Ma agrees with the age of other Spektakel Suite granites showing similar field relationships. It classifies geochemically as a highly potassic monzogranite with more evolved compositions than all other plutons of the Spektakel Suite.

Variscan thrusting in I- and S-type granitic rocks of the Tribeč Mountains, Western Carpathians (Slovakia): evidence from mineral compositions and monazite dating

The Tribeč granitic core (Tatric Superunit, Western Carpathians, Slovakia) is formed by Devonian/Lower Carboniferous, calc-alkaline I- and S-type granitic rocks and their altered equivalents, which provide a rare opportunity to study the Variscan magmatic, post-magmatic and tectonic evolution. The calculatedP-T-Xpath of I-type granitic rocks, based on Fe-Ti oxides, hornblende, titanite and mica-bearing equilibria, illustrates changes in redox evolution. There is a transition from magmatic stage atTca. 800–850 °C and moderate oxygen fugacity (FMQ buffer) to an oxidation event at 600 °C between HM and NNO up to the oxidation peak at 480 °C and HM buffer, to the final reduction at ca. 470 °C at ΔNN= 3.3. Thus, the post-magmatic Variscan history recorded in I-type tonalites shows at early stage pronounced oxidation and low temperature shift back to reduction. The S-type granites originated at temperature 700–750 °C at lower water activity and temperature. TheP-Tconditions of mineral reactions in altered granitoids at Variscan time (both I and S-types) correspond to greenschist facies involving formation of secondary biotite. The Tribeč granite pluton recently shows horizontal and vertical zoning: from the west side toward the east S-type granodiorites replace I-type tonalites and these medium/coarse-grained granitoids are vertically overlain by their altered equivalents in greenschist facies. Along the Tribeč mountain ridge, younger undeformed leucocratic granite dykes in age 342±4.4 Ma cut these metasomatically altered granitic rocks and thus post-date the alteration process. The overlaying sheet of the altered granites is in a low-angle superposition on undeformed granitoids and forms “a granite duplex” within Alpine Tatric Superunit, which resulted from a syn-collisional Variscan thrusting event and melt formation ~340 Ma. The process of alteration may have been responsible for shifting the oxidation trend to the observed partial reduction.

The geochronology of some granitic bodies from eastern Newfoundland and its bearing on Appalachian evolution

Whole-rock Rb–Sr isochron ages from nine granitic plutons that intrude the Central Mobile Belt and the Avalon Zone of Newfoundland are consistent with the view that most of the magmatic activity for this segment of the Appalachian Orogen is related to the Acadian Orogeny. Most of the granites are between 400 Ma and 340 Ma old, although two from the Avalon Zone are significantly different. One, the foliated Swift Current granite, is late Cambrian – early Ordovician (500 ± 30 Ma), whereas the other, the peralkaline St. Lawrence granite, is Carboniferous (315 ± 5 Ma).Several foliated granites from the Gander Zone of the Central Mobile Belt, formerly considered to be older than Mid-Ordovician, are Silurian or Devonian in age. Five megacrystic biotite granites have ages ranging between 420 ± 20 Ma and 340 ± 10 Ma, and a leucocratic granite, the Middle Ridge Pluton, gives an age of 370 ± 15 Ma. None of the granites from the Gander Zone is older than 420 ± 20 Ma, and geochronological evidence for a Precambrian basement in the Central Mobile Belt has still to be found.The initial 87Sr/86Sr ratios show no correlation with age, although the initial ratio of 0.722 for the St. Lawrence granite is much higher than the ratios of between 0.704 and 0.709 for the other granites; this high ratio is consistent with derivation of the granite by partial melting of the continental crust.The isotopic data impose certain constraints on models proposed for the evolution of Newfoundland, the most important being: (i) extensive Devonian magmatism; (ii) intense post-400 Ma cataclasis; (iii) generation of petrographically similar granites at different times; and (iv) Carboniferous magmatism.