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| Titel: |
Paleoproterozoic Magmatic Provenance of Detrital Zircons, Porongos Complex Quartzites, Southern Brazilian Shield |
| Auteur: |
Hartmann, Leo Philipp, Ruy Liu, Dunyi Wan, Yusheng Wang, Yangin Santos, Joao Vasconcellos, Marcos |
| Verschenen in: |
International geology review |
| Paginering: | Jaargang 46 (2004) nr. 2 pagina's 127-157 |
| Jaar: |
2004-02 |
| Inhoud: |
Provenance investigation using detrital zircon U-Pb SHRIMP (sensitive high-resolution ion microprobe) age dating for six quartzite samples (167 spot determinations on 166 grains) indicates that metasedimentary rocks of the Porongos Complex from the southern Brazilian shield were derived almost entirely from Paleoproterozoic sources. Intensive study of igneous and metamorphic rocks in this portion of southwestern Gondwana employing SHRIMP geochronology over the past five years provides important evidence for provenance investigation of zircon grains. Ages of magmatic sources of the zircon detritus in the deformed quartz sandstones show an eight-peak distribution (known equivalent rocks in parenthesis): 2470 Ma (Neto Rodrigues Gneiss), 2350 Ma (Santa Maria Chico Complex Granodiorite), 2200 Ma (Encantadas Complex Tonalite), 2140 Ma (Paso Severino Dacite, Miseria Mylonite), 2100 Ma (Sierra Azul Granite), 2080 Ma (Chacofy Tonalite, Villa Monica Monzogranite), and 2040 and 2020 Ma (Itapema Monzogranite). Ages between 2260 and 2000 Ma correspond to the Trans-Amazonian Cycle, and were already known in the basement of the region, but the 2470 Ma and 2350 Ma ages are a new contribution regarding the tectonic evolution of the southern Brazilian shield. Tectonic scenarios for the Porongos basin fill are restricted to two different environments: (1) cratonic cover, if the fill is near 1500 Ma; or (2) passive margin if the age of fill is near 1000 Ma (this explanation requires additional studies). We have thus elucidated a major problem in the provenance of detrital zircon in the southern Brazilian shield, because we now know that the protolith sediments of the Porongos Complex were deposited under stable tectonic conditions and derived from Paleoproterozoic sources. This occurred after cratonization of the crust and formation of supercontinent Columbia following the end of the Trans-Amazonian Cycle. Nearly all zircon analyses have Th/U ratios higher than 0.2. Thus, only magmatic crystals survived the sedimentary processes. We show that detailed study of detrital zircon is a powerful tool for the understanding of the provenance of sandstones. |
| Uitgever: |
Taylor & Francis |
| Bronbestand: |
Elektronische Wetenschappelijke Tijdschriften |
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