UWCScholar :: Browsing by Author "Frei, D"

Browsing by Author "Frei, D"

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Dating detrital zircon from the gold-bearing Ventersdorp contact reef in the Ventersdorp supergroup of South Africa
(Elsevier, 2021) Frei, D; Cornell, D.H; Meintjes, P.G
The Ventersdorp Contact Reef (VCR) at the base of the >4.5 km-thick volcanosedimentary Ventersdorp Supergroup unconformably overlies the Witwatersrand Supergroup and is the second youngest mineable reef in the Witwatersrand Goldfields. The volcanic rocks of the Ventersdorp Supergroup are predominantly mafic, affected by low-grade thermal metamorphism and difficult to date. Only the Makwassie Formation in the upper Platberg Group of the main Ventersdorp repository has been reliably dated on four felsic volcanic samples at 2720 ± 2 Ma. The actual timing of Ventersdorp volcanism and the duration of the three recognised lithostratigraphic groups remains enigmatic, despite much research and heroic attempts to synthesize the available data. In this work detrital zircon grains from VCR conglomerates were U-Pb dated in order to improve the time constraints on the Klipriviersberg Group at the base of the Ventersdorp Supergroup. The six youngest grains in VCR samples were reliably dated at 2799 ± 9 Ma. The Klipriviersberg Group and the Ventersdorp Supergroup is thus younger than 2808 Ma and the supergroup is older than the 2642 Ma Vryburg Formation at the base of the Transvaal Supergroup. Comparisons of detrital grain dates confirm that the VCR was largely derived from erosion products of the underlying Witwatersrand Supergroup, however the youngest VCR grains are ~20 Ma younger and may have been derived directly from magmatic rocks in the provenance or a felsic facet of the synchronous komatiitic Klipriviersberg volcanism. Multi-grain analyses of discordant grains show that recent lead loss is predominant.
Depositional age and provenance of high-grade paragneisses from the Mérida Andes, Venezuela: Implications for the Ediacaran–Cambrian tectonic setting of northwestern Gondwana
(Elsevier, 2021) Tazzo-Rangel, MD; Weber, B; Frei, D; González-Guzmán, R
Isotopic, geochemical, and geochronological data are provided to constrain sedimentary sources and depositional ages of high-grade paragneisses of the Iglesias Complex in the Mérida Andes (Venezuela). U-Pb geochronology of detrital zircons suggests a maximum depositional age of 540–530 Ma, whereas age spectra reveal sources from Pan-African-Brasiliano belts, the Amazon Craton, and Oaxaquia-Putumayo basement. Reworking of such Gondwanan sources is also reflected in whole-rock Nd and Hf crustal residence ages of ~2.0–1.3 Ga. Post-depositional disturbance of the U-Pb system by metamorphism during the early Paleozoic and Permo–Triassic is assessed through in-situ analysis of zircon rims and discordant analyses. Field evidence, geochemical and isotopic signatures are consistent with pelitic-psammitic and volcanic-volcaniclastic compositions for sedimentary protoliths. The former show provenances from continental intermediate sources, whereas the latter are probably sourced in an immature volcanic arc, suggesting the initiation of subduction of the Iapetus oceanic crust beneath northwestern Gondwana around 530 Ma. Sedimentary precursors were likely deposited during the latest Ediacaran–earliest Cambrian in an extensive continental shelf, fed by detritus draining from the topographic highs of the Pan-African-Brasiliano belts, across the Amazon craton and into the Iapetus Ocean. Thus, the main direction of sediment flow was opposite to the present-day transport from the Andes Cordillera by the Pacific Ocean to the Amazon fan in the Atlantic Ocean. Chemical trends in gneisses suggest fluvial sedimentary protoliths and cold-climate weathering at high latitudes. Metasedimentary rocks in the Mérida Andes show similarities with equivalent units in Peri-Gondwanan crustal fragments such as the Acatlán Complex and the Santander Massif, but no correlation with Ediacaran metasedimentary rocks of the Maya Block. Ediacaran–Cambrian fossiliferous sedimentary rocks in the Eastern Venezuela Basin and the Colombian foreland basin are possible equivalents but these units remained unmetamorphosed, as its paleoposition was far from the subduction trench during the Early Ordovician climax of the Famatinian orogeny. © 2021 Elsevier B.V.
Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss
(Bureau for Scientific Publications, 2021) Doggart, S; Macey, P; Frei, D
The Twakputs Gneiss is a garnetiferous, K-feldspar megacrystic, biotite granite-granodiorite orthogneiss. It represents a major unit in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Metamorphic Province extending about 250 km between Riemvasmaak in South Africa and Grünau in southern Namibia. The Twakputs Gneiss occurs as foliation-parallel, sheet-like bodies tightly infolded together with granulite-facies paragneisses into which it intrudes along with a variety of other pre-tectonic granite and leucogranite orthogneisses. These rocks were subsequently intruded by late-tectonic garnet-leucogranites, granites and charnockites. The Twakputs Gneiss is a distinctive unit characterised by large ovoid to elongate megacrysts of twinned perthitic K-feldspar, set in a coarse-grained matrix of garnet, biotite, quartz and feldspar. It contains a penetrative foliation defined by the alignment of K-feldspars and streaks of biotite that developed during the main phase D2 of the Namaqua Orogeny (~1.2 to 1.1 Ga). The foliation and an accompanying elongation lineation are more intensely developed along lithological contacts, especially at the margins of the mega-scale F3 domes and basins that refold the regional fabrics. U-Pb zircon dating of the Twakputs Gneiss has yielded concordia ages of between ~1192 and 1208 Ma. Wholerock geochemistry shows consistent major, trace and REE elemental trends, and thus reflect chemical variability from a single fractionating magma. The Twakputs Gneiss has a granitic to granodiorite composition and is strongly peraluminous. The geochemistry and the ubiquitous presence of garnet and pelitic xenoliths indicate an S-type granite protolith. The Twakputs Gneiss is the most voluminous and widespread member of the Eendoorn Suite which comprises seven textural variants of garnetiferous, K-feldspar-megacrystic granitoid orthogneiss of the same age.
Lithostratigraphy of the Naros Granite (Komsberg Suite), South Africa and Namibia
(Bureau for Scientific Publications, 2021) Thomas, R; Macey, P; Frei, D
The Naros Granite occurs as a large, northwest-trending ovoid batholith roughly 30 km long and 15 km wide straddling the Orange River border between South Africa and Namibia, 25 km northeast of Onseepkans. It consists mainly of a leucocratic to mesocratic grey, coarse-grained equigranular hornblende-biotite granite-granodiorite that is locally mildly feldspar porphyritic. Small, ovoid mafic autoliths are common and characteristic of the Naros Granite. The composition of the unit varies from granite to granodiorite with a minor leucogranitic phase observed along the southern margin of the batholith. Hornblende and biotite are ubiquitous mafic minerals but small amounts of orthopyroxene occur locally. The Naros Granite has yielded tightly-constrained U-Pb zircon ages between 1 114 Ma and 1 101 Ma. The Naros Granite is generally unfoliated to weakly deformed with only localised shearing along contacts with the surrounding country rocks giving rise to orthogneissic fabrics. It has an intermediate to felsic composition (mean SiO2: 63.9 ± 2.2 wt.%) and is strongly metaluminous. This, together with its biotite-hornblende ± orthopyroxene mineral assemblage and the abundance of mafic autoliths, suggests it is an I-type granitoid, with the source magma produced by partial melting of older igneous rocks that had not undergone any significant chemical weathering. The Naros Granite is the youngest and most evolved member of the ~1.11 Ga Komsberg Suite, a collection of late-to post-tectonic I-type metaluminous, intermediate to felsic, biotite ± hornblende granitoids and their charnockitic equivalents that have intruded the older pre-tectonic gneisses of the Kakamas Domain of the Namaqua Metamorphic Sector
Petrological, geochemical (major, trace, and rare earth elements), and U–Pb zircon data of the Tamatán Group, NE Mexico
(Elsevier, 2021) Frei, D; Casas-Peña, J.M; Ramírez-Fernández, J.A
From samples of the Paleozoic Tamatán Group (Huizachal–Peregrina Anticlinorium, Tamaulipas, Mexico), petrographic (qualitative and modal) and geochemical analyses (major, trace, and rare earth elements) were conducted. The first U–Pb geochronological data on detrital zircons of the Tamatán Group were generated using four samples. The data presented here contains a broad overview of photomicrography, recalculated modal point-count data, raw geochemical data, and simple statistics of selected geochemical parameters. The data presented in this article are interpreted and discussed in the research article titled “Provenance and tectonic setting of the Tamatán Paleozoic sequence, NE Mexico: Implications for the closure of the Rheic Ocean at the northwestern part of Gondwana” [1].
Provenance and tectonic setting of the Paleozoic Tamatán Group, NE Mexico: Implications for the closure of the Rheic Ocean
(2021) Frei, D; Casas-Pena, J.M; Ramirez-Fernandez, J.A
The Huizachal–Peregrina Anticlinorium in northeastern Mexico comprises a wide variety of Precambrian and Paleozoic basement units. In this work, Silurian-to-Permian unmetamorphosed siliciclastic successions (i.e., Cañón de Caballeros, Vicente Guerrero, Del Monte, and Guacamaya formations) forming the Tamatán Group is described; the group overlies the Novillo metamorphic complex, which is the northernmost exposed region of the Oaxaquia microcontinent. In this paper, a provenance model for the Tamatán Group is proposed based on new petrological data, whole-rock geochemical data, and U–Pb LA-ICP-MS detrital zircon age data from these marine fossiliferous strata. The results show a shallowmarine to deep-water clastic succession classified as feldspatho-lithic-quartzose and litho-feldspathoquartzose (Cañón de Caballeros and Vicente Guerrero formations) and feldspatho-quartzo-lithic and litho-quartzo-feldspathic (Del Monte and Guacamaya formations). The petrological and geochemical data indicate a derivation from felsitic, intermediate, and basic rocks, intense to moderate weathering, and high to moderate recycling of the source area. The geochronological data can be organized into three main detrital zircon U–Pb age groups: (1) a Meso-Neoproterozoic group, likely derived from Oaxaquia and the Maya block, which is also the probable source of 1.5–1.6 Ga zircon grains; (2) an Ordovician–Silurian group that is likely sourced by igneous Maya Block intrusions or from local units such as the Ordovician Peregrina Tonalite; and (3) Pennsylvanian and Permian zircon that can be derived from magmatic arc-related intrusions from the diachronous Rheic closure. Additionally, major, trace, and rare earth element concentrations and ratios from the Tamatán Group indicate a passive margin that evolved into magmatic arc sources within the upper continental crust. The integration of our data indicates that the Tamatán Group was deposited in an unreported back-arc basin from the northernmost part of Oaxaquia near the Maya Block along the northwestern Gondwanan margin during the closure of the Rheic Ocean priori to complete Pangea.
Provenance of metasedimentary rocks of the western Dom Feliciano Belt in Uruguay: Insights from U–Pb detrital zircon geochronology, Hf and Nd model ages, and geochemical data
(Elsevier, 2021) Frei, D; Oyhantçabal, P; Oriolo, S
New isotopic and geochemical data for the Paleo- Meso- and Neoproterozoic metasedimentary cover of the southern Dom Feliciano Belt (Brasiliano/Pan-African) are presented and evaluated combined with published information. Whole-rock major and trace element geochemistry indicates that the dominant source for all the units had a composition similar to average upper continental crust. The geochemistry is similar even for late Ediacaran successions with a source component from slightly older Ediacaran granites, due to the crustal origin of these granites. Age constraints based on detrital zircon, fossil content, interbedded volcanic rocks and isotope geochemistry confirm Paleo-, Meso-, and Neoproterozoic successions, despite uncertainties remain in some cases. Detrital zircon data show the dominance of Archean (3.3–2.8 Ga) to Paleoproterozoic (2.2–1.9 Ga), and subordinated Mesoproterozoic (1.5–1.3 and 1.1 Ga) and Neoproterozoic (0.5–0.6 Ga) ages. Lu–Hf zircon and Sm–Nd whole-rock model ages confirm that the Archean and the Paleoproterozoic were the major crustal growth periods for the source areas. U–Pb detrital zircon age distributions and model ages demonstrate that the Nico Pérez Terrane and not the Río de la Plata Craton was the source for most metasedimentary rocks of the western Dom Feliciano Belt. Likewise, comparison of Archean, Paleo-, Meso- and Neoproterozoic events supports that the Nico Pérez Terrane could represent a fragment of the Congo Craton.

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