Mesoproterozoic Zircon Grains Research Articles

Modern sand provenance and transport across the western Gulf of Mexico margin

Petrographic and detrital zircon U-Pb analysis of modern beach sands and river sands from major catchments in northeastern Mexico draining to the Gulf of Mexico provides evidence for a minimum of 650 km of littoral sand transport southward from the mouth of the Rio Grande at the Mexico-U.S. border to the central part of the state of Veracruz, Mexico. Principal tracers of Rio Grande sand include: (1) quartzose composition that contrasts with lithic compositions of sand in eastern Mexico rivers and (2) detrital zircon ages with Mesoproterozoic modes at 1.8−1.5 Ga and 1.4 Ga, age groups that are typical of basement and derivative sediment of the SW United States but are uncommon to rare in Mexican river catchments. In contrast, abundant Miocene and younger grains in beach sands of Veracruz indicate primary sediment derivation from active and recently active volcanoes in the Trans-Mexican volcanic belt in central Mexico. A proportional decrease in sand of Rocky Mountain provenance with distance southward along the coast from the mouth of the Rio Grande and absence of Miocene and younger zircon grains in beaches north of rivers draining the Trans-Mexican volcanic belt indicate net littoral sand transport southward along the eastern coast of Mexico, demonstrating that wintertime shoreline-parallel surface currents rather than north-directed summertime currents dominate sediment transfer. Sand samples of Tamaulipas beaches in northeastern Mexico commonly have equal or higher proportions of U.S.-derived Mesoproterozoic zircon grains than are present in river bar sand of the lower Rio Grande and the Rio Grande delta, and thus require that littoral processes rework and incorporate coastal dune and beach sands of northeastern Mexico that are enriched in predam Rio Grande sediment. Implied coastal erosion may be related to Holocene transgression or interruption of sediment supply to the coastal sediment transport system by dams in the Rio Grande drainage basin. Such coastal erosion is impacting long-term shoreline stability and viability of the littoral environment.

Provenance of pre- and Carboniferous sequences of the Esquel-Arroyo Pescado-Tepuel regions (Argentine Patagonia): A combined U–Pb and Hf isotope study of detrital zircon and constraints on depositional setting

Provenance and stratigraphic age of pre-Carboniferous or pre-glacial (Esquel and Arroyo Pescado formations) and Carboniferous glacial (Valle Chico Formation) metasedimentary units exposed in the southwestern sector of the North Patagonian Argentine region are analyzed by sedimentary petrology and combined U–Pb and Lu–Hf isotope on detrital zircons. Additionally, a sample of the Jaramillo Formation (Tepuel Group) allowed comparison and correlation to the Carboniferous deposits in the type area of the Tepuel-Genoa basin.Despite differences, samples of all units studied indicate deposition linked to an arc tectonic environment using ternary petrological diagrams. U–Pb detrital zircon age distribution of the Esquel Formation show a dominance of Famatinian, Pampean-Brazilian and Mesoproterozoic ages, whereas the Arroyo Pescado Formation displays a predominant Mesoproterozoic data cluster. On the other hand, the Valle Chico Formation records a main input of post-Famatinian or Chanic (Devonian) ages and minor Gondwanic (Carboniferous) ages. The Jaramillo Formation is unique due to the remarkable unimodal concentration of Gondwanic ages (Carboniferous) and the scarcity of older than Famatinian detrital zircon ages. The maximum sedimentation age is Lochkovian (Lower Devonian) for the Esquel Formation and, Tournaisian and Tournaisian-Visean (lower Carboniferous) for the Valle Chico and Jaramillo formations, respectively; the youngest records for the Arroyo Pescado Formation indicate a middle Cambrian to Middle Ordovician age. Most Mesoproterozoic zircon grains show a remarkable juvenile Hf isotopic character and a slight age difference between U–Pb and Hf TDM data. These records are equivalent to those identified in different units of the Paleozoic of the Patagonian region and the Malvinas/Falkland Islands, where the Mesoproterozoic detrital zircons with positive εHf (t) values indicative of a juvenile character are present. The conspicuous participation of Mesoproterozic zircons ages within the pre-Carboniferous units supports the probable existence of an unexposed Mesoproterozoic crust in the North Patagonian region, as mentioned by previous authors.The allochthonous or autochthonous character of South Patagonia is tested using combined U–Pb ages and initial Hf-isotope compositions of single zircons. The allochthonous alternative of South Patagonia in the pre-Carboniferous stage, favours contributions from eastern Antarctica, South Africa, Malvinas/Falkland Islands and local sources such as the Deseado Massif and the suspected Chaitenia terrane. An autochthonous scenario suggests that the North Patagonian, Deseado Massifs, Malvinas/Falkland Islands and local areas (Chaitenia terrane) were the main source for the pre-Glacial and Glacial studied units, with minor influence from distant regions of South America.

Paleogeographic constraints on source area and depositional systems in the Neoproterozoic Irecê Basin, São Francisco Craton

Neoproterozoic carbonate sequences overlying the Middle Cryogenian glacial deposits in the Sao Francisco Craton are included in the Salitre Formation within the Irecê Basin. Stratigraphic, elemental, and stable isotope geochemistry, and SHRIMP U–Pb detrital zircon analyzes of the Salitre Formation's Unit B are presented. The lower and intermediate sections of Unit B register three different depositional zones: basin, outer ramp, and middle ramp settings. The carbon isotope record is close to 0.0‰ to slightly negative for δ13C, suggesting an ocean isotopic signature. Detrital zircon grains occur in two different age distributions: (a) zircon grains from lower Unit B have ages of ca. 3.38, 2.56, and 2.07 Ga; and (b) zircon grains from the middle Unit B have the same ages plus 1.78 and 1.50 Ga. The Archean and Paleoproterozoic detrital zircon age distribution resembles the age distribution of the Gavião Block within the São Francisco Craton, which is the substratum for the Irecê Basin deposition. This implies a local source area, which includes granite–gneiss and greenstone belt associations. The Mesoproterozoic zircon grains in the São Francisco Craton are chrono-correlated to volcanic rocks preserved in the Espinhaço Group. The change in the detrital zircon age distribution is followed by a change in the carbonate facies, suggesting a possible unconformity from lower to middle Unit B. The youngest detrital zircon grain indicates the maximum depositional age of Salitre Formation of ca. 670 Ma. Results demonstrate that the Irecê Basin formed in a gulf-like basin as an extension of the Neoproterozoic passive margin at the end of the Sturtian or Marinoan glaciation.

Arenópolis sequence, evolution of a marginal basin in the Neoproterozoic Goiás magmatic arc, central Brazil

The Goiás Magmatic Arc in central South America and the Arabian-Nubian shield record major phases of Tonian subduction during a period marked otherwise by dominant tectonic quiescence. Several basins were formed in convergent tectonic settings and their rock record may enlighten some aspects of the tectonic history of the Rodinia Supercontinent, such as provenance patterns and pathways and subsidence mechanisms. In this study, we present the results of geologic mapping and geochronologic analyses of detrital zircon grains of the Tonian volcanic-sedimentary Córrego do Santo Antônio unit, Arenópolis Sequence, of the Goiás Magmatic Arc, Brasília Belt, central Brazil. It is formed of several mafic-ultramafic rocks exposed along eastwards dipping thrust sheets. These bodies are covered by thin layers of metachert and marble. Between the thrust faults, voluminous mica schist and quartzite packages are exposed. Two samples of siliciclastic metasedimentary rocks from an eastern thrust sheet show multimodal age spectra for detrital zircon grains, with a major peak with ages between 1.9 and 2.1 Ga, and minor peaks of 0.99 Ga, 1.2 Ga, ca. 1.4 Ga, 2.45 Ga, 2.75 Ga and ca. 3.0 Ga. One sample of quartzite from the westernmost thrust sheet presents a similar pattern, but with major peaks of 1.2 Ga, 1.55 Ga, 1.85 Ga, and 2.1 Ga and minor peaks of 0.9 Ga and 2.55 Ga. Another sample of metagraywacke from the westernmost thrust sheet displays a unimodal pattern, with a peak of 893 Ma and a minor contribution of Mesoproterozoic zircon grains. Geologic field data suggest that the Córrego do Santo Antônio unit represents a Tonian trench basin, which might have received low rates of sedimentation. Aeromagnetic data confirm that these rocks occur along the limit of blocks, corroborating formation in a trench basin. Geochronologic data from one sample corroborate this interpretation. However, the U–Pb isotopic data of the other samples indicate that the sedimentation history is quite more complex. We suggest three possibly complementary hypotheses to explain both geologic and geochronologic data: 1) Metasedimentary rocks were formed in distinct tectonic environments and later were tectonically imbricated; 2) Older metasedimentary rocks were the preferred source for the basin; 3) The basin preferentially received sediments from older terrains by longitudinal turbidity currents. We conclude that the basin, which originated the Córrego do Santo Antônio unit had a link with cratonic areas and received sediments from longitudinal currents.

Detrital zircon U-Pb ages of the late Mesoproterozoic–Neoproterozoic Qiaotou formation in the Liao-Ji area of the North China Craton: implications for Rodinia reconstruction

ABSTRACT The Qiaotou Formation comprises the late Mesoproterozoic to early Neoproterozoic strata in southern Jilin and Liaoning Provinces at the eastern margin of the North China Craton (NCC). The depositional age of the formation is 1.1–0.9 Ga, as constrained from the ages of the youngest detrital zircons and later mafic intrusions. U-Pb age dating of detrital zircons yielded age peaks at 1.16, 1.58, and 1.74 Ga, and a minor peak of 2.47 Ga. The late Neoarchaean to Palaeoproterozoic detrital zircons were derived from the ca. 2.5 Ga Longgang and Liaonan blocks, ca.1.9 Ga magmatic/metamorphic rocks and late Palaeoproterozoic extensional-related magmatic rocks in NCC. The abundance of Mesoproterozoic zircon grains is less easy to explain. The possible source may represent (i) possibly concealed/vanished Grenvillian magmatic rocks in NCC, or (ii) derivation from the Qinling Microcontinent or (iii) Fennoscandian shield, or (iv) São Francisco–Congo Craton.

Dating of detrital zircons and tracing the provenance of quartzites from the Bystrzyckie Mts: implications for the tectonic setting of the Early Palaeozoic sedimentary basin developed on the Gondwana margin

We studied the petrographical characteristics and bulk chemical composition along with the age of detrital zircon grains from quartzites exposed in the western part of the Orlica-Śnieżnik dome in the Saxothuringian zone. Age spectra of detrital zircons were obtained by U–Pb laser ablation inductively coupled plasma mass spectrometry dating. The zircon ages define a few clusters of which the most numerous are Cambro-Ordovician and Neoproterozoic, and less frequent are Paleoproterozoic and Archean. A distinctive feature of the investigated samples is the lack of Mesoproterozoic zircon grains. The youngest age cluster documented in the investigated quartzites points to a Late Cambrian maximum depositional age and detritus derivation from erosion of Cambro-Ordovician metagranitoids. Bulk chemical composition of the quartzites indicates that they originated owing to erosion of granite within sedimentary basins developed on a passive continental margin. Presented data prove the similarity of the examined rocks to quartzites known from the eastern part of the Orlica-Śnieżnik dome and quartz metasandstones documented in other parts of the Variscan Belt of Europe known as the Armorican quartzites or their equivalents exposed in the Saxothuringian zone. Our results support the suggestion that the Orlica-Śnieżnik dome has an affinity to the West African Craton of the Gondwana margin and represents a fragment of the Saxothuringian zone.

Provenance of the Newfoundland Appalachian foreland basins

The tectonic history of the Appalachian orogen is recorded in the adjacent foreland. Offshore seismic data suggest that an initial, Middle Ordovician foreland basin was filled by sources in Newfoundland, whereas the Late Ordovician foreland basin was loaded and filled by sources to the SW, in the Quebec segment of the orogen, where NW-vergent Taconian arc-continent collision may have continued later than in Newfoundland. We test this hypothesis using U/Pb ages of detrital zircon within foreland basin successions in western Newfoundland. Previously published results from the oldest foreland succession, the Middle Ordovician Goose Tickle Group, demonstrate ages similar to units preserved with the Humber Arm Allochthon, emplaced during Middle Ordovician Taconic orogenesis, including a predominant Paleoproterozoic peak at 1.85 Ga. In this study we investigated U/Pb geochronology of detrital zircon from younger foreland successions from Upper Ordovician to Devonian. The largest proportion of analyses within all foreland successions fall between 0.95 and 1.3 Ga, with largest peaks occurring between 1.0 and 1.1 Ga, typical of zircon derived from the Grenville Orogen. Earlier Mesoproterozoic and Paleoproterozoic ages range from 1.3 to 2.0. The abundance of Mesoproterozoic grains and conspicuous lack of 1.85 Ga Paleoproterozoic zircon in the overlying Upper Ordovician Long Point Group and latest Silurian to Early Devonian Clam Bank Formation indicates that these sediments were not derived from units within the Humber Arm Allochthon. Probability density plots of continental margin units in the Quebec/New England segment of the orogen demonstrate a similar strong Mesoproterozoic and weak Paleoproterozoic signature, suggesting derivation of the Long Point Group from the Quebec segment of the orogen. Within the mid-Paleozoic Clam Bank - Red Island Road succession, typical Gondwanan ages are absent and 1.0 Ga grains derived from the Grenville Orogen are abundant. This is consistent with underthrusting of the Gondwanan microcontinents Ganderia and Avalonia during Salinic and Acadian orogenesis. Only Mesoproterozoic zircon grains were found in the Early Devonian Red Island Road Formation, consistent with derivation from Mesoproterozoic Grenville massifs in western Newfoundland or Cape Breton Island which were exhumed during Devonian Acadian inversion.

The impact of the end-Ordovician glaciation on sediment routing systems: A case study from the Meseta (northern Morocco)

Assessment of sediment redistribution by end-Ordovician ice sheets is crucial for the reconstruction of Lower Paleozoic source-to-sink patterns. Focusing on the ice-distal, deepwater Tazekka depocenter (Moroccan Meseta), we performed a provenance study that combined whole-rock geochemistry, petrography and insights from high-resolution detrital zircon ages. The results show that the glacigenic sediments are compositionally — mineralogically and geochemically — more mature than preglacial strata. This observation points to a preferential cannibalization of the “great Lower Paleozoic quartz-rich sandstone sheet”, with a limited input of first-cycle, far-travelled clastic sediments. Differentiation of glacial units is not straightforward, yet the glaciation acme is typified by a highly mature sedimentary source and an age spectrum lacking Mesoproterozoic zircon grains, both features strongly indicating derivation from the Cambrian–Lower Ordovician cover of the Tuareg Shield. More regional sources are expressed during the earlier glaciation stages, during which lowstand remobilizations unrelated to subglacial erosion are also suspected. Subordinate but notable late Tonian (∼0.8 Ga) and latest Stenian to early Tonian (∼1 Ga) zircon populations are also evidenced in Morocco, which may have implications for future paleogeographic reconstructions.

U–Pb (zircon) ages and provenance of the White Rock Formation of the Rockville Notch Group, Meguma terrane, Nova Scotia, Canada: evidence for the “Sardian gap” and West African origin

The White Rock Formation is the lowermost formation of the Rockville Notch Group, an assemblage of Silurian–Devonian rocks preserved in five areas along the northwestern margin of the Meguma terrane of Nova Scotia. The formation consists mainly of mafic and felsic metavolcanic rocks, interlayered with and overlain by marine metasedimentary rocks. Felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range (with errors) of about 446–434 Ma. These dates confirm a 30 Ma hiatus after deposition of the Early Ordovician Hellgate Formation in the underlying Halifax Group. This hiatus is coeval with the “Sardian gap” in the Lower Palaeozoic of peri-Gondwanan Europe. The metavolcanic–metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses; some of the latter were previously interpreted to be Ordovician tillite, an interpretation no longer viable. New detrital zircon data from metasedimentary samples indicate that the major sediment sources for the White Rock Formation have ages of ca. 670–550 and ca. 2050 Ma, similar to ages from the underlying Goldenville and Halifax groups. A smaller population of Mesoproterozoic zircon grains indicates that the Meguma terrane interacted with a terrane composed mainly of Mesoproterozoic crust during the Silurian and Devonian. The occurrence of the “Sardian gap” and the detrital zircon record constrain the palaeoposition of the Meguma terrane to have been close to Cadomia and West Africa in the Early Cambrian to Early Silurian.

The onset of flysch sedimentation in the Kaoko Belt (NW Namibia) – Implications for the pre-collisional evolution of the Kaoko–Dom Feliciano–Gariep orogen

Detrital zircon provenance study of a metamorphosed sedimentary succession in the eastern part of the Kaoko Belt in Namibia has revealed two distinct sources for the Neoproterozoic sedimentation along the southwestern Congo Craton margin. The lower part of the succession shows detrital zircon ages consistent with erosion of Paleoproterozoic basement of the Congo Craton with an inferred Mesoproterozoic volcano-sedimentary cover. Within the middle part of the succession, which includes glaciogenic sediments correlated with the Sturtian (717–660Ma) glaciation, the Mesoproterozoic zircon grains disappear and the signal is dominated by ages known from the Congo Craton basement. The sedimentation in these parts of the succession is interpreted as related to the early Neoproterozoic rifting.The sedimentary rocks in the top part of the profile contain only subordinate proportion of the Paleoproterozoic–Archaean zircon grains and the populations are dominated by three age groups of ca. 1.0–1.2Ga, ca. 800–750Ma and ca. 650Ma, consistent with erosion of the Punta del Este–Coastal Terrane exposed in the centre of the Kaoko–Dom Feliciano–Gariep orogen. An associated glaciogenic horizon interpreted as reflecting the Marinoan (645–635Ma) glaciation constrains the sedimentation in the upper part of the succession and suggests a short time span between the high-grade metamorphism/magmatism in the Punta del Este–Coastal Terrane and its exhumation. Sedimentary rocks with such detrital zircon pattern appear also in the Damara and Gariep belts. Their source in the western part of the Kaoko–Dom Feliciano–Gariep orogen suggests that they represent an early orogenic flysch that originated during early collision in the western part of the orogen.The short time span between the metamorphism/magmatism in the Punta del Este–Coastal Terrane and deposition of the early orogenic flysch derived from it suggests that the Coastal Terrane was never separated from the Congo Craton by an oceanic domain. The estimated time span between the end of lithospheric stretching and sedimentation of the early flysch suggests that the hypothetical Adamastor Ocean separating the western and eastern forelands of the Kaoko–Dom Feliciano–Gariep orogen must have been small.

Crustal nature and origin of the Russian Altai: Implications for the continental evolution and growth of the Central Asian Orogenic Belt (CAOB)

The Central Asian Orogenic Belt is a gigantic tectonic collage of numerous accreted terranes. However, its geodynamic evolution has been hotly debated primarily due to incomplete knowledge on the nature of these enigmatic terranes. This work presents new detrital zircon U–Pb and Hf isotopic data to constrain the crustal nature and origin of the Russian Altai, a critical segment of Altai-Mongolian terrane. The youngest zircon 206Pb/238U ages of 470Ma constrain that the Terekta Formation, previously envisaged as Precambrian basement, was actually deposited after the Middle Ordovician. As for the three more sedimentary sequences above the Terekta Formation, they have youngest zircon 206Pb/238U ages of 425Ma, 440Ma and 380Ma, respectively, indicating their depositions likely in the Late Silurian to Devonian. From all analyses, it is noted that many zircon U–Pb ages cluster at ca. 520Ma and ca. 800Ma, and these zircons display oscillatory zoning and have subhedral to euhedral morphology, which, collectively, suggests that adjacent Neoproterozoic to Paleozoic igneous rocks were possibly dominant in the sedimentary provenance. Additionally, a few rounded Archean to Mesoproterozoic zircon grains are characterized by complex texture, which are interpreted as recycling materials probably derived from the Tuva–Mongolian microcontinent. Precambrian rocks have not been identified in the Russian Altai, Chinese Altai and Mongolian Altai so far, therefore, Precambrian basement may not exist in the Altai-Mongolian terrane, but this terrane probably represents a large subduction–accretion complex built on the margin of the Tuva–Mongolian microcontinent in the Early Paleozoic. Multiple episodes of ridge–trench interaction may have caused inputs of mantle-derived magmas to trigger partial melting of the newly accreted crustal materials, which contributed to the accretionary complex. During accretionary orogenesis of the CAOB, formation of such subduction–accretion complex is likely ubiquitous, indicating continental crust growth by both lateral accumulation and vertical basaltic injection.

Provenance characteristics and regional implications of Neoproterozoic, Timanian-margin successions and a basal Caledonian nappe in northern Norway

The autochthonous/parautochthonous lithostratigraphic successions of Varanger Peninsula, NE Norway, range in age from Tonian to Cambrian and constituted a passive (Baltican) margin depositional system throughout most of the Neoproterozoic. Sediment sources were mainly from the south on the Fennoscandian Shield. U–Pb zircon age spectra from sandstones of eight formations show significant input of Late Palaeoproterozoic and Mesoproterozoic age, as well as a prominent Neoarchaean peak. These mostly reflect derivation from rock complexes and terranes that are exposed on the present-day Baltic Shield, although the abundance of Mesoproterozoic zircon grains is less easy to explain. Possible sources may represent (i) possible basement of this age now concealed beneath the Caledonian nappes, (ii) a northward continuation of the Sveconorwegian orogen, (iii) recycling of a sandstone-dominated thrust sheet derived from the Rodinian margin and emplaced in the Tonian or (iv) recycled material from pre-existing extensive river systems farther south on the Fennoscandian Shield. One major exception to the above age-spectral pattern is provided by the Late Ediacaran to earliest Cambrian Stáhpogieddi Formation in the immediately overlying Gaissa Nappe Complex. Sandstones in this unit are derived from a northeasterly source, and show a major zircon age peak at c. 550Ma. The formation is considered to represent deposition in a foreland basin ahead of the rising Timanian orogen.

Age provinces in the Antarctic craton: Evidence from detrital zircons in Permian strata from the Beardmore Glacier region, Antarctica

The Antarctic Gondwana sequence crops out along the Transantarctic Mountains. Permian strata at Mt. Bowers and Mt. Weeks in the central part of the range were mainly derived from the ice-covered East Antarctic craton, for which the geology is inferred from coastal outcrops and the Miller Range in the Transantarctic Mountains. Analysis of detrital zircons from Permian sandstones at those two localities demonstrate a slowly changing zircon age-probability distribution for four samples from the lower part of the sequence, with an abrupt change for the upper Buckley Formation sandstone. The stratigraphically lowest sample, from a unit informally designated the Mt. Bowers sandstone, is dominated by zircons in the age range 610–540Ma, with minor components at ∼1.0Ga and∼2.7Ga. Upward, the Pagoda Formation sandstone is likewise dominated by zircons of 600–550Ma age together with a significant Grenville-age cluster (1.3–1.0Ga) and a minor 1.5–1.4Ga cluster. The Fairchild Formation sandstone has a preponderance of grains in the range 600–530Ma, with a scatter of older ages. Zircon grains in the lower Buckley Formation sandstone are principally in the range 575–520Ma with another cluster at 470–450Ma and a scattering of older ages. Most zircons in the upper Buckley Formation sandstone are Permian in age, but with a very small component in the range 650–475Ma and a trace of older ages. Archean to early Mesoproterozoic zircon grains are few in number and may have been recycled from distal outcrops. The 1.5–1.4Ga grains suggest an unexposed Mesoproterozoic source within the East Antarctic craton, which could have been linked to Laurentian North America. The Grenville-age source is considered to be, similarly, a sub-glacial belt in the interior of East Antarctica. Zircons with a late Neoproterozoic to Ordovician signature (600–460Ma) were derived from a Ross Orogen foreland sequence, and/or an early and inland extension of the orogen itself. Permian zircons in the upper Buckley sandstone reflect the development of a magmatic arc on the Gondwana Pacific margin.

Peri-Amazonian provenance of the Proto-Pelagonian basement (Greece), from zircon U–Pb geochronology and Lu–Hf isotopic geochemistry

The basement of the Pelagonian zone of the Hellenides, in the Eastern Mediterranean realm, has been shaped by mid-Neoproterozoic (700Ma) and Variscan (300Ma) igneous activities. In the present study, detrital zircon U–Pb geochronology and Lu–Hf isotope geochemistry of ca. 700Ma-aged granites and of pre-700Ma metasediments from the Pelagonian zone allow a genuine perspective into the provenance and origin of this terrane which hosts one of the oldest sedimentary sequences known in SE Europe. Pelagonian crustal vestiges comprising 700Ma granitoids and their hosting metasediments are termed here “Proto-Pelagonian”.Except from several concordant Tonian-aged detrital zircon grains (as young as 980Ma), which set the Early Tonian as a maximum sedimentation age, a preponderance of the detrital zircon grains separated from four Proto-Pelagonian metasedimentary samples yielded Mesoproterozoic–Paleoproterozoic ages peaking at 1.5Ga. Neoproterozoic-aged detrital zircon grains are generally absent. The detrital zircon isotopic data is thus consistent with an Amazonian source for this sequence. In terms of their detrital zircon age spectra, Proto-Pelagonian metasediments exhibit close similarity to pre-Avalonian metasedimentary sequences (e.g. Gamble Brook Formation) which underlie Late Neoproterozoic "typical Avalonian" successions in Atlantic Canada.The detrital zircon age signal obtained from the pre-700Ma metasediments is however markedly different from that previously obtained from African-derived terranes which are imbricated with the Pelagonian zone in the Eastern Mediterranean region and which are usually devoid of Mesoproterozoic detrital zircon grains (other than Stenian). Likewise, the Proto-Pelagonian basement also differs from Avalonian-type terranes previously identified in the Eastern Mediterranean–SE European region (Istanbul block of the Pontides, Pirgadikia terrane of the Internal Hellinides, North and Central Dobrogea and Danubian basement), which alongside abundant Mesoproterozoic zircon grains contain also Neoproterozoic-aged detrital zircons. Thus, from the point of view of the pre-Variscan geological history, the Pelagonian zone is exotic with respect to surrounding terranes of both, North African and Avalonian origins.

Proveniência de metassedimentos das sequências Arroio Areião, Cerro Cambará e Quartzo Milonitos no Complexo Metamórfico Porongos, Santana da Boa Vista, RS

Estudos de proveniência em grãos de zircão detríticos através do método U-Th-Pb (LA-ICPMS) dos metassedimentos do Complexo Metamórfico Porongos determinaram a existência de duas áreas-fontes principais de idades paleoproterozóicas e mesoproterozóicas para os protólitos dos xistos Arroio Areião, Cerro Cambará e quartzo milonitos associados. Foram obtidas idades entre 1041 ± 46 Mae 2414 ± 31 Ma para a sequência Arroio Areião, entre 1010 ± 17 Ma e 2652 ± 32 Ma para a sequência Cerro Cambará, e idades entre 1619 ± 39 Ma e 2910 ± 24 Ma para os quartzo milonitos associados. A idade máxima de deposição das sequências Arroio Areião e Cerro cambará é neoproterozóica, enquanto os quartzo milonitos possuem idade máxima de deposição mesoproterozóica. A origem dos grãos de idade paleoproterozóica pode ser correlacionada às rochas do Complexo Encantadas, que faz parte do embasamento das unidades supracrustrais, além de rochas do embasamento do cráton Kalahari. Entretanto, áreas-fontes de idade mesoproterozóica (entre 1041 e 1619 Ma) não foram ainda reconhecidas no Cinturão Dom Feliciano. Estes grãos de zircão podem ser derivados de associações de rocha pertencentes aos terrenos mesoproterozóicos (tipicamente associados com as orogêneses Elzeverian e Greenville) associados ao sistema geodinâmico que inclui os crátons Amazonas, Kalahari, Congo – São Francisco e Laurencia e cinturões associados. Este trabalho demonstra que do ponto de vista isotópico e estratigráfico, não existem diferenças significativas de idades detríticas entre as sequências metassedimentares que afloram nos flancos da Antiforme de Santana da Boa Vista.

Significance of U–Pb detrital zircon ages in quartzite from peri-Gondwanan terranes, New Brunswick and Nova Scotia, Canada

Detrital ages have been determined for zircon grains from quartzite clasts in Neoproterozoic conglomerate units in the Avalonian Mira and Caledonia terranes and from quartzite units in both Avalon terrane (Gamble Brook Formation) and the adjacent Brookville–Bras d’Or terrane (Green Head Group). These data are compared to published detrital zircon data from other sedimentary units in the Avalon and Brookville–Bras d’Or terranes, and from paragneissic units in the latter terrane. The aim of this study was to determine the source of the clasts and the general provenance of metasedimentary rocks in these terranes, data that could help to interpret whether the terranes were assembled by the Neoproterozoic or later during the Paleozoic. Studied quartzite units in the Avalon terrane show no ages younger than about 1000 Ma, in contrast to stratigraphically younger conglomerate units, which contain a high proportion of detrital ages around 600 Ma, the peak of igneous activity in Avalon terrane. Quartzite from the Brookville terrane and quartzite clasts from the Mira and Caledonia (Avalon) terranes have no ages younger than about 1200 Ma. Detrital zircon ages from these units are concentrated in the range 2000–1200 Ma, and most samples also show some Late Archean ages. Paragneissic units in Brookville–Bras d’Or terrane show age peaks at ca. 700–650 Ma that slightly pre-date the main pulse of Avalon magmatism, and contain Mesoproterozoic zircon grains younger than those in the quartzite samples. The broad range of Mesoproterozoic to Late Archean ages and general similarity of the age distributions suggest that Brookville–Bras d’Or terrane and Avalon terrane sensu stricto developed near the same or similar large continental landmasses, possibly over an extended period of time. On the other hand, the differences between the terranes, together with lithological and metamorphic contrasts, suggest that quartzite and paragneiss in the Brookville–Bras d’Or terrane are not directly related to each other or to sandstone/quartzite units in the Avalon terrane sensu stricto. Some components of their detrital age signatures point toward Amazonia as the associated landmass, but if so, Trans-Amazonian (2300–2070 Ma) rocks constituted only a very minor proportion of the source area. Baltica may be a more suitable source to explain the data. Quartzite clasts in the Mira and Caledonia terranes (Avalon terrane sensu stricto) show somewhat unique features, such as a concentration of ages around 2000 Ma and the presence of detrital metamorphic zircon at around 680 Ma. This metamorphic zircon may have been derived from a unit such as the Hammondville metamorphic suite in New Brunswick, which has been interpreted to represent the remains of a subduction complex in the Avalon terrane.