Search results for "Proterozoic"
showing 10 items of 50 documents
Geochronology of the medium to high-grade metamorphic units of the Peloritani Mts., Sicily
2003
The Peloritani Mountains are a fragment of an orogen variably attributed to the Alpine or Hercynian orogeny. On the basis of 39 Ar- 40 Ar, U-Pb and Rb-Sr dating, the main metamorphism of the two medium-high grade metamorphic units, the Mela and Aspromonte Units, and most of the thrusting responsible for stacking the orogenic edifice are seen to be Hercynian. The main thrusting of the Aspromonte Unit over the lower grade units took place at 301€2 Ma. Brittle deformation during Tertiary reactivation of Hercynian thrust planes did not generate any rejuvenation of white micas in the studied sector. Our dataset shows a great complexity and we propose to unravel it by considering different levels…
Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka
2003
¶Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg ex…
Proterozoic Tectonostratigraphy and Paleogeography of Central Madagascar Derived from Detrital Zircon U‐Pb Age Populations
2004
Abstract Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor f…
Revision of "Leptite-gneisses" in the Menderes Massif: a supracrustal metasedimentary origin
2001
The Pan-African basement of the Menderes Massif comprises a regular lithological succession that reaches a thickness of 8 km, the oldest units of which are, in ascending order, so-called leptite-gneisses, a quartzite-sequence transition zone and mica schists. New findings suggest that the protoliths for the leptite-gneisses, proposed to be of volcanic origin in previous studies, were predominantly elastic sediments of litharenitic composition, Geochemical data indicate that the protoliths for the leptite-gneisses were of cratonic provenance. Because they have undergone Pan-African polymetamorphism and are intruded by approximately 550 Ma gneisses of granitic origin, it is believed that the …
Detrital zircon and micropalaeontological ages as new constraints for the lowermost tectonic unit (Talea Ori unit) of Crete, Greece
2007
The Talea Ori unit is the lowermost known tectonic unit of Crete and the most external part of the Hellenides. Its stratigraphy ranges from Late Carboniferous to Oligocene and outcrops of the lower part are only known in the Talea Ori mountains (central Crete). In this area, a black sandstone at the base of the Galinos Beds, thought to be the oldest formation, contains zircons which were dated using the single grain evaporation method. The majority of these grains yielded Late Carboniferous ages (Variscan), while a small group yielded Early Proterozoic ages. The age distribution of these zircons suggests that, at the Carboniferous–Permian boundary, not much of the older North Gondwanan base…
Age and origin of the Böllsteiner Odenwald
2001
The granitoid protoliths of the gneisses of the Bollsteiner Odenwald, a part of the Mid German Crystalline Rise, intruded at 405 ± 3 Ma. This age was obtained by single zircon 207Pb/206Pb evaporation measurements. It is supported by an upper discordia intercept age of 410 ± 11 Ma from single zircon U/Pb data. These granitoids were derived from hybrid magmas according to their geochemical characteristics. Inherited zircon grains with apparent ages of 882 and 1138 Ma further constrain the source heterogeneity. The isotope data with Nd model ages for the granitoid gneisses between 1.3 and 1.7 Ga also underline the involvement of Proterozoic material into the magma sources. Since the geochemica…
Nitrogen isotope evidence for stepwise oxygenation of the ocean during the Great Oxidation Event
2019
24 pages; International audience; The Earth’s oxygenation represents one of the most important environmental drivers of life’s evolution, with the first rise, known as ‘the Great Oxidation Event’ (GOE), corresponding to unpreceded accumulation of atmospheric O2, changes in the flux of marine nutrients and possibly global glaciations. However, the detailed evolution of the GOE is still debated, as for instance the accumulation trends of oceanic versus atmospheric oxygen and the nature of biogeochemical responses to oxygenation. Here, we combine organic carbon and bulk nitrogen isotope compositions with major element concentrations and iron speciation data of sedimentary rocks recovered from …
Recycling in the subduction factory: Archaean to Permian zircons in the oceanic Cretaceous Caribbean island-arc (Hispaniola)
2018
Abstract Little mineralogical evidence is left of the recycling of continental and oceanic crust into the mantle at subduction zones. Zircon, because of its exceptional robustness, is probably the only surviving phase and the best mineral tracer of this global-scale process. This article combines new in-situ U-Pb dating and O and Hf isotope analyses on Cretaceous (co-magmatic) and pre-Cretaceous (inherited) zircons separated from Albian-Aptian arc-related igneous rocks from the Dominican Republic. The O and Hf systematics of Cretaceous zircons reflect derivation from predominantly juvenile sources and variable mixing with evolved melts, as expected for an oceanic island-arc. Inherited zirco…
The Proterozoic Franceville Basin (S.E. Gabon): an example of interaction between marine sedimentation and extensional faulting.
2006
30 pages; International audience
Archean and Proterozoic ancestry in late Precambrian to early Paleozoic crustal elements of southern Turkey as revealed by single-zircon dating
1990
Detrital zircon ages and paleontology limit the age of the oldest known metasedimentary rocks in the Menderes-Taurus block of southwestern Turkey to between 657 {plus minus}5 Ma and Middle Cambrian (ca. 533 Ma). A mylonitic granite, also part of the basement, yielded a date of intrusion of 543 {plus minus}7 Ma. The scatter of both detrital and xenocryst zircon ages between 612 {plus minus}6 and 3,140 {plus minus}2 Ma virtually precludes northeastern Africa and Arabia as their provenance, but is compatible with a source in the Angara craton of Siberia. These results suggest that the Pan-African evolution in the Middle East may have ended by Angara's collision with Gondwana in the Early Cambr…