Search results for "Archean"
showing 10 items of 87 documents
Characterization, provenance, and tectonic setting of Fig Tree greywackes from the Archaean Barberton Greenstone Belt, South Africa
1999
Abstract In this study, mineralogical and geochemical data, as well as Sr and Nd isotopic compositions of early Archaean greywackes from the Fig Tree Group are presented to provide further information about the evolution of the Barberton Greenstone Belt (BGB) of the Kaapvaal Craton, South Africa. The chemical data on the collected greywackes from the BGB suggest limited chemical modification of the source rocks. The Eu/Eu * anomaly is negative with an average value of 0.76 which is slightly higher than that of Eu-depleted crustal rocks (0.65). High Cr and Ni concentrations and a Cr/Ni ratio of about 1.6 indicate derivation from ultramafic sources without fractionation. Although rounded zirc…
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…
High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa
2018
Abstract The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swazi…
Formation Ages and Environments of Early Precambrian Banded Iron Formation in the North China Craton
2016
The North China Craton (NCC) has had a long geological history back to ca. 3.8 Ga ago, but the most important tectonothermal event occurred at the end of the Neoarchean, the most important period of BIF formation. There are three ancient terranes (>2.6 Ga) in the NCC. Most BIFs are distributed along the western margin of the Eastern Ancient Terrane, accounting for about 89 % of the total identified BIF iron ore resources in the NCC. They are considered to have formed on a continental basement in terms of rock association of the BIF-bearing supracrustal sequences which were intruded by slightly younger crustally derived granites. Most BIFs in the NCC show positive Y anomalies, implying that …
Metamorphic P–T paths for the Archean Caozhuang supracrustal sequence, eastern Hebei Province, North China Craton: Implications for a sagduction regi…
2020
Abstract Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P–T path involving pre-peak isobaric heating to peak conditions at 780–800 °C and 10–11 kbar (medium-P/T type), followed by decompression to 5–6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions …
Reply to comment by Ngako and Njonfang on “The Adamawa-Yade domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Ba…
2018
Abstract V. Ngako and E. Njonfang addressed in their comment an interesting and debated issue about the collision geodynamics of the Pan-African Central African Orogenic Belt (CAOB) in Cameroon. However, the reason for this comment is still not clear to us since it mainly deals with issues far away from the scope of our study, namely, the pre-collisional set up of the belt. The comment raises two main points – the reworking process during plate collision and the Pan-African strain pattern – not discussed in our paper for the simple reason that it was not our goal and because our data set does not even allow such discussion. Having said that, the comment gives us the opportunity to clarify o…
Eastern Ancient Terrane of the North China Craton
2016
Based on the spatial distribution of ancient rocks and zircons, three ancient terranee older than ca. 2.6 Ga have recently been identified in the North China Craton, namely the Eastern, Southern, and Central Ancient Terranee. The Eastern Ancient Terrane is the best studied and understood of the three ancient terranes. It has a long geological history back to ca. 3.8 Ga ago and includes the areas of Anshan-Benxi, eastern Hebei, eastern Shandong and western Shandong. In Anshan-Benxi, several different types of 3.8 Ga rocks were discovered together with 3.1–3.7 Ga rocks, whereas 2.9–3.0 Ga K-rich granites and 2.5 Ga syenogranite occur on larger scales. In eastern Hebei, 3.0–3.4 Ga rocks and ol…
The Adamawa-Yadé domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Bafia area, Cameroon)
2017
Abstract New field observations, ages and geochemical data are presented for the Adamawa-Yade domain, forming part of Central African Orogenic Belt in Cameroon. This belt delineates the northern margin of the Congo craton and is part of the Braziliano/Pan-African orogenic system, which was formed during amalgamation of the West Gondwana continent at the end of the Neoproterozoic. U–Pb dating indicates that the protoliths of some orthogneisses from the Adamawa-Yade domain were emplaced during three distinct periods. Tonalite-Trondhjemite-Granodiorite (TTG) suites in the Makenene area intruded at ca. 3.0–2.5 Ga, and were affected by partial melting at ca. 2.08–2.07 Ga, during the Eburnean oro…
Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China
2016
Abstract We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99 Ma intraplate basaltic rock suite hosted in a subduction–accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431–99 Ma; n = 31; peak at 256 Ma), meta-granitoid-derived (ca. 449–113 Ma; n = 24; peak at 251 Ma) and gabbro-derived (436–242 Ma; n = 15; peaks at 264 and 244 Ma) grains. Each textural type is characterized by a distinct zirc…
The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: An example from the late Archean TTG terrane of western …
2017
Abstract There are different viewpoints on metamorphic and anatectic zircons recording ages of 2.45–2.48 Ga or even younger in some areas of the North China Craton where both late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed. These are: 1) partial resetting of the U-Pb isotopic system in the late Neoarchean zircons, 2) metamorphism lasting from the late Neoarchean to the earliest Paleoproterozoic, and 3) earliest Paleoproterozoic metamorphism as separate different event. Western Shandong Province is an area where the late Neoarchean tectono-thermal event is widely developed but the late Paleoproterozoic event has not been identified. This provides an opport…