Search results for "Felsic"
showing 10 items of 55 documents
Geochronological evidence for Archaean and Palaeoproterozoic polymetamorphism in the Central Zone of the Limpopo Belt, South Africa
2018
Abstract The Central Zone of Limpopo Belt is a polymetamorphic and complexly deformed Precambrian terrane whose tectonic evolution is still uncertain because details on the timing of its evolution are not well established. We report zircon ages for newly discovered rocks in the Central Zone which document three distinct high-grade tectono-metamorphic events at 3.22, 2.62 and 2.02 Ga. The youngest of these has been well established, but the older two events were so far largely based only on rare zircon overgrowth rims. Most of our samples were collected from a large enclave in the 2.61 Ga Bulai pluton and constitute a strongly deformed, brecciated high-grade assemblage of supracrustal rocks,…
Multiple zircon growth within early Archaean tonalitic gneiss from the Ancient Gneiss Complex, Swaziland
1988
Abstract U Pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at3644 ± 4Ma(2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at3504 ± 6Ma and3433 ± 8Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite plu…
Petrogenesis of synorogenic diorite–granodiorite–granite complexes in the Damara Belt, Namibia: Constraints from U–Pb zircon ages and Sr–Nd–Pb isotop…
2015
The synorogenic Palmental complex (central Damara Belt, Nambia) consists of ca. 545 Ma old quartz diorites and rare granodiorites and ca. 520 Ma-old leucogranites, representing one of the earliest and most primitive phase of crustal plutonism predating the main high-T regional metamorphism. Most quartz diorites and one granodiorite evolved through multistage, polybaric evolutionary processes involving fractionation from a lithospheric mantle-derived melt, followed by fractional crystallization of mainly hornblende, plagioclase and apatite which is shown by decreasing MgO, FeO, CaO, TiO2 and P2O5 with increasing SiO2. Assimilation of felsic basement gneisses was also important during formati…
Carboniferous high-potassium I-type granitoid magmatism in the Eastern Pontides: The Gümüşhane pluton (NE Turkey)
2010
The Gumushane pluton, a high-K calc-alkaline I-type granodionte/granite complex, forms an important component of the pre-Liassic basement of the Eastern Pontides (NE Turkey) In its eastern part, the pluton shows a compositional zonation ranging from biotite-hornblende granodiorite in the NW through biotite-hornblende granite to leucogranite/granophyre in the SE Numerous mafic microgranular enclaves (up to similar to 40 cm in diameter) suggest the former presence of globules of mafic melt during crystallization Emplacement of the pluton occurred during the latest Early Carboniferous, as shown by the 320 +/- 4 Ma Ar-40-Ar-39 bionte/homblende and 324 +/- 6 Ma LA-ICP-MS U-Pb zircon ages. In Har…
Chronology of early Archaean granite-greenstone evolution in the Barberton Mountain Land, South Africa, based on precise dating by single zircon evap…
1991
We report precise 207Pb/206Pb single zircon evaporation ages for low-grade felsic metavolcanic rocks within the Onverwacht and Fig Tree Groups of the Barberton Greenstone Belt (BGB), South Africa, and from granitoid plutons bordering the belt. Dacitic tuffs of the Hooggenoeg Formation in the upper part of the Onverwacht Group yield ages between 3445 +/- 3 and 3416 +/- 5 Ma and contain older crustal components represented by a 3504 +/- 4 Ma old zircon xenocryst. Fig Tree dacitic tuffs and agglomerates have euhedral zircons between 3259 +/- 5 and 3225 +/- 3 Ma in age which we interpret to reflect the time of crystallization. A surprisingly complex xenocryst population in one sample documents …
Evidences of magma mixing in the "Daly gap" of the alkaline suites: a case study from the enclaves of Pantelleria (Italy)
2006
The island of Pantelleria consists of trachytes, pantellerites and minor mildly alkaline basalts. Rocks of intermediate composition (falling in the so-called ‘Daly Gap’) such as mugearites, benmoreites and mafic trachytes occur only in the form of enclaves in trachytes and pantellerites inside the main caldera of the island (Caldera ‘Cinque Denti’), which collapsed during the ‘Green Tuff ’ ignimbrite eruption at 50 ka. The enclaves include volcanic, subvolcanic and intrusive rock types. The enclaves in host trachyte contain traces of glass; devitrified glass occurs within enclaves in host pantellerites. Minerals in the enclaves show regular compositional variations with whole-rock silica co…
Archaean Intracrustal Differentiation from Partial Melting of Metagabbro--Field and Geochemical Evidence from the Central Region of the Lewisian Comp…
2012
The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic^ultramafic bodies that exhibit macroscopic features diagnostic of anatexis. In situ partial melting of metagabbroic rocks produced patches and sheets of coarse-grained plagioclase-rich leucosome containing euhedral peritectic clinopyroxene.These leucosomes connect with larger, laterally continuous tonalite or trondhjemite sheets that record segregat…
Geochemistry, zircon U–Pb ages and Lu–Hf isotopes of early Paleozoic plutons in the northwestern Chinese Tianshan: Petrogenesis and geological implic…
2013
Abstract The northwestern Chinese Tianshan is a key part of the southern Central Asian Orogenic Belt. Voluminous arc-related igneous rocks in this region have recorded the opening and closure of several oceanic basins that may represent southern branches of the Paleo-Asian Ocean. We have conducted geochemical, geochronological and Hf-in-zircon isotopic studies on mafic and felsic intrusive rocks in the northwestern Chinese Tianshan to provide important clues for the evolution of the Junggar Ocean and to understand the early Paleozoic crustal growth in the Central Asian Orogenic Belt. The mafic plutons are composed of metagabbro (453.8 ± 2.0 Ma) and fine-grained diorite (461.1 ± 2.0 Ma). The…
A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt
1991
The late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60–75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either h…
U-Pb zircon and Sm-Nd dating of Moldanubian H P /H T granulites from South ohemia, Czech Republic
1994
Conventional multigrain U-Pb systematics of zircon and single-grain evaporation data of zircons in combination with Nd model ages are presented for pyroxene-free, felsic, garnetiferous granulites and orthopyroxene-bearing granulites of felsic, mafic and intermediate composition from the Blansky Les mountains granulite complex, S Bohemia. Zircons from mafic and intermediate granulites yield Concordia intercept ages between 346 ± 5 and 351 ± 6 Ma as well as a single-grain evaporation age of 346 ± 12 Ma. These ages define the time of Variscan high-grade metamorphism in S Bohemia and are supported by an internal Sm-Nd garnet-zircon-plagioclase-whole rock isochron age of 343 ± 21 Ma. Euhedral zi…