Search results for "Peridotite"
showing 10 items of 48 documents
Tertiary Ultrapotassic Volcanism in Serbia: Constraints on Petrogenesis and Mantle Source Characteristics
2005
The Serbian province of Tertiary ultrapotassic volcanism is related to a post-collisional tectonic regime that followed the closure of the Tethyan Vardar Ocean by Late Cretaceous subduction beneath the southern European continental margin. Rocks of this province form two ultrapotassic groups; one with affinities to lamproites, which is concentrated mostly in the central parts of the Vardar ophiolitic suture zone, and the other with affinities to kamafugites, which crops out in volcanoes restricted to the western part of Serbia. The lamproitic group is characterized by a wide range of Sr/Sri (0 70735–0 71299) and Nd/Ndi (0 51251–0 51216), whereas the kamafugitic group is isotopically more ho…
Constraints on the sources of post-collisional K-rich magmatism: The roles of continental clastic sediments and terrigenous blueschists
2017
Abstract The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30–40 million years after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in …
Chemical heterogeneities in the mantle: The equilibrium thermodynamic approach
2016
Abstract This study attempts to answer a simple and yet fundamental question in relation to our understanding of the chemical evolution of deep Earth and planetary interiors. Given two initially separate assemblages (lithologies) in chemical equilibrium can we predict the chemical and mineralogical compositions of the two assemblages when they are put together to form a new equilibrated system? Perhaps a common perception is that given sufficient time, the two assemblages will homogenize chemically and mineralogically, however from a chemical thermodynamic point of view, this is not the case. Certain petrological differences in terms of bulk composition, mineralogy and mineral abundance rem…
Hydrothermal origin and age of jadeitites from Sierra del Convento Mélange (Eastern Cuba)
2012
Jadeitite (jadeite jade) from Sierra del Convento (eastern Cuba) occurs in a subduction-related serpentinite-matrix melange associated with a variety of high-pressure tectonic blocks including garnet-amphibolites and related anatectic trondhjemites. The eastern Cuban jadeitite is massive and characterized by rare quartz inclusions and omphacite exsolution in jadeite crystals, as well as replacement or infilling by omphacite. Minor minerals include epidote, biotite, albite, phengite, titanite, rutile, zircon, and apatite. Oscillatory zoning in jadeite crystals and zircon ages suggest hydrothermal crystallization in veins formed in serpentinized peridotite, probably of the mantle wedge. Al-Na…
Origin and crystallization history of Permian tholeiites from the Saar-Nahe trough, SW Germany
1973
The Hirschberg and Rodern diatremes, within the Permian Saar-Nahe trough, SW Germany, are composed chiefly of basaltic tuffs, with associated small intrusions of K-rich tholeiites. Several tholeiite bodies carry 2–20 mm crystals of magnesian clinopyroxene and orthopyroxene, the latter containing up to 5.5% Al2O3 and often extensively resorbed and rimmed by fine-grained olivine and clinopyroxene. Experimental duplication of these pyroxenes has been achieved under conditions of Pload=6–10 kb, T=1280–1080° C and 2–4 wt.-% H2O, confirming that they represent a rare occurence of high pressure phenocrysts in tholeiitic basalts. These conditions of pyroxene crystallization also place constraints o…
Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr-Nd-Hf-Pb isotope systematics of primitive alkaline basalts and lamprophyr…
2016
A new high-precision Ar-40/Ar-39 anorthoclase feldspar age of 176.7 +/- 0.5 Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr-Nd-Hf-Pb isotope dataset for representative basanite and nephelinite samples (>8 wt.% MgO) from 16 subvolcanic necks of the 30 by 40 km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (Sr-87/(86)Sri = 0.7034-0.7048; epsilon Nd-i = +4.4 to +5.2; epsilon Hf-i = +4.7 to +8.1; Pb-206/(204)Pb…
Traces of ancient mafic layers in the Tethys oceanic mantle
2014
Abstract Oceanic basalts are formed by melting of a chemically and isotopically heterogeneous mantle source. The oceanic mantle probably resembles a marble cake containing layers of mafic rock – perhaps recycled ocean crust – stored in the mantle for >1 billion years. Many questions about the nature and distribution of these mantle heterogeneities remain. Here we show that lithological and isotopic traces of ancient mafic layers can still be seen in mantle rocks that have melted to form oceanic crust at a spreading centre in the Tethys Ocean. We have found centimetre-scale heterogeneity in initial osmium isotope ratios in mantle rocks from the Pindos Ophiolite. Deformed pyroxenite layers ha…
Patterns and origin of igneous activity around the Tanzanian craton
2012
Abstract Tertiary and later igneous activity is common on and around the Tanzanian craton, with primitive magma compositions ranging from kimberlites and varieties of picrites through nephelinites, basanites and alkali basalts. This review focuses on elucidating the conditions of origin of the melts, addressing the question of the state and involvement of the Tanzanian cratonic lithosphere in magma genesis. The Tanzanian craton is anomalous with a surface elevation of >1100 m reflecting buoyancy supported by a subcratonic plume whose effects are seen in the volcanics of both western and eastern rift branches. Magmatism on the craton and at its edge has high K/Na and primitive melts show fra…
Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone, northern Italy: a microtextural, petro…
1995
The microtextural, petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of Finero/Ivrea Zone (northern Italy) reveals the long-term deformation of this zone. The zone is divided into a protomylonitic and an ultramylonitic part. Both parts reflect different periods of deformation, although the orientation of the mineral lineations does not change. In the coarse-grained part (period 1) the deformation started under granulite facies conditions (about 775°C). Olivine, ortho- and clinopyroxene and phlogopite recrystallized dynamically. In the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two-stage deformation history …
Constraints on Earth accretion deduced from noble metals in the oceanic mantle
1998
If the Earth's mantle were in equilibrium with its core, the mantle would contain three orders of magnitude less of the noble metals (platinum-group elements Pt, Os, Ir, Ru, Pd and Rh, plus Au and Re) than are observed. An explanation put forward to account for this disparity has been that the last 1% of the Earth's accretion occurred after the iron-rich core had separated from the mantle1,2. Recent debate has accordingly centred on which meteorite class or classes made up this ‘late veneer’ of accretion3. Here we present analyses of noble-metal concentrations in oceanic peridotites (plutonic rocks which are thought to represent samples of the Earth's upper mantle). We find that the average…