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Reply to the Comment by M. Garcia and G. Herail on 'Geochronology (Ar-Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from northern Chile (18-22 degrees S): implications for magmatism and tectonic evolution of the central Andes' by Worner et al. (2000)
No full textRidge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm
No full textMantle wedge-derived are volcanism ceased in southern Costa Rica after ca, 8 Ma because of subduction of the aseismic Cocos Ridge beneath the Central American are and the subsequent opening of a slab window. Geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5.8 and 2 Ma identify a source derived from the Galapagos plume. The presence of this source is explained by an influx of Pacific upper mantle into the Caribbean mantle wedge through a slab window, where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted Cocos Ridge. By using geochemical and isotopic signatures, we trace this upper mantle flow beneath Central America from southern Costa Rica northward at a rate of 40 mm/yr
Reply to the Comment by M. Garcia and G. Herail on 'Geochronology (Ar-Ar, K-Ar and He-exposure ages) of Cenozoic magmatic rocks from northern Chile (18-22 degrees S): implications for magmatism and tectonic evolution of the central Andes' by Worner et al. (2000)
No full textPetrogenesis of basanitic to tholeiitic volcanic rocks from the Miocene Vogelsberg, Central Germany
No full textThe Miocene Vogelsberg volcano in Central Germany produced mafic magmas ranging in composition from basanite to quartz tholerite and limited amounts of evolved magmas. Trace element and Nd, Sr and Pb isotopic compositions reveal the presence of three distinct mantle sources: (1) a trace element enriched, asthenospheric plume-type source, similar to the European Asthenospheric Reservoir composition inferred for many other Tertiary volcanic provinces in Central Europe; (2) a depleted mantle source, located in the lithospheric mantle or uppermost asthenosphere; (3) a veined lithospheric mantle source. The oldest basanites of the Vogelsberg volcano have distinctly higher Ti, Al, Sc and V contents than younger basanites. These high-Ti basanites may have been produced by partial melting of a veined lithospheric mantle source, formed during the earliest stages of uplift of the Rhenish Shield, similar to 70 Myr ago. Younger basanites were generated by small degrees of partial melting of the European Asthenospheric Reservoir, whereas alkali basalts and tholeiites formed by mixing of variable proportions of melts derived from the European Asthenospheric Reservoir and depleted mantle sources, respectively. These magmas then interacted with metasomatized sub-continental lithospheric mantle, which explains the observed range in Sr, Nd and Pb isotopic compositions. Subsequently the most depleted tholeiites were contaminated by lower-crustal rocks. The distinct stratigraphic position of the various lava groups in the 656.5 m 'Forschungsbohrung Vogelsberg 1996' borehole and the correlation of their chemical stratigraphy with palaeomagnetic reversals reflects an episodic temporal evolution of magmas and mantle sources. During Stage I, melts from the veined lithospheric mantle source were pooled in crustal magma chambers and evolved to erupt a range of differentiated lavas. In Stage II melts were formed in the depleted mantle source and up-section gradually mixed with melts from the asthenospheric mantle. In Stage III the depleted mantle source was exhausted and pure asthenospheric melts were erupted
Sources and fluids in the mantle wedge below Kamchatka, evidence from across-arc geochemical variation
No full textMajor and trace element and Sr Nd Pb isotopic variations ill mafic volcanic rocks hve been studied in a 220 km transect across the Kamchatka are from the Eastern Volcanic Front, over the Central Kamchatka Depression to the Sredinny Ridge in the back-arc. Thirteen volcanoes and lava fields, from 110 to 400 km above the subducted slab, were sampled. This allows its to characterize spatial variations and the relative amount and composition of the slab fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO. display a strong increase in large ion lithophile, light rare earth and high field strength elements from the arc front to the back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constant across the arc, which suggests a similar fluid input for Ba and Pb. La/Yb and Nb/Zr- increase from the are front to the back-arc. Rocks from the, Central Kamchatka Depression range in Sr-87/Sr-86 from 0.70334 to 0.70366, but have almost constant Nd isotopic compositions (Nd-141/Nd-144 0.51307-0.51312). This correlates with the highest U/Th ratios in these rocks. Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like but decrease slightly from the volcanic front to the back-mv. The initial mantle source ranged from N-MORB-like ill the volcanic front and Central Kamchatka Depression to more enriched in the back-arc. This enriched component is similar to all ocean-island basalt (OM) source. Variations in (CaO)(6.0)-(Na2O)(6.0) show that degree of melting decreases fi-om the arc front to the Central Kamchatka Depression and remains constant from there to the Sredinny Ridge. Calculated fluid compositions have a similar trace element pattern across the arc, although minor differences are implied. A model is prevented that quantifies the various mantle components (variably depleted, N-MORB-mantle and enriched OIB-mantle) and the fluid compositions added to this mantle wedge. The amount of fluid added ranges from 0.7 to 2.1%. The degree of melting changes from similar to 20% at the arc front to < 10% below the back-an, region. 77if, xocksfioni volcanoes qj'thc northern part of the Central Kamchatka Depression to the north of the transect considered in this study - are significantly, different in their trace element composition) compared with the other rocks of the transect and their source appear) to have been enriched by a component derived from melting of the edge of the ruptured slab
Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei, Italy).
No full textBehaviour of high field strength elements in subduction zones: constraints from Kamchatka-Aleutian arc lavas
No full textModels explaining the characteristic depletion of High Field Strength Elements (HFSE) relative to elements of similar compatibility in subduction zone magmas invoke either (1) the presence of HFSE-rich minerals in the subduction regime or (2) a selectively lower mobility of HFSE during subduction metasomatism of the mantle. In order to investigate the properties of HTSE in subduction regimes closer, we performed high precision measurements of Nb/Ta, Zr/Hf, and Lu/Hf ratios together with Hf-176/Hf-177 analyses on arc rocks from Kamchatka and the western Aleutians. The volcanic rocks of the Kamchatka region comprise compositional end members for both fluid and slab melt controlled mantle regimes, thus enabling systematic studies on the HFSE mobility at different conditions in the subarc mantle. Hf-Nd isotope and systematic Zr/Hf and Lu/Hf covariations illustrate that Zr-Hf and Lu are immobile in fluid-dominated regimes. Hf-Nd isotope compositions furthermore indicate the presence of "Indian" type depleted mantle beneath Kamchatka, as previously shown for the Mariana and Izu-Bonin arcs. In addition to a depleted mantle component, Hf-Nd isotope compositions enable identification of a more enriched mantle wedge component in the back-arc (Sredinny Ridge) that most likely consists of mantle lithosphere. The ratios of Nb/Ta and Zr/Hf are decoupled in rocks from fluid-dominated sources, indicating that Nb and Ta can be enriched in the mantle by subduction fluids to a small extent. In contrast to the fluid-dominated regime in Central Kamchatka, the budget of HFSE and Lit in rocks from the Northern Kamchatka Depression and in adakitic rocks from the western Aleutians is significantly affected by slab melts that originate from subducted oceanic crust. Compositions of the rocks with the highest slab melt components in their source (Sr/ Y>30) provide no evidence that either Nb/Ta or Zr/Hf ratios are fractionated at a globally significant scale during melting of subducted oceanic crust. Subduction processes are therefore an unlikely candidate to explain the terrestrial Nb-Ta paradox (i.e., the subchondritic Nb/Ta ratios in all accessible terrestrial silicate reservoirs). (C) 2004 Elsevier B.V. All rights reserved
Synkinematic emplacement of granitoids in a Pan-African shear zone in Central Cameroon
No full textThe magmatic complex of Koala area that is part of the central Domain of the North Equatorial fold belt outcrops within the Central Cameroon Shear zone (CCSZ). This complex comprises four plutonic groups of rocks: (i) monzodiorites, (ii) quartz-monzodiorites, (iii) quartz-monzonites and (iv) granites. The elongate shape of the plutons, variable orthogneissification and foliation strikes parallel to the CCSZ point to syntectonic, magma emplacement. The rocks are metaluminous, high-K, and calc-alkaline to shoshonitic ferro-potassic, with mineralogical and geochemical characteristics of I-type granitoids. The plutonic rocks are characterised by high Sr, moderate Sigma REE concentrations and low Ni and Cr contents. They also display chondrite-normalised REE patterns characterised by variable LREE enrichment, moderate to minor HREE fractionation with moderate negative Eu anomalies (Eu/Eu : 0.66-0.93 (quartz-monzonites), 0.17-0.96 (quartz-monzodiorites), 0.76-0.93 (monzodiorites)) and significant positive Eu anomalies (1.27-5.2 (granites)). Trace element distribution patterns show that all these rocks are distinctively depleted in Th, Nb, Ba, Ti and Ta relative to other trace elements and are enriched in LILE compared to HFSE. Each plutonic group shows distinct evolutionary trends and the geochemical data indicate that the groups were derived from different crustal protoliths. The source materials for the four plutonic groups were similar and possibly included amphibolitised high-K calc-alkaline basaltic andesites. The plutonic rocks of Koata area resemble other Neoproterozoic high-K calc-alkaline syntectonic plutons in western Cameroon. They also display strong similarities with high-K calc-alkaline plutons of the Pernambuco shear zone in NE Brazil. (c) 200.6 Elsevier Ltd. All rights reserved
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