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The calc-alkaline association of the Hercynian Sardinia-CorsicaBatholith consists of multiple coalescent granitoid plutonsand minor gabbroic complexes. Isotopic and trace element dataare presented for selected gabbros and I-type granitoids representativeof the parental mantle- and crust-derived magmas, respectively.The gabbros belong to normal calc-alkaline suites and have markedrelative enrichments in Rb, Ba, K and Pb in primitive mantle-normalizedtrace element diagrams. The granitoids belong to high-K calc-alkalinesuites and have fairly uniform trace element compositions resemblingvolcanic arc granitoids (VAG). A significant overlap in Sr andNd isotope compositions is observed between gabbros and granitoids. Geochemical and isotopic data provide evidence for the originof the gabbros from mantle sources enriched in incompatibletrace elements through recycling of sediments via subductionzones, whereas the granitoids were derived from crustal sourcescomposed mainly of igneous protoliths with relatively homogeneouscomposition. Sr and Nd isotope compositions of gabbros and granitoidsare consistent with both the mantle enrichment process and theformation of the igneous crustal sources occurring at 450 Ma,during the earlier calc-alkaline igneous activity. The connection between Hercynian and Ordovician igneous activityhas important and new implications for the Palaeozoic evolutionof the Sardinia and Corsica lithosphere, and permits the Hercynianorogeny to be placed in a wider geodynamic setting, consistingof three main phases. The Ordovician precollisional phase wascharacterized by a N-NE-dipping subduction of an oceanic plateunder a continental plate with emplacement of acid and subordinatebasic-intermediate volcanic and intrusive rocks. The subcontinentalmantle underneath Sardinia and Corsica experienced enrichmentin incompatible trace elements through recycling of sediments.Major crustal accretion also occurred with underplating of basalticmagmas. The Devonian collisional phase was characterized bythe collision of two continental plates after the total consumptionof the oceanic plate. Crustal thickening processes took placetogether with regional metamorphic events that recorded a clockwiseP-T-t path. The Carboniferous post-collisional phase was characterizedby isostatic and thermal readjustments following crustal thickeningthat caused extensive partial melting. Large quantities of I-typegranitoids and subordinate gabbroic complexes were emplacedin the middle-upper crust and formed the mainframe of the Sardinia-CorsicaBatholith. This geodynamic model is consistent with the Palaeozoic evolutionof other sectors of Western Europe suggested on the basis ofgeological, geochronological and palaeomagnetic data. The palaeomagneticrestoration of the Late Palaeozoic position of Sardinia andCorsica close to Southern France suggests that Sardinia andCorsica could have been portions of the southern edge of theArmorican plate that, during Siluro-Devonian, collided withthe Ibero-Aquitanian plate after the total consumption of theLate Cambro-Ordovician South Armorican and/or Massif CentralOcean. KEY WORDS: crustal growth; Hercynian orogeny; mantle enrichment; radiogenic isotopes; Sardinia-Corsica Batholith
*Corresponding author. Present address Dipartimento di Scienza del Suolo e Nutrizione della Pianta, Piazzale Delle Cascine i6, 50144 Firenze, Italy 相似文献
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Intra-Grain Sr Isotope Evidence for Crystal Recycling and Multiple Magma Reservoirs in the Recent Activity of Stromboli Volcano, Southern Italy 总被引：3，自引：1，他引：3
FRANCALANCI LORELLA; DAVIES GARETH R.; LUSTENHOUWER WIM; TOMMASINI SIMONE; MASON PAUL R. D.; CONTICELLI SANDRO 《Journal of Petrology》2005,46(10):1997-2021
Over the last several hundred years, Stromboli has been characterizedby steady-state Strombolian activity. The volcanic productsare dominated by degassed and highly porphyritic (HP-magma)black scoria bombs, lapilli and lava flows of basaltic shoshoniticcomposition. Periodically (about one to three events per year),more energetic explosive eruptions also eject light colouredvolatile-rich pumices with low phenocryst content (LP-magma)that have more mafic compositions than the HP-magma. An in situmajor and trace element and Sr isotope microanalysis study ispresented on four samples chosen to characterize the differentmodes of activity at Stromboli: a lava flow (1985–1986effusive event), a scoria bomb from the ‘normal’present-day activity of Stromboli (April 1984), and a scoriaand coeval pumice sample from a recent more explosive eruption(September 1996). Plagioclase (An62–90) and clinopyroxene(Mg-number between 0·69 and 0·91) phenocrystsin all samples record marked major element variations. Largeand comparable Sr isotope variations have been detected in plagioclaseand clinopyroxene. HP-magma crystals have resorbed cores, witheither high 87Sr/86Sr (0·70635–0·70630)or low 87Sr/86Sr (0·70614–0·70608); thelatter values are similar to the values of the outer cores.Mineral rims and glassy groundmasses generally have intermediate87Sr/86Sr (0·70628–0·70613). Similarly,mineral growth zones with three groups of 87Sr/86Sr values characterizeminerals from the LP-pumice, with the lowest values presentin mineral rims and groundmass glass. These results define amixing process between HP- and LP-magmas, plus crystallizationof clinopyroxene, plagioclase and olivine, occurring in a shallowmagma reservoir that feeds the present-day magmatic activityof Stromboli. An important observation is the presence of athird component (high 87Sr/86Sr in mineral cores) consideredto represent a pre-AD 1900 cumulus crystal mush reservoir situatedjust below the shallow magma chamber. These cumulus phases areincorporated by the LP-magma arriving from depth and transportedinto the shallow reservoir. A rapid decrease of 87Sr/86Sr inthe replenishing LP-magma immediately prior to eruption of theAD 1985 lava flow is associated with an increased volume ofLP-magma in the shallow magma chamber. The HP-magma in the shallowreservoir is not fully degassed when it interacts with the LP-magma,making efficient mixing possible that ultimately produces awell overturned homogeneous magma. Further degassing and crystallizationoccur at shallower levels as the HP-magma moves through a conduitto the surface. KEY WORDS: isotopic microsampling; mineral recycling; mixing; Sr isotope disequilibria; Stromboli 相似文献
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Geochronology and Petrogenesis of the Cretaceous Antampombato-Ambatovy Complex and Associated Dyke Swarm, Madagascar 总被引：1，自引：0，他引：1
MELLUSO LEONE; MORRA VINCENZO; BROTZU PIETRO; TOMMASINI SIMONE; RENNA MARIA ROSARIA; DUNCAN ROBERT A.; FRANCIOSI LUIGI; D'AMELIO FOSCO 《Journal of Petrology》2005,46(10):1963-1996
The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km2. It has an 40Ar/39Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO2 (2·2wt %) and total iron (14 wt % as Fe2O3), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar 相似文献
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Model for the Origin and Significance of Microgranular Enclaves in Calc-alkaline Granitoids 总被引：8，自引：1，他引：8
The origin and significance of microgranular enclaves as indicatorsof the occurrence of magma mixing and/or mingling episodes betweenbasic magmas and anatectic acid magmas, either I- or S-type,are re-evaluated. A model for studying microgranular enclaveand host rock associations has been developed, based on thegeochemical characteristics and the outcrop relationships observedin microgranular enclave-bearing granitoid suites. The model consists of three main stages, concerning the injection(stage 1), the evolution (stage 2), and the mixing (stage 3)processes that basic magmas experience when injected into anatecticcrustal environments. In stage 1, an acid magma is intrudedby one or more injections of almost completely liquid basicmagma which is hotter and less viscous than the acid magma.The two systems do not mix easily, but remain as discrete entitiesuntil thermal equilibrium and comparable viscosities are reached,and freezing of the basic magma and superheating of the acidmagma are operative along their boundaries. In stage 2, thebasic magma experiences both the physical processes of stretching,convective stirring, and mingling with the acid magma, and thechemical processes of crystal fractionation, and contaminationwith the acid magma (CFC process). Repeated cycles of thesephysical and chemical processes result in the formation of bothmicrogranular enclaves and an evolved liquid which is thermallyequilibrated with the acid magma. Accordingly, microgranularenclaves record the steps of evolution of the basic magma. Instage 3, the evolved products of the basic magma (tonaliticto granodioritic in composition) and the acid magma participatein a two-endmember mixing process which accounts for the geochemicalevolution of granitoid plutons bearing microgranular enclaves. The model sheds new light on the magmatic processes occurringin plutonic environments during the formation of composite batholiths,and also suggests some ideas on the petrogenesis of tonaliticplutons. Finally, the observed scale-independent property ofmicrogranular enclaves suggests that fractal geometry, a relativelynew topic of mathematics, can play a determinant role in theunderstanding of the chaotic flow mechanics of viscous fluids,i.e., the kinematics of the mingling and stretching of basicmagmas. 相似文献
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