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11.
Distinct equilibration temperatures, deformation and trace elementcharacteristics are observed in amphibole-bearing and amphibole-freeperidotite xenoliths from Nushan, Sino-Korean Craton, easternChina. Amphibole-free peridotites are predominantly deformed,fine-grained ( 相似文献
12.
Several spinel peridotite xenoliths from Spitsbergen have SrNdisotopic compositions that plot to the right of the mantlearray defined by oceanic basalts and the DM end-member(depleted mantle, with low 87Sr/86Sr and high 143Nd/144Nd).These xenoliths also show strong fractionation of elements withsimilar compatibility (e.g. high La/Ce), which cannot be producedby simple mixing of light rare earth element-depleted peridotiteswith ocean island basalt-type or other enriched mantle melts.Numerical simulations of porous melt flow in spinel peridotitesapplied to SrNd isotope compositions indicate that thesefeatures of the Spitsbergen peridotites can be explained bychemical fractionation during metasomatism in the mantle. Chromatographiceffects of melt percolation create a transient zone where thehost depleted peridotites have experienced enrichment in Sr(with a radiogenic isotope composition) but not in Nd, thusproducing SrNd decoupling mainly controlled by partitioncoefficients and abundances of Sr and Nd in the melt and theperidotite. Therefore, SrNd isotope decoupling, earlierreported for some other mantle peridotites worldwide, may bea signature of metasomatic processes rather than a source-relatedcharacteristic, contrary to models that invoke mixing with hypotheticalSr-rich fluids derived from subducted oceanic lithosphere. Pbisotope compositions of the Spitsbergen xenoliths do not appearto be consistently affected by the metasomatism. KEY WORDS: Spitsbergen; lithospheric mantle; metasomatism; radiogenic isotopes; theoretical modelling 相似文献
13.
GARRIDO CARLOS J.; BODINIER JEAN-LOUIS; BURG JEAN-PIERRE; ZEILINGER GEROLD; HUSSAIN S. SHAHID; DAWOOD HAMID; CHAUDHRY M. NAWAZ; GERVILLA FERNANDO 《Journal of Petrology》2006,47(10):1873-1914
We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of LaN, YbN and(La/Sm)N with Eu* [Eu* = 2 x EuN/(SmN + GdN), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)Nwith Eu*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclasegarnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 2530 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middleupper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middleupper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the arc paradox of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth 相似文献