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Toothill J.; Williams C. A.; MacDonald R.; Turner S. P.; Rogers N. W.; Hawkesworth C. J.; Jerram D. A.; Ottley C. J.; Tindle A. G. 《Journal of Petrology》2007,48(1):3-42
St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963950°C at fO2 NNO + 1 (whereNNO is the nickelnickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between 87Sr/86Sr or 143Nd/144Nd and SiO2 rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and 208Pb/204Pb and between 208Pb/204Pband SiO2 are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and SrNdPbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by 238U excesses that indicate that fluid additionof U occurred <350 kyr ago; UTh isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 1368 ka. However, plagioclase is consistentlydisplaced above these isochrons, with apparentages of 39236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of 226Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: SrNdPb isotopes; U-series isotopes; crystal size distribution; petrogenesis 相似文献
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