Contrasting Fractionation Trends in Coexisting Continental Alkaline Magma Series; Cantal, Massif Central, France   总被引：4，自引：3，他引：1  

WILSON  MARJORIE; DOWNES  HILARY; CEBRIA  JOSE-MARIA 《Journal of Petrology》1995,36(6):1729-1753

Two coexisting series of strongly alkaline (basanite-tephritephonolite)and weakly alkaline (alkali basalt-trachyandesite-trachyte-rhyolite)lavas occur in the Cantal volcano (French Massif Central). Theparental magmas appear to be derived by variable degrees ofpartial melting of a common asthenospheric mantle source. Derivativetrachyandesites and feldspathoidbearing tephrites show depletionsand enrichments in trace elements which indicate that they havebeen generated by broadly similar fractionation processes, relatedto the removal of a mineral extract, from the parental alkalibasalts and basanites respectively, dominated by olivine, clinopyroxene,amphibole, apatite and titaniferous magnetite±plagioclase.In the most extreme differentiates (trachytes, rhyolites andphonolites) fractionation of zircon, sphene and alkali feldsparexerts a major control on the trace element characteristicsof the magmas. Sr-Nd-Pb isotopic data for the two magma series suggest theimportance of combined assimilation-fractional crystallizationprocesses (AFC) within the lower crust in their evolution. Modellingdemonstrates that the AFC process amplifies the original compositionaldifferences between the parent magmas. After 55% crystallizationin the strongly alkaline series and 65% in the weakly alkalineseries crustal contamination ceases, although fractional crystallizationcontinues beyond this point to produce the most evolved differentiates(phonolites and rhyolites). This may reflect progressive sealingof the lowercrustal magma reservoirs. The tendency of the magmasto follow over- or under-saturated evolutionary trends, producingrhyolitic and phonolitic residua respectively, appears to beestablished at the early stages of magmatic differentiation,reflecting inherent differences in the compositions of the parentalmagmas. KEY WORDS: alkaline magmas; Massif Central; Cantal; AFC; magmatic differentiation  相似文献