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Phase Equilibria of the Lyngdal Granodiorite (Norway): Implications for the Origin of Metaluminous Ferroan Granitoids 总被引:2,自引:0,他引:2
BOGAERTS MICHEL; SCAILLET BRUNO; AUWERA JACQUELINE VANDER 《Journal of Petrology》2006,47(12):2405-2431
The Proterozoic (950 Ma) Lyngdal granodiorite of southern Norwaybelongs to a series of hornblendebiotite metaluminousferroan granitoids (HBG suite) coeval with the post-collisionalRogaland AnorthositeMangeriteCharnockite (AMC)suite. This granitoid massif shares many geochemical characteristicswith rapakivi granitoids, yet granodiorites dominate over granites.To constrain both crystallization (P, T, fO2, H2O in melt) andmagma generation conditions, we performed crystallization experimentson two samples of the Lyngdal granodiorite (with 60 and 65 wt% SiO2) at 42 kbar, mainly at fO2 of NNO (nickelnickeloxide) to NNO + 1, and under fluid-saturated conditions withvarious H2OCO2 ratios for each temperature. Comparisonbetween experimental phase equilibria and the mineral assemblagein the Lyngdal granodiorite indicates that it crystallized between4 and 2 kbar, from a magma with 56 wt % H2O at an fO2of NNO to NNO + 1. These oxidized and wet conditions sharplycontrast with the dry and reduced conditions inferred for thepetrogenesis of the AMC suite and many other rapakivi granitesworldwide. The high liquidus temperature and H2O content ofthe Lyngdal granodiorite imply that it is not a primary magmaproduced by the partial melting of the crust but is derivedby the fractionation of a mafic magma. Lyngdal-type magmas appearto have volcanic equivalents in the geological record. In particular,our results show that oxidized high-silica rhyolites, such asthe Bishop Tuff, could be derived via fractionation of oxidizedintermediate magmas and do not necessarily represent primarycrustal melts. This study underlines the great variability ofcrystallization conditions (from anhydrous to hydrous and reducedto oxidized) and petrogenetic processes among the metaluminousferroan magmas of intermediate compositions (granodiorites,quartz mangerites, quartz latites), suggesting that there isnot a single model to explain these rocks. KEY WORDS: ferroan granitoids; crystallization conditions; experiments; Norway; Sveconorwegian; Bishop Tuff 相似文献
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