PHASE EQUILIBRIA OF THE LYNGDAL GRANODIORITE (NORWAY): IMPLICATIONS FOR THE ORIGIN OF METALUMINOUS FERROAN GRANITOIDS

Abstract

The Proterozoic (950 Ma) Lyngdal granodiorite of southern Norway belongs to a series of hornblende-biotite metaluminous ferroan granitoids (HBG suite) coeval with the post-collisional Rogaland Anorthosite-Mangerite-Charnockite (AMC) suite. This granitoid massif shares many geochemical characteristics with rapakivi granitoids, yet granodiorites dominate over granites. To constrain both crystallization (P, T, fO2, H2O in melt) and magma generation conditions, we performed crystallization experiments on two samples of the Lyngdal granodiorite (with 60 and 65 wt % SiO2) at 4-2 kbar, mainly at fO2 of NNO (nickel-nickel oxide) to NNO + 1, and under fluid-saturated conditions with various H2 O-CO2 ratios for each temperature. Comparison between experimental phase equilibria and the mineral assemblage in the Lyngdal granodiorite indicates that it crystallized between 4 and 2 kbar, from a magma with 5-6 wt % H2O at an fO2of NNO to NNO + 1. These oxidized and wet conditions sharply contrast with the dry and reduced conditions inferred for the petrogenesis of the AMC suite and many other rapakivi granites worldwide. The high liquidus temperature and H2O content of the Lyngdal granodiorite imply that it is not a primary magma produced by the partial melting of the crust but is derived by the fractionation of a mafic magma. Lyngdal-type magmas appear to have volcanic equivalents in the geological record. In particular, our results show that oxidized high-silica rhyolites, such as the Bishop Tuff, could be derived via fractionation of oxidized intermediate magmas and do not necessarily represent primary crustal melts. This study underlines the great variability of crystallization conditions (from anhydrous to hydrous and reduced to oxidized) and petrogenetic processes among the metaluminous ferroan magmas of intermediate compositions (granodiorites, quartz mangerites, quartz latites), suggesting that there is not a single model to explain these rocks. © 2006 Oxford University Press.