Looking up from the Base of a Thickened Arc — Evaluating Eclogite Fractionation Depths

Abstract:

The Western Fiordland Orthogneiss (WFO) in New Zealand comprises deep-arc magmatic rocks crystallised and variably deformed and re-equilibrated at high-P. Two garnet-bearing components of the WFO, the Malaspina Pluton and Breaksea Orthogneiss, crystallised from monzodioritic magma at 1.2 and 1.8 GPa, respectively. These units are dominated by monzodioritic gneiss with minor decimetre- to centimetre-scale cumulate layers including garnetite, clinopyroxenite, eclogite and hornblendite. Diopside in the Malaspina Pluton compared with omphacite in the Breaksea Orthogneiss is the key difference between the monzodioritic hosts and eclogite components, reflecting a c. 20 km difference in emplacement levels. Mineral rare earth element (REE) characteristics of garnet and clinopyroxene within each monzodioritic host and included cumulate eclogite layers are remarkably similar for the different crustal levels; most garnet and clinopyroxene in each unit has a common igneous origin. Trace element mineral fractionation models based on the observed igneous chemical concentrations and accumulated proportions within both units predict eclogite (garnet-clinopyroxene) fractionation as significant in producing middle and upper crustal magmatism on the Cretaceous Gondwana margin. The results suggest fractionation of magmatic eclogitic residue is of significance in crustal differentiation of overthickened continental arcs at depths greater than 1.2 GPa. The similar trace element compositions of garnet and clinopyroxene for rocks emplaced at c. 40 km and c. 60 km suggest that once an arc is > 40 km thick, eclogitic fractionation will be produce similar patterns regardless of how thick the arc is.