Crustal Differentiation in a Thickened Arc—Evaluating Depth Dependences

Cretaceous plutonic and volcanic rocks of the c. 126–112 Ma Western Fiordland Orthogneiss (WFO) and related Separation Point Suite (SPS), New Zealand, reflect a major flux of arc magmatism along the Late Cretaceous Gondwana margin and provide an opportunity to evaluate processes controlling magmatic differentiation in thickened arc crustal settings. Trace element mineral fractionation models are evaluated for key parts of the WFO based on relict crystal chemistry and cumulate material in garnet-bearing monzodioritic plutons. Garnet pyroxenite (garnet–diopside) adcumulate formed in the Malaspina Pluton at 1·2 GPa, together with two-pyroxene, garnet-absent monzodioritic components and xenoliths of hornblendite. Adcumulate garnet–omphacite (eclogite), garnetite and clinopyroxenite material crystallized in the Breaksea Orthogneiss protoliths at 1·8 GPa. Mineral rare earth element (REE) characteristics of garnet and clinopyroxene are remarkably similar between all cumulate layers and both host-rock plutonic bodies, despite distinctions of inferred emplacement depth and clinopyroxene type. Fractionation of magmatic garnet pyroxenite and/or eclogite can account for a diverse (40–75% SiO2) majority of WFO and SPS rocks, and control crustal differentiation in overthickened continental arcs (P >1·2 GPa, >40 km). However, the natural REE data do not clearly discriminate between fractionation controlled by garnet pyroxenite or eclogite in thickened arc settings. The structural level of differentiation is dependent on the lower limit of plagioclase stability, which acts as a buffer forcing most accumulation towards the arc base.