Evaluating the importance of metamorphism in the foundering of continental crust

Timothy Chapman,Sandra Piazolo,Nathan R. Daczko,Geoffrey L. Clarke

doi:10.1038/s41598-017-13221-6

Abstract

The metamorphic conditions and mechanisms required to induce foundering in deep arc crust are assessed using an example of representative lower crust in SW New Zealand. Composite plutons of Cretaceous monzodiorite and gabbro were emplaced at ~1.2 and 1.8 GPa are parts of the Western Fiordland Orthogneiss (WFO); examples of the plutons are tectonically juxtaposed along a structure that excised ~25 km of crust. The 1.8 GPa Breaksea Orthogneiss includes suitably dense minor components (e.g. eclogite) capable of foundering at peak conditions. As the eclogite facies boundary has a positive dP/dT, cooling from supra-solidus conditions (T > 950 ºC) at high-P should be accompanied by omphacite and garnet growth. However, a high monzodioritic proportion and inefficient metamorphism in the Breaksea Orthogneiss resulted in its positive buoyancy and preservation. Metamorphic inefficiency and compositional relationships in the 1.2 GPa Malaspina Pluton meant it was never likely to have developed densities sufficiently high to founder. These relationships suggest that the deep arc crust must have primarily involved significant igneous accumulation of garnet–clinopyroxene (in proportions >75%). Crustal dismemberment with or without the development of extensional shear zones is proposed to have induced foundering of excised cumulate material at P > 1.2 GPa.

Highlights

The foundering and removal of dense continental crust[1] is part of the gross-scale geochemical and geodynamic cycling in magmatic arcs[2,3]
We present a case study of well-exposed deep arc crust to evaluate the efficiency of closed-system metamorphism to produce crust that is denser than the upper mantle
We primarily evaluate the capacity for the Breaksea Orthogneiss to founder as it reflects the highest-P conditions, many relationships are similar for the Malaspina Pluton

Summary

Introduction

The foundering and removal of dense continental crust[1] is part of the gross-scale geochemical and geodynamic cycling in magmatic arcs[2,3]. Metamorphism can increase the density of lower arc crust through: (i) its conversion to eclogite[9,11]; (ii) anatectic melt-extraction and the production of restite[13]; or (iii) large-scale metasomatic alteration[14]. The high-P Breaksea Orthogneiss was a composite layered pluton of monzodiorite and gabbro that was patchily deformed and recrystallised at ~850 °C and 1.8 GPa21,23 It experienced limited partial melting during metamorphism, as it lacks peritectic phase assemblages and maintains incompatible-element-rich whole-rock compositions[21]. In SW Fiordland, it is largely garnet monzodioritic gneiss with discontinuous layers of garnet pyroxenite, that reflect limited partial melting, patchy deformation and recrystallization at ~750 °C and 1.2 GPa24 These WFO plutons present a striking natural laboratory with rock compositions[18,19] appropriate to evaluate the importance of metamorphic densification at P–T conditions commonly posited for lower crustal foundering. Two models are evaluated in the context of observed field relationships: foundering being linked to crustal extension, or counter-flow diapirism

Methods

Results

Conclusion