Model for the development of kyanite during partial convective overturn of Archean granite–greenstone terranes: the Pilbara Craton, Australia

Abstract

Restricted occurrences of early, syn‐ and late‐kinematic kyanite adjacent to large domal batholiths in the Archean granite–greenstone terrane of the east Pilbara craton, Australia, are considered to result from partial convective overturn of the crust. The analogue models of Dixon & Summers (1983) and thermo‐mechanical models of Mareschal & West (1980), involving gravitionational overturn of dense greenstone crust that initially overlay sialic basement, successfully explain the geometry, dimension, kinematics and strain patterns of the batholiths and greenstone rims. Application of these models suggests that andalusite and sillimanite are the stable aluminosilicate polymorphs in domal crests and rims, where prograde clockwise P–T–t paths, with small pressure changes, should be recorded. Both aluminosilicates are predicted to overprint kyanite, which is observed locally around the east Pilbara domes. Kyanite is the predicted aluminosilicate polymorph in the deeper parts of domal rims and within sinking greenstone keels, reflecting rapid, near‐isothermal burial. The narrow zones of kyanite‐bearing schists adjacent to some batholiths in the Pilbara craton are metamorphosed, highly strained equivalents of altered felsic volcanic rocks in the low‐grade greenstone succession, dragged to mid‐crustal depths (6 kbar) during greenstone sinking. The schists rebounded as an arcuate tectonic wedge along the southern Mount Edgar batholith rim, during the later stages of doming, and were juxtaposed against regional, greenschist facies, low‐strain greenstones. Thus, kyanite was preserved: if the walls had remained at depth, it would have been overprinted by the higher‐temperature aluminosilicate polymorphs during thermal recovery.Kyanite growth in the Pilbara craton is unlikely to have resulted from ballooning of plutons, mantled gneiss doming, metamorphic core complex formation, or early crustal overthickening. The typical subvertical foliations and lineations of the tectonic wedge suggest that subvertical fabrics extended to mid‐crustal depths (c. 20 km) before rebound, providing a three‐dimensional glimpse of Archean dome‐and‐keel structures. The general occurrence of large granitoid domes in Archean granite–greenstone terranes, restriction of rare kyanite to the adjacent, high‐strain batholith margins, and its absence from the batholiths, suggest that partial convective overturn of the crust may have been a common process at this early stage of Earth history.