Wj. Collins et Mj. Van Kranendonk, Model for the development of kyanite during partial convective overturn ofArchean granite-greenstone terranes: the Pilbara Craton, Australia, J METAMORPH, 17(2), 1999, pp. 145-156
Restricted occurrences of early, syn- and late-kinematic kyanite adjacent t
o large domal batholiths in the Archean granite-greenstone terrane of the e
ast Pilbara craton, Australia, are considered to result from partial convec
tive overturn of the crust. The analogue models of Dixon & Summers (1983) a
nd thermo-mechanical models of Mareschal & West (1980), involving gravition
ational overturn of dense greenstone crust that initially overlay sialic ba
sement, successfully explain the geometry, dimension, kinematics and strain
patterns of the batholiths and greenstone rims. Application of these model
s suggests that andalusite and sillimanite are the stable aluminosilicate p
olymorphs in domal crests and rims, where prograde clockwise P-T-t paths, w
ith small pressure changes, should be recorded. Both aluminosilicates are p
redicted to overprint kyanite, which is observed locally around the east Pi
lbara domes. Kyanite is the predicted aluminosilicate polymorph in the deep
er parts of domal rims and within sinking greenstone keels, reflecting rapi
d, near-isothermal burial. The narrow zones of kyanite-bearing schists adja
cent to some batholiths in the Pilbara craton are metamorphosed, highly str
ained equivalents of altered felsic volcanic rocks in the low-grade greenst
one succession, dragged to mid-crustal depths (6 kbar) during greenstone si
nking. The schists rebounded as an arcuate tectonic wedge along the souther
n Mount Edgar batholith rim, during the later stages of doming, and were ju
xtaposed 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 durin
g thermal recovery.
Kyanite growth in the Pilbara craton is unlikely to have resulted from ball
ooning of plutons, mantled gneiss doming, metamorphic core complex formatio
n, or early crustal overthickening. The typical sub-vertical foliations and
lineations of the tectonic wedge suggest that subvertical fabrics extended
to mid-crustal depths (c. 20 km) before rebound, providing a three-dimensi
onal 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 abs
ence from the batholiths, suggest that partial convective overturn of the c
rust may have been a common process at this early stage of Earth history.