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

Citation
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
Citations number
58
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF METAMORPHIC GEOLOGY
ISSN journal
02634929 → ACNP
Volume
17
Issue
2
Year of publication
1999
Pages
145 - 156
Database
ISI
SICI code
0263-4929(199903)17:2<145:MFTDOK>2.0.ZU;2-J
Abstract
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.