Géochronologie U-Pb par ablation laser et ICP-MS (LA-ICP-MS ...

Contrib Mineral Petrol (1999) 136: 258 – 272 Ó Springer-Verlag 1999
O. Bruguier á D. Bosch á R.T. Pidgeon
D.I. Byrne á L.B. Harris
U-Pb chronology of the Northampton Complex,
Western Australia ± evidence for Grenvillian sedimentation,
metamorphism and deformation and geodynamic implications
Received: 7 January 1998 / Accepted: 10 March 1999
Abstract Conventional and SHRIMP U-Pb analyses of
zircon, monazite, titanite and apatite from the high
grade rocks of the Northampton Complex in Western
Australia provide constraints on the timing of metamorphic
processes and deformation events in the
northern Darling Mobile Belt (western margin of the
Archean Yilgarn Craton). Paragneisses and mafic volcanics
and/or intrusions have undergone granulite facies
metamorphism in a probable extensional tectonic setting
prior to formation of W- to NW-verging folds and
thrusts cut by normal shears (interpreted as late collapse
structures) during the main deformation event (D 1 ).
These structures are folded by open to tight folds with
NW-striking axial surfaces developed in a second,
NE-SW contractional event (D 2 ). Zircons from a mafic
granulite provide an age of 1079 ± 3 Ma attributed to
new zircon growth prior to, or at the peak of regional
granulite facies metamorphism. Metamorphic monazites
extracted from a paragneiss yield an identical age of
1083 ± 3 Ma. The similarity of ages between zircons
from the mafic granulite (1079 ± 3 Ma) and monazites
from the paragneiss (1083 ± 3 Ma) is interpreted to
reflect fast cooling and/or rapid uplift, which is consistent
with thrusting of the gneissic units during the first
deformation event (D 1 ) associated with the onset of
O. Bruguier (&) á D. Bosch
ISTEEM, Universite´ Montpellier 2-CNRS,
cc 066, Place Eugène Bataillon,
34095 Montpellier Cedex 5, France
e-mail: bruguier@dstu.univ-montp2.fr
R.T. Pidgeon
School of Applied Geology,
Curtin University of Technology,
GPO Box U 1987, Perth, WA 6001, Australia
D.I. Byrne á L.B. Harris
Tectonics Special Research Centre,
Department of Geology and Geophysics,
The University of Western Australia,
Nedlands, WA 6907, Australia
Editorial Responsibility: T.L. Grove
retrograde metamorphism. Granitic activity at
1068 ± 13 Ma was followed by intrusion of post-D 2
pegmatite (989 ± 2 Ma), which constrains the end of
metamorphism and associated deformation. Cooling of
the complex to about 500 °C is timed by the apatite age
of 921 ± 23 Ma. SHRIMP U-Pb ages of detrital zircons
from a paragneiss sample yield a maximum age of
2043 Ma, with no evidence of an Archean Yilgarn signature.
A majority of ages between 1.6 and 1.9 Ga are
consistent with derivation from the Capricorn Orogen
on the northern margin of the Yilgarn Craton. Younger
detrital zircons with 1150–1450 Ma ages, however, indicate
an additional source that had undergone early
Grenvillian igneous or metamorphic event(s) and also
places a maximum age constraint upon deposition. The
source of this clastic material may have been from within
the southern Darling Mobile Belt or from Greater India
(adjacent to the Northampton Complex in Rodinia reconstructions).
This study documents an extended
Grenvillian history, with basin formation, sedimentation,
granulite facies metamorphism, contractional tectonics
(two periods with orthogonal directions of
shortening) and late pegmatite emplacement taking
place between 1150–989 Ma on the western margin of
the Yilgarn Craton. Ages recorded in this study indicate
that the proposed global distribution of Grenvillian belts
during assembly of the Rodinia supercontinent should
be reassessed to include the Darling Mobile Belt.
Introduction
The Northampton Complex has been included in the
Darling Mobile Belt (or Pinjarra Orogen after Myers
et al. 1996) and represents an isolated Proterozoic
basement inlier within the Phanerozoic Perth Basin
(Fig. 1a). It is dominated by granulite facies paragneisses
that show polyphase deformation, and is intruded
by granitoids. A major question concerns the
relationships between the Complex and the adjacent