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ELSEVIER Earth and Planetary Science Letters 141 (1996) 187-198
Inherited zircon and titanite U-Pb systems in an Archaean syenite
from southwestern Australia: implications for U-Pb stability of
titanite
R.T. Pidgeon a, * , D. Bosch b, 0. Bruguier b
a School of Applied Geology, Curtin University of Technology, Bentley. Western Australia. 6102
b Laboratoire’ de GPochronologie-Giochemie-Petrologie, U.R.A. 1763, Case courrier 066, U.M. II, place E. Bataillon, 34095 Montpellier
Cedex 5, France
Received 20 July 1995; accepted 22 March 1996
Abstract
Inherited zircon and titanite have been identified in a syenite from the Archaean of southwestern Australia. Conventional
and SHRIMP analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 f 5
Ma for the syenite. In addition, SHRIMP analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca.
3250 Ma inherited component. Conventional U-W ages on titanite also fall between ca. 3250 Ma and ca. 2650 Ma,
demonstrating that inherited titanite as well as zircon is present in the syenite. The syenite has been affected by regional
upper amphibolite facies metamorphism at an estimated temperature of 625-650°C. Retention of the inherited radiogenic Pb
in the titanite is evidence that the closure temperature for titanite is greater than 650°C. The presence of inherited titanite and
zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of
crustal rocks by a potassic magma at < 75O”C, rather than from a homogeneous high T magma.
Keywords: Archean; U/Pb; zircon; titanite; diffusion;
SHRIMP data; isotopes
1. Introduction
Detailed studies of U-Pb ages on coexisting accessory
minerals such as zircon, titanite, monazite,
garnet and apatite are providing a new dimension in
the understanding of tectonic and metamorphic processes.
However, although considerable information
exists, more case histories are needed to determine
the U-PI, stability of these minerals under magmatic
and metamorphic conditions. In this report we pre-
_ Corresponding
author. E-mail: tpidgeon@cc.curtin.edu.au
sent the results of a TIMS and SHRIMP study of the
U-Pb systems of zircon and titanite from an Archaean
syenite from the Jimperding Metamorphic
Belt in the Yilgam Craton of southwestern Australia
(Fig. 1) [l]. Th e o nginal . purpose of the project was
to investigate the timing of emplacement of the
syenite within the context of the complex igneous
and metamorphic events associated with the evolution
of the Jimperding Metamorphic Belt. Results of
this broader study will be presented elsewhere [2]. In
this contribution our main purpose is to consider the
implications of the presence of inherited U-Pb systems
in zircon and titanite on estimates of the block-
0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved
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