Relationships between Zircon U-Pb SHRIMP Ages and Leucosome Type in Migmatites of the Halls Creek Orogen, Western Australia

Abstract

Field relations, and zircon zoning and SHRIMP U–Pb patterns INTRODUCTION from an outcrop of deformed pelitic migmatites reveal relationships Both the amount and isotopic variability of accessory between leucosome type and the amount of zircon growth during minerals incorporated into granitic magmas and their migmatization. Zircons from the oldest, stromatic migmatites show constituent trace elements are a complex function of several Pb/Pb intragrain analyses with 1850–1840 Ma source and crystallization processes (Bea et al., 1994; anatectic ages and distinctive cathodoluminescence (CL) patterns, Brown, 1994; Ayres et al., 1995; Barbey et al., 1995; Bea, although the sample is dominated by inferred pre-1855 Ma detrital 1996). It is recognized that initial bulk compositions (X ) zircons. Subsequent thick ‘contact–sheath’ leucosomes and ‘backand pressure (P )–temperature (T ) conditions of anatexis veins’ developed at 1845–1830 Ma in response to intrusion of are major controls on the chemistry and subsequent mafic dykes into the stromatic migmatites. Zircons show CL evidence behaviour of granitic magmas (e.g. Clemens, 1990; for substantial anatectic growth and overgrowths with post-1850 Brown, 1994). Also, there is increasing awareness that Ma ages, and a single population age 20 my younger than the deformation in the source region can also control the early other migmatites. Zircons from the structurally youngest, shearand physical pathways and chemical evolution of magmas (e.g. vein-hosted leucosomes show few overgrowths and retain a large Sawyer, 1991; Hand & Dirks, 1992; Brown et al., 1995; detrital population. Correlation between shear zones and minimal Rushmer, 1995; Watt et al., 1996; Oliver & Barr, 1997), anatectic zircon growth is inferred to reflect rapid deformationwhich in turn may have a profound impact on subsequent induced melt extraction from the protolith, and/or rapid quenching. granitic magma evolution. For a given bulk source comThe earlier contact–sheath and stromatic leucosomes probably had position, magma extracted at low melt fractions from a combination of slow melt segregation, larger interacting melt an actively deforming partially molten zone may differ volume and longer residence times at high temperatures. Results substantially in composition from one extracted at high suggest that rapid melt extraction by deformation will increase the melt volumes from rocks undergoing little deformation tendency for preservation of source rock isotopic patterns in some (Rushmer, 1995). The link between ‘frozen’ migmatites granites. and complete granite bodies may not be direct (Clemens