RELATIONSHIPS BETWEEN MAGMATISM, METAMORPHISM AND DEFORMATION IN THE WESTERN MOUNT ISA-INLIER, AUSTRALIA

A structural and geochronological study of a granitoid and two pegmati tes is used to clarify age relationships between magmatism, deformatio n and metamorphism in the Mount Isa Inlier. Analyses of zircon U-Pb is otopic systems date the emplacement age of the Sybella Batholith at 16 60 +/- 4 to 1655 +/- 5 Ma. Similar analyses of two pegmatite generatio ns demonstrate high-U zircons with complex U-Pb systematics which resu lt from alteration and multi-stage Pb loss, A concordant group of zirc on analyses from the older pegmatite displays a bimodal distribution i ncluding groups at 1532 +/- 7 and 1565 +/- 5 Ma which are interpreted to represent the crystallisation age and inherited xenocrystic zircon, respectively. Replicate analyses of a less altered zircon grain from the younger pegmatite are near concordant at 1480 +/- 14 Ma. This is i nterpreted as a minimum age for crystallisation of the younger pegmati te. Neither pegmatite is genetically related to the Sybella Batholith. Granoblastic quartz textures and cleavage overprinting relationships indicate the 1532 +/- 7 Ma pegmatites intruded prior to or (at latest) during low-pressure (P), high-temperature (T) metamorphism, Partial m elting during peak metamorphism provides a likely source for the pegma titic melts. This implies that metamorphism and concurrent polyphase d eformation occurred at similar to 1532 Ma, similar to 130 Myr after em placement of the Sybella Batholith. Thus the batholith did not contrib ute to the high heat flow responsible for low-P, high-T metamorphism n or did it intrude during the associated period of regional crustal sho rtening. Age constraints instead suggest that the batholith was emplac ed during, or soon after, extension and rift-related sedimentation and felsic volcanism.