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

A structural and geochronological study of a granitoid and two pegmatites is used to clarify age relationships between magmatism, deformation and metamorphism in the Mount Isa Inlier. Analyses of zircon U-Pb isotopic systems date the emplacement age of the Sybella Batholith at 1660 +/- 4 to 1655 +/- 5 Ma. Similar analyses of two pegmatite generations demonstrate high-U zircons with complex U-Pb systematics which result from alteration and multi-stage Pb loss, A concordant group of zircon analyses from the older pegmatite displays a bimodal distribution including 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 interpreted as a minimum age for crystallisation of the younger pegmatite. 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 melting during peak metamorphism provides a likely source for the pegmatitic melts. This implies that metamorphism and concurrent polyphase deformation occurred at similar to 1532 Ma, similar to 130 Myr after emplacement of the Sybella Batholith. Thus the batholith did not contribute to the high heat flow responsible for low-P, high-T metamorphism nor did it intrude during the associated period of regional crustal shortening. Age constraints instead suggest that the batholith was emplaced during, or soon after, extension and rift-related sedimentation and felsic volcanism.