Relationships between magmatism, metamorphism and deformation in the western Mount Isa Inlier, Australia

Abstract

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 pegmatities 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 ∼ 1532 Ma, ∼ 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.