Evolution of a post-batholith dike swarm in central coastal Queensland, Australia: arc-front to backarc?

A swarm of felsic and mafic dikes cuts a Late Carboniferous-Permian batholi th called the Urannah Suite in central coastal Queensland. Late Permian-Tri assic westward thrusting (Hunter-Bowen Orogeny) exposed this mid-crustal Su ite and the crosscutting dikes, thus enabling examination of dikes that ran ge in age from syn- to post-batholithic. Although both mafic and felsic dik es have the same dominant northerly strike, field, geochronologic and geoch emical examination reveal that the swarm is composite; felsic dikes are old er (285 Ma) and geochemically and isotopically distinct from mafic dikes (s imilar to 273-229 Ma). Dike compositions are compared to those of the host plutonic rocks, and to volcanic rocks the same age as the dikes, Whereas th e felsic (older) dikes are compositionally similar to their host granites ( initial Sr-87/Sr-86 > 0.7045), the mafic (younger) dikes are isotopically ( Sr, Nd, Pb) less radiogenic. Moreover, several different types of mafic dik es are evident based on geochemistry; most of these have mixed characterist ics in terms of tectonic classification. All but two dikes of basalt and ba saltic andesite composition classify as 'with-in plate' on Ti-Zr-Y tectonic classification plots. Many of the basalts have high TiO2 contents (1.5-3.0 wt.%). Most of these have REE and spider diagram patterns like calc-alkali ne tholeiites, the exceptions being a few all;ali basalts recognized by the ir alkali content, and high Ti, Ce, Nh and Zr contents. When put into the c ontext of all plutonic rocks in the area (late Paleozoic and Mesozoic), the se dikes record a transition at similar to 280 Ma, after which time, all ma gnatism in the region is less isotopically evolved (initial Sr-87/Sr-86 = 0 .7033-0.7044). A model of slab retreat and hinge movement to the east in th e latest Permian explains the change of geochemical signature from are-fron t to backare at about 280 Ma. (C) 2000 Elsevier Science B.V. All rights res erved.