ISOTOPIC EVIDENCE FOR THE DEPENDENCE OF RECURRENT FELSIC MAGMATISM ON NEW CRUST FORMATION - AN EXAMPLE FROM THE GEORGETOWN REGION OF NORTHEASTERN AUSTRALIA

U-Pb zircon, Sm-Nd, and Rb-Sr isotopic data, together with previously accumulated geological and chemical evidence show that the Georgetown inlier of northeast Queensland and its immediate environs were subjected to three widespread, temporally discrete episodes of felsic magmatism. The earliest of these, at about 1550 Ma, produced widespread anatexis within the metasedimentary rocks of the inlier, which have a poorly constrained depositional age, but which were derived by the degradation of ca. 2000-2500 Ma crust. Contemporaneous I-type (trondhjemitic) magmas contained more radiogenic Nd (ε{lunate} = -0.1 compared with - 1.9 to -4.7), and are thought to have formed from mixing of newly formed crustal material with the igneous precursors of the metasediments. A 420 Ma event was also characterised by two isotopically distinct magmas (with ε{lunate}Nd = -5.6 and about -16), but in this case both were of I-type. The more radiogenic magma is represented by the Dido Granodiorite, which is likely to have formed by the melting of ca. 1550 Ma crust, whereas the other magma type (which includes three different granitoids) was apparently derived from the same (2000-2500 Ma) igneous crust from which the metasediments were formed. I-type 300 Ma igneous rocks had a uniform isotopic composition (ε{lunate}Nd = -7.5, 87Sr 86Sr = 0.710) consistent with their formation by different degrees of partial melting (perhaps combined with fractional crystallisation) from a Dido Granite-like precursor. A younger source component was at least partially involved in the origin of spatially associated A-type volcanics, which are about 30 Ma younger and have more primitive Nd compositions. Most of the Palaeozoic igneous rocks reflect at least two episodes of intracrustal melting. The data are best explained in terms of successive addition from below of new crustal material (via underplating or emplacement into the lower crust) at about 1550 Ma, 420 Ma, and 300 Ma. The model requires that such newly accreted material does not necessarily melt and mobilise the preceeding underplate. Often it is a still earlier underplate that is activated. These regions of the lower crust can remain dormant for well over a billion years before they produce widespread magmatism destined for the upper crust. © 1990.