Carbonatites as recorders of mantle-derived magmatism and subsequent tectonic events: An example of the Gifford Creek Carbonatite Complex, Western Australia
Abstract The Gifford Creek Carbonatite Complex (GCCC), Western Australia contains a diverse suite of alkaline igneous rocks, including magnesiocarbonatites, ferrocarbonatites, phoscorites, fenites, magmatic-hydrothermal peralkaline dykes, and ironstones. This study employs U-Pb, Sm-Nd, and Lu-Hf radiogenic isotope techniques on monazite – (Ce), fluorapatite, and zircon to determine the origin, age, and history of the GCCC. Zircon crystals found in glimmerite alteration selvages adjacent to ferrocarbonatites exhibit pyramidal crystal morphologies, eHf values of −1.8 to −4.3, high Th/U, and variable Zr/Hf, all of which are indicative of carbonatitic zircon sourced from an enriched mantle component. Uranium-Pb dating of these zircons returned a definitive magmatic age of ~1370 Ma for the GCCC. Monazite hosted in the ferrocarbonatites, phoscorites, and fenite alteration assemblages yielded variable U-Pb ages ranging from ca. 1250 Ma to 815 Ma. Neodymium isotope isochrons determined from coexisting monazite and apatite gave ages between ca. 1310 Ma to ca. 1190 Ma, but all with similar initial 143 Nd/ 144 Nd values of 0.51078–0.51087. The 1370 Ma age of the GCCC does not correspond to any known mantle plume activity, but does broadly correlate with the separation of the North China Craton from the West Australian Craton as part of the greater breakup of Nuna. The monazite and apatite eNd data illustrate that the multiple younger U-Pb monazite and Nd isotope isochron ages are not recording multiple magmatic intrusions into the complex, but rather represent partial recrystallisation/resetting of REE-bearing minerals during the protracted tectonic history of the Western Australia Craton from ~1300 Ma to 815 Ma and its involvement in the breakup of Nuna and assembly and disassembly of Rodinia. The age variability in the U-Pb and the Sm-Nd isotope systems in monazite and apatite reveal that tectonically-induced hydrothermalism can contribute to the isotopic resetting of phosphate minerals. This age resetting, if properly identified, can be used as a thorough geochronological record of tectonism affecting alkaline igneous complexes after initial magmatic emplacement.