Nd isotope and petrogenetic constraints for the origin of the Mount Angelay igneous complex: implications for the origin of intrusions in the Cloncurry district, NE Australia

The Mesoproterozoic Mount Angelay igneous complex contained intrusions that were emplaced into amphibolite facies metasedimentary rocks during two periods of similar to 1550 and post-1540 Ma magmatism. Sm-Nd isotopic analysis together with mineralogical and chemical considerations suggest that the intrusions were produced from a Pale-oproterozoic crustal source with a T-2 model age similar to 2200 Ma. On geochemical and petrological grounds, the similar to 1550 Ma trondhjemitic intrusions are interpreted to have been produced by melting of amphibolite under garnet-stable conditions ( > 8-10 kbar). The late-syn to post-peak metamorphic timing of these intrusions suggested that they were associated with the tectono-thermal event that produced regional peak metamorphic mineral assemblages. The post-1540 Ma intrusions are K-rich and consist of two groups of synchronously emplaced intrusions, (1) a high-K monzodiorite and monzogranite suite that range between 51 and 77 wt.% SiO2; and (2) a high-K, Na-enriched hornblende monzonite. The chemistry and mineralogy of these intrusions suggested that they were derived via plagioclase-stable and garnet-unstable melting (<8-10 kbar). The high-K monzodiorite and monzogranite are interpreted to have formed from a plagioclase-bearing source that contained abundant K-feldspar, biotite and/or amphibole. These intrusions are relatively enriched in K, Ca, LREE, Ba, Sr, Zr, Cl and F, and depleted in Na2O, P2O5, Cr, V and Zn compared with slightly younger high-K monzonite, which is interpreted to have formed via one of two mechanisms, (1) melting of a low-K amphibole- and plagioclase-rich source; or (2) melting of residual material that produces a potassic and incompatible element-rich melt. These magmas likely contained mantle-derived material, particularly the K-rich intrusions of mafic composition. The heat required for the production of post-1540 Ma intrusions appears to have been generated by the intrusion of high-T, mantle-derived, mafic material into the crust (<similar to>25-30 km; similar to8-10 kbar). This model is consistent with the synchronous emplacement of mafic and felsic magma and the lack of a consanguineous regional metamorphic association, and suggests high-T, high-degree partial melting in localised pockets within fertile source regions in the crust. An increase in Sm-Nd model source age and decrease in epsilon Nd with increasing SiO2 in the K-rich intrusions suggests the incorporation of juvenile material in the more mafic rocks. The origin of this component is unknown, but it may represent either the incorporation of mantle-derived material during melting, or the partial melting of crust with a younger mafic component. On a district scale, the > 30 million year period over which the K-rich post-1540 Ma intrusions were emplaced suggested that mantle-derived material continued to be injected into the crust. A mantle component to these rocks, and the global distribution of Proterozoic intrusions with similar geochemical affinities, strongly suggests a world-wide period of mantle-induced crustal melting at that time. The dominant Paleoproterozoic isotopic composition of most of these intrusions suggests melting of similarly composed and matured source rocks. (C) 2001 Elsevier Science B.V. All rights reserved.