Timing relationships between pegmatite emplacement, metamorphism and deformation during the intra-plate Alice Springs Orogeny, central Australia

Abstract

In the Harts Range (central Australia), the upper amphibolite facies to lower granulite facies, c. 480-460 Ma Harts Range Metamorphic Complex (HRMC), and the upper amphibolite facies, c. 340-320 Ma Entia Gneiss Complex are cut by numerous, generally peraluminous pegmatites and their deformed equivalents. The pegmatites have previously been interpreted as locally derived partial melts. However, SHRIMP U-Pb monazite and zircon dating of 29 pegmatites or their deformed equivalents, predominantly from the HRMC, reveal that they were emplaced episodically throughout almost the entire duration of the polyphase, c. 450-300 Ma intra-plate Alice Springs Orogeny. Episodes of pegmatite intrusion correlate with the age of major Alice Springs-age structures and with deposition of syn-orogenic sedimentary rocks in the adjacent Centralian Superbasin. Similar Alice Springs ages have not been obtained from anatectic country rocks in the HRMC, suggesting that the pegmatites were not locally derived. Instead, they are interpreted as highly fractionated granites, and imply that much larger parental Alice Springs-age granites exist at depth. The mechanism to allow repeated felsic magmatism in an intraplate setting, where all exposed rock types had a previous high-temperature history, is enigmatic. However, we suggest that episodic underthrusting and dehydration of unmetamorphosed Centralian Superbasin sedimentary rocks allowed crustal fertility to maintained over a c. 140 Ma interval during the intra-plate Alice Springs Orogeny.