Dating lower crust and upper mantle events: an ion microprobe study of xenoliths from kimberlitic pipes, South Australia

Abstract

Zircons separated from five xenoliths (three quartzofeldspathic granulites, one mafic granulite and one eclogite) of lower crustal and uppermost mantle derivation were dated using the SHRIMP ion microprobe. The zircon data indicate that different types of xenoliths had different origins and formed at different times. The xenoliths are from the Calcutteroo kimberlitic pipes (167–174 Ma) that outcrop on the eastern margin of the Australian craton. The combined data-set of all zircon UPb ages from all the xenoliths records four major tectonic events associated with zircon formation, recrystallization or Pb loss. These events at about 1600-1500, 780, 620 or earlier and 330 Ma, correlate with large-scale tectonic events recorded in adjacent crustal and mantle rocks: the ∼ 1600 magmatism and tectonism of the exposed Willyama Complex and Gawler Block (in New South Wales and South Australia), the 850-650 Ma rifting and faulting of the Adelaidean Fold Belt, the 650-575 Ma extension, uplift and collision episodes at the Tasman suture line and the 500-300 Ma metasomatic events in the mantle in western Victoria. However, more detail is provided by the data from individual xenolith types. A considerable number of zircons from the quartzofeldspathic granulite xenoliths grew during the granulite facies metamorphism which occurred at some time between 1600 and 1500 Ma. A small number of zircons with ages up to 1800 Ma reflect the xenoliths' prior history and other zircons grew at 610 Ma or earlier. Zircons from the mafic granulite xenolith record an event at ∼ 780 Ma, interpreted as the age of the granulite facies metamorphism of the protolith, probably shortly after the underplating and differentiation of the mantle-derived mafic magma. Zircons from the eclogite xenolith have very low U (4–13 ppm) and Th (<3 ppm) contents, similar to those observed in zircons from some exposed eclogites of N-type MORB magma derivation. Eclogite facies metamorphism not later than 620±46 Ma is inferred from the oldest 206Pb 238 U zircon age obtained from the xenolith. An event at ∼ 330 Ma caused Pb loss in zircons from all three types of xenoliths. The coincidence of the zircon age clusters with large-scale crustal or mantle events indicates the coupling of major mantle and crustal tectonic episodes in this region. The identification of multiple zircon-forming events and the inferred different times of formation of different xenolith assemblages show that SmNd isochron ages based on an assumption of a cogenetic relationship are invalid.