DATING LOWER CRUST AND UPPER-MANTLE EVENTS - AN ION MICROPROBE STUDY OF XENOLITHS FROM KIMBERLITIC PIPES, SOUTH-AUSTRALIA

Zircons separated from five xenoliths (three quartzofeldspathic granul ites, one mafic granulite and one eclogite) or lower crustal and upper most 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 Calc utteroo kimberlitic pipes ( 167-174 Ma) that outcrop on the eastern ma rgin of the Australian craton. The combined data-set of all zircon U-P b ages from all the xenoliths records four major tectonic events assoc iated with zircon formation, recrystallization or Pb loss. These event s at about 1600-1500, 780, 620 or earlier and 330 Ma, correlate with l arge-scale tectonic events recorded in adjacent crustal and mantle roc ks: the approximately 1600 magmatism and tectonism of the exposed Will yama 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 sut ure line and the 500-300 Ma metasomatic events in the mantle in wester n Victoria. However, more detail is provided by the data from individu al xenolith types. A considerable number of zircons from the quartzofe ldspathic granulite xenoliths grew during the granulite facies metamor phism which occurred at some time between 1600 and 1500 Ma. A small nu mber of zircons with ages up to 1800 Ma reflect the xenoliths' prior h istory and other zircons grew at 610 Ma or earlier. Zircons from the m afic granulite xenolith record an event at approximately 780 Ma, inter preted as the age of the granulite facies metamorphism of the protolit h, probably shortly after the underplating and differentiation of the mantle-derived mafic magma. Zircons from the eclogite xenolith have ve ry low U (4-13 ppm) and Th ( < 3 ppm) contents, similar to those obser ved in zircons from some exposed eclogites of N-type MORB magma deriva tion. Eclogite facies metamorphism not later than 62O +/- 46 Ma is inf erred from the oldest Pb-206/U-238 zircon age obtained from the xenoli th. An event at approximately 330 Ma caused Pb loss in zircons from al l three types of xenoliths. The coincidence of the zircon age clusters with large-scale crustal or mantle events indicates the coupling of m ajor mantle and crustal tectonic episodes in this region. The identifi cation of multiple zirconforming events and the inferred different tim es of formation of different xenolith assemblages show that Sm-Nd isoc hron ages based on an assumption of a cogenetic relationship are inval id.