Combined U-Pb dating and Sm-Nd studies on lower crustal and mantle xenoliths from the Delegate basaltic pipes, southeastern Australia |
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| Authors: | Y D Chen Suzanne Y O'Reilly W L Griffin T E Krogh |
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| Institution: | (1) Geochronology Laboratory, Royal Ontario Museum, Toronto, Ontario, Canada M5S 2C6, CA;(2) GEMOC National Key Centre, School of Earth Sciences, Macquarie, Sydney, NSW, Australia 2109, AU;(3) CSIRO Exploration and Mining, North Ryde, NSW 2113, Australia, AU |
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| Abstract: | Mafic rocks dominate the lower crustal and upper mantle xenolith suites within the Jurassic Delegate basaltic diatremes in
the Paleozoic Lachlan Fold Belt, SE Australia. Two upper mantle mafic xenoliths from the Delegate pipes, a garnet pyroxenite
and a garnet granulite (equilibrated at 1060 and 1140 °C, and 40–50 km), yield garnet-clinopyroxene Sm-Nd ages of 160 ± 4
Ma and 153 ± 10 Ma, respectively. Both ages are indistinguishable from the time of eruption of the diatremes, and are interpreted
as showing continuous isotopic equilibrium within the mantle of Sm and Nd between garnet + clinopyroxene at temperatures ≥ 1050 °C.
A lower crustal, 2-pyroxene granulite xenolith (equilibrated at 810–850 °C and ca. 25 km) yields a clinopyroxene + plagioclase + whole
rock Sm-Nd isochron ages of 283 ± 26 Ma. This age probably reflects partial resetting of the isotopic systems of much older
granulite during slow cooling, or after a heating event in the lower crust associated with the Jurassic magmatic activity
represented by the basaltic host rock. Metamorphic zircons from the 2-pyroxene granulite xenolith were dated by the U-Pb method
at 398±2 and 391 ± 2 Ma. These ages are considered to date granulite facies metamorphic events in the lower crust of the region.
The age gap between the granulite facies metamorphism and granitoid plutonism in the region (420–410 Ma) indicates that the
dated granulite is unlikely to represent residue after partial melting and magma extraction that generated the regional granitoids.
It is suggested that these ages may record a relatively slow cooling following the cessation of mafic magmatic intrusion that
formed the xenolith protoliths and that was probably the heat source responsible for granite production. At about 25 km, this
thermal relaxation accounts for the change from an olivine + plagioclase + 2-pyroxene gabbroic assemblage into the granulite
facies 2-pyroxene + plagioclase + spinel field.
Received: 17 May 1995 / Accepted: 24 March 1997 |
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