U-Pb ages and morphologies of zircon in microgranitoid enclaves and peraluminous host granite: evidence for magma mingling |
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Authors: | M A Elburg |
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Institution: | (1) Department of Earth Sciences, Monash University, Clayton 3168, Australia Now at: Department of Geology and Geophysics, University of Adelaide, Adelaide 5005, Australia, AU |
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Abstract: | Granites of the S-type Wilson's Promontory Batholith (Lachlan Fold Belt, Australia) contain zircons which are euhedral and
relatively large; their age is 395 Ma, which can be considered as the best available estimate of the crystallysation age of
the granites. Contrary to their dominance in other S-type granites of the Lachlan Fold Belt, very few zircon cores give inherited
ages, varying between 500 and 1700 Ma. Microgranitoid enclaves contained within the granites contain a zircon population that
is dominated by relatively small, anhedral or elongated crystals. These give ages that are indistinguishable from the crystallisation
age of the granite. Some enclaves, which are characterised by the presence of megacrysts, contain a proportion of larger,
euhedral zircons. These zircons give inherited ages similar to the zircons from the granitic host rocks. The data are in agreement
with a magma mingling origin for the microgranitoid enclaves. The large euhedral zircons are interpreted to have been introduced
into the “enclave magma” during a hybridisation event which also introduced quartz and plagioclase megacrysts into the magma.
The relatively high proportion of inherited cores within the “large” zircon population of the enclaves is related to the timing
of mixing between “enclave” and host magma. This mixing event took place before the majority of the magmatic zircons crystallised
in the granitic magma. The small, anhedral zircons within the enclaves crystallised during quenching of the globules of enclave
magma against the cooler granitic magma.
Received: 21 August 1995 / Accepted: 9 October 1995 |
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