Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites |
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Authors: | Tod E Waight Roland Maas Ian A Nicholls |
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Institution: | Danish Lithosphere Centre, ?ster Voldgade 10, DK-1350 Copenhagen K, Denmark E-mail: tew@dlc.ku.dk, DK School of Earth Sciences, La Trobe University, Bundoora, VIC 3083, Australia, AU Department of Earth Sciences, Monash University, Clayton, VIC 3168, Australia, AU
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Abstract: | Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia
has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording
feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to
the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified
form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave
and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation.
Feldspar rims have higher 87Sr/86Sri and lower ɛNd(i) than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests
that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high
87Sr/86Sr, low ɛNd(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases
in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma
and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral
scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars
as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest
that much heterogeneity in plutonic systems may be overlooked on a whole rock scale.
Received: 28 September 1998 / Accepted: 29 December 1999 |
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