TY - JOUR

T1 - Fingerprinting feldstar phenocrysts using crystal isotopic composition stratigraphy

T2 - Implications for crystal transfer and magma mingling in S-type granites

AU - Waight, Tod E.

AU - Maas, Roland

AU - Nicholls, Ian A.

PY - 2000/9/9

Y1 - 2000/9/9

N2 - 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/86Sr(i) and lower ε(Nd)(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 ε(Nd)(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also preserved 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.

AB - 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/86Sr(i) and lower ε(Nd)(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 ε(Nd)(i) component such as metasedimentary wall rocks. Isotopic heterogeneities are also preserved 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.

UR - http://www.scopus.com/inward/record.url?scp=0033845916&partnerID=8YFLogxK

U2 - 10.1007/s004100000128

DO - 10.1007/s004100000128

M3 - Journal article

AN - SCOPUS:0033845916

VL - 139

SP - 227

EP - 239

JO - Contributions to Mineralogy and Petrology

JF - Contributions to Mineralogy and Petrology

SN - 0010-7999

IS - 2

ER -