Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I-type plutons of the Lachlan Fold Belt, southeast Australia

TY - JOUR

T1 - Sr and Nd isotopic investigations towards the origin of feldspar megacrysts in microgranular enclaves in two I-type plutons of the Lachlan Fold Belt, southeast Australia

AU - Waight, T. E.

AU - Dean, A. A.

AU - Maas, R.

AU - Nicholls, I. A.

PY - 2000/1/1

Y1 - 2000/1/1

N2 - New Sr and Na isotopic cata are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bricle Track biutons, along with analyses of their host enclave grounamass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Felaspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave ground-mass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equillibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher 143Nd/122Na9) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.

AB - New Sr and Na isotopic cata are presented for several large feldspar crystals occurring in microgranular enclaves in the Swifts Creek and Bricle Track biutons, along with analyses of their host enclave grounamass and adjacent granitoid. In the Swifts Creek Pluton several previous studies have concluded that the microgranular enclaves represent admixed, more mafic and more primitive magmas, and new data presented here confirm that. Felaspar megacrysts in the microgranular enclaves have Sr and Nd isotopic signatures that are distinct from the surrounding enclave ground-mass and from other enclaves in the pluton and therefore cannot have crystallised in situ. Isotopic compositions of these feldspars are more consistent with their having crystallised in a reservoir similar in composition to the most primitive granitoid analyses. The crystals were then physically transferred from the granitoid magma into the enclave while the latter was still partially liquid, thus invalidating arguments for a porphyroblastic origin. Field, petrographic and geochemical data are consistent with microgranular enclaves in the Bridle Track pluton also originating as admixed, more mafic magmas. However, Sr isotopic compositions of the enclaves are identical, within error, to the host granite and indicate that significant Sr isotopic equillibration has occurred. Nd isotopic compositions of the enclaves extend to slightly higher 143Nd/122Na9) and are consistent with a mingled magma origin followed by major isotopic equilibration. Feldspar phenocrysts in the studied enclave have isotopic compositions indistinguishable from both the enclave groundmass and host granite, preventing an interpretation of their origin using isotopic evidence alone.

KW - Enclave

KW - Granite

KW - Isotopes

KW - Lachlan Fold Belt

UR - http://www.scopus.com/inward/record.url?scp=0034503082&partnerID=8YFLogxK

U2 - 10.1046/j.1440-0952.2000.00831.x

DO - 10.1046/j.1440-0952.2000.00831.x

M3 - Journal article

AN - SCOPUS:0034503082

SN - 0812-0099

VL - 47

SP - 1105

EP - 1112

JO - Australian Journal of Earth Sciences

JF - Australian Journal of Earth Sciences

IS - 6

ER -