ZIRCON GEOCHRONOLOGY AND PETROLOGY OF THE MIGRATING TARANA INTRUSIVE COMPLEX, BATHURST BATHOLITH, AUSTRALIA

Understanding how magma plumbing systems operate at depth is a fundamental aspect of determining the causes and patterns of volcanic eruptions. Plutonic complexes, representing cross-sections of magma plumbing systems at depth, display different geometrical and compositional patterns. One puzzling pattern is that of migrating plutons and their possible link to migrating volcanic centers. This study is examining both the temporal and compositional patterns of a spectacular example of a migrated pluton, the Tarana Intrusive Complex (TIC) of the ca. 30 myr long lived, Carboniferous arc in the Lachlan fold and thrust belt, eastern Australia.

The ca. 420 km² TIC is composed of a series of 15 separately mapped bodies, starting with 3 main units in the western, asymmetrically nested Bathurst batholith, evolving to ca. 9 crescent-shaped units that are concave to the east and end in 3 nested units at the eastern end. The units' bow-shaped bodies and crosscutting relationships suggest that as magma ascent migrated to the east, each new unit intruded as a roughly elliptical pluton while truncating and removing parts of the older unit. This west to east migration is perpendicular to the structural grain (defined by subparallel, steeply-dipping stratigraphic units, bedding, cleavage and faults) of the host rocks. The migrated section of the TIC appears to have intruded over four magmatic cycles that range from more mafic, magnetic, hornblende-bearing equigranular granitoids to porphyritic granites that are more felsic in composition eastward.

New LA-ICP-MS U/Pb zircon geochronology confirms the west-to-east migration pattern: the westernmost sample in the migrated section yielded an age of 334.26± 0.97 Ma, a unit halfway between the two end units yielded an age of 333.95±0.77 Ma, and the easternmost and youngest unit yielded an age of 327.49±0.72 Ma. The eastern sample also contains inherited cores ranging from 1.083 to 1.092 Ga. Published Rb/Sr biotite cooling ages in the area fall between 315-330 myr (Shaw and Flood, 1993). The 6.8 myr age difference between the oldest and youngest samples occurs over a distance of ca. 47 km thus giving a migration rate of ca. 7 km/myr.

Petrography and XRF element geochemistry currently in progress will allow the examination of the cyclic nature of the rock compositions and the causes of migration.