HR: 1400h
AN: V53B-10    [Abstracts]
TI: Construction of the Tuross Head Pluton, Coastal NSW, Australia
AU: * Wiebe, R A
EM: bwiebe@fandm.edu
AF: Robert A. Wiebe, Department of Earth and Environment Franklin and Marshall College, Lancaster, PA 17604, United States
AU: Collins, W J
EM: bill.collins@jcu.edu.au
AF: William J. Collins, School of Earth Sciences James Cook University, Townsville, QD 4811, Australia
AB: The Tuross Head pluton, roughly 8 km in diameter, consists dominantly of tonalite with layers, lensoid zones and enclaves of diorite that define a steep basinal structure. Way up structures indicate that these rocks accumulated on an aggrading magma chamber floor, which sagged inward as accumulation occurred. Hence, rocks young inward, and coastal exposures provide stratigraphic sections with clear records of compositional variation through time. Extensive magma mingling and hybridization between tonalite and diorite is apparent from field relations and mixed populations of complexly zoned feldspars. Synmagmatic dikes are dominantly basaltic and dacitic and include composite dikes with chilled mafic pillows in a felsic host. Dikes with intermediate compositions also occur. The chemical and isotopic compositions of the dikes closely match those of the plutonic rocks and suggest they were feeders to the pluton. Isotopic compositions of dikes range widely and correlate broadly with SiO2 (eNd varies from about +8 to +3; Sri varies from 0.7032 to 0.7042). The dikes include both basaltic magmas derived from asthenosphere and dacitic magmas probably derived from Neoproterozoic- Cambrian lower crust. Intermediate dikes appear to represent magmas that mixed prior to emplacement. The Tuross Head pluton formed by multiple emplacements of compositionally varied magmas into one or more magma chambers of unknown size. Chemical and isotopic variations in the plutonic rocks appear to reflect accumulation of early-forming crystals from these contrasted and contemporaneous magmas onto an aggrading chamber floor. Although chemical variation, as viewed on Harker diagrams, might appear consistent with simple fractional crystallization, the stratigraphic occurrences of these samples indicate that tonalite compositions did not become more evolved through time. There is subtle variation upward through the stratigraphic column: the silicic host becomes more mafic, and mafic enclaves become more silicic, until they reach a zone of high strain in the magmatic state, where enclaves are so strongly attenuated that the rocks appear gneissic, and it is impossible not to sample a mixed magma. Above this zone, at the top of the Tuross section, the 'tonalitic' rocks have compositions intermediate between tonalite and diorite end-members.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8434 Magma migration and fragmentation
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly