Australia: The Land Where Time Began

A biography of the Australian continent 

Southeast Australia – Multiple Felsic Events in Volcanism Later than 10 Ma: Inputs in Appraising Magmatic Models that are Proposed

In this paper Southerland et al. present the results of their study of felsic episodes in young SE Australian volcanism using combined zircon U-Pb, feldspar ⁴⁰Ar-³⁹Ar and fission track dating. An 8 Ma age range for the felsic sequences was yielded by trachytes, xenocrysts in basalts and derived crystals in the Macedon-Trentham, about 8-5 Ma, and Western District, <5-0.0 Ma, provinces of Victoria. Zircon and feldspar ages of 6.3-6.1 ± 0.1 Ma at Newham agree with the stratigraphy of the local basalt, while zircon dating near Trentham suggests there was felsic activity at about 8.3 Ma and 6-5 Ma. Following amphibolite crystallisation in the mantle (6.3 Ma in the Brimbank Complex) zircons crystallised in high-temperature crustal trachytes that had evolved from alkali basalts. The 8-5 Ma felsic episodes have been attributed to the passage of lithosphere over an asthenospheric upwelling of plume-like type that is now centred beneath Bass Strait. Quartz-normative trachyte near Creswick (⁴⁰Ar-³⁹Ar age about 2.4 ± 0.04 Ma) is included in the Western District Province, near Daylesford, zircon megacrysts in basalt (U-Pb age 1.8 ± 0.3 Ma) and at Bullenmerri maar zircon megacrysts in tuff (U-Pb zircon age of 0.28 ± 0.04 Ma). Sutherland et al. suggest fractionation of icelandites during the peak of volcanic activity in the Western District, may be represented by the Creswick and Daylesford felsic phases. In NW Victoria-SE New South Wales bentonitic beds of trachyandesite affinities lie in strata dated to about 2 Ma and may mark a separate distal phase of the peak felsic volcanism in the Western District. The E-W trend of the post-5 Ma basaltic volcanism in the Western District has been attributed to lithospheric edge-driven or Tasman Fracture Zone fault-driven magmatic upwelling. It is suggested, however, by new tomographic modelling of the sublithospheric upper mantle that the Bassian asthenospheric inputs may explain young felsic components in adjacent basalts. Greater appraisal of the young volcanic genesis and eruptive risks of the area has been allowed by multiple felsic inputs.

Sources & Further reading

1. Sutherland, F. L., I. T. Graham, J. D. Hollis, S. Meffre, H. Zwingmann, F. Jourdan and R. E. Pogson (2014). "Multiple felsic events within post-10 Ma volcanism, Southeast Australia: inputs in appraising proposed magmatic models." Australian Journal of Earth Sciences 61(2): 241-267.