Multiple felsic events within post-10 Ma volcanism, Southeast Australia: inputs in appraising proposed magmatic models

Abstract

Felsic episodes in young SE Australian volcanism were studied using new combined zircon U–Pb, feldspar ⁴⁰Ar–³⁹Ar and fission-track dating. Trachytes, xenocrysts in basalts and derived detrital crystals yielded an 8 Ma range for felsic sequences in the Macedon–Trentham (ca 8–5 Ma) and Western District (< 5–0.0 Ma) provinces of Victoria. At Newham, zircon and feldspar ages of 6.3–6.1 ± 0.1 Ma agree with the local basalt stratigraphy, while near Trentham zircon dating suggests felsic activity at ca 8.3 Ma and 6–5 Ma. Zircons crystallised in high-temperature crustal trachytes that evolved from alkali basalts, following amphibole crystallisation in the mantle (6.3 Ma Brimbank complex). The 8–5 Ma felsic episodes are attributed to lithospheric passage over an asthenospheric plume-like upwelling, now centred under Bass Strait. The Western District Province includes quartz-normative trachyte near Creswick (⁴⁰Ar–³⁹Ar age ca 2.4 ± 0.4 Ma), zircon xenocrysts in basalt near Daylesford (U–Pb age 1.8 ± 0.3 Ma) and zircon megacrysts in tuff at Bullenmerri maar (U–Pb zircon age 0.28 ± 0.04 Ma). The Creswick and Daylesford felsic phases may represent fractionation of basaltic icelandites during peak Western District volcanic activity. Bentonitic beds of trachyandesite affinities in NW Victoria–SE New South Wales lie in strata dated at ca 2 Ma and may mark a separate distal phase of peak Western District felsic volcanism. The E–W trend of post-5 Ma Western District basaltic activity has been attributed to lithospheric edge-driven or Tasman Fracture Zone fault-driven magmatic up-wells. However, new tomographic modelling of sublithospheric upper mantle suggests that Bassian asthenospheric inputs may explain young felsic components in adjacent basalts. Multiple felsic inputs allow greater appraisal of the young volcanic genesis and eruptive risks for the area.

采用新型复合锆石铀-铅、长石40Ar-³⁹Ar和裂变径迹测年法，我们研究了澳大利亚东南部的近代火山活动的长英质幕。对粗面岩、玄武岩中捕虏晶和衍生碎屑晶体进行的年龄测定，将Macedon-Trentham长英质序列（约8-5Ma）和维多利亚西区（< 5-0.0 Ma）的年代确定为8Ma范围。在Newham，锆石和长石年龄6.3-6.1 ± 0.1Ma与当地的玄武岩地层一致而Trentham附近的锆石年龄表明酸性活动发生在约8.3Ma和6-5Ma。在地幔内（6.3Ma Brimbank复合体）角闪石结晶之后，锆石在从碱性玄武岩演变而来的高温地壳粗面岩内结晶。8-5Ma的长英质幕是由于岩石圈的通道通过一个软流圈地幔柱上涌。西区省包括Creswick附近的石英规范的粗面岩（40Ar-³⁹Ar年龄约 2.4 ±0.4 Ma），Daylesford附近的玄武岩内锆石捕虏晶（U-Pb年龄为1.8 ±0.3 Ma）和Bullenmerrimaar地区的凝灰岩内锆石巨晶（锆石U-Pb年龄为0.28± 0.04 Ma）。Creswick和Daylesford的长英质相可能代表西区火山活动高峰期玄武质铁安山岩的分馏作用。在维多利亚西北部至新南威尔士东南部，粗安岩类的膨润土岩层位于年龄约为2Ma的地层中，可能标志着西区长英质火山活动高峰的一个远端相。西区玄武活动5Ma之后的东 - 西走向已被归因于岩石圈边缘驱动或塔斯曼断裂带断层驱动的岩浆上涌。然而，新的断层造型亚岩石圈上地幔表明Bassian软流圈的介入可以解释相邻的玄武岩内的年轻长英质组份。多次长英质介入可以更好地解释该地区年轻火山成因和火山喷发的风险。

KEY WORDS:

• Southeast Australia
• late Cenozoic
• felsic magma
• zircon U–Pb dating
• feldspar ⁴⁰Ar–³⁹Ar dating
• basaltic fractionation
• lithosphere
• asthenosphere
• tectonics

ACKNOWLEDGEMENTS

Fons VandenBerg, Geological Survey of Victoria, supplied background data on the Trentham and Lancefield 1: 50 000 and Romsey 1: 100 000 mapping and stratigraphic notes. D. H. Taylor and C. E. Wilman and associates provided data from the 1: 100 000 Creswick and Castlemaine map sheet projects. Bernie Joyce, Geography Department, University of Melbourne provided data on Victorian volcano distribution, regolith and eruptive risk. Astrid Carlton, Geological Survey of NSW, Maitland, assisted with airborne geophysical survey reports and unpublished data from SW New South Wales. Alan Reid and Anthony Mason, Arumpo Bentonite Pty Ltd, Mildura, provided data and discussion on the bentonite deposits. Hugo Corbella, National Natural Sciences Museum, Buenos Aries, Argentina assisted with fieldwork in central Victoria.

The Australian Museum Trust supported fieldwork, collection use and analytical work. Jo-Anne Wartho assisted with ⁴⁰Ar/³⁹Ar data for the Niggl Road trachyte sample (413) analysed at Curtin University. The geology departments of the University of Pretoria and the University of Cape Town, South Africa, assisted with analytical results through Maggie Loubser and Andreas Spath. Greg Yaxley, Research School of Earth Sciences, Australian National University, Canberra, provided data on East Australian zircon megacrysts and Ti-in-zircon thermometry. The School of Biological, Earth and Environmental Sciences, University of New South Wales, helped with microscope photography and chemical analyses (Irene Wainwright, UNSW Analytical Centre). School of Science, University of Western Sydney provided EMPA access through Simon Hager. Ben Cohen, Earth Sciences, University of Queensland gave access to his 2007 PhD thesis on Ar–Ar dating of East Australian volcanic rocks.

Sabin Zahirovic, Earth Byte Group, University of Sydney, was instrumental in developing tomographic mantle models. Figure preparation was aided by Francesca Kelly, St Peters, Sydney. The manuscript was read by Graziella Caprarelli, School of the Environment, University of Technology Sydney. Constructive reviews were made by Bob Kitch of R. B. Kitch & Associates, Brisbane and Bill Birch, Museum Victoria, Melbourne.

SUPPLEMENTARY PAPERS

Supplementary Table 1 Zircon U–Pb age analyses, central to west Victorian megacrysts.

Supplementary Table 2 Analyses of selected elements (ppm) on age-analysed zircon spots.

Supplementary Table 3 Analyses (LA-ICP-MS, ppm)*. Bullenmerri zircon (core to rim).

Supplementary Table 4 ⁴⁰Ar/³⁹Ar geochronology feldspar fraction (413), Niggl Rd trachyte