Petrogenesis of Victorian Cambrian Tholeiites and Implications for the Origin of Associated Boninites |
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| Authors: | CRAWFORD ANTHONY J; KEAYS REID R |
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| Institution: | 1Geology Department, University of Tasmania P.O. Box 252C, Hobart, Tasmania, Australia 7000
2Geology Department, University of Melbourne Parkville Victoria, Australia 3052 |
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| Abstract: | In Victoria, tholeiitic basalts and subvolcanic dolerites andgabbros in the Heatheote and Mount Wellington Greenstone Beltsoccur stratigraphically above boninites, and in the latter beltcontain interbedded arc-derived andesitic detritus. Intrusiverocks form concordant, sill-like bodies up to 1 km thick whichare often strongly differentiated, chemically and mineralogically.The volcanic section in both greenstone belts, between 1000and 1500m thick, is dominated by aphyric or sparsely augitephyrictholeiites which trend toward ferrotholeiite compositions andare comagmatic with underlying intrusives. The tholeiites are a low-K2O suite with chondritic REE patterns,initial  Nd= +4?5 to +5?0, and trace element characteristicsmost like those shown by modern basalts erupted during the earlieststages of arc rifting and backarc basin opening. From virtuallyidentical parental magmas, the basalts in the Heathcote GreenstoneBelt show a very rapid increase in TiO2, Zr, Y, and La withincreasing differentiation relative to those in the Mount WellingtonGreenstone Belt. This is explained using O'Hara's (1977) modelfor open system fractionation, in which magma supply and eruptionrate is significantly higher for the Heathcote belt magma chambers. The proposed petrogenetic model for the Victorian Cambrian tholeiitesinvolves subduction of an oceanic spreading centre beneath anearly Cambrian forearc region. Continued magma generation atthe subducted spreading centre resulted in diapirs of 1400?CMORB-source mantle piercing into the over-riding plate and providingheat to locally generate boninites from shallow, refractory,hydrous upper mantle beneath the forearc. Subsequent partialmelting of the MORB-source diapirs yielded tholeiitic magmas,which ascended to erupt upon the very slightly older boninites,and eventually split the arc and open a backarc basin as magmasupply became more voluminous. In this forearc setting, thesource-to-eruption crustal thickness through which the earliesterupted tholeiites had to traverse was notably greater thanat typical spreading centres, and basaltic magmas were ableto differentiate through to ferrobasaltic compositions. Theboninite-tholeiite sections of the Heathcote and Mount WellingtonGreenstone Belts are therefore considered to represent a recordof the earliest stages of magrnatism accompanying ridge subductionand splitting of an arc to form a backarc basin. |
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