|
|||||
|
|
Petrology and Isotope Geochemistry of Tertiary Lavas from the Northern Flank of the Tweed Volcano, Southeastern Queensland |
|
| Authors: | EWART A; OVERSBY V M; MATEEN A |
| Institution: | 1Department of Geology and Mineralogy, University of Queensland Brisbane, Queensland 4067, Australia 2 School of Earth Sciences, Australian National University Canberra, A.C.T. 2600, Australia 3 Department of Geology and Mineralogy, University of Queensland Brisbane, Queensland 4067, Australia Permanent address: Pakistan Atomic Energy Commission, Lahore, Pakistan |
| Abstract: | The generalized stratigraphic sequence (20–21.8 m.y.)of the northern flank of the Tweed Volcano is: Beechmont Basalt(base)—Rhyolite (composed of two distinct units, the Springbrookand Binna Burra rhyolites)—Hobwee Basalt. In addition,comendite occurs as a postrhyolite intrusive phase. Chemicallyand mineralogically, the ‘basalts’ are tholeiiticandesites, which are conveniently divided into olivine-normativeand quartz-normative types. Phenocryst mineralogy is olivineand labradorite (microphenocrystic) in the olivine-normativelavas, and plagioclase plus rare augite in the quartz-normativelavas. Rhyolites (which constitute some 7 vol. per cent of the TweedShield volume) are of the potassic two-feldspar type; theseare characterized by highly fractionated trace element patterns,which are most extreme in the Binna Burra rhyolites. The latter,for example, have low K/Rb (<100) and La/Yb, highly depletedEu, Ba, Sr, V, Ni, Cr, and variably enriched Rb, U, Th, Pb,Nb, and Zn. Phenocryst phases are: quartz, oligoclase, sanidine,ilmenite, ferrohypersthene (Springbrook rhyolite), and quartz,sanidine, oligoclase, ilmenite, rare Fe-rich fluor-biotite,and very rare resorbed grains of extremely ulvöspinel-richtitanomagnetite (Binna Burra rhyolite). Phenocryst equilibrationtemperatures are estimated to be in the range  900–1050°C for the Springbrook rhyolite and 800–950 °Cfor the Binna Burra rhyolite, at oxygen fugacities in proximityto the WM buffer. The comendites are characterised by sanidine,quartz, fluor-arfvedsonite, minor acmite, and ilmenite. Pb isotopic compositions indicate at least two distinct groupsof mafic lavas; certain olivine-normative tholeiitic andesiteswith compositions less radiogenic than modern oceanridge basalts(possibly indicative of lower crustal contamination), and asecond more radiogenic group including the remaining isotopicallyanalysed tholeiitic andesites. Sr isotopes reveal small differencesbetween the Beechmont and Hobwee Groups. Pb and Sr isotopiccompositions of the three rhyolitic groups are distinct, andall more radiogenic than the mafic lavas. It is concluded that the tholeiitic andesites represent thefractionation products from an olivine-normative tholeiiticbasalt, and calculations suggest that olivine-plagioclase-aluminousclinopyroxene-Fe-Ti oxides were the likely fractionating mineralphases. The potassic rhyolites are interpreted also in termsof fractionation from a basaltic parent, although the geochemistryof the Binna Burra rhyolite has been further modified by continuedfractionation at the quartz-feldspar minimum. Chemical and mineralogicaldata, however, suggest some modification of the rhyolitic magmasby crustal equilibration (possibly lower crust). Although thecomendite is isotopically distinct from the exposed rhyolites,various least squares mixing calculations suggest that the comenditemay have developed by continued late stage quartz-feldspar ternaryminimum fractionation of rhyolitic magma. |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ | |