Geochemistry of the Jemba Rhyolite, northeastern Victoria
Titel:
Geochemistry of the Jemba Rhyolite, northeastern Victoria
Auteur:
Oates, C.J. Price, R.C.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 30 (1983) nr. 1-2 pagina's 41-57
Jaar:
1983-07
Inhoud:
The Jemba Rhyolite, of Upper Silurian age, outcrops south of Walwa in the Upper Murray Valley, and is temporally and spatially associated with leucogranitoids of similar chemistry and with an extensive dyke swarm. The dyke swarm is litho-logically bimodal, rhyolite and basalt being the dominant lithologies. The basalts are transitional alkalic in chemistry. The Jemba Rhyolite shows systematic vertical variation in geochemistry and mineralogy. Earlier eruptive activity resulted in the emplacemnt of rhyolitic flows, air-fall tuffs, and thin ash-flow tuffs, but the bulk of the outcrop represents a single eruptive event that deposited a single, vertically-zoned, ash-flow sheet. The vertical zonation resulted from eruption inversion of a magma chamber, in which a small degree of gravity controlled crystal fractionation had occurred. The rhyolites and leucogranitoids of the Upper Murray Valley are dissimilar to most of the granitoids of SE Australia, and have high SiO2 and total alkalies; are exceptionally depleted in Mg, V, Cr, Sc, Ti, Ca, Sr, Zr, Etf and Ba; and are unusually enriched in Nb, HREE, Rb and Ga. The magmas they represent were relatively anhydrous and F-rich, and originated as partial melts from source-rocks with unusual chemistry. Subtle variations in trace-element chemistry and variations in strontium isotopic characteristics among the leucogranitoids and rhyolites reflect differences in the source material for each geochemically different suite. The source-rocks are believed to have been refractory residual materials, which remained in the crust after the granitoids of the Corryong Batholith were extracted as partial melts. The partial melting event, which produced the rhyolites and leucogranitoids, was associated with emplacement of alkalic basaltic magmas into the crust during a rifting event that post-dated the principal deformation and the major batholith-producing magmatism affecting the Wagga Zone in Lower Palaeozoic times.
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