A Volcano-Plutonic Association in Northeastern New South Wales

Abstract

In the Emmaville-Tenterfield region of northeastern New South Wales, Late Permian comagmatic volcanic and high-level intrusive rocks are spatially associated with large cauldron structures. Major economic concentrations of cassiterite, molybdenite and wolframite are associated with specific leucogranitoid plutons which were intruded to subvolcanic levels in regions between discrete cauldrons. Chemical, mineralogical and field data, used to relate the associated calc-alkaline volcanic and intrusi ve rocks and the various types of mineralization, have led to the recognition of a volcano-plutonic association displaying features common to volcanoplutonic associations developed in evolved geosynclinal environments in many other parts of the world. Igneous activity in the Emmaville-Tenterfield region commenced with the accumulation of large volumes of leucoadamellite and adamellite magma generated by minimum and non-minimum progressi ve partial melting of lower crustal rocks of presumed adamellitic composition. Intrusion of these magmas to high levels within Early Permian silicified siltstones and sandstones induced doming and fracturing of the brittle sedimentary rocks above specific magma chambers, leading to the upward venting of magmas along arcuate fractures and eventual eruption in the form of pyroclastic volcanism. The Emmaville Volcanics so formed, consist of a large number of individual ash flows, minor lava flows and rare ash-fall units which comprise a thick sequence of dominantly rhyolitic volcanics ranging in composition from dacite to high-silica rhyolite. Rare andesites and associated mafic rhyodacites are also present and constitute a geochemically distinct group apparently unrelated to the bulk of the Emmaville Volcanics. The evacuation of large volumes of magma from the high-level chambers caused subsidence of the overlying sedimentary and fresh volcanic rocks, forming simple cauldrons of the Glencoe type.