Genesis of magma for acid calc-alkaline volcano-plutonic formations

Abstract

Abstract In the Upper Palaeozoic igneous province of the Georgetown inlier, north Queensland, high-level adamellite plutons, extensive rhyodacite ash flows in large cauldrons, and ring complexes containing both extrusive and intrusive phases are developed with equal importance. The igneous rocks were intruded in a semi-stable block adjacent to the Tasman geosyncline during its late-orogenic to post-orogenic stage of development, and the tectonic setting of the province is typical of most large volcano-plutonic formations. The magma for volcano-plutonic formations appears to have been generated by fractional melting (partial fusion) of basic rocks in the lower crust due to the increase in heat flow from the mantle during the orogenic phase of geosynclinal development. In the tectonically unstable zone near the geosyncline, where temperatures were higher, calc-alkaline trachyandesite, andesite, and basalt magmas formed, but away from the geosyncline, beneath the adjacent semi-stable continental block, the lower crust would be less affected by the orogenic forces. Here, pockets of acid magma formed by fractional melting would coalesce and accumulate to form a widespread magma chamber in the lower crust, doming the crust above. The field evidence in the Georgetown inlier indicates that the acid magma was rhyodacitic and rich in volatiles, and that pockets of magma formed up to 200 miles away from the geosyncline. The relaxation of compressional forces accompanying the waning of the orogenic phase, and the doming effect of the magma generated, placed the semi-stable block in a state of tension. During the post-orogenic phase, major basement faults in the block were reactivated; they extended downwards into the lower crust, and finally tapped the acid magma chamber. Magma rose along the faults and was extruded as viscous flows. This caused a drop in pressure in the magma chamber and allowed volatiles to concentrate near the top of the chamber. The volatile-rich magma was erupted mainly as huge ash flows. The cauldron subsidence areas were initiated by the subsidence of the crust above areas where the magma was depleted most. The cauldrons in the Georgetown inlier range from a few miles to 70 miles long, and contain about 2,000 ft. of ash-flow deposits. Before the gas-rich eruptions, ring complexes were initiated in the major zones of weakness within the semi-stable block, and small high-level magma chambers formed in the upper crust by underground cauldron subsidence. Magma differentiated in these chambers was erupted at the surface through a series of ring fractures now occupied by large ring dykes. As the cauldrons developed the excess of volatiles in the lower crustal magma chamber escaped in eruptions through the peripheral fracture zones containing the ring structures, and this initiated the caldera phase of ring complex development: thus the volatile content of magma in the lower crust was maintained at a level suitable for ash-flow eruptions in the cauldrons. Later, during the underground cauldron subsidence phase, magma from the lower crust was intruded as ring dykes and stocks in the ring complexes. The movement of crustal blocks during ash-flow eruptions in the cauldrons opened additional fractures and allowed more magma to rise into the crust. The adamellite emplaced in the Georgetown inlier has a uniform composition; so it was only slightly, if at all, contaminated with basic rocks in the lower crust. Magma rose along the sub-volcanic conduits until it reached a high level in the crust, where it intruded by underground cauldron (block) subsidence. Once this was accomplished, blocks higher in the crust subsided into the high-level magma chambers and the magma rose rapidly to occupy the space of the foundered blocks with very little assimilation. At various stages volatile-rich magma was erupted as ash flows at the surface. Finally, the still-liquid magma filled the spaces remaining after crustal blocks below the base of the ash-flow deposits had subsided. Here, volatiles were lost through small fissures in the roof and the magma crystallized, generally with an even, medium grain size, under an insulating mantle of ash flows about 2,000 ft. thick.