Abstract

In the Tibchi Complex, an initial gas build-up in the ascending felsic magma was most probably accompanied by updoming of the overlying basement rocks. Paroxysmal volcanic eruptions followed and culminated in a central shield volcano whose conduit was eventually sealed. Magmatic activity was renewed, with consequent increase in magma pressure but the volcano's load effect hindered a new central updoming or reopening of the already welded volcanic neck. Instead there resulted a peripheral stress concentration around the volcano to form a master cone-fracture that was apparently complete as a ring-fracture and which in attitude was inward-dipping at depth, steepening towards the ground surface. Fluidization along the ring-fracture brought about surface cauldron subsidence of the early volcanic edifice, and intra-caldera extrusion of crystal-rich ignimbrites. Fluidization waned as the magma became depleted in gas; the feeders to the crystal-rich ignimbrites were frozen in the ring-fracture as quartz porphyries. Gas-poor magma corresponding to the present granite porphyry ring-dyke displaced the frozen quartz porphyries by ring-fracture stoping and marked the transition to the plutonic phase of intrusion.

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