Peperite in Oligocene Lower Diatremes of Chubut, Argentina

MARTIN U; NEMETH K; HALLER MJ; ALRIC VI

Potsdam

Congreso; 19th Colloquium on Latin American Geosciences; 2005

In the centre of the Patagonian mafic Cenozoic plateau lava fields newly discovered diatremes stand about 100m above the surrounding plane exposing the lower diatremes of former phreatomagmatic volcanoes and their feeding dyke systems.  This remotely located erosion remnants cut through Paleozoic granitoids and Jurassic and Cretaceous siliciclastic continental successions that are generally easily erodable.  Plateau lava fields are generally located a few hundred meters above the top level of the present tops of the preserved diatremes suggesting a complex erosional history and potential interrelationships between the newly identified diatremes and the surrounding lava fields. Uprising magma from the underlying feeder dyke into the diatreme root zone intruded the clastic debriss in the diatremes, inflated them and mingled with the debris to form subterranean peperite. Lack and/or changing position of further explosions across the diatreme pipe, but continued rise and intrusion of magma caused emplacement of such a peperite masses as plugs. Dyke margins are chilled and well-developed forming up to a few centimeters ovoide bulbous, irregular black glassy zones around the otherwise reddish brown aphanitic intrusions.  Toward the feeder dyke centres more pronounced horizontally oriented jointing pattern and a general massive texture is characteristic.  In the marginal zone of the feeding dykes, dyke wall parallel elongated millimeter size vesicles just bellow the glassy rims are common.  Master feeding dykes are commonly traceable tens of kilometers in the desert floor forming a few hundreds o meters of irregular, curving dyke sections.  Individual sections form a array of “en-echelon_-like distribution where diatremes are seemingly located in major steps between dyke segments pointing toward the possible relationship between diatreme formation, and structural as well rheological control of syn-eruptive hydrogeology of the host rock units.  Close to the feeder dyke there are zones of mixed basalt/sediment mingling in the liquid state-  The intimate mingling took place between basalt lava and fluidized sedimentary material.  Such mingling can be recognized from millimeter to meter scale.  The recognition of the mingling of host siliciclastic as well of the diatreme filling volcanoclastic sediments indicates the host units water saturated and loose state.  The significance of the recognition of diatremes in Patagonia are twofold, 1) indicating that in syn-eruptive paleoenvironment water was available in various “soft sediment”, commonly porous media aquifer sources and 2) the identifierd and abundant diatremes that form diatreme fields are good candidates for sources of extensive lava fields, especially considering that phreatomagmatism may facilitate magma rise with effective opening of fissures before major lava effusions.