科研之友
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摘要
The Valles Caldera complex in north-central New Mexico, USA, represents the type example of a resurgent caldera system, characterized by eruption of two voluminous high-silica rhyolite ignimbrites, the Otowi and Tshirege Members of the Bandelier Tuff. Central resurgence of similar to 1000 m occurred within 27 +/- 27 kyr following eruption of the Tshirege, or Upper Bandelier Tuff (UBT). This process was accompanied by small-volume eruptions of the Deer Canyon Rhyolite, followed closely by the Redondo Creek Rhyodacite. The Cerro del Medio Rhyolite lava dome complex is a product of ring fracture volcanism following resurgence. We have combined cathodoluminescence (CL) imaging and titanium-in-quartz geothermometry techniques for single quartz crystals from (1) different stratigraphic horizons of the UBT ignimbrite, (2) samples of the Deer Canyon Rhyolite and (3) the Cerro del Medio Rhyolite lavas to build an evolutionary model of the Valles Caldera lifecycle. CL imaging reveals that similar to 70% by volume of the UBT ignimbrite contains unzoned quartz crystals (average concentration 28 +/- 2 ppm Ti), recording relatively stable conditions. The abrupt appearance of compositionally zoned quartz crystals within the mid-to-late erupted UBT ignimbrite units 3-5 reveals evidence for interaction with hotter magma. Corresponding titanium-in-quartz measurements of outer, high-intensity CL rims (average of 71 +/- 9 ppm Ti) reveal temperature increases of between similar to 35 and 140 degrees C relative to the initial, volumetrically large Upper Member eruptions (flow units 1 and 2). These temperature increases mimic those originally reported in the literature. Compared with the temperature increases, changes in pressure appear to play a secondary role. We have also discovered an interesting heterogeneity within the Deer Canyon Rhyolite lavas, with strong spatial control on the eruption of porphyritic lavas containing complexly zoned quartz crystals on the western parts of the resurgent dome. Conversely, crystal-poor to aphyric lavas containing small, unzoned quartz crystals are confined to eastern parts of the resurgent dome. The Cerro del Medio Rhyolite lavas are sparsely porphyritic to aphyric, and contain unzoned quartz with titanium concentrations more than 40 ppm higher than the cores of UBT quartz. The quartz-free Redondo Creek Rhyodacite is the most primitive silicic material erupted during the Valles Caldera cycle. Intrusion of this hotter magma into a residual UBT crystal mush zone may have facilitated eruption and geochemical-thermal heterogeneity within the Deer Canyon Rhyolite, resurgence of the caldera, and expulsion of hot, crystal-poor rhyolite batches from the mush zone, which were subsequently erupted as the Cerro del Medio complex.
- 出版日期2013-4
