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摘要
Igneous thermobarometry may contribute to locate the upper-crust heat source/s representing one of the main factors required to establish the potential of high enthalpy geothermal areas through models of the heat flux. To test this possibility, we applied amphibole thermobarometry to some of the youngest lavas and pyroclastics (andesite to rhyolite) of Apacheta and La Torta (Chile) and Chachimbiro (Ecuador) areas representing medium to high enthalpy geothermal systems located in two different crustal contexts along the Andean active continental margins of South America. We used the robust application of Ridolfi and Renzulli (2012) allowing to estimate the intrinsic physico-chemical parameters with reasonably low uncertainties (T +/- 24 degrees C, P +/- 12%, fO(2) +/- 0.4 log units) and the composition of the melt in equilibrium with Mg-rich calcic amphibole in a wide range of conditions, up to 1130 degrees C and 2.2 GPa. In addition, analyses of the intra-crystalline compositional and textural variations allowed us to propose a method to distinguish between chemically homogeneous and heterogeneous amphiboles, avoiding some incorrect applications of the Ridolfi and Renzulli (2012)'s equations and thus unreliable results.
Thermobarometric constraints indicate a polybaric crystallization of magmas below the Chilean geothermal areas of Apacheta and La Torta. In the last 150 ka, the feeding systems of these areas mainly consisted of shallow silicic magma chambers at depths of 4-8 km (740-840 degrees C), giving rise to dacite domes (tortas). Amphiboles in the rhyolite lava flows, erupted earlier (Pliocene-Pleistocene) during the activity of the nearby Apacheta-Aguilucho Volcanic Complex, show a similar T-depth range (780-810 degrees C, 4-6 km). At deeper levels (10-20 km), the depths estimated using amphiboles from the magmatic enclaves (in the dacite domes) and from the andesite lavas are approximately aligned along the upper morphologic margins of the well geophysically constrained Altiplano-Puna Magma Body (APMB; a partially melted zone in the upper crust where the S waves velocities are <2.9 km/s), with estimated temperatures of 850-1010 degrees C. Heat source contributions for the investigated Chilean geothermal areas should therefore be represented by both the shallower magma bodies and the huge APMB.
The Holocene products of the Ecuadorian Chachimbiro geothermal area indicate amphibole crystallization from 7 to 17 km depth at temperature conditions in the range of 800-910 degrees C. The occurrence of two separated regions of magma storage at approximately 230 +/- 30 MPa and 380 +/- 60 MPa is discussed. They should be taken into account as heat sources for this Ecuadorian geothermal area.
- 出版日期2018-6-1
