PLUTONS, PRESSURE, AND PORPHYRY SYSTEMS—GEOBAROMETRY AS A TOOL FOR ECONOMIC GEOLOGY

In the 1980s, E-an Zen and I published on Al in hornblende and the presence of magmatic epidote as indicators of pluton emplacement depths for intermediate igneous rocks (TTG, tonalite-trondhjemite-granodiorite). E-an went on to note that the exhumation of magmatic epidote-bearing plutons in mobile belts in the North American cordillera implies anomalously thick crust at the time of formation. Since then, many experimental and field studies investigated and refined the pressure-dependencies of these two minerals in magmatic rocks, as summarized by Schmidt and Poli in the 2004 Reviews in Mineralogy and Geochemistry volume on epidotes. These studies show that textural criteria alone cannot unambiguously distinguish magmatic epidote and that oxygen fugacity, in particular, affects pressure interpretations. Epidote in H₂O saturated tonalitic magmas at high f_O2 (HM) is stable at pressures as low as 0.3 GPa in contrast to the 0.5 GPa minimum pressures observed at lower f_O2 (NNO ). The most commonly reported examples of magmatic epidote are in TTG plutons emplaced at intermediate to deep crustal levels in mobile belts in post-collisional tectonic settings. Epidote-free plutons of these same general compositions emplaced at shallow crustal levels (~0.1 Gpa) in arc and post-collisional settings commonly are associated with porphyry copper systems. Epidote-bearing calcic to calc-alkaline TTG plutons share some geochemical characteristics of adakite-like calc-alkalic porphyry-related plutons (oxidized, water-rich, low initial Sr ratios). Some mobile belts (the Urals, Gangdese Belt of southern Tibet) host both deeply emplaced epidote-bearing plutons (barren) and younger, epidote-free shallowly emplaced (porphyry copper-associated) plutons now exposed at the earth’s surface. This assemblage may be indicative of a post-convergent setting of exhumed older arc and second-stage, porphyry-related magmas. Identification of magmatic epidote and hornblende geobarometry of plutons can distinguish emplacement pressure regimes amenable to preservation of porphyry systems from those that are too deep. Similarly, hornblende geobarometry can contribute to identification of tilted and dismembered large porphyry copper systems.