Complex and dynamic magmatic processes captured in plagioclase and their implications for petrogenesis of Taupo Volcanic Zone rhyolites
Abstract
The Okataina Volcanic Centre in the Taupo Volcanic Zone (TVZ) is a frequently-active rhyolitic caldera and intra-caldera magmatic system that is understood to be driven by basaltic recharge. To understand how mantle and crustal inputs into magma genesis have varied temporally and spatially, we investigated Sr- and Pb- isotopic ratios in plagioclase in conjunction with mineral textures and geochemistry from ten rhyolitic deposits. Units include the oldest caldera event (Utu, 0.56 Ma), three units from a dome-building episode (Haparangi, Kakapiko, Round Hill, ~200 ka), the youngest caldera event (Rotoiti, 45 ka), a syn-caldera eruption (Earthquake Flat, 45 ka), and four recent eruptions from the two intra-caldera dome complexes (Okareka, 21.9 ka, Rotoma, 9.5 ka, Whakatane, 5.6 ka, and Kaharoa, 0.7 ka). Texturally, crystals across the deposits consist of four normally-zoned groups, two reversely-zoned groups, and one group that exhibits both types of zoning. Compositionally, relative to increasing Anorthite contents (An19-77), analyses from all units reveal an increase in FeO (0.14-0.47 wt%), Mg (20-370 ppm), Ti (40-220 ppm), and Sr (370-1250 ppm), decrease in Ba (90-1300 ppm), and no relationship to Rb (<4 ppm) and light rare Earth element concentrations. Isotopically, crystals were analysed via two methods: (1) whole-crystal analyses (single- or multi-crystal Pb and Sr), which revealed subtle differences between caldera events, older domes, and recent eruptions; and (2) in-situ spot analyses of cores and rims, which showed a wide range of 87Sr/86Sr ratios (0.7042-0.7065), but with significant single-crystal homogeneity, and the same 87Sr/86Sr range regardless of unit or textural group. However, eight crystals (out of 117 analysed) exhibit core versus rim heterogeneity, where higher-An zones are more radiogenic. These crystals are only found in pre-45 ka units or younger units from the southern intra-caldera complex. The heterogeneity of An contents and isotopic ratios indicate different sources, and the higher-An yet more radiogenic compositional zones suggest high crustal input in the associated melts. Data from this study support previously presented hypotheses that TVZ rhyolites have a complex origin with significant crustal contribution and multiple recharge events on top of fractionation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V51F0128S
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3651 Thermobarometry;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY