| Abstract |
- The ~1 Myr history of the Purico-Chascon volcanic complex (PCVC) records significant changes in the production and storage of magmas in the crust. At ~1 Ma activity at the PCVC initiated with the eruption of a large 80-100 km³ crystal-rich dacite ignimbrite with restricted whole rock ⁸⁷Sr/⁸⁶Sr isotope ratios between 0.7085-0.7090. In-situ analyses of plagioclase from the Purico ignimbrite have ⁸⁷Sr/⁸⁶Sr=0.7087-0.7090. The dacite magma accumulated and evolved at relatively low temperatures around 800-850 °C in the upper crust at 4-8 km depth. Minor andesite and rhyolite pumice late in the sequence have similar restricted whole rock ⁸⁷Sr/⁸⁶Sr=0.7089-0.7091. The radiogenic isotopes of this 0.98 Ma activity are consistent with all these compositions resulting from 50 to 70% crustal assimilation by parental Central Andean "baseline" magmas at depths between 15-30 km.
The final eruptions at the PCVC occurred <0.18 Ma producing three small < 5 km³ crystal-rich dacite lava domes with whole rock ⁸⁷Sr/⁸⁶Sr ratios 0.7075 to 0.7081 containing abundant basaltic-andesite enclaves with whole rock ⁸⁷Sr/⁸⁶Sr ratios of 0.7057- 0.7061. Plagioclase and amphibole from samples from the largest of these domes, Cerro Chascon, record two distinct magmatic environments; an upper crustal environment identical to the Purico ignimbrite and a second deeper, ~15-20 km depth, higher temperature (~922-1001 °C) environment consistent with conditions recorded in the basaltic andesite enclaves. Accordingly, plagioclase cores in the host dacite lava and enclaves have enriched in-situ ⁸⁷Sr/⁸⁶Sr isotopic compositions of 0.7083 to 0.7095 while plagioclase rims and microphenocrysts in the enclaves have ⁸⁷Sr/⁸⁶Sr isotope ratios from 0.7057 to 0.7065 and 0.7062 to 0.7064 respectively. Lavas from Cerro Chascon also contain abundant Fo82 olivine with spinel and basaltic melt inclusions that crystallized in a deep crustal environment (>1250 °C) consistent with a lower crustal MASH zone. The high baseline isotopic ratios observed in bulk rock and plagioclase crystals from Cerro Chascon (0.7057-0.7065) are consistent with MASH processes.
The evolution of the PCVC is a microcosm of the Andean arc in this region where, from 10 - 1 Ma, dominantly dacitic upper crustal magmatism of the Altiplano-Puna Volcanic Complex ignimbrite flare-up persisted until ~1 Ma, when smaller volume, more heterogeneous and less isotopically enriched basaltic andesite to dacite composite volcanoes signal a return to steady state arc volcanism. I suggest that the PCVC captures the transition of the Andean arc from flare-up to steady state. The temporal trend at the PCVC is consistent with a waning thermal flux. High magmatic fluxes during the flare-up would have resulted in elevated geothermal gradients and efficient crustal processing leading to a dominantly dacitic upper crust (0 to 35 km) that fed the large volume Purico ignimbrite. As magmatic flux and thermal energy wanes, crustal isotherms relax resulting in greater thermal contrast between parental magmas, crust and remnant upper crustal dacite magma. This manifests in more heterogeneity and the survival of less isotopically enriched magmas in the upper crust. These arc scale magma dynamics are recorded even at the intra-crystalline scale.
Individual crystals from Cerro Chascon also record vital information on the crystallization and evolution of mantle-derived magmas in continental magmatic arcs. Fo₈₂ olivine, olivine hosted spinel, and basaltic melt inclusions record the crystallization of olivine at >1250 °C in conditions consistent with a lower crustal (~70 km depth) MASH zone. Another significant crystallization event appears to have occurred at ~20 km depth, characterized by the crystallization of high An plagioclase (An₇₂₋₈₄) at ~1100-1050 °C followed by high-Al amphibole (~12-15 wt.% Al₂O₃) at ~1000-950 °C. The appearance of amphibole on the liquidus appears to have resulted from a nearly 2-fold increase in melt water content following ~45% crystallization of high An plagioclase. Following this extensive crystallization the highly crystalline mafic magma ascended into the upper crust and interacted with the remnant crystal mush from the Purico ignimbrite magma reservoir. Low An plagioclase (An₃₉₋₅₅), low Al amphibole (~6-9 wt.% Al₂O₃), sanidine, and biotite retain the chemical composition of the Purico ignimbrite magma, whereas, olivine, high An plagioclase, and high Al amphibole record the mafic recharge magma. The textures and compositions observed in Cerro Chascon are common in both continental and oceanic magmatic arcs worldwide and I propose that multiple crystallization events and upper crustal assimilation are fundamental processes intrinsic to arc magmatism.
I have also used in situ ⁸⁷Sr/⁸⁶Sr isotope ratios in plagioclase from andesite, dacite, and rhyolite pumice from the ~1 Ma Purico ignimbrite to determine the cause for compositional zoning in the Purico ignimbrite magma reservoir. Andesite pumice contains two texturally, compositionally, and isotopically distinct types of plagioclase, small (<500 μm) subhedral to euhedral crystals with high MgO (130-490 ppm) and low ⁸⁷Sr/⁸⁶r crystals (0.7076-0.7084) record a hot (>900 °C) andesite magma derived from an ~20 km deep magma reservoir. In contrast, the second type of plagioclase in the andesite appear to broken fragments of larger crystals and have significantly lower MgO (90-240 ppm), higher ⁸⁷Sr/⁸⁶Sr (0.7096-0.7114), and appears to be derived from the lower temperature (crystallized at ~800-900 °C), upper crustal (<10 km) plutonic basement. Dacite pumice also contains two texturally and compositionally distinct types of plagioclase. However, both types have very restricted MgO (b.d.l.-200 ppm) and ⁸⁷Sr/⁸⁶Sr (0.7085-0.7095) ratios and appear to have grown at ~850°C. These crystals are also significantly larger (>1000 μm) than plagioclase from the andesite pumice and have clear euhedral rims. Rhyolite pumice from the Purico ignimbrite also contains distinct types of plagioclase. Both types of plagioclase are similar in size (<500 μm) and appear to be fragments of larger crystals. One type is characterized by low MgO (b.d.l.-240 ppm) and restricted ⁸⁷Sr/⁸⁶Sr isotope ratios (0.7088-0.7095) similar to plagioclase in the dacite pumice, and the other has significantly higher ⁸⁷Sr/⁸⁶Sr ratios (0.7095-0.7103) consistent with the upper crustal ignimbrite basement. The compositional variations observed in plagioclase crystals from the Purico ignimbrites are consistent with the recharge of a previously emplaced upper crustal (4-8 km depth) dacite magma reservoir by a hotter, deeper (20 km deep) andesite. During ascent, the andesite incorporated crystals from the surrounding upper crustal plutonic bodies before pooling against the residence dacite magma and crystallizing. Crystallization of the andesite resulted in the expulsion of a rhyolite interstitial melt that ascended through the dacite reservoir and pooled at the top of the reservoir. The rhyolite melt incorporated crystals from the dacite magma during ascent as well as crystals from the roof rock, which in the case of the Purico ignimbrite represents the plutonic remnants from other large silicic magmatic systems associated with the APVC. Thus, the compositional variations observed in the Purico ignimbrite results from a combination of crustal assimilation, crystallization, and melt extraction all initiated by mafic recharge.
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