As the mid-Tertiary ignimbrite flare-up is a spatially- and temporally-massive event, I will focus on the San Juan volcanic field in southwestern Colorado as an example of a caldera typical in the flare-up events. By using techniques listed in the previous petrogenesis section, researchers are addressing the questions of identifying the parent magmas of the lavas and ignimbrites, in addition to determining the magmatic processes that modified the parent magmas.
Askren et al. (1997) conducted a petrologic study of the lavas and tuffs associated with the San Juan volcanic field in Colorado, and also the Central Nevada and Indian Peak volcanic fields in Nevada and Utah, respectively (Figure 1). These volcanic fields erupted rhyolitic and dacitic ash flow sheets and contemporaneous andesite lava flows between the Oligocene and Miocene.
Figure 1. Location map of volcanic fields (figure from Askren et al., 1997)
of the rocks from the three volcanic fields straddles the alkalic/sub-alkalic
boundary with the majority of the samples either an andesite or trachyandesite
Figure 2. Weight percent oxide plot of samples from three volcanic fields (figure from Askren et al., 1997).
Trace element analysis shows similar spiderdiagrams for the three volcanic fields. The Windows Butte tuff formation from the Central Nevada volcanic field and the Carpenter Ridge tuff from the San Juan volcanic field show similar spiderdiagram traces, although there are excursions of Sr and P (Figure 3). These spiderdiagrams show patterns similar to subduction-related lavas (decrease in Nb, Ta, Ti; high value of Ba/Ti).
Figure 3. Trace element spiderdiagram for three volcanic fields; four samples from San Juan (SJ), seven samples from Central Nevada (CN), 10 samples from Indian Peaks (IP). Weight percent oxide plot of samples from three volcanic fields (figure from Askren et al., 1997) The Windows Butte tuff formation is part of the Central Nevada volcanic field and the Carpenter Ridge tuff is part of the San Juan volcanic field. Shaded regions on diagram represent the envelope that contains all data for a particular field.
Implementing petrological numerical models to explain a decrease in weight percentages of oxides (i.e., CaO, MgO, Ti02, P2O5), Askren et al. (1997) determined that the parent magmas of the andesites were formed by partial melting of mantle material. The andesitic magmas were then modified by crystal fractionation and magma mixing in magma chambers.
Figure 4. Proposed caldera evolution at the San Juan volcanic field (figure from Riciputi et al., 1995). In lower portions of the figures, arrows in the square boxes indicate the isotopic evolution of ash flow tuffs, Nd indicates end, Sr represents 87Sr/86Sr, and Pb indicates 206Pb/204Pb. The value of an isotopic ratio increases upward, time evolves to the right.
Riciputi et al. (1995) use Sr, Nd, and Pb isotopes to constrain magma origin and movement (figure 4). end values range from about -10 to +0.3. They conclude that two stages of magma evolution are required before eruption. Fractional crystallization of mantle-derived basalts and assimilation of the lower crustal into melts are key processes. In the first stage (A), mantle -derived basalts interact with the lower crust and undergo fractional crystallization and assimilation. In the second stage (B), magma ascends into the upper crustal magma chamber where more assimilation and crystal fractionation occurs. After the caldera eruption (C), the upper and lower crust obtain an isotopic signature more closely related to the mantle due to mixing of magmas - this results in the production of crustal-mantle hybrid crust.
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