The Temporal Record of Ignimbrite Flare-ups - Filtered Signals From Deep

Large silicic volcanic fields (10,000's of km<2>; 1000s- 10,000's of km<3>) are constructed during ignimbrite "flareups": intense periods of volcanism dominated by eruptions of ignimbrites with volumes in excess of 1000 km3. Concordantly, it is being increasingly recognized that cordilleran batholiths are also constructed during these events. Given the strong spatial, geochemical and petrological, and geophysical kinship between silicic volcanic and plutonic rocks it is reasonable to believe that these ignimbrite flare-ups must be accompanied by significant (and more voluminous) plutonic activity at depth and that most long-lived and productive magmatic provinces generate co-genetic volcanic and plutonic rocks, with connections to deeper levels of the crust. Flare-ups thus account for a significant portion of what can be collectively termed "crustal magmatism" - mantle-powered but significantly crust-modified. While the resolution of the temporal record preserved in the plutonism maybe compromised by late stage processes during the waning of a magmatic system, the temporal record of the volcanism provides a higher resolution record of the crustal magmatism. This is explored through the temporal record of the Altiplano-Puna Volcanic Complex of the Central Andes. This record of the flare-up reveals three types of signals. The broadest and dominant signal is that of the flare-up itself, a transient pulse magmatism of the order of 10<7> years that punctuates a background or baseline level of activity. This represents the waxing and waning of the primary thermal "drive" which is the mantle power input. The second signal is system-wide episodicity that operates on a time scale roughly an order of magnitude faster. This is manifested by intense pulses of eruption that last from 10<5> to 10<6> years separated by periods of 2x10<6> years. This is the result of crustal filtering or modulation of the primary mantle signal. The third signal is recorded in individual volcanic systems. Close correspondence between the timing of activity at the three main long-lived caldera complexes, La Pacana, Guacha, and Pastos Grandes is evidenced by major eruptions at 5.5 ± 0.15 Ma, 3.4 ± 0.5 Ma and 1.5 ± 0.2 Ma from each of these. This connotes a systemic "drive" for volcanic activity in these three systems. However, the magmatic systems of each of these centers is unique and any connection must be at depth and is not recorded materially in the erupted products, only temporally. Broad concordance of time scales of operation of the individual caldera systems and system-wide episodicity suggests that the main control on the timing of magmatism and volcanism is crustal modulation. The geophysically imaged low-seismic velocity zone, the Altiplano-Puna Magma Body, at 17 to 20 km, is the most likely modulator where thermal and chemical signals from the mantle/lower crust MASH zone are buffered and eventually translated to pre-eruptive upper crustal magma chambers.