The geochemistry, stratigraphy and volcanology of the Cerro Galán caldera system, NW Argentina

The Cerro Galán caldera, located in northwestern Argentina in the Central Andean Volcanic Zone of the Andean continental margin volcanic arc of South America, is often cited as an example of one of the largest caldera-forming “super eruptions” on Earth. No super-eruptions have been witnessed in human history; little is known concerning their frequency, eruption mechanisms and emplacement processes of erupted material. This thesis uses new geochemical and petrological techniques combined with mapping and stratigraphy to shed new light into these relatively unknown features of Cerro Galán and other supervolcanic eruptions. The Cerro Galán volcanic system has erupted at least nine crystal-rich ignimbrites from ~6 to 2 Ma. Starting with the earliest eruption, these consist of the Blanco Ignimbrite (date unknown; volume ~10 km3), the Merihuaca Formation Ignimbrites, consisting of the Lower, Middle and Upper Members (~5.1 to 4.9 Ma; total volume ~65 km3), the Pitas Ignimbrite (~4.8 Ma; 190 km3), the Real Grande Ignimbrite (~4.7 Ma; 390 km3) and the Vega Ignimbrite (~4.5 Ma; volume unknown). Together, these ignimbrites are known as the Toconquis Group and were exclusively deposited from the south-west to the north of the modern caldera. There was then a hiatus in ignimbrite eruptions until the Cueva Negra Ignimbrite (~3.8 Ma; 50 km3) was deposited to the east of the caldera, followed by the climactic eruption of the Cerro Galán Ignimbrite (CGI; ~2.1 Ma; 630 km3) which is found radially around the caldera. The eruption of the CGI facilitated a caldera collapse event, producing the modern volcano-tectonic depression preserved today. Collapse occurred via a piecemeal trapdoor process with the hinge located along the eastern margins of the caldera. Post-CGI resurgence uplifted the central portion of intracaldera fill to produce a resurgent dome; this period also witnessed small-volume pyroclastic surges and ignimbrite eruptions (<2 Ma) with associated domes and block-and-ash flows close to the structural margins of the caldera. The climactic CGI is found 40 km in all directions from the structural margins, with a maximum distal extent of 79 km (to the north). Based on detailed mapping and field observations, the currently-preserved areal extent of the CGI is estiamted to be ~2400 km2. Magma volumes increase with subsequent eruptions in the Galán volcanic system, perhaps reflecting the thermomechanical evolution of the upper crust in response to increased volumes of magma ‘priming’ the crust for formation of large batholith-sized magma chambers. By applying a number of analytical techniques across the spectrum of spatial scales in juvenile components, this thesis has shown that the Cerro Galán volcanic system has repeatedly erupted magmas with nearly identical geochemistries over an almost 4 Myr time period. All ignimbrites are calc-alkaline, high-K rhyodacitic in composition (68-71 wt. % SiO2). The mineralogy is broadly constant throughout the sequence with plagioclase, quartz, biotite, Fe-Ti oxides, apatite and titanite. Early ignimbrite magmas also contained amphibole, while the CGI erupted a magma that contained rare amphibole, but significant sanidine. Each major ignimbrite contains two main juvenile clast types; dominant ‘white’ pumice and ubiquitous but subordinate ‘grey’ pumice. The ignimbrites also contain rare dense, crystal-rich juvenile clasts. Fe-Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry reveal the grey pumice originated from potentially hotter and deeper magmas (800-840°C, 3-5 kbar) than the more voluminous white pumice (770-810°C, 1.5-2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galán system emplaced into the upper crust from a deeper storage zone. Crystal-rich juvenile clasts are thought to represent crystal cumulate or developing crystal-mush portions of the magma chambers. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 volume % on a vesicle-free basis. This is supported by geochemical modelling showing subtle bulk-rock variations in Ta, Y, Nb, Dy and Yb between the Galán Ignimbrites can be reconciled with small differences in amounts of crystal fractionation from the ‘grey’ parental magma. The amount of fractionation is inversely correlated with volume; the largest volume CGI and Real Grande Ignimbrites return higher F values (amount of liquid remaining) than the older, smaller volume Toconquis Ignimbrites, implying less crystal fractionation took place during their upper-crustal evolution. This relationship is attributed to variations in magma chamber geometry; the largest volume ignimbrites came from flat slab-like magma chambers, reducing the relative proportion of chamber margin crystallisation and fractionation compared to the smaller volume ignimbrite eruptions. The grey pumices are physical evidence of silicic recharge throughout the history of the Cerro Galán system, and recharge days prior to eruption has previously been suggested based on reversely-zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin anorthite (An)-rich rims in many juvenile clasts in the older ignimbrites support the importance of recharge in the evolution and potential triggering of eruptions. At Cerro Galán, upper crustal magma chambers serve only to modify the geochemistry of the magmas. A geochemical ‘buffer’ zone is hypothesised to exist between mantle magma input and magma evolution in upper crustal chambers, which imparts the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (Melting, Assimilation, Storage and Homogenisation) zone is proposed to act as this buffer zone. New oxygen isotope values generated from quartz crystals in ignimbrites from the Central Andes yield high δ18O(Quartz) values (+8.1 to +9.6 ‰) with little variation throughout the lifetime of individual volcanic systems. Rhyodacitic ignimbrites from the Cerro Galán system yield the highest δ18O(Quartz) values (+8.8 to +9.6 ‰). The most primitive basaltic andesite lavas from the Galán system possess elevated δ18O(bulk rock) values (+8.2 to +8.5 ‰). Geochemical modelling shows that the elevated δ18O common to all magmas produced in the Galán system since ~6 Ma can be produced by progressively contaminating mantle magmas with crustal melts. The δ18O and 87Sr/86Sr values of the Galán Ignimbrites require ~40% assimilation of crustal material. Geochemical signatures of ignimbrites from the Cerro Galán magmatic system are compared to those in the nearby Altiplano-Puna Volcanic Complex (APVC). These two regions have erupted large-volume crystal-rich ignimbrites with very similar stable and radiogenic isotopic compositions, indicating common magma generation and evolution processes underpinning these two regions. Ignimbrites from this Andean Central Volcanic Zone (CVZ) are then compared to large-volume ignimbrites from the western US. Ignimbrites from the western US yield lower δ18O(Quartz) values (+6.4 to +8.4 ‰). These differences are controlled by a combination of factors including; 1) the thicker and more thermomechanically mature crust in the Central Andes, favouring increased crustal melting and assimilation, 2) the high crystallinities and long residence periods of these CVZ ignimbrites in the upper crust, and 3) the higher elevation, aridity and evaporation rates in the CVZ region compared to the western US. 238U/206Pb ages obtained from SIMS (Secondary Ion Mass Spectrometry) analyses of zircon crystals in ignimbrites from the Cerro Galán system show evidence for complex magma dynamics and crystallisation histories. The dominant zircon population in each ignimbrite consists of phenocrysts that are shown to have crystallised up to 500 kyr prior to eruption, supporting the long residence times of crystal ‘mushes’ in the upper crust that give rise to crystal-rich ignimbrites such as those observed at Cerro Galán. Many ignimbrites also contain zircon antecrysts that yield ages corresponding to their growth from the preceding ignimbrite magma in the Galán stratigraphy. These crystals provide evidence for the recycling of non-erupted material from older ignimbrite eruptions. Finally, rare zircon xenocrysts (~540 to 500 Ma) with ages predating the initiation of the modern Galán system are also observed in the ignimbrites. These are inferred to be derived from local basement outcrops comprising the regional upper crust into which the Galán magmas were repeatedly intruded and stored. These zircon crystals in the Galán Ignimbrites help to explain many features common to other large ‘monotonous’ ignimbrites including their high crystallinities, lack of geochemical variations and isotopic signatures including high O, Sr and Pb isotopic ratios and low Nd isotopic ratios. This study has redefined the eruptive stratigraphy of the Cerro Galán ignimbrites along with producing a new geologic map of the region and better constrained, more realistic volume estimates. The factors that act to reduce the original volumes of erupted material to that which is preserved today are discussed in more quantitative ways than are often applied to these large volume systems. It has also extended the notion that large volumes of nearly identical silicic magmas can be repeatedly generated, producing continued geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. The magmas produced in the Galán system share a general chemical familiarity with other large-volume magmas produced in the Central Andean region, suggesting the same large-scale regional magmatic system was operating beneath the Central Andes from 10 to 2 Ma.