UPPER CRUSTAL ASSIMILATION-FRACTIONAL CRYSTALLIZATION IN THE NEOGENE CENTRAL ANDES - A MODEL FOR CONTINENTAL SILICIC VOLCANO-PLUTONIC PROVINCES (Invited Presentation)

Extending from the base of the crust to pre-eruptive levels in the upper crust, transcrustal magmatic systems are the foundries and foundations of the continental crust. Here progressive thermal and chemical evolution produces the batholiths and their complementary caldera-related magma systems in the upper 9 to 5 km. Rather than being merely a zone of stagnation, accumulation, eventual eruption, or viscous death of magmas whose character has been set at deeper levels, the evidence from the Central Andes is for the shallow pre-eruptive levels to be where adolescent magmas mature to their final chemical and rheological character. Typically erupting as crystal-rich dacites, derivative crystal-poor rhyolites and their plutonic equivalents, these calc-alkaline, metaluminous and peraluminous magmas have been forged by upper crustal assimilation fractional crystallization.

Evidence for upper crustal evolution includes: Quartz-hosted melt inclusion with H₂O and CO₂ contents (2.1 to 6.0 wt %, 36 to 630 ppm, respectively) indicating quartz crystallization took place at pressures of ~200 -100 MPa; δ¹⁸O from quartz and zircon crystals from + 8.1 to + 9.6‰ and +6.7 to +7.8‰ respectively; carryover of zircon antecrysts from previous magmatic episodes in long-lived co-axial, volcanic centers; and xenocrysts (zircon) of upper crustal assimilant lithologies.

Whole-rock isotopic ratios of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and Pb-isotopes, coupled with δ¹⁸O are best reconciled with a two- stage assimilation–fractional crystallization (AFC) model. Stage 1 initiates with parental melts from the 30 to 15km deep Altiplano–Puna Magma Body (APMB) fractionating and assimilating crustal lithologies in the upper crust (10–25 km depth) to generate the andesitic magma that represent the parental magmas. These magmas subsequently accumulate and undergo a second stage of AFC in the uppermost crust (~800–850°C and 5–9 km depth) to produce the dacitic to rhyolitic compositions erupted as ignimbrites. The parental magmas are represented by ubiquitous andesitic compositions in most ignimbrites and associated lavas.