THOLEIITIC (TH) AND CALC-ALKALINE (CA) SERIES AT FRONTAL-ARC VOLCANOES OF THE 33.3-41.1°S PORTION OF THE ANDEAN SOUTHERN VOLCANIC ZONE (SVZ), CHILE: ASSESSING THE IMPACT OF CRUSTAL THICKNESS, PARENT MAGMA COMPOSITIONS, DIFFERENTIATION MECHANISMS, AND CONDITIONS OF MAGMA EVOLUTION

The genesis of divergent magmatic series that originate from subalkaline, basaltic parent magmas has been a preoccupation for igneous petrologists since Fenner and Bowen, neither of whom appreciated the central role that convergent-margin magmatism plays in the creation and evolution of the continental crust. Petrologic studies of volcanic centers within a portion of the Andean SVZ (Tupungato to Calbuco) have contributed data and ideas pertinent to the consequences of intra-crustal differentiation, mantle heterogeneity, and subduction erosion for continental-arc magmas. Crustal thickness, which nearly doubles from south to north (~30-55 km), and structural arc-segmentation (33.3-34.3°S - 34.8-36°S - 36.2-41.1°S) define the framework for examining contrasting magma evolution paths on regional and local scales. Mantle-source compositions do not vary regularly as a function of latitude. These issues are addressed in terms of factors related to CA versus TH differentiation with a compilation of ~2800 WR analyses (TH:CA ~40:60). Extreme TH differentiation requires nearly closed-system fractional crystallization of H₂O-poor and relatively reduced magmas at low pressures. Late Holocene to historic lavas from Lonquimay (38.4°S) define such a trend. Comparably extreme closed-system CA magmas (e.g.,Holocene Longaví: 36.2°S) are fractionation products of oxidized and H₂O-rich magmas at mid- to lower-crustal depths. The remaining large majority of analyses defines a gradational body of data that occupies the compositional space between these extremes, straddles the Miyashiro Discriminant (MD), and manifests variable open-system contributions. Magma mixing and crustal assimilation are expressed as convergence of evolved compositions on the MD, from both directions. Data subsets for segments and individual volcanoes display tendencies for: (1) higher fractions of CA analyses and (2) greater open-system contributions with increasing crustal thickness. These are consistent with greater average and maximum depths of magma evolution and a higher probability for more extensive crust-magma interactions as ascent-path lengths increase. These first-order observations are derived from combinations of non-traditional plotting parameters that provide new perspectives on this long-standing issue.