Secular variations of magma source compositions in the North Patagonian batholith from the Jurassic to Tertiary: Was mélange melting involved?

Abstract

This study of Sr-Nd initial isotopic ratios of plutons from the North Patagonian batholith (Argentina and Chile) revealed that a secular evolution spanning 180 m.y., from the Jurassic to Neogene, can be established in terms of magma sources, which in turn are correlated with changes in the tectonic regime. The provenance and composition of end-member components in the source of magmas are represented by the Sr-Nd initial isotopic ratios (87Sr/86Sr and 143Nd/144Nd) of the plutonic rocks. Our results support the interpretation that source composition was determined by incorporation of varied crustal materials and trench sediments via subduction erosion and sediment subduction into a subduction channel mélange. Subsequent melting of subducted mélanges at mantle depths and eventual reaction with the ultramafic mantle are proposed as the main causes of batholith magma generation, which was favored during periods of fast convergence and high obliquity between the involved plates. We propose that a parental diorite (= andesite) precursor arrived at the lower arc crust, where it underwent fractionation to yield the silicic melts (granodiorites and granites) that formed the batholiths. The diorite precursor could have been in turn fractionated from a more mafic melt of basaltic andesite composition, which was formed within the mantle by complete reaction of the bulk mélanges and the peridotite. Our proposal follows model predictions on the formation of mélange diapirs that carry fertile subducted materials into hot regions of the suprasubduction mantle wedge, where mafic parental magmas of batholiths originate. This model not only accounts for the secular geochemical variations of Andean batholiths, but it also avoids a fundamental paradox of the classical basalt model: the absence of ultramafic cumulates in the lower arc crust and in the continental crust in general.

Glacier fluctuations in the northern Patagonian Andes (44°S) imply wind-modulated interhemispheric in-phase climate shifts during Termination 1

“…The foundation of our chronology comes from one of these tributary valleys located ~ 15 km from the southern shore of the lake, at the foot of Cerro Riñón (~ 1790 m asl). The Cerro Riñón valley (informal name) is carved into the North Patagonian batholith 18 , which was the source of the abundant glacially-transported granitoid boulders found on the local landscape.

Figure 1 Glacial geomorphology and chronology of the ( a ) Lago Palena/General Vintter ice lobe with 10 Be ages (n = 4) obtained from the innermost moraine ridge (PV6) and ( b ) the Cerro Riñón valley glacier with 10 Be ages (n = 26) obtained from CR1-CR5 moraine complexes and the additional perched boulder.

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Climatic control on the location of continental volcanic arcs

“…[43][44][45][46]. In this sector, the Patagonian Batholith is exposed, which allows for reconstructions of the magmatic arc migration since the Cretaceous 47 . Conversely, batholiths are not exposed in the Central Andes where deformed volcanic and sedimentary rocks are preserved at the surface 39,41 .…”

Generation of Magmatism Under Active Continental Margins: A Thermodynamic Study of Subduction and Translithospheric Diapirs

“…Moreover, there seems to be a decoupling between the major element geochemistry of the plutonic rocks and their trace element and isotopic composition, where plutons with a dominantly granodioritic and tonalitic composition have a wide range in the Sr -Nd system, and where more Nd-radiogenic rocks rocks can be richer in silica than less Nd-radiogenic rocks (e.g. Castro et al, 2021;Hervé et al, 2007;Pankhurst et al, 1999). However, there is generally no direct access to the source regions of this magmatism (except for slab-melted blocks in subduction channels, e.g.…”

“…The most recent model (Behn et al, 2011;Castro, 2014;Castro et al, 2010;Gerya and Yuen, 2003;Hacker et al, 2011;Vogt et al, 2012Vogt et al, , 2013, termed 'mélange diapir model' by Castro et al (2021), explains the Cordilleran magmatism as a consequence of melts derived from mélanges detached from the subducted plate (Fig. 2).…”

“…According to this model, the crustal and mantellic contributions come from the subducting slab, which would be an inexhaustible source of these materials, thus explaining the long duration of the Cordilleran magmatism (for instance, 150 Myr for the South Patagonian batholith, Hervé et al, 2007;80 Myr for the Peruvian Coastal Batholith, Martínez Ardila et al, 2019). The decoupling between the major element and isotopic compositions could be explained as premelting mixing of the basaltic and crustal components, as assimilation of crustal material by mantle melt would couple the SiO 2 content and a more contaminated isotopic signature (Castro et al, 2021). Finally, the mélange diapir model supports the andesite model of Taylor (1967) and Taylor and McLennan (1985) for the growth of continents in that the initial melt composition is intermediate (andesitic) and separation of the silica-rich melt from the silica-poor residuum produces the compositional variation of the continental crust in a silicic upper crust and a more mafic lower crust (Castro et al, 2010).…”