Thick crust, hydrous magmas, and the paradox of voluminous cold magmatism

Cin-Ty Lee,Boda Liu

doi:10.30909/vol.04.02.227238

Abstract

Andesites are refined and “cold” magmas compared to their basaltic parents, yet large volumes of andesites are generated at continental arcs. We show that large andesitic plutons are favored when arc crust attains a thickness of ~60 km while mafic plutons are small and favored when arc crust is thin. Using simple thermal models, we show that large, long-lived and relatively cold partially molten zones, sustained by recharge of hydrous basaltic magmas, are favored at depth when arc crust is thick due to the reduced efficiency of heat loss with increasing crustal thickness. Thin crust and drier magmas favor hotter and thinner partially molten zones. Our study provides an explanation for the apparent paradox that the most voluminous magmas in continental arc settings are cold. The origin of andesites may be linked to the interplay between magmatic differentiation, the availability of water, and the processes that control crustal thickness.

Highlights

The origins of intermediate magmas, such as andesites and dacites and their intrusive equivalents, have long been debated because of their importance in arc systems
Other models include mixing between mafic and felsic magmas [Reubi and Blundy 2009; Blatter et al 2017], watersaturated melting of the mantle [Kelemen et al 2007; Grove et al 2012], re-melting of the lower crust [Draut et al 2002; Collins et al 2016; 2020], re-melting of subducted oceanic crust [Rapp et al 2003; Martin et al 2005], melting of hybridized mantle [Lara and Dasgupta 2020], and enhanced melt productivity at the intermediate temperatures that favor andesitic melts [Reubi and Blundy 2009]. Andesites, dacites and their intrusive equivalents have long been thought to be most prevalent in thick continental arcs like the modern Andes, an observation confirmed by the fact that the silica contents of arc volcanics correlate with elevation and crustal thickness [Farner and Lee 2017]
We propose that the lower crust of active magmatic arcs is represented by long-lived partially molten zone (PMZ) sustained by inefficient heat loss through thick crust and by recharge of hydrous basaltic magmas

Summary

Introduction

The origins of intermediate magmas, such as andesites and dacites and their intrusive equivalents (granodiorites and tonalites), have long been debated because of their importance in arc systems. Other models include mixing between mafic and felsic magmas [Reubi and Blundy 2009; Blatter et al 2017], watersaturated melting of the mantle [Kelemen et al 2007; Grove et al 2012], re-melting of the lower crust [Draut et al 2002; Collins et al 2016; 2020], re-melting of subducted oceanic crust [Rapp et al 2003; Martin et al 2005], melting of hybridized mantle [Lara and Dasgupta 2020], and enhanced melt productivity at the intermediate temperatures that favor andesitic melts [Reubi and Blundy 2009] Whatever their origin, andesites, dacites and their intrusive equivalents have long been thought to be most prevalent in thick continental arcs like the modern Andes, an observation confirmed by the fact that the silica contents of arc volcanics correlate with elevation and crustal thickness [Farner and Lee 2017]. As we will show here, these relatively cold, intermediate magmas form the largest

Methods and Results

Thermal models

Limitations of the model

Controls on PMZ thickness and melt composition

Conclusions and future work