A MASH Zone Revealed: the Mafic Complex of the Sierra Valle Fértil

Abstract

The Sierra Valle Fe ´rtil Complex of west-central Argentina represents a section of the Ordovician ($470 Ma) Famatinian arc and exposes a continuous, tilted crustal arc section ranging in depth from $12 to 32 km ($4-8 kbar pressure). This arc section exposes the complete compositional architecture from ultramafic and mafic rocks to upper crustal granodiorites. Field and compositional data are presented to document the deep ($6-8 kbar) mafic complex of the Sierra Valle Fe ´rtil. The mafic complex is composed of many tens to hundreds of plutonic cumulate bodies in a complex and non-regular arrangement. There is no simple compositional, kinematic or age relationship between neighboring plutons throughout the section, as expressed by cumulate compositions, emplacement horizon, size, composition, texture or style of contact. Amphibole gabbronorites and mafic tonalites dominate, but norites, amphibole websterites, troctolites and minor anorthosites are present. Amphibole is common but always as a replacement phase, and is never observed undergoing subsequent dehydration melting. Hence there is no evidence that voluminous tonalites were produced by dehydration melting of mafic precursors. A field-based, cumulateremoval fractionation model is presented that produces the observed compositional variations in five steps. Isotopic compositions of Sr and Nd deviate significantly from primitive mantle values, indicating a crustal contribution; however, this hybridization appears to have played a minor role in the major element evolution of the mafic complex. We interpret this isotopic and elemental decoupling as a byproduct of prolonged, punctuated MASH (melting, assimilation, storage, homogenization) processes in the lower crust. Isotopes may be the only residual evidence of assimilation within the mafic zone. This requires that melt removal from the cumulates was extraordinarily efficient.

Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)

“…Karakas and Dufek, 2015) and thus call for fractionation as the primary means for producing silicic magmas (e.g. Jagoutz, 2010;Lee and Bachmann, 2014;Rioux et al, 2010;Walker et al, 2015). Whereas we do not contest that the latter may be the case in several geologic settings, we stress that the efficiency of assimilation is strictly dependent on the thermal evolution and the compositional and tectonic conditions of the intruded crust.…”

Birth, life, and demise of the Andean–syn‐collisional Gissar arc: Late Paleozoic tectono‐magmatic‐metamorphic evolution of the southwestern Tian Shan, Tajikistan

“…The greater diversity of applied pressure‐estimation techniques for granitoids and paragneisses from the Garm massif (Al‐in‐Hbl, GASP, and pseudosection modeling) compared to granitoids from the Gissar batholith (only Al‐in‐Hbl) reflects the greater lithologic diversity of the Garm massif. We speculate that exposures of rare, deformed mafic and ultramafic rocks reported in the Garm massif [ Budanov , ; Brookfield , ] may represent the upper MASH zone of the Gissar arc [e.g., Walker et al , ].…”

Interaction between mantle-derived magma and lower arc crust: quantitative reactive melt flow modelling using STyx

“…6A). Although, there is not much data (to our knowledge) on the actual thickness of the MASH zone, available studies suggest that it is most likely comprised between 1 and 10 km thick (e.g., Jackson et al, 2002;Walker et al, 2015). Jackson et al (2002) showed that 2 km was enough to strongly influence the compositional evolution of the magmatic products segregated from the MASH zone.…”

“…In migmatitic orthogneiss from the Gföhl Unit (Bohemian Massif), Hasalová et al (2008a) described 100-m scale pervasive flow from an external source produced migmatitic textures of high melt fraction. In the Ordovician Sierra Valle Fertil Complex of west-central Argentina, Walker et al (2015) studied an exposed lower crustal arc section. Based on petrological and geochemical studies, the authors show that the mafic section (mainly gabbronorites) was the locus of prolonged periods of (cyclic) magma injection, crystallization, and assimilation, with very efficient melt segregation at the kilometer scale.…”