Devonian continental arc intermediate-felsic magmatism in the Gorny Altai terrane, northwestern Central Asian Orogenic Belt: Heterogenous crustal melting and input of mantle melts

Abstract

Devonian intermediate-felsic igneous rocks are widespread in the Gorny Altai terrane and they were suggested to form in a continental arc setting that was built on the late Neoproterozoic to early Ordovician Kuznetsk-Altaiintra-oceanicsubduction-accretion system (KAIOSS).1 Petrogenesis of these rocks will not only provide information on geodynamic mechanisms for continental arc magmatism but also shed light on how accretionary orogens can be differentiated and matured, both of which are pivotal in understanding the continental generation and evolution. This study is focused on igneous suites from the early Devonian Ongudai and middle Devonian Kuratin complexes. The former complex is dominated by andesites and basaltic andesites. Zoning patterns of clinopyroxene phenocrysts and whole-rock geochemical modeling suggest that these volcanic rocks formed mainly by magma mixing with substantial input of mantle-derived mafic melts, signifying significant crustal growth. The latter complex comprises a volcanic suite (ca. 383–382 Ma) ranging from andesites to rhyolites and minor granitic intrusions (ca. 385–382 Ma). The rhyolitic endmembers of the volcanic suite were probably sourced from the late Neoproterozoic to early Ordovician meta-sedimentary sequences of the KAIOSS. In contrast, the andesitic endmembers possibly formed by partial melting of mixed crustal components comprising the late Neoproterozoic magnesian andesites (or their geochemical equivalents) and more depleted basaltic rocks of the KAIOSS. Geochemical data demonstrate that the granitic intrusions were most likely products of melts from the late Neoproterozoic to Cambrian arc-type basaltic rocks of the KAIOSS. Taken together, the whole igneous suite of the Kuratin complex documents strong crustal differentiation. Of particular importance is that the highly heterogenous crustal components involved likely mirror the pre-Devonian crustal architecture that was induced by prolonged subduction-accretion of the KAIOSS. Our study suggests that heterogeneous crustal melting by underplating of hydrous basaltic melts in continental arcs is a plausible way to transform previous immature oceanic materials in accretionary orogens into differentiated and mature continental crust. This process contributes significantly to the magmatic diversity in subduction zones and crustal growth and differentiation in accretionary orogens.