Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian-Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc - Davle volcanic complex (DVC) situated in the Tepl ' a-Barrandian unit, Bohemian Massif - that formed during the Neoproterozoic Avalonian-Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite-trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (epsilon Nd +6.0 to +8.7 and epsilon Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhje-mites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as low-pressure intra-oceanic arc granitoids or LP IOAG. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite-trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth.

Automated summary

The processes leading to the formation of siliceous magmatic rocks from juvenile sources within intra-oceanic island arcs play a significant role in the formation and growth of continental crust. The study reveals that low-pressure intra-oceanic arc granitoids can form in a nearly closed system with little assimilation of older crustal material.