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Abstract

In the early Earth, significant production of sodic continental crust (i.e., trondhjemite–tonalite–granodiorite—TTG) occurred during the Archean eon by partial melting of basaltic (mafic) crust, at depth between 25 and 50 kilometres. Fluid-fluxed melting has recently been invoked as an important trigger for TTGs formation1, but the geodynamic settings and processes involved are highly debated. Important questions about the source of the melt-triggering fluids and how they were transported to deeper crustal levels have remained difficult to ascertain. In this contribution, we combine petrographic observations, major and trace element whole-rock geochemical data, in-situ oxygen isotope analysis, and U-Pb dating of zircon from TTG gneisses from the Lewisian Gneiss Complex (LGC), in NW Scotland, to highlight the differences between two distinct groups of TTGs present in this region: hornblende-bearing (i.e., central region of the LGC) and biotite-bearing (northern and southern regions) TTGs. Our results show that hornblende-TTGs are commonly primitive, Na-rich tonalitic magmas derived from partial melting of low-K mafic rocks and yield δ18O values of 5–6‰ indicating that these magmas were hydrated by mantle-derived fluids rather than from a sedimentary or hydrothermal source. By contrast, less sodic, more ‘’mature’’, biotite-TTGs reflect more evolved trondhjemite to granodiorite compositions. The biotite-TTGs yield δ18O values that, in places, are slightly above that of the average mantle zircon (5.3 ± 0.6‰ 2SD)2. This may indicate that TTG magma production in the northern and southern regions of the Lewisian Gneiss Complex was triggered by mantle-derived fluids that were previously contaminated with a supracrustal source to account for the small positive O isotopic excursions. These results highlight the hybrid sources of fluids which play an important role in the evolution of intracrustal recycling and crust-mantle interaction during crust formation in the Archean. 21st Swiss Geoscience Meeting, Mendrisio 2023 REFERENCES 1. Pourteau, A. et al. TTG generation by fluid-fluxed crustal melting: Direct evidence from the Proterozoic Georgetown Inlier, NE Australia. Earth and Planetary Science Letters 550, 116548 (2020). 2. Valley, J. W. et al. 4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon. Contributions to Mineralogy and Petrology 150, 561– 580 (2005).