hide
Free keywords:
-
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).
