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Abstract:
By the end of the Archean Eon, a significant portion of Earth’s
continental crust had formed through partial melting of hydrated
mafic rocks, producing sodic granitoids of the tonalite-
trondhjemite-granodiorite (TTG) suite. While extensive research
has focused on the geodynamic environments of Archean crustal
formation, the mechanisms and sources of water required for
TTG generation at mid- to low- crustal depths remain poorly
understood. To address this, petrography, bulk-rock
geochemistry, in-situ O and Hf isotopes, and zircon
petrochronology from TTG gneisses of the Lewisian Gneiss
Complex (LGC), NW Scotland, are used to identify two
compositionally distinct groups of TTGs—hornblende- (hbl-
TTGs) and biotite-bearing (bt-TTGs)— and constrain the origin
of the water involved in their formation.
In-situ U–Pb geochronology of magmatic zircon cores from
both hbl- and bt-TTG yields crystallization ages between 2.7 and
2.9 Ga, coinciding with a major phase of crustal growth in the
LGC at ca. 2.8 Ga. Hbl-TTGs are primitive, sodic
(K 2O/N 2O 0.26) tonalitic magmas derived from partial melting
of low-K mafic rocks, with characteristic Nb/Ta 20, Zr/Sm 85,
Gd/Yb 4 and Sr/Y 121. Hbl-TTGs between 2.94 to 2.83 Ga
show εHf values between +3 and −3, while zircon δ18O values
(5.4–6.4‰) suggest interaction with a predominantly mantle-
derived water source and a subordinate contribution from
isotopically fractionated supracrustal fluids. In contrast, bt-TTGs
are less sodic (K2O/N 2O 0.48) and more evolved, derived from
partial melting of low-K mafic and tonalitic sources, with lower
Nb/Ta 12, Zr/Sm 65, and higher Gd/Yb 5 and Sr/Y 164,
compared to the hbl-TTGs. Zircon εHf (+4 to −6) and mantle-
like δ18O values (5.2–6.0‰) from 2.7 to 2.8 Ga bt-TTGs indicate
addition of juvenile material to reworked crust.
Petrography and geochemistry identify two distinct TTG
groups with different source characteristics. Hbl-TTGs are
primitive magmas that originated from the interaction of a mafic
source with a dominantly mantle-derived fluid. In contrast, the
slightly younger bt-TTGs formed by partial melting of a more
evolved source that interacted with juvenile material and
reworking of older crust. These findings suggest that the water
required for crustal formation processes was likely already present and bound within the mafic crust in the early Earth.
