Generation of Archaean TTG Gneisses Through Amphibole‐Dominated Fractionation

Abstract

The early continental crust is characterized by the tonalite-trondhjemite-granodiorite gneisses (TTG) of Archaean terrains. There is a consensus that TTGs were formed by partial melting of hydrated mafic rocks. High-pressure melts tend to be more trondhjemitic with higher Sr/Y ratios due to the presence of garnet and absence of plagioclase, whereas low-pressure melts are more tonalitic with relatively lower Sr/Y ratios. Here we present a geochemical study of diorites, TTGs, and hornblendites from Eastern Hebei, North China Craton. Geochemical analyses show that dioritic parental magmas may evolve to tonalites and trondhjemites through a maximum of 27% fractional crystallization of amphibole and apatite. We also identified the cumulate apatite-bearing hornblendite, a predicted solid fractionate and chemical complement to the TTGs. Mixing of the trondhjemite and the hornblendite can generate similar chemical compositions to the diorites in this region. The mixing trends are consistent with those predicted for fractional crystallization. Based on these analyses, we conclude that these trondhjemites and tonalites can be formed by fractional crystallization of amphiboles and accessory minerals from the calc-alkaline diorites. Partial melting is not a unique solution to the problem of how to form the Archean TTG gneisses and how to make an Archean continent. Thick crust or lithosphere (in the garnet stability field) is also not a unique requirement. Amphibole fractionation may also cause an abrupt increase of Eu/Eu* of Archean granitoids at 3.2-3.0 Ga, which may be linked to a transition period in tectonic style at~3 Ga in Earth's evolution.

Tracing the Early Emergence of Microbial Sulfur Metabolisms

“…This would, of course, depend on the environment, but if there was no external source of P outside the periphery of the volcanic vent, the rate of productivity would be limited by the concentration of phosphorus species in the vent fluids (Hao et al 2020). Figure 8 depicts P-percent weight ratios in Archean carbonate sediments over time and, while levels remain somewhat low, there is a large spike in the maximum concentrations around 3.5 Ga. Hf isotope ( Guitreau et al 2012) and Eu rare earth element (REE) anomalies (Liou et al 2019) also indicate that major crust forming events occurred during this time that would have brought large quantities of phosphorus to the surface. The phosphorus laden igneous rocks would then be weathered, washing large quantities of phosphorus into the ocean (Horton, 2015) thus fertilizing it and possibly driving the sudden "appearance" of photosynthesis~3.5 Ga (Olson et al 2006).…”

Tonalite-Dominated Magmatism in the Abitibi Subprovince, Canada, and Significance for Cu-Au Magmatic-Hydrothermal Systems

“…The bulk of HREE-depleted rocks display primitivemantle normalized negative Ta, Nb, and Ti anomalies likely inherited from the primitive magma, i.e., Ta, Nb, and Ti were retained in the source by a Ti bearing residual phase(s). That their REE contents decrease with increasing SiO2 suggests a fractional crystallization process controlled by amphibole and, possibly, apatite [38,39]. The Eu anomaly is influenced by the fractionation and accumulation of feldspar and amphibole, which preferentially incorporate Eu 2+ and Eu 3+ , respectively [40,41].…”

“…The major and trace elements content of TTG suites is generally interpreted in terms of depth of partial melting, with the most HREE-depleted and Na-rich magmas produced by higher-pressure melting of the hydrated mafic crust [10]. However, recent studies show that these magmas differentiate significantly upon ascent [39,44]. Thus, the HREE-depleted rocks of the Abitibi Subprovince likely have a homogeneous source, and their noted compositional diversity (e.g., Yb content) is a consequence of lower to upper crustal differentiation-fractional crystallization likely-and is not indicative of a variable depth of partial melting.…”

Deep Into the Chibougamau Area, Abitibi Greenstone Belt: Structure of a Neoarchean Crust Revealed by Seismic Reflection Profiling

“…A matter raised by the latter models is whether basalts hydrated by sea water can be buried fast enough to produce the H 2 O‐rich source of the TTG suite. Another matter that remains to be investigated is whether tonalites are HREE depleted because they come from the melting of a basaltic source in the stability field of garnet (Moyen & Martin, 2012) or whether the HREE depletion is mostly due to differentiation controlled by amphibole and apatite (Liou & Guo, 2019).…”