Partial melting of metasedimentary rocks at the Archean-Paleoproterozoic boundary and formation of strongly peraluminous granites: A theoretical study

Sediment characters of the early Earth show a significant change in the later part of the Archean era due to the emergence of continents above sea level for the first time. This broadly coincides with dramatic changes in metamorphic styles and magmatic characters, and these secular changes are linked to the change in tectonic settings from a stagnant lid to a peeling-off type during this transition. Continents are mostly composed of felsic igneous rocks whose composition also evolved from a tonalite-trondhjemite-granodiorite to strongly peraluminous granite broadly at the same time frame. The present study examines the partial melting behaviour of Late Archean and Early Proterozoic sediments using phase equilibria modelling and explores its role in changing the composition of the felsic continental rocks along with the mineralogical diversity of the residual products of partial melting. The results show that strongly peraluminous granites could originate from mudstone, shale, and greywacke protoliths across the Archean-Proterozoic boundary without significant differences in melting temperatures. It is also important to note that significant melting (>40 vol.%) might have occurred at temperatures exceeding 750 degrees C at lower crustal conditions (similar to 8 kbar), and the melt could ascend to higher levels as granite plutons. A large number of such highly aluminous K-rich granites near the Archean-Proterozoic boundary could, thus, originate from the partial melting of sediments eroded from the continental highlands. The mineralogical diversity of the residual granulites could be explained by the diversity of original sediment characters (greywacke vis-a-vis pelite) as well as the pressure, temperature, and fluid compositions of melting.