Can fractional crystallization, mixing and assimilation processes be responsible for Jamaican-type adakites? Implications for generating Eoarchaean continental crust.

Abstract

Understanding how the Earth’s first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet’s primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite–tonalite–granodiorite (TTG) suite. These can be divided into (1) a mid- to late Archaean (∼3·5–2·5 Ga) suite with low SiO2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3·5 Ga) suite with higher SiO2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid- to late Archaean (∼3·5–2·5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd–Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase- and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge; (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times.