Fig. 6: Compositional comparisons between the Black Flag Group (BFG) and global high pressure TTG and medium pressure TTG.
From: No evidence for high-pressure melting of Earth’s crust in the Archean
Variations in selected major and trace elements and trace element ratios (Mg# (a), Na2O (b), Zr (c), Sr/Y (d), Sr/Nb (e)] with SiO2 (calculated volatile free), Ce/Sr (f) and Sr/Y (g) with Y, and Sr (h) and Nb (i) with SiO2 (calculated volatile free). Dashed lines in a–h outline kernel density plots comprising 90% of the data (see Methods). Trends lines in h and i are approximations of highest data density. High pressure (HP) TTG forms the evolved end member of the BFG array, critically also in terms of the compositional attribute used to distinguish medium-pressure (MP) and HP TTG. HP TTG can clearly be subdivided compositionally (6g) – one subdivision (HP TTG 1) being characterized by very high-Sr and typically low-Nb, is intersected by the BFG trend. The second subdivision (HP TTG 2), is distinct from silica-rich BFG but indistinguishable from MP TTG in terms of (for example) Sr and Nb (used to infer stability/instability of plagioclase and rutile, respectively).
