The genesis of Neoarchean TTG in the Shangyi area of the northern margin of North China Craton: melt from basalt of oceanic crust at varying depth

Shangyi basalt is a component part of Neoarchean oceanic crust in Shangyi-Chicheng fault zone, and its geochemical characteristics suggest an enriched mantle source. The basalt can be divided into two types: TH1 (Flat-REE) and TH2(Fractionated-REE). Shangyi TTG is mainly of the medium-potassium, metaluminous and calc-alkaline type, and similar to low-Al2O3 TTG owing to its low Al2O3 content. In addition, Shangyi TTG shows features of adakite by slab melting according to trace elements Rb, Sr, Y and REE. Based on features of major and trace elements (SiO2, K2O, Na2O, Al2O3, Rb, Sr, Y, REE) and trace element equilibrium melting calculation, the authors have reached the conclusion that Shangyi TTG was formed throughout the low-high pressure range and was probably produced from partial melting of oceanic crust basalts (TH1 type) at varying depth, with the low-Al2O3 type formed under low pressure.