Abstract: The TTG petrotectonic assemblage (or igneous series) characterizes subduction of oceanic crust. In this paper, we suggest four subtypes of the TTG series: (1) The magnesium andesite (MA) lower pressure subtype is formed at a subduction zone of very young oceanic crust at highest temperature, ≤15001600 MPa pressure and ≤5060 km depth, for example, at the margins of the slab-window of the active mid-oceanic ridge subduction. (2) The magnesium andesite (MA) higher pressure subtype is formed at a subduction zone of relatively young oceanic crust at high temperature, ≥15001600 MPa pressure and ≥5060 km depth. (3) The lower (or non) magnesium andesite (LMA) lower pressure subtype is formed at a lower crust of the intra-oceanic island arc at ≤15001600 MPa pressure and ≤5060 km depth. And (4) The lower (or non) magnesium andesite (LMA) higher pressure subtype is formed at the mountain root of the continental arc at ≥15001600 MPa pressure and ≥5060 km depth. Using a combination of the four TTG assemblage subtypes and non-magma generation at the subduction zone of older oceanic crust at low temperature with or without magma generation at the mantle wedge, we may reconstruct the crust-mantle and thermal structures of the magmatic arcs and provide a geologic basis for oceanic subduction related mineralization.

Key words:  tonalite-trondhjemite-granodiorite (TTG), magmatic arc and oceanic subduction, subtypes of the TTG group, magnesium andesite (MA) and lower (or non) magnesium andesite (LMA), higher pressure and lower pressure