Trace element discrimination of arc, slab failure, and A-type granitic rocks

Abstract

Although widely used, classification schemes for granitic rocks have until now failed to separate slab failure from arc magmatism in collisional orogens. Our previous work showed that Cretaceous Cordilleran-type batholiths are composed of a pre-collisional belt of mesozonal to epizonal arc plutons, commonly emplaced into their own cover, and a post- accretion tract of what we interpret as “slab failure” plutons, emplaced into the tectonically thickened collisional hinterland. Histograms based on new geochemical compilations of pre- and post-collisional igneous rocks were employed to derive trace element criterion to distinguish between these two plutonic types. Using rocks over the SiO2 range of 55–70 wt. % and aluminum saturation index of <1.1, slab failure rocks are readily separated from arc rocks based on Sr/Y > 20, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Gd/Yb > 2, and Sm/Yb > 2.5. Also, we found that A-type magmas can be distinguished from other post-collisional igneous rocks based on Ta + Yb>6 ppm and Nb + Y > 60 ppm. These trace element differences between our pre-collisional arc and post-collisional slab failure plus A-type groups allowed us to build composite discrimination diagrams for destructive margin igneous rocks. Although most S-type granites are also post-collisional, they should not be plotted on our diagrams, for some of their geochemical features overlap those of our slab failure group. Unlike our arc and slab failure groups, A-type granites are formed in diverse tectonic settings such that their classification is more related to geochemical signature and magma sources than to tectonic setting. Lastly, we show how our discrimination diagrams delineate examples of slab failure magmatism through time and suggest links with porphyry Cu-Au deposits.