TRACE ELEMENT DISCRIMINATION OF ARC, SLAB FAILURE, AND A-TYPE GRANITIC ROCKS

Our work on Cretaceous Cordilleran-type batholiths showed that they are composed of a pre-collisional belt of mesozonal to epizonal arc plutons commonly emplaced into their own cover, and a post-accretionary tract of slab failure plutons emplaced into the tectonically thickened collisional hinterland.

It is important to separate the two suites because they represent different tectonic regimes, yet commonly lie adjacent to one another in collisional orogens, with slab failure suites sitting just inboard, or even intruding rocks of the accreted arc. The failure to recognize slab failure plutons led to the misconception that the slab failure rocks represent arc magmas that assimilated cratonic crust when a subducting slab shallowed beneath it. This, in turn, led geologists to create models for many collisional orogens with reversed (and incorrect) subduction polarity.

Currently used geochemically based classification schemes for granitic rocks fail to separate post-collisional slab failure magmatism from arc magmatism, so we compiled and used geochemical data from pre- and post-deformational suites to create histograms from which we derived values to separate the arc and slab failure rocks. By using rocks over the SiO2 range of 55-70 wt. % and aluminum saturation index of <1.1, slab failure rocks are readily distinguished from arc rocks based on Sr/Y>20, Nb/Y>0.4, Ta/Yb>0.03, La/Yb>10, Gd/Yb>2, and Sm/Yb>2.5.

Discrimination is possible because arc magmas are derived from partial melting of spinel- plagioclase source rocks with residual pyroxene, plagioclase and rutile, whereas slab failure magmas are derived from garnet-bearing, plagioclase-free sources, which we suggest to be the metamorphosed basaltic-gabbroic portion of the subducted oceanic plate at p > 2GPa. The strong partitioning of heavy rare earth elements into residual garnet and the absence of plagioclase result in the distinctive high La/Yb, Sm/Yb and Gd/Yb and Sr/Y ratios of slab failure magmas. Instability of a Ti phase such as rutile, plus residual garnet, yields their high Nb/Y and Ta/Yb ratios.

We also found that A-type plutons could be distinguished from both arc and slab failure rocks based on Ta + Yb>6 ppm and Nb + Y>60 ppm, which allowed us to build composite discrimination diagrams that can be used to separate arc, slab failure, and A-type rocks.