A model for Trondhjemite‐Tonalite‐Dacite Genesis and crustal growth via slab melting: Archean to modern comparisons

The petrogenesis of trondhjemite‐tonalite‐dacite (TTD) involves all major petrologic models in various tectonic settings. A specific subtype of TTD, high‐Al type, is the one most commonly associated with Archean gneiss terranes. During the Archean, continental crust formation was operating at an elevated rate relative to the Phanerozoic, and the generation of high‐Al TTD played an integral role in its nucleation and growth. High heat flow, rapid convection, and subduction of hotter, smaller plates were unique tectonic elements to the Archean which optimized conditions required for transformation of subducted oceanic crust into sial via partial melting. Anatexis of Archean mid‐ocean ridge basalt (MORB) under eclogitic to garnet amphibolitic conditions produced weakly peraluminous to metaluminous high‐Al TTD with low heavy rare earth elements (HREE), Y, Nb, K/Rb, and Rb/Sr and high La/Yb and Sr/Y. This study demonstrates that Archean TTD crustal generation processes are also present in selected high‐Al Phanerozoic TTD terranes. The Cenozpic high‐Al TTD suites are commonly found in tectonic settings which are thought to recreate the elevated Archean thermal gradients, i.e., at sites of young, hot oceanic plate subduction. These relationships imply a petrologic continuity of TTD generation through time. A fertile zone of melting is envisioned at 23–26 kbar (75–85 km) and 700–775°C, where wet partial melting of the subducting slab occurs concurrently with dehydration reactions. At this depth, the converting mantle wedge continuously feeds hot mantle material to the wedge‐slab interface, creating strong temperature gradients, intraslab fluid migration, and slab melting. In summary, in modern arc terranes where young (< 20–30 Ma) oceanic crust is being subducted, high‐Al TTD generation via slab melting is considered likely; whereas when older (>30 Ma) oceanic crust is subducted, mantle‐derived magmas are dominant, giving rise to basaltandesite‐dacite‐rhyolite (BADR) fractionation suites. This study introduces the importance of subducted oceanic crustal age on arc petrogenesis.