FORWARD MODELING OF TRACE ELEMENTS IN ARCHEAN TTGS: THE IMPORTANCE OF OPEN-SYSTEM PROCESSES

Tonalite-trondhjemite-granodiorite (TTG) plutonic suites dominate preserved Archean crust; however, the geodynamic setting in which they formed is a subject of ongoing debate. The trace element compositions of TTGs are commonly used to determine the depth of melting of their metabasite sources and to infer the geodynamic processes linked to TTG genesis. Garnet, plagioclase, and rutile are important reservoirs for key trace elements (e.g., Y, Sr, and Nb) and understanding the behavior of these minerals in anatectic metabasites is crucial for the interpretation of trace element compositions of TTGs. Phase equilibria modeling assuming closed-system behavior is commonly used to predict the stability and modal proportions of these minerals at various P–T conditions, but the role of open-system processes such as melt loss and fractionation of trace elements into growing peritectic porphyroblasts has received little attention in the context of TTG petrogenesis.

Here, we use forward phase equilibria modeling of two mafic source rocks to evaluate the sensitivity of trace element concentrations of TTGs to open-system processes during anatexis, including the loss of melt and fractionation of trace elements into peritectic garnet. P–T pseudosections and modal proportions of phases along different P–T gradients were coupled with trace element partition coefficients to calculate the predicted trace element concentrations of TTG melt. Trace element ratios commonly used as proxies for depth of melting, Sr/Y and La/Yb, were calculated for anatectic melt produced along these gradients. The results of our modeling show that although these key trace element ratios generally increase with increasing pressure, the elevated values observed in TTGs are only replicated in scenarios including garnet fractionation, regardless of starting bulk composition. Furthermore, the full ranges of Sr/Y and La/Yb values observed in natural samples are reproduced at pressures lower than those typically inferred for the generation of TTGs. These results have important implications for the use of trace elements in TTGs to infer tectonic processes responsible for their petrogenesis in Archean cratons.