Trace element evidence for growth of early continental crust by melting of rutile-bearing hydrous eclogite

摘要

The early continental crust is composed dominantly of Archean tonalite-trondhjemite-granodiorite (TTG) gneisses and is generally explained as the product of melting of metabasalts in the subducted crust. However, whether the melting occurs in shallow-level amphibolite facies or in relatively deep eclogite facies is debated. Here I present experimental partition coefficients (Ds) for 27 trace elements between garnet/amphibole and tonalitic melts. They are used together with published mineral/melt trace element Ds to model the melting of metabasalt in order to delimit the conditions for TTG production. The results clearly show that model melts with trace element characteristics that completely mimic the TTG are in equilibrium with rutile-bearing anhydrous and hydrous (amphibole bearing) eclogitic residues, but not rutile-free, amphibole-dominated residues. Rutile appears to be a necessary residual phase to account for the characteristic negative Nb-Ta anomaly in the TTG. These results thus suggest that the early continental material was produced under eclogite facies conditions. Based on the modeling results obtained using appropriate partition coefficients and the Archean geotherm, the preferred process for the TTG production is the melting of rutile-bearing hydrous eclogite, triggered by the release of H₂O from the progressive breakdown of amphibole. The pressure-temperature (P-T) conditions for TTG production via this process are constrained to 1.5–2.5 GPa (50– 80 km) and 850–1050 °C by the P-T stability boundaries of amphibole and rutile in the basalt system.