First-order control of basaltic sources on the diversity of Archean TTG magmatism

Abstract

Abstract Tonalite-trondhjemite-granodiorite (TTG) gneisses, the major components of Archean continental crust, show a broad compositional range. Most were viewed as crust-derived, and the inferred melting P-T conditions shaped our knowledge about the origin of early continental crust. While one single basaltic source (e.g., median of Archean tholeiites) was commonly invoked for different TTG gneisses, the potential influences of wide-ranging basaltic sources remain not well constrained. Late Neoarchean tonalitic to trondhjemitic gneisses, covering the compositional spectrum of global database, were identified from the Jiaobei terrane, North China Craton. Thermodynamic and trace element modeling demonstrates that (1) their chemical diversity is primarily determined by different basaltic sources with variable TiO2 and heavy rare earth element (HREE) contents; and (2) the inferred melting pressures of most tonalitic to trondhjemitic gneisses would reduce dramatically if a different basaltic source with slightly lower TiO2 and HREE contents was assumed. Integrated with synchronous chemical changes of global Archean TTG and basaltic rocks, a source-based twofold classification of TTG gneisses (i.e., low- and high-Ti-HREE types) is put forward. The low-Ti-HREE TTG gneisses necessitate both low-Ti-HREE basaltic sources and high melting pressures (~1.8 GPa), and their outburst after ~3.2 Ga may mark the development of thickened continental crust globally.