Geochemistry characteristics and tectonic significance of the ‘ “mafic” S-type granites’

Abstract

<p>Taking account of their importance in inferring continental geodynamic processes, S-type granites have been the subject of considerable research since their discovery and identification. With their high A/CNK (Mol. Al2O3/(Na2O + K2O + CaO) > 1.1) and low Na2O (<3.2 wt.%) and MgO + FeO^T contents (generally with maficity < 4 wt.%), S-type granites were originally considered to be largely derived from partial melting of metasedimentary sources in orogeny zone settings. However, it is hotly debated why many S-type granites show significantly higher maficity [here defined as the weight percent of MgO + FeO^T up to 12 wt.% and lower silica (SiO2 <65 wt.%), known as low- to medium-silica ‘mafic’ S-type granites (L-MSMST granites). Hence, the interpretation of these distinctive S-type granites has provided valuable insights into anatectic and metamorphic processes, tectonic thermal events, and the evolutionary dynamics of the crust along convergent margins. Nevertheless, with the increasing number of reported ‘special S-type granites ’,the absence of standardized terminology has become increasingly evident. In light of this, the present study critically examines advancements in defining maficity S-type granites and formally introduces widely accepted classifications and genetic models associated with them. A comprehensive compilation of published articles on maficity S-type granites was utilized to construct a robust database. This database facilitated the screening of 404 samples, applying various geochemical definitions to refine the selection process. We recognize that (1) the L-MSMST granites are inconsistent with originally defined S-type granite (ST granite); (2) the schematic models of the addition of mantle derived materials can explain the formation of high silica ‘non-mafic’ S-type granites (HSNMST granites) and L-MSMST granites; and (3) L-MSMST granites were may be formed in a typical extensional tectonic background.</p>