Tracing magmatic differentiation of peralkaline granites by using K stable isotopes

Abstract

The genesis of peralkaline granites has fundamental implications for understanding their source characters and tectonic settings, yet it remains highly controversial after decades of research. Two major challenges in studies of peralkaline granites are how the peralkaline magmas evolve and how to trace their magmatic evolution. Here we use K isotopes in granites from Zhoushan archipelago of southeast China to explore these two issues. The Zhoushan archipelago hosts two groups of Cretaceous high-silica granites, which are the peralkaline rocks (PAG) and the calc-alkaline rocks (CAG). The CAG rocks exhibit a small variation (δ41K = −0.54 ± 0.06 ‰ to −0.36 ± 0.06 ‰) in K isotopic compositions, in contrast to the larger variation for the PAG rocks (δ41K = −0.67 ± 0.07 ‰ to −0.27 ± 0.05 ‰). The PAG rocks show strong correlations between δ41K values and various whole-rock major elements (i.e., Al2O3, Na2O, and K2O contents), but these correlations are absent for the CAG rocks. We propose that the separation of Na-rich alkali-feldspar, rather than low-Na alkali-feldspar and/or plagioclase, controlled the K isotope variability in PAG. The alkali-feldspar in PAG is enriched in Na, among which the alkali-feldspar with Or (orthoclase; mol. %) <15 is characterized by high δ41K ranging from −0.34 ‰ to 0.36 ‰. Geochemical modeling demonstrates that 23–33 % fractional crystallization of such Na-rich alkali-feldspar is required to generate the geochemical trends observed in PAG. Those findings suggest that crystallization of Na-rich alkali-feldspars dominates the magma evolution for peralkaline magmatism, supporting strong fractional crystallization in high-silica peralkaline magma. K isotopes show promise to trace detailed magmatic processes and provide constraints on the generation of peralkaline granites.