Elemental and Sr–Nd isotope geochemistry of microgranular enclaves from peralkaline A-type granitic plutons of the Emeishan large igneous province, SW China

Abstract

Microgranular enclaves are common within intermediate to felsic granitic rocks that have I- and S-type affinity however they are rare within alkaline anorogenic granitoids of A-type affinity. The Permian (~ 260 Ma) Emeishan large igneous province (ELIP) of southwest China contains two peralkaline silica saturated A-type granitic plutons that host microgranular enclaves. The enclaves from the Baima pluton are intermediate in composition and have lower SiO 2 and higher TiO 2, CaO and Mg# (SiO 2 = 57.2 to 63.0 wt.%; TiO 2 = 0.8 to 1.8 wt.%; CaO = 1.7 to 3.3 wt.%; Mg# = 28 to 44) than their host (SiO 2 = 62.6 to 67.8 wt.%; TiO 2 = 0.5 to 1.4 wt.%; CaO = 0.4 to 1.8 wt.%; Mg# = 15 to 31). The enclaves from the Taihe pluton are more felsic (SiO 2 = 63.8 to 71.3 wt.%; TiO 2 = 0.3 to 0.6 wt.%; CaO = 0.6 to 2.3 wt.%; Mg# = 8 to 22) but are still less evolved than their host (SiO 2 = 69.8 to 75.1 wt.%; TiO 2 = 0.2 to 0.6 wt.%; CaO = 0.4 to 0.8 wt.%; Mg# = 3 to 12). In both cases, the enclaves have very similar εNd (T) values (Baima εNd (T) = + 2.8 to + 3.2; Taihe εNd (T) = + 1.0 to + 2.0) as their hosts (Baima εNd (T) = + 3.0 to + 3.2; Taihe εNd (T) = + 1.5 to + 1.9). The major and trace element trends of the enclave-host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal. The enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma.