Sr-rich apatite from the Dangzishan leucitite-ijolite xenoliths (Heilongjiang Province): Mineralogy and mantle-fluid metasomatism

Abundant apatite grains with elevated SrO content were found in leucitite-ijolite xenoliths in the Dangzi Mountain, Heilongjiang Province. Morphological and mineralogical associations, and compositional analysis of the Sr-rich apatite, were carried out using an electron microprobe. Two principal types of apatite were distinguished. One type occurs as short-prismatic crystals with Sr-rich rims, while another type with acicular or finely columnar shapes constitutes Sr-rich apatite element maps and systematic quantitative analysis of apatite revealed great variations in SrO content (0.42%-26.79%). The calculated structural formula of the measured Sr-rich apatite is: (Ca_3.15-4.963Sr_0.019-1.510Ba_0.00-0.030Na_0.006-0.108REE_0.106-0.153) (P_2.84-3.028Si_0.009-0.094)O₁₂(F_0.675-1.079, Cl_0.000-0.256, OH_0.084-0.297). Mineralogical textures of apatite implied a replacement mechanism for the Sr-rich apatite. Sr-rich rims of apatite are suggested to have formed after metasomatism (isomorphic replacement) of early fluorapatite in a Sr-, light rare-earth elements (LREE), lithophile light elements (LILE)- and volatile-rich residual melt during late magmatism. However, the acicular apatite probably directly crystallized from a Sr-rich residual melt. The Sr-Nd isotopic composition of the apatite was distinctive from that of leucitite-ijolite and its host rock. These differences, combined with mineralogical features of apatite, imply that Sr-LREE-LILE-volatile-rich components in the residual melt were derived from deep in the asthenosphere, and subsequently were enriched at a late stage in magma evolution.