Geochemistry and petrogenesis of carbonatites from South Nam Xe, Lai Chau area, northwest Vietnam

This paper presents a study of the petrography, mineral chemistry, geochemistry, and Sr-Nd-Pb-C-O isotope systematics of carbonatite dykes and associated rocks from the northeastern part of the Song Da intracontinental rift in South Nam Xe (northwest Vietnam) aimed at constraining the origin of the carbonatite magmas. The carbonatites are characterized by SiO2 < 12.18 wt.% and by wide ranges in FeO, MgO and CaO content that define them as calciocarbonatite and ferrocarbonatite. On U-Th-Pb isochron diagrams, whole rocks and mineral separates from the ferrocarbonatites form linear arrays corresponding to ages of 30.2-31.6 Ma (Rupelian, Oligocene). The South Nam Xe carbonatites are extremely enriched in Sr, Ba, and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) (e.g. Ti, Nb, Ta, Zr and Hf). The age-corrected Sr-Nd-Pb isotope ratios and C isotope data are relatively uniform (Sr-87/Sr-86((t)) = 0.708193-0.708349; Nd-143/Nd-144((t)) = 0.512250-0.512267; epsilon(Nd(t)) = -6.46 to -6.80; Pb-206/Pb-204((t)) = 18.26-18.79; Pb-207/Pb-204((t)) = 15.62-15.64; Pb-208/Pb-204((t)) = 38.80-39.38; delta C-13(V-PDB) = -2.7 aEuro degrees to -4.1 aEuro degrees). These isotopic compositions indicate source contamination that occurred before the production of the carbonatite magmas, and did not change noticeably during or after emplacement. The variation in oxygen isotopes is consistent with the change in mineral compositions and trace element abundances: the lower delta O-18 values (9.1-11.0 aEuro degrees) coupled with Sr-rich, Mn-poor calcite, and igneous textures such as triple junctions among calcite grain boundaries, define a magmatic origin. However, the elevated delta O-18 values of the ferrocarbonatites (12.0-13.3 aEuro degrees) coupled with a volatile-bearing mineral assemblages (including REE fluorcarbonates, sulfates, sulfides and fluorite) may be due to interaction with meteoric water during low-temperature alteration. High delta C-13 values and Sr-Pb ratios, and low Rb/Sr (0.00014-0.00301), Sm/Nd (0.089-0.141) and Nd-143/Nd-144 ratios, coupled with very high Sr-Nd concentrations, suggest the involvement of an enriched mantle component, which probably resulted from metasomatism due to the migration of subducted material. Because of the lack of tectonic data and the limited number of samples studied, this conclusion is still ambiguous and requires further study.