Abstract

Nam Nậm Xe carbonate dikes in Phong Thổ district, Lai Châu province distribute in the Northeastem part of the Sông Đà structural zone, including coarse-grained calciocarbonatite, folliated calciocarbonatite and ferrocarbonatite. The rocks intrude Permian íbrmations of Na Vang limestones and Viên Nam basalts. The coarse-grained calciocarbonatites are composed of typical magmatic mineral association such as calcite, large biotite and aegirine crystals, arfvedsonite, magnetite and íluorapatite; the small- grain foliated calciocarbonatites are composed of small to medium calcite grains, biotite, fibrous or radial arfvedsonite, and pyrite. Accessory minerals mainly include monazite, barite, celestine, ancylite, parisite, strontianite and cordylite. Their calcites have ralatively constant values of δ¹³C and δ¹⁸O (δ¹³C_V-PDB = -3.10~-3.93‰, δ^l8O_V- SMOW = +9.09 ~ +11.04‰). These isotope values are higher than those of typical carbonatites, but completely different from those of vicinity Permian fme-grain grey limestones of Na Vang and Triassic greenish grey limestones of Đồng Giao Formations. The mineralogical features and C-O isotopic compositions are strong evidence of an existence of carbonatitic magma in the Northeastem part of Sông Đà zone in the Middle-Late Paleogene period.