PETROGEOCHEMICAL FEATURES OF LAMPROITE FORMATIONS
IN P̀N HỒ - SIN CAO AREA, TAM ĐƯỜNG, LAI CHÂU PROVINCE
Nguyễn Thị Bích Thủy1, Nguyễn Thị Xuân2, Bùi Thế Anh2
1General Department of Geology and Minerals of Vietnam
2Vietnam Institute of Geosciences and Mineral Resources
Abstract: Lamproite formations in Ṕn Hồ - Sin Cao area were found in Giang Ma commune, Tam Đường, Lai Châu province, they occurred under forms of intrusive dykes and pyroclastic lamproite tuffs. Petrographic compositions of lamproite dykes are mainly of 2 types: 1) Olivine- clinopyroxene lamproites with olivine phenocrysts, and 2) Phlogopite - clinopyroxene lamproites with phenocrysts of phlogopite and clinopyroxene. The pyroclastic lamproite tuff is phlogopite-pyroxene lamproite tuff with a rare occurrence of olivine. Chemically analytical results of rock-forming minerals indicated that olivine, proxene and phlogopite have relative high contents of MgO and low TiO2; some K-feldspar minerals have high concentration of BaO. All of analytical lamproite samples belong to high magnesium series and ultra-high potassium rocks, relatively high Al2O3 (>10 wt%) and low TiO2 (<1 wt%) contents. These geochemical features of lamproites are similar with those of lamproites from Murcia-Almenia (Spain), West Australia and Roman lava (Italy). Additionally, the lamproites are characterized by enrichment in both incompatible (Rb, K, U, Pb) and compatible (Cr>700 ppm, Ni>200 ppm, V>100 ppm) elements. These imply that lamproite formations were formed by partial melting of lithospheric mantle sources, which have been metasomatized in the past relating to an ancient subduction event.
Keywords: Lamproite, lamproite tuff, Ṕn Hồ - Sin Cao, Tam Đường.
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