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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