Petrogenesis of scapolite-rich gabbro from the alkaline Cho Don complex in north-eastern Vietnam - mineralogical and geochemical implications

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A unique scapolite-rich gabbro has been found in the Cho Don complex (north-eastern Vietnam) and investigated using cathodoluminescence microscopy and spectroscopy, electron-microscopic observations, and Raman micro-spectroscopy. The scapolite, occupying up to ca. 30 vol% of rock matrix, displays distinctive brownish to greenish (S2_activated) luminescence, while its chemical composition is marked by the enrichment in Ca (~Me73–80) and CO2 (~4.60 wt%). Meionitic scapolite is accompanied by such magmatic phases as: clinopyroxenes (partially altered to uralite), oligoclase-andesine, K-feldspars, biotite, as well as fluorapatite and titanite. On the basis of micro-textural observations and phase-relations including interstitial form of scapolite, the high Ca content relative to adjacent Na-rich plagioclase and high-angle contacts between pyroxenes and scapolites, it may be inferred that the investigated Me-scapolite represents a high-temperature igneous phase. The presence of wollastonite, vesuvianite and calcite within the rock matrix gives a clue to the interaction between parental melt and country rocks that could act as a source for the formation of a specific CO2-enriched magma. Furthermore, scapolite breaks down into: (1) intergrowths of Ca-rich plagioclase with worm-like calcite that may reflect isobaric cooling following the high-temperature stage of rock evolution, and (2) prehnite-vuagnatite clusters, which in turn correspond to the late-stage low-temperature hydrothermal alteration. The formation of the latter could be induced by alteration of primary clinopyroxenes due to the release of Ca2+ and Ti4+. According to major and trace element data of whole-rock samples, the scapolite-rich gabbro originated from slightly evolved alkaline melt, but also shows enrichment in LREE relative to HREE ([La/Yb]CN ~ 10.24), as well as strong depletion in Nb-Ta-Ti relative to U and Th content. These geochemical signatures point to the enriched lithospheric mantle source of parental magma, which is likely to have been imprinted by subduction components (possibly sediment-derived melts) and/or crustal contamination.