Abstract:

Picrite, as one of the evident manifestations of mantle plume activity, has garnered significant attention from researchers. However, many picrites contain olivine crystals exhibiting diverse crystallization habits, indicating the incorporation of xenocrystic olivine. Thus, characterizing the properties of olivine crystals is crucial for understanding the petrogenesis of picrites and their geological implications. This study presents compositional profile analyses of representative olivine grains from picrite sills in the Kuangshanliangzi Fe deposit, western Sichuan Province. We identify four distinct populations of olivine crystals: coarse-grained olivine, aggregate olivine, inclusion olivine, and spinifex olivine. Statistical analyses reveal that coarse-grained olivine exhibits a major peak at ~Fo₈₄ and a minor peak at ~Fo₇₇. Aggregate olivine shows a major peak at ~Fo₉₂ and a minor peak at ~Fo₇₇. Inclusion olivine (~Fo₈₂) and spinifex olivine (~Fo₇₇) each display a single peak, with the former being compositionally uniform and the latter exhibiting a steep compositional gradient. By combining compositionally similar coarse-grained and inclusion olivines, three distinct olivine populations are recognized in the Kuangshanliangzi picrites. These three types are also identified in Emeishan basalt and Dabanshan gabbro, suggesting that magma mixing is a ubiquitous process within the magmatic system of the Emeishan Large Igneous Province. Thermodynamic and fluid dynamic analyses indicate that aggregate olivine crystallized at ~4 GPa and ~1600 ℃, coarse-grained olivine (and inclusion olivine) mainly formed at ~2-2.5 GPa and ~1450-1550 ℃, and spinifex olivine crystallized at ~0.1 GPa and ~1200 ℃. Accordingly, spinifex olivine is interpreted to have crystallized from the carrier magma, whereas the other two types represent antecrysts transported from deeper magma chambers by the ascending melt. The NiO content of olivine generally increases with Fo content but exhibits four distinct secondary trends. These trends are characterized by modest increases in Fo accompanied by sharp rises in NiO, implying that the magma was a volatile-fluid-supersaturated system. Furthermore, the volumetric proportion of olivine varies markedly with elevation, and the occurrence of aggregate olivine suggests rapid magma ascent. Based on these findings, we propose that the Kuangshanliangzi magmatic system comprised at least three deep-seated magma chambers, and that the picrites represent mixtures of melt, crystals, and fluids rather than products of direct consolidation from a primitive picritic magma.

Key words: Pingchuan iron deposit, picrite sill, olivine crystal population, Transmagmatic fluid, Panzhihua Fe deposit