Evolution of Alkalic Magma Systems: Insight from Coeval Evolution of Sodic and Potassic Fractionation Lineages at The Pleiades Volcanic Complex, Antarctica

摘要

The magmatic evolution of The Pleiades, a Quaternary alkalic volcanic complex in Northern Victoria Land (NVL), Antarctica, is investigated using major and trace element, and Sr, Nd and Pb isotopic data. The volcanic rocks can be subdivided into two distinct magmatic lineages based on petrography and whole-rock compositions: (1) a sodic silica-undersaturated lineage with abundant kaersutite phenocrysts and (2) a potassic and mildly-alkalic, silica-saturated to slightly under-saturated lineage containing olivine phenocrysts but no kaersutite. The pressure-temperature paths estimated by clinopyroxene-liquid thermobarometry are similar in each lineage. Mass-balance calculations using whole-rock and mineral compositions show that kaersutite fractionation without olivine has played a major role in magmatic differentiation of the sodic lineage, whereas the compositional variations of the potassic lineage can be ascribed to fractionation of an assemblage of plagioclase, clinopyroxene, olivine, titanomagnetite and apatite, combined with about 10% lower crustal assimilation. The higher Sr-87/Sr-86 (> 07035), lower Nd-143/Nd-144 (< 051285), and Pb-206/Pb-204 (< 193) ratios of the evolved potassic lavas compared to the mafic lavas support crustal assimilation. The mafic lavas from both lineages are characterized by elevated Pb-206/Pb-204 (>195) ratios and narrow ranges of Sr-87/Sr-86 (070313-070327) and Nd-143/Nd-144 (051289-051290) ratios, which is consistent with a high mu-like (HIMU, where mu=(U-238/Pb-204)(t=0)) component typical of Cenozoic volcanic rocks in Antarctica and Zealandia. This HIMU-like isotopic signature of The Pleiades volcanic rocks, together with elevated Nb concentrations and negative K anomalies in primitive mantle-normalized diagrams, suggests an amphibole-bearing metasomatized lithospheric mantle source. We suggest that the primary magmas of the two lineages were formed by partial melting of metasomatic hydrous veins in the lithospheric mantle with varying degrees of reaction with the surrounding, anhydrous peridotite. The drier potassic magma experienced greater peridotite assimilation relative to the more hydrous sodic magmas. This hypothesis is supported by lower contents of Al2O3, TiO2, K2O, Rb, and Nb in the mafic potassic lavas compared to the sodic ones. This initial difference was intensified by crustal assimilation in the potassic magma suite, resulting in a silica-saturated alkalic trend which is distinct from the trend of the sodic silica-undersaturated alkalic magmas.