Reconstruction of primary alkaline magma composition from mineral archives: Decipher mantle metasomatism by carbonated sediment

Abstract

Alkaline magmas play a key in understanding crust-mantle interaction and deep carbon recycling. Here, petrography and in situ mineral analyses of evolved alkaline rocks from the North China Craton are reported to decipher mantle metasomatism by recycled carbonated sediments via reconstruction of primary magma composition. The alkaline rocks are nepheline syenites for which new, consistent zircon and titanite UPb ages of ~235 Ma are presented. The rocks are highly evolved (SiO2 = 58.1–60.0 wt%) with negative anomalies for Sr, Ba, Ti, P and Eu, and relative depletion of MREE, indicating fractional crystallization of feldspar, apatite, titanite and amphibole. Core-rim variation of 87Sr/86Sr (0.7052–0.7092) of late-crystallized low Mg# clinopyroxene and feldspar are attributed to crustal assimilation.The origin of primary alkaline magma is recorded in early high Mg# (70–84) clinopyroxenes with weak Eu anomalies (Eu/Eu* = 0.94–1.29), suggesting that they crystallized from a little-differentiated mantle-derived magma (Mg# > 60). Their variable 87Sr/86Sr ratios (0.7031–0.7060) correlate positively with Sr/Y and (La/Yb)N and negatively with Ti/Eu ratios, but are not correlated with Mg# or Eu/Eu*. Moreover, the REE patterns of melt equilibrated with high 87Sr/86Sr clinopyroxene is similar to coeval alkaline rocks from adjacent areas. These features indicate that the primary alkaline magma was derived from the mixing of depleted mantle and enriched mantle metasomatized by carbonatite melts from subducted crustal materials. Low zircon εHf(t) values (−1.4 to −2.8) imply that the subducted crustal material was carbonated sediments. Calcite and CO2 inclusions occur in early-crystallized olivine, clinopyroxene and zircon, and amphibole and fluorapatite both reveal a volatile-rich (H2O, F and CO2) alkaline magma derived from mantle enriched by carbonate metasomatism. The mineral archives of alkaline rocks not only record mantle metasomatism by recycled carbonated sediments, but also fractional crystallization and assimilation during magma evolution.