The Kanasi potassic magnesian-rich dacites, Altai, northwestern Chinese: Metasomatism by the melts of subducted sediments

Abstract

Potassic magnesian-rich dacites occur to the south of the Kanasi, Altai, northwestern Chinese. U-Pb dating of zircons from these rocks yields an early Devonian age of 411 ± 4 Ma. The dacites are enriched in SiO 2 (64.71 ̃ 67.77 wt%), MgO (3.28 ̃ 3.53 wt%) and K 2O (2.12 ̃ 2.75 wt%), and depleted in Na 2O (1.70 ̃ 1.85 wt%). The rocks have low CaO content(1.12 ̃ 1.28 wt%), high Al 2O 3 content (14.12 ̃ 15.33 wt%) and K 2O/Na 2O ratios (1.2 ̃ 1.6), and ASI indexes larger than 1.8, indicating features of peraluminous potassic calc-alkali igneous rocks, which are clearly different to primitive High-Mg andesites. The dacites possess moderate Cr(134 × 10 -6 ̃ 153 × 10 -6), Co(17.5 × 10 -6 ̃ 18.7 × 10 -6), and Ni (72 × 10 -6 ̃ 81 × 10 -6) contents. These rocks are enriched in LREEs, large ion lithophile elements(e.g. Rb, K) and high field strength element (e. g. U, Th, Pb) with moderate Sr, Ba, and "TNT" ( Ta, Nb, Ti) negative anomalies, showing geochemical characteristics of igneous rocks in subduction zone. Their high Mg #(51.7 ̃ 52.5) and low ε Nd(t)ratios(-4.6 ̃ -4.2) imply partial melting of metasomatic peridotites in the mantle wedge. The relative high Ba/La, Pb/La and low Th/Yb, Nb/Y ratios suggest that the metasomatism of mantle peridotites was predominated by the melts of subducted sediments. Therefore, the above geochemistry indicates that the potassic magnesian-rich dacites were originated from the partial melting of metasomatic mantle peridotites mainly altered by the melts of subducted sediments, showing an active continental margin in the Devonian. In addition, the timing of the potassic magnesian-rich dacites provides a good constrain on the minimum depositional age of the underlying Habahe group and suggests the sediments of the group deposited before the early Devonian.