Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China

Abstract

Palaeocene (c.55^58 Ma) adakitic andesites from the Yanji area, NE China, are typically clinopyroxene-bearing sodic andesites containing 60Á9^62Á2% SiO 2 and 4Á02^4Á36% MgO, with high Mg-number [100 Mg/(Mg þ ÆFe) atomic ratio] from 65Á5 to 70Á1.Whole-rock geochemical features include high Cr (128^161ppm) and Ni (86^117 ppm) concentrations, extremely high Sr (2013^2282 ppm), low Y (10^11ppm) and heavy rare earth elements (HREE; e.g.Yb ¼ 0Á79^1Á01ppm), and mid-ocean ridge basalt (MORB)-like Sr^Nd^Pb isotopic compositions [e.g.87 Sr/ 86 Sr(i) ¼ 0Á70298^0Á70316, e Nd (t) ¼ þ3Á8 to þ6Á3 and 206 Pb/ 204 Pb ¼ 17Á98 ^18Á06], analogous to high-Mg adakites occurring in modern subduction zones.However, mineralogical evidence from clinopyroxene phenocrysts and microcrystalline plagioclase clearly points to magma mixing during magma evolution.Iron-rich clinopyroxene (augite) cores with low Sr, high Y and heavy REE contents, slightly fractionated REE patterns and large negative Eu anomalies probably crystallized along with low-Ca plagioclase from a lower crustal felsic magma.In contrast, high Mg-number clinopyroxene (diopside and endiopside) mantles and rims have higher Sr and lower HREE and Y concentrations, highly fractionated REE patterns (high La/Yb) and negligible Eu anomalies, similar to those found in adakites from subduction zones.The Yanji adakitic andesites can be interpreted as a mixture between a crust-derived magma having low Mg-number and Sr, and high Y and HREE, and a mantle-derived high Mg-number adakite having high Sr and low Y and HREE concentrations.During storage and/or ascent, the mixed magma experienced further crustal contamination to capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional