Generation of palaeocene adakitic andesites by magma mixing; Yanji Area, NE China

Feng Guo, Eizo Nakamuru, Weiming Fan, Katsura Kobayoshi, Chaowen Li

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214 Citations (Scopus)

Abstract

Palaeocene (c. 55-58 Ma) adakitic andesites from the Yanji area, NE China, are typically clinopyroxene-bearing sodic andesites containing 60·9-62·2% SiO2 and 4·02-4·36% MgO, with high Mg-number [100 Mg/(Mg + ΣFe) atomic ratio] from 65·5 to 70·1. Whole-rock geochemical features include high Cr (128-161 ppm) and Ni (86-117 ppm) concentrations, extremely high Sr (2013-2282 ppm), low Υ (10-11 ppm) and heavy rare earth elements (HREE; e.g. Υb = 0·79-1·01 ppm), and mid-ocean ridge basalt (MORB)-like Sr-Nd-Pb isotopic compositions [e.g. 87Sr/86Sr(i) = 0·70298-0·70316, εNd(t) = +3·8 to +6·3 and 206Pb/204Pb = 17·98 - 18·06], 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 Υ 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 Υ concentrations, highly fractionated REE patterns (high La/Υb) 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 Υ and HREE, and a mantle-derived high Mg-number adakite having high Sr and low Υ 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 tectonic regime in the Yanji area and surrounding regions suggest that these Palaeocene adakitic andesites were formed during post-subduction extension that followed the late Cretaceous Izanagi-Farallon ridge subduction. Generation of these adakitic andesites does not require contemporaneous subduction of a young, hot oceanic ridge or delamination of eclogitic lower crust as suggested by previous models.

Original languageEnglish
Pages (from-to)661-692
Number of pages32
JournalJournal of Petrology
Volume48
Issue number4
DOIs
Publication statusPublished - Apr 1 2007

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Keywords

  • Adakitic andesites
  • Magma mixing
  • NE China
  • Palaeocene

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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