Mineralogical and geochemical constraints on magmatic evolution of Paleocene adakitic andesites from the Yanji area, NE China

Feng Guo, Eizo Nakamura, Weiming Fan, Katsura Kobayashi, Chaowen Li, Xiaofeng Gao

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

Paleocene (55-58 Ma) adakitic andesites from the Yanji area of NE China are subdivided into clinopyroxene andesites and amphibole andesites. Relative to the clinopyroxene andesites, the amphibole andesites contain higher SiO2, K2O and lower MgO, FeOT, Al2O3, CaO, TiO2, Cr, Ni, Sr, Y and Nb, and have evolved Sr-Nd-Pb isotope compositions. Compositional variation between the clinopyroxene andesites and amphibole andesites involves systematic decreases in MgO, Sr/Nd and εNd(t) accompanied by increases in 87Sr/86Sr(i) and 206Pb/204Pb(i), and suggests a role for crustal contamination. The compositional variations recorded in clinopyroxene and amphibole phenocrysts indicate that the primary mantle-derived adakitic magma experienced complex crustal-level processes, including magma mixing, fractional crystallization and crustal contamination or assimilation. The low Na2O contents and adakitic trace element features (high Sr/Y and Nd/Yb) in Mg-rich parts of clinopyroxene phenocrysts, and negative Sr and Eu anomalies in the Fe-rich clinopyroxene cores, suggest that all clinopyroxenes crystallized in equilibrium with little or no garnet, and argues against magmatic evolution involving differentiation of basaltic magmas. Combined mineralogical and geochemical data indicate that the clinopyroxene andesites were generated by magma mixing, clinopyroxene fractionation and limited degrees of crustal assimilation; whereas the amphibole andesites underwent magma mixing, fractionation of clinopyroxene + amphibole +/- plagioclase and higher degree of crustal assimilation. The difference in mineralogical assemblage between the two rock types was influenced by magma temperature, i.e., 900-950 °C for the amphibole andesites and > 950 °C for the clinopyroxene andesites. Our results provide the following important constraints on high-MgO adakitic magma (or low-SiO2 adakite) petrogenesis: (1) the primary magma of such adakites is probably produced through melting of slab melt-modified mantle rather than being a slab melt variably hybridized by peridotite: (2) The complex magmatic evolution recorded in the Yanji adakitic andesites suggests that such adakitic rocks are far from melts in equilibrium with mantle, and magmatic process needs to be carefully examined before the petrogenetic or geodynamic significance can be assessed: (3) The perquisites for creating high-MgO or low-SiO2 adakites include slab melt-mantle interaction during oceanic slab subduction and melting of this metasomatized mantle, but this melting event need not necessarily be related in time to the subduction event.

Original languageEnglish
Pages (from-to)321-341
Number of pages21
JournalLITHOS
Volume112
Issue number3-4
DOIs
Publication statusPublished - Oct 1 2009

Keywords

  • Adakitic andesites
  • Geochemistry
  • Magmatic evolution
  • Mineralogy
  • NE China
  • Paleocene

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

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