Journal of Petrology Advance Access originally published online on February 2, 2007
Journal of Petrology 2007 48(4):661-692; doi:10.1093/petrology/egl077
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Generation of Palaeocene Adakitic Andesites by Magma Mixing; Yanji Area, NE China
1Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Wushan, Guangzhou, 510640, China
2Institute for Study of the Earth's Interior (ISEI), Okayama University, Yamada 827, Misasa, Tottori 682-0193, Japan
RECEIVED JUNE 16, 2006; ACCEPTED DECEMBER 14, 2006
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Abstract |
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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 + 
Fe) 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 Y (10–11 ppm) and heavy rare earth elements (HREE; e.g. Yb = 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, 
Nd(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 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 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.
KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China
* Corresponding author. Telephone: + 86-20-85290280. Fax: + 86-20-85290130. E-mail: guofengt{at}263.net
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