Widespread late Neoarchean reworking of Meso- to Paleoarchean continental crust in the Anshan-Benxi area, North China Craton, as documented by U-Pb-Nd-Hf-O isotopes

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2015
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Abstract
We present an integrated study of zircon U-Pb dating and O-Hf isotopic analyses combined with whole-rock geochemistry and Nd isotopic systematics on widespread late Neoarchean syenogranites (Qidashan Pluton) and other granitoids (granodiorite, quartz monzonite, monzogranite) in the Anshan-Benxi area, eastern North China Craton. All these rocks were emplaced at *ca*. 2.5 Ga according to SHRIMP zircon U-Pb dating and other indirect methods. The syenogranites are characterized by high SiO~2~ and K~2~O/Na~2~O and low CaO, FeOt, MgO, TiO~2~ and P~2~O~5~. However, they differ in trace element and REE compositions and can be roughly subdivided into two types. Type 1 syenogranite has strongly negative Eu anomalies and Ba depletion, with large variations of LREE to HREE differentiation; type 2 syenogranite has no negative Eu anomaly and Ba depletion. Other granitoids are also rich in K~2~O and show similar trace element and REE compositional features as the type 2 syenogranite. Most syenogranite samples have whole-rock ε~Nd(t)~ values of --10.5 to --2.7 and t~DM(Nd)~ ages of 2.96 to 3.90 Ga, whereas the other granitoids have ε~Nd(t)~ values of --5.2 to --2.2 and t~DM(Nd)~ ages of 2.95 to 3.19 Ga. Magmatic zircon from the syenogranites has ε~Hf(t)~ values and Hf crustal model ages ranging mainly from --11.0 to +4.4 and 2.70 to 3.46 Ga, respectively; those from other granitoids have ε~Hf(t)~ values and Hf crustal model ages of --16.0 to +2.4 and 2.81 to 3.72 Ga, respectively. All these rocks contain Meso- to Paleoarchean xenocrystic zircon grains with ε~Hf(t)~ values and Hf crustal model ages ranging from --14.3 to +8.3 and 2.79 to 3.93 Ga, respectively. Decoupling of whole-rock Nd and Hf-in-zircon isotopes occurs in some samples. Most low-U magmatic zircon (U\<200 ppm) from all granitoids has δ^18^O values in the range of +4 to +7.2 permil, whereas high-U magmatic zircon (U≥200 ppm) shows a larger variation with the lowest δ^18^O value being 1.6. The xenocrystic zircon grains have δ^18^O values in the normal range of Archean magmatic zircon. Combined with previous studies, we conclude that the 2.5 Ga granitoids formed by recycling of mature, old continental crust, including metasedimentary sources, probably in an extensional tectonic environment.
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Authors Y. Wan, Mang Ma, Chenyin Dong, Hongzhe Xie, Shi-Shuai Xie, Peng Ren, D. Liu
Journal american journal of science
Year 2015
DOI
10.2475/07.2015.02
URL
https://ajs.scholasticahq.com/article/65679.pdf
Keywords Keywords not found

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