Partial Melting Time of the Sulu UHP Terrane: Constraints from Zircon U-Pb Age, Trace Element and Lu-Hf Isotope Composition of Leucosome in Rongcheng Granitic Gneiss
详细信息
- 责任者：Xu Haijin (State Key Laboratory of Geological Processes and Mineral Resources ; Faculty of Earth Sciences ; China University of Geosciences ; Wuhan 430074 ; China) ; Song Yanru
- 刊名：Acta Petrologica Sinica
- 出版年：2013
- 卷期：29(5)
- 关键词：花岗质片麻岩 ; granitic gneiss ; 铀—铅计时 ; U-Pb dating ; 苏鲁造山带 ; Su-Lu Orogenic Belt
- 页码：p.1594-1606
- 图表：illus., 3 tables, 49 refs.
- 分类号：0802
- issnNo：ISSN1000-0569
- isbnNo：CN11-1922/P
摘要
In order to decipher the partial melting time of the Sulu ultrahigh pressure ( UHP) terrane during exhumation,we carried out a combined study of zircon internal structure,U-Pb age,trace element and Lu-Hf isotopic compositions coupled with fieldwork on leucosome of the Baocun grantic gneiss in Rongcheng region. Our works suggest that the leucosome,mainly composed of quartz, plagioclase,K-feldspar,and biotite,was the product of partial melting. Zircon from the leucosome display core-rim structure in cathodoluminescence ( CL) images: inherited magmatic core and new growth magmatic rim. Most age data of the inherited cores are concordant with apparent 206 Pb / 238 U ages ranging from 620 ± 8Ma to 784 ± 7Ma with a weighted mean age of 701 ± 33Ma. The inherited cores have the characteristics of typical magmatic zircons,e. g. ,high HREE and Y contents,strong HREE enrichment on chondrite-normalized REE plot,very large positive Ce and negative Eu anomalies,and very low ( Gd / Lu) N and Hf / Y ratios. The inherited cores have ε Hf ( t =700Ma) values of -13. 0 to -8. 0 ( mean = -10. 9 ±0. 8) and t DM2 of 2120Ma to 2429Ma ( mean = 2303 ± 51Ma) . The new growth rims have concordant 206 Pb / 238 U ages ranging from 211 ± 6Ma to 244 ± 5Ma with a weighted mean age of 226 ± 10Ma. Although the new growth rims are greatly different from the inherited cores in trace element compositions,they still display features of the magmatic zircon,e. g. ,strong HREE enrichment on chondrite-normalized REE plot,large positive Ce and negative Eu anomalies, and very low ( Gd / Lu) N and Hf / Y ratios. The new growth rims have ε Hf ( t =225Ma) values of -17. 3 to -14. 2 ( mean = -15. 2 ± 0. 6) and t DM2 between 2152Ma and 2347Ma ( mean = 2214 ± 33Ma) . These results suggest that protolith of the leucosome are Mid-Neoproterozoic ( 701 ± 33Ma) magmatic rocks which were from Paleoproterozoic ( 2. 2 ～ 2. 3Ga) continental crust. The partial melting is expected under eclogite-facies to granulite-facies conditions during the early stage hot exhumation of the Rongcheng UHP felsic gneisses,and the partial melting time might be 226 ± 10Ma. The K-feldspar- and quartz-rich pegmatitic veins ( ca. 220 ～ 210Ma) in the Weihai-Rongcheng region might be from fractional crystallization of the partial melts during late stage cool exhumation under amphibolite facies conditions.
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