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| Geochemical and Pb-Sr-Nd isotopic compositions of Indosinian granitoids from the Bikou block,northwest of the Yangtze plate:Constraints on petrogenesis,nature of deep crust and geodynamics | |
| 作者姓名： | ZHANG HongFei XIAO Long ZHANG Li YUAN HongLin & JIN LanLan State Key Laboratory of Geological Processes and Mineral Resources China University of Geosciences Wuhan China Faculty of Earth Sciences China University of Geosciences Wuhan China State Key Laboratory of Continental Dynamics Northwest University Xi’an China |
| 作者单位： | ZHANG HongFei1，2，XIAO Long2，ZHANG Li1，2，YUAN HongLin3 & JIN LanLan1 1 State Key Laboratory of Geological Processes and Mineral Resources，China University of Geosciences，Wuhan 430074，China; 2 Faculty of Earth Sciences，China University of Geosciences，Wuhan 430074，China; 3 State Key Laboratory of Continental Dynamics，Northwest University，Xi’an 710069，China |
| 基金项目： | 国家自然科学基金;教育部长江学者和创新团队发展计划 |
| 摘 要： | This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma),Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3:14.56%―16.48%) and Sr (352 μg/g―1047 μg/g),and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g),resulting in high Sr/Y (36.3―150) and (La/Yb)N (7.8―36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (ISr) from 0.70419 to 70752,εNd(t) values from-3.1 to -8.5,and initial Pb isotopic ratios 206Pb/204Pb=17.891-18.250,207Pb/204Pb=15.494-15.575,and 208Pb/204Pb=37.788-38.335. Their geochemi-cal signatures indicate that the granitoids are adakitic. However,they are distinct from some adakites,generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust,be-cause they have high K (K2O: 1.49%―3.84%) and evolved Nd isotopic compositions,with older Nd iso-topic model ages (TDM=1.06―1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thick-ened basaltic lower crust. Combined with regional analyses,a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand,based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks,it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type. |
| 收稿时间： | 24 February 2006 |
| 修稿时间： | 11 August 2006 |
Geochemical and Pb-Sr-Nd isotopic compositions of Indosinian granitoids from the Bikou block,northwest of the Yangtze plate: Constraints on petrogenesis,nature of deep crust and geodynamics |
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| Zhang?HongFei,Xiao?Long,Zhang?Li,Yuan?HongLin,Jin?LanLan.Geochemical and Pb-Sr-Nd isotopic compositions of Indosinian granitoids from the Bikou block,northwest of the Yangtze plate:Constraints on petrogenesis,nature of deep crust and geodynamics[J].Science in China(Earth Sciences),2007,50(7):972-983. | |
| Authors: | Zhang HongFei Xiao Long Zhang Li Yuan HongLin Jin LanLan |
| Institution: | 1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China 2. Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074, China 3. State Key Laboratory of Continental Dynamics, Northwest University, Xi'an 710069, China 4. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China |
| Abstract: | This paper reports geochemical and Pb-Sr-Nd isotopic compositions of the Indosinian Yangba (215 Ma), Nanyili (225 Ma) and Mopi granitoids from the Bikou block of the northwestern margin of the Yangtze plate. These granitoids are enriched in Al (Al2O3: 14.56%–16.48%) and Sr (352 μg/g–1047 μg/g), and depleted in Y (<16 μg/g) and HREE (e.g. Yb<1.61 μg/g), resulting in high Sr/Y (36.3–150) and (La/Yb)N (7.8–36.3) ratios and strongly fractionationed REE patterns. The Indosinian granotoids show initial Sr isotopic ratios (I Sr) from 0.70419 to 70752, ε Nd(t) values from −3.1 to −8.5, and initial Pb isotopic ratios 206Pb/204Pb=17.891−18.250, 207Pb/204Pb=15.494−15.575, and 208Pb/204Pb=37.788−38.335. Their geochemical signatures indicate that the granitoids are adakitic. However, they are distinct from some adakites, generated by partial melting of subducted oceanic slab and/or underplated basaltic lower crust, because they have high K (K2O: 1.49%–3.84%) and evolved Nd isotopic compositions, with older Nd isotopic model ages (T DM=1.06–1.83 Ga). Geochemical and Sr-Nd isotopic compositions suggest that the magmas of the Insoninian adakitic rocks in the Bikou block were derived from partial melting of thickened basaltic lower crust. Combined with regional analyses, a lithospheric delamination model after collision between the North China and South China plates can account for the Indosinian adakitic magma generation. On the other hand, based on the Pb-Sr-Nd isotopic probing to the magma sources of the adakitic rocks, it is suggested that there is an unexposed continent-type basement under the exposed Bikou Group volcanic rocks. This can constrain on the Bikou Group volcanic rocks not to be MORB-or OIB-type. Supported by the National Natural Science Foundation of China (Grant Nos. 40234052 and 40521001) and the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT0441) |
| Keywords: | adakitic rocks geochemistry Pb-Sr-Nd isotopes petrogenesis geodynamics Bikou block |
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