Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China
Pb and Nd isotopic composition of the Jigongshan granite: constraints on crustal structure of Tongbaishan in the middle part of the Qinling–Tongbai–Dabie orogenic belt, Central China
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DOI：
10.1016/j.lithos.2003.12.006
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发表时间：
2004-04
期刊：
影响因子：
3.5
通讯作者：
Hongfei Zhang;Z. Zhong;Shan Gao;Ben-ren Zhang;Li Zhang;Shenghong Hu;Q. Hou
中科院分区：
文献类型：
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作者：
Hongfei Zhang;Z. Zhong;Shan Gao;Ben-ren Zhang;Li Zhang;Shenghong Hu;Q. Hou
秦岭-大别-苏鲁变质带是世界上最大的超高压变质带。大别-苏鲁超高压变质作用的年龄为220-245 Ma，而秦岭超高压变质作用的年龄为507 Ma。桐柏山位于西秦岭造山带和东大别-苏鲁超高压变质带之间。是研究秦岭与大别-苏鲁带构造关系和超高压变质作用穿时性的关键地区。鸡公山花岗岩体（t=128 Ma），总面积1200 km ~ 2，主要侵位于桐柏岩体中，为桐柏岩体的出露基底。鸡公山花岗岩的SiO_2 =69.85- 72.35%，K_2O/Na_2O =0.87-1.13，A/CNK=0.91-1.03，Rb/Sr=0.14-0.25，Th/U=3.3-12。稀土元素组成具有强烈的分馏模式，（La/Yb）N=14-58，Eu*/Eu=0.79 ~ 1.05。花岗岩具有低放射成因铅同位素组成的特点。现今全岩Pb同位素比值为206 Pb/204 Pb =16.707- 17.055，207 Pb/204 Pb =15.239-15.326，208 Pb/204 Pb =37.587-37.853，与大陆下地壳的Pb同位素比值相近。它们的εNd（t）值介于−16 ~ −20之间，亏损地幔Nd模式年龄（TDM）介于1.8 ~ 2.2 Ga之间。上述证据表明，鸡公山花岗岩的岩浆来源于陆壳的部分熔融。鸡公山花岗岩的Pb、Nd同位素组成与大别核杂岩相似，但与桐柏核杂岩不同。大别核杂岩是鸡公山花岗岩的岩浆源区。结果表明，大别核杂岩向西延伸，构成桐柏杂岩下伏的未出露基底。
The Qinling–Dabie–Sulu belt is the world's largest ultrahigh pressure (UHP) metamorphic belt. The UHP metamorphism is well dated at 220–245 Ma in the Dabie–Sulu belt but at ∼507 Ma in the Qinling belt. The Tongbaishan is located between the Qinling orogenic belt to the west and the Dabie–Sulu UHP metamorphic belt to the east. It is the key area for studying the tectonic relation between the Qinling and Dabie–Sulu belts and the diachronous UHP metamorphism. The Jigongshan granitic pluton (t=128 Ma) with a total area of 1200 km2, composed of monzogranite, was mostly emplaced into the Tongbai complex, an exposed basement in the Tongbaishan. The Jigongshan granites have SiO2=69.85–72.35%, K2O/Na2O=0.87–1.13, A/CNK=0.91–1.03, Rb/Sr=0.14–0.25 and Th/U=3.3–12. Their REE compositions show strongly fractionated patterns with (La/Yb)N=14–58 and Eu*/Eu=0.79–1.05. The granites are characterized by low radiogenic Pb isotopic composition. The present-day whole-rock Pb isotopic ratios are206Pb/204Pb=16.707–17.055,207Pb/204Pb=15.239–15.326 and208Pb/204Pb=37.587–37.853, which are similar to that of the continental lower crust. Their εNd(t) values range from −16 to −20, and depleted-mantle Nd model ages (TDM) from 1.8 to 2.2 Ga. The above evidence indicates that the magma of the Jigongshan granites was derived from the partial melting of the continental crust. The Pb and Nd isotopic compositions of the Jigongshan granites resemble those of the Dabie core complex in the Dabieshan but are distinct from those of the Tongbai complex in the Tongbaishan. Thus, the Dabie core complex would be the magma source of the Jigongshan granites. The result implies that the Dabie core complex is extended to the west and constitutes the unexposed basement underlaying the Tongbai complex in the Tongbaishan.