Petrogenesis of Neoproterozoic Granitoids from the Wulian Region in the Sulu Orogen: Sr-Nd Isotopic Constraints
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摘要: 五莲新元古代花岗岩类分布于五莲断裂以北, 动力变形强, 但变质程度弱, 仅达到绿片岩相.岩性以黑云正长花岗质为主, 少数为石英正长质.岩石中Sr同位素初始值的变化范围很广, 从0.697 306~0.753 765, 除了部分是由于源区的不均一或多源不同比例混合外, 岩浆侵位过程中或固结后的次生扰动可能也起着一定的作用.而它们的εNd (750 Ma) 均为负值且变化范围广(-3.1~-24.3), Nd同位素二阶段模式年龄从1.7~3.4 Ga, 均显示它们的源区以古老的地壳为主, 但具有多源混合的特征.推测五莲新元古代花岗岩类最可能的成因是太古代的TTG片麻岩(主要是其中的英云闪长质组分) 在压力约0.8 Gpa (相当于~25 km的中下地壳) 条件下发生部分熔融, 并混入了不同比例的年轻地幔来源物质.地幔来源物质的加入对于五莲新元古代花岗岩类的成因有重要的影响.Abstract: The Neoproterozoic granitoids in Wulian region are located north of the Wulian fault. They have undergone strong dynamic deformation but are slightly metamorphosed (green-schist facies). The granitoids are mainly biotite-syenogranitic in composition with minor quartz syenitic. The initial 87Sr/86 Sr ratios (ISr) are wide, ranging from 0.697 306 to 0.753 765. This may partially be ascribed to the inhomogeneity of magma source material or multi-source mixing with different proportions. Secondary disturbed after the granitic body's intrusion and concretion may also play a role, to some degree. The εNd (750 Ma) values of the granitoids are all lower than 0 and with wide ranges (-3.1 to-24.3), and the model ages (TDM) of two stages ranges from 1.7 to 3.4 Ga. This indicates that the parental magma of the Neoproterozoic granitoids in the Wulian region was primarily derived from crustal materials but with the mixed features of multi-sources. Therefore, the Neoproterozoic granitoids in Wulian have most probably been generated by partial melting of Archaean TTG gneisses (major tonalitic component thereinto) at pressures of about 0.8 Gpa (i.e. within the middle or the lower crust), and the addition of different proportions of mantle-derived juvenile materials. The addition of mantle-derived materials has been an important influence on the genesis of the Neoproterozoic granitoids in the Wulian region.
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Key words:
- granitoid /
- Sr and Nd isotope /
- partial melting /
- Wulian /
- Sulu orogen
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图 2 五莲片麻岩的地球化学成分图解
a.石英(Q)-碱性长石(A)-斜长石(P) 三角分类图解(Le Maitre, 1989); b.A/NK-A/CNK关系图; c. (Nb+Y)-Nb构造环境判别图(Pearce et al., 1984); d.La/Yb~Th/Yb判别图(Condie, 1989); e.稀土元素球粒陨石标准化曲线(球粒陨石数值据Boynton, 1984); f.不相容元素原始地幔标准化蛛网图(原始地幔数值据McDonough and Sun, 1995); 侵入岩系列的演化趋势据Lameyre and Bonin (1991); A.强碱性; B.适度碱性; C.二长质; D.钙碱性.资料来源: 薛怀民等(2005b); 黄洁等(2005)
Fig. 2. Geochemical compositions of representative samples from the Wulian gneisses
图 3 五莲新元古代花岗岩类的同位素组成
a.147Sm/144Nd与143Nd/144Nd关系图; b.Nd同位素初始值投影在εNd-时间演化图上(扬子基底TTG演化趋势据Gao et al., 1999); c.Sr同位素初始值与1/Sr关系图; d.Nd同位素初始值与1/Nd关系图.扬子基底TTG片麻岩资料据Gao et al. (1999)
Fig. 3. Sr and Nd isotopic compositions of the Neoproterozoic granitoid from Wulian
图 4 五莲新元古代花岗岩类的εNd (750 Ma) 与ISr关系图解
将扬子地台北缘随州地区晚元古代末期的辉长岩(Sr=166 μg/g, ISr=0.704 5, Nd=14.16 μg/g, εNd (750 Ma) =+4.6) 代表地幔组分.2个地壳组分, 地壳1为崆岭地区太古代的TTG片麻岩(Ames et al., 1996; Sr=143 μg/g, ISr=0.708 456, Nd=9.05 μg/g, εNd (750 Ma) =-33.9), 地壳2为韩国的一类地壳组分(Kwon et al., 1999)
Fig. 4. εNd (750 Ma) vs. ISr diagram showing mixing correlation between the presumed mantle component and crustal components for the Neoproterozoic granitoid from Wulian
表 1 五莲新元古代花岗岩类的Rb-Sr和Sm-Nd同位素组成
Table 1. Rb-Sr and Sm-Nd isotopic compositions of Neoproterozoic granitoids from Wulian
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