大兴安岭北端洛古河东花岗岩的地球化学、SHRIMP锆石U-Pb年龄和岩石成因
Geochemistry, zircon SHRIMP U-Pb age and petrogenesis of the East Luoguhe granites at the northern end of the Great Hinggan Range
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摘要: 大兴安岭北端漠河县洛古河东岩体主要岩石类型为二长花岗斑岩、正长花岗斑岩和石英二长斑岩，内部可见闪长质微粒包体，属高钾钙碱性I型花岗岩。花岗岩的元素地球化学和锆石SHRIMP铀-铅年代学研究结果表明，洛古河东岩体形成于早白垩世，其花岗斑岩体的锆石SHRIMP铀-铅年龄为129.8±2.2Ma。花岗岩的SiO2含量介于68.03%~74.32%之间，Al2O3含量介于13.06%~14.55%之间，Na2O/ K2O介于0.45~0.86之间，铝饱和指数为0.94~1.11，Mg#指数介于18~42之间且多小于30。稀土元素总量为160.00×10-6 ~ 235.15×10-6，δEu介于0.31~0.52，(La/Yb)N介于8.99~17.87，为轻稀土富集型。岩体Sr含量低，介于118×10-6 ~ 268×10-6之间，而Y含量高，介于16.9×10-6~ 26.1×10-6之间，Sr/Y比值低，介于5.62~13.81之间，属低锶高钇型岩石。在原始地幔标准化的微量元素蛛网图中，Rb、Th、U、K、Zr、Hf和轻稀土元素（如La、Ce、Nd和Sm等）富集，Ba、Sr、P和Ti等元素强烈亏损，Nb和Ta具有中等-弱亏损。主量、稀土和微量元素特征表明，岩石具后碰撞花岗岩类的地球化学特征，属后碰撞花岗岩。岩体εNd (t)值介于-3.45~-2.64，平均-3.01；亏损地幔Nd模式年龄介于969~1131Ma之间，平均1018Ma；锶初始比值 (ISr)介于0.702486~0.707269之间，平均0.705434；钾长石206Pb/204Pb、207Pb/204Pb和208Pb/204Pb比值变化范围分别为18.5939~18.6721、15.6019~15.6058和38.4058~38.5249，平均值分别为18.6426、15.6035和38.4613；岩体中的钾长石氧同位素组成很低，δ18O (‰)值介于-8.1 ~ 4.1之间，多为负值，表明洛古河东岩体为低18O花岗岩。Nd、Sr、Pb和O同位素组成显示洛古河东岩体形成于含有较多幔源成分的源区物质的部分熔融作用，推测源区主要为Rodinia超大陆会聚过程中（中元古代—新元古代之交）形成的初生地壳。由于古亚洲洋和蒙古-鄂霍茨克洋分别于古生代末期和二叠纪—中侏罗世闭合，因此大兴安岭北端早白垩世花岗岩应该形成于中朝-蒙古大陆与西伯利亚大陆碰撞造山过程的后碰撞阶段。
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关键词:
- 洛古河东早白垩世花岗岩 /
- 锆石SHRIMP铀-铅年龄 /
- 元素地球化学 /
- 同位素地球化学 /
- 大兴安岭
Abstract: The East Luoguhe intrusion in Mohe County, Heilongjiang Province, located in the northern end of the Great Hinggan Range, is mainly composed of monzogranite-porphyry, syenogranite-porphyry and quartz monzonite-porphyry, with minor dioritic microgranular enclave. The intrusion belongs to I-type granite, with affinity to high-K calc-alkaline series. The SHRIMP zircon U-Pb age of the granite-porphyry is 129.8±2.2Ma, indicating that the intrusion formed in Early Cretaceous. Its SiO2 and Al2O3 contents range 68.03%~74.32% and 13.06%~14.55%, respectively. The Na2O/K2O ratios range 0.45~0.86, ASI values from 0.94 to 1.11(mostly# index from 18 to 42 (mostly-6and 235.15×10-6, with δEu of 0.31~0.52 (average 0.41) and (La/Yb)N ratios of 8.99~17.87(average 13.82). The East Luoguhe intrusion is characterized by low-Sr (118 ×10-6~ 268 ×10-6), high-Y (16.9 ×10-6~ 26.1 ×10-6, generally >18 ×10-6), and low Sr/Y ratios (5.62~13.81). The intrusion is enriched in Rb, Th, U, K, Zr, Hf and LREE (e.g. La, Ce, Nd and Sm), but strongly depleted in Ba, Sr, P and Ti, with notable depletion of Nb and Ta. The geochemical signatures above are similar to those of post-collisional granitoids, suggesting that the East Luoguhe intrusion can be classified into the group of post-collisional granites. The intrusion has low initial 87Sr/86Sr ratio (0.702486~0.707269, average 0.705434), clear negative εNd (t) value (-3.45~ -2.64, average -3.01), young Nd-model age (969~1131Ma, average 1018Ma), and extremely lowδ18O ratio (-8.1‰to 4.1‰ for K-feldspar). And the 206Pb/204Pb (18.5939~18.6721, average 18.6426), 207Pb/204Pb (15.6019~15.6058, average 15.6035) and 208Pb/204Pb (38.4058~38.5249, average 38.4613) ratios for K-feldspar are relatively high.The Nd-Sr-Pb-O isotope systematics shows that the intrusion originated from partial melting of a source with remarkable mantle-derived components. The source is likely the juvenile crust formed during the convergence of Rodinia supercontinent at the transition from Mesoproterozoic to Neoproterozoic. Given that the Paleo-Asian Ocean was finally closed at the end of Late Paleozoic, followed by closing of the Mongol-Okhotsk Ocean during Permian-Middle Jurassic, the East Luoguhe intrusion of Early Cretaceous age must be formed in the collision regime between the Siberia and Mongol-Sinokorea continents, especially in a tectonic transition setting from compression to extension. -
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