Geochronology and geochemistry of the Early Jurassic granitoids in the central Lesser Xing'an Range, NE China and its tectonic implications
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摘要:
本文对小兴安岭中部3个花岗岩和1个黑云母二长岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究，以便了解其形成时代、成因以及它们所揭示的区域构造背景。研究区4个代表性岩石中的锆石均呈自形-半自形，显示出典型的岩浆震荡生长环带，暗示其为岩浆成因。测年结果表明研究区所讨论的花岗岩与黑云母二长岩均为早侏罗世(175~185Ma)岩浆事件的产物。这些花岗岩的SiO2含量为67.32%~75.61%，Al2O3含量为12.76%~16.71%，全碱(Na2O+K2O)值为8.07%~10.40%，Na2O/K2O值为0.94~1.67，属于高钾钙碱性系列。此外，该期花岗岩的A/CNK值均小于1.1，且A/NK均大于1，显示I型花岗岩的特点；且以富集大离子亲石元素(LILEs)和轻稀土元素(LREEs)，亏损重稀土元素(HREEs)和高场强元素(HFSEs)为特征。锆石εHf(183~185Ma)=+6.3~+10.8，对应的二阶段模式年龄为754~1092Ma，显示它们的岩浆起源于新增生陆壳的部分熔融。该期花岗岩与区域上同时代的镁铁质-超镁铁质侵入岩组成典型的双峰式火山岩组合，揭示该区在早侏罗世处于一种伸展环境。结合区域早侏罗世火山岩的空间展布特征，小兴安岭中部早侏罗世花岗岩的形成应与古太平洋板块向欧亚大陆下俯冲作用和蒙古-鄂霍茨克洋向额尔古纳地块之下俯冲作用--即双向俯冲作用的弧后伸展环境相对应。
Abstract:LA-ICP-MS zircon U-Pb dating and geochemical data are reported for three granites and one biotite monzonite in the central Lesser Xing'an Range in order to constrain its formation time, petrogenesis and the regional tectonic setting. Zircons from four representative intrusive rocks are euhedral-subhedral in shape and display oscillatory growth zoning in CL images, implying their magmatic origin. The dating results indicate that the granites and biotite monzonite from the northwestern Lesser Xing'an Range formed in the Early Jurassic (175~185Ma). The granitoids have SiO2=67.32%~75.61%, Al2O3=12.76%~16.71%, Na2O+K2O=8.07%~10.40%, Na2O/K2O=0.94~1.67, A/CNK=0.91~1.06, A/NK=1.09~1.27, and belong chemically to high-potassium calc-alkaline I-type ones. Moreover, these granitoids are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). Their zircon εHf(183~185Ma) values range from +6.3 to +10.8, and Hf two-stage model ages vary from 754Ma to 1092Ma, implying that their primary magmas were derived from partial melting of the juvenile lower crust. The Early Jurassic granitoids, together with coeval mafic-ultramafic intrusive rocks in the Lesser Xing'an-Zhangguangcai Range, constitute a typical bimodal igneous rock association, suggesting an extensional environment. Combined with the spatial compositional variation of the Early Jurassic igneous rocks in NE China, it is suggested that the Early Jurassic granitoids from the Lesser Xing'an Range formed under an extensional environment similar to back-arc basin which could be related to double-subductions of the Paleo-Pacific plate beneath the Eurasian continent and the Mongol-Okhotsk oceanic plate beneath the Erguna Massif.
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Key words:
- Early Jurassic /
- Granitoid /
- Geochronology /
- Geochemistry /
- Tectonic setting /
- Central Lesser Xing'an Range
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图 1
研究区地质简图(a, 据Wu et al., 2007修改)
Figure 1.
Geological sketch map of the studied area (a, modified after Wu et al., 2007)
图 2
研究区早侏罗世侵入岩的镜下显微照片
Figure 2.
Photomicrographs of the Early Jurassic intrusive rocks
图 3
研究区早侏罗世侵入岩部分测定锆石的CL图像
Figure 3.
CL images of the selected zircons from the Early Jurassic intrusive rocks in the study area
图 4
研究区早侏罗世侵入岩锆石U-Pb年龄谐和图
Figure 4.
Zircon U-Pb concordia diagrams for the Early Jurassic intrusive rocks in the study area
图 5
研究区早侏罗世侵入岩SiO2-(Na2O+K2O)和SiO2-K2O图解(据Irvine and Baragar, 1971; Rickwood，1989)
Figure 5.
Plots of total alkali versus SiO2 (TAS) and SiO2 versus K2O for the Early Jurassic intrusive rocks in the study area (after Irvine and Baragar, 1971; Rickwood, 1989)
图 6
研究区早侏罗世侵入岩的A/CNK-A/NK图解(据Maniar and Piccoli, 1989)
Figure 6.
Plot of A/CNK versus A/NK for the Early Jurassic intrusive rocks in this study (after Maniar and Piccoli, 1989)
图 7
研究区早侏罗世侵入岩的稀土元素球粒陨石标准化配分图解(a, 标准化值据Boynton, 1984)和微量元素原始地幔标准化蛛网图(b, 标准化值据Sun and McDonough, 1989)
Figure 7.
Chondrite-normalized REE patterns (a, normalization values afterBoynton, 1984) and primitive mantle-normalized trace element spider (b, normalization values after Sun and McDonough, 1989) diagrams for the Early Jurassic intrusive rocks
图 8
研究区早侏罗世侵入岩的锆石εHf(t)-t图解(a, 据Yang et al. 2006)
Figure 8.
Zircon Hf isotopic features for the Early Jurassic intrusive rocks in the study area (a, after Yang et al., 2006)
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