Geochronology and geochemistry of the Early Palaeozoic granitoids in Baoshan block, western Yunnan and their implications
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摘要:
位于滇西特提斯构造带保山地块的平河岩体，岩石类型主要为花岗岩、二长花岗岩，其次有少量的花岗闪长岩。4件样品的锆石U-Pb年龄变化于480~486Ma，表明这些花岗岩类侵位于早奥陶世。平河花岗岩类的K2O/Na2O值大于1，铝饱和指数(A/CNK) 为1.07~1.1，属高钾钙碱性过铝质花岗岩。岩石总体上富集大离子亲石元素和Pb，亏损高场强元素。明显富集轻稀土元素[(La/Yb)N=4.33~7.05]，显示明显的负Eu异常(δEu=0.25~0.48)。4件样品58个测点的锆石εHf(t) 值变化范围较大(主要集中于-12.4~-3.0之间)，对应的Hf同位素地壳模式年龄集中于2.2~1.7Ga。这些地球化学特征指示平河花岗岩类为S型花岗岩，主要来源于古老地壳物质(如砂屑岩) 的重熔，并不同程度地混入了幔源物质。平河花岗岩类与出露于中部拉萨地体的变质酸性火山岩存在可比性，可能代表了早古生代冈瓦纳大陆原特提斯边缘岩浆弧的一部分。
Abstract:The Pinghe granitoid pluton, located in the Baoshan block of the Tethyan belt, western Yunnan, consists mainly of granite and monzogranite with subordinate granodiorite. Zircon U-Pb ages of 480~486Ma for four samples from this pluton indicate that this pluton was emplaced during the Early Ordovician. The Pinghe granitoids are high-K calc-alkaline and peraluminous, with K2O/Na2O>1 and A/CNK=1.07~1.13. These rocks are enriched in large-ion lithophile elements (LILEs), light rare-earth elements (LREEs)[(La/Yb)N=4.33~7.05] and Pb, and depleted in high field-strength elements (HFSEs). They show similar chondrite-normalized REE patterns, with negative Eu anomalies (δEu=0.25~0.48). 58 analyses from four samples have a wide range zircon εHf(t) values (-12.4~-3.0) and varying Hf-isotope crustal model ages (2.2~1.7Ga). Such geochemical signatures indicate that the Pinghe granitoids are S-type granite in character and were derived mainly from anatexis of ancient crustal materials (such as greywacke) with varying contributions from mantle-derived components. The Pinghe granitoids are geochemically comparable to the coeval silicic metavolcanic rocks reported in the central Lhasa Terrane. It is suggested that the Baoshan block may represent a part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean.
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Key words:
- Early Palaeozoic granitoids /
- Geochemistry /
- Zircon Hf isotope /
- Baoshan block /
- Western Yunnan
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图 1
滇西保山地块构造地质简图(a，据Shen et al., 2002修改)、保山地块平河花岗岩类的地质简图(b，据金世昌和庄凤良，1988修改) 和花岗岩类标本及显微镜下特征(c-f)
Figure 1.
Simplified tectonic map of the Baoshan block in western Yunnan Province (a, modified after Shen et al., 2002), simplified geological map of the Pinghe granitoids in Baoshan block (b, modified after Jin and Zhuang, 1988) and rock photograph and microphotographs of the Pinghe granitoids (c-f)
图 2
平河花岗岩类锆石U-Pb年龄谐和图(a-d) 和阴极发光图像(e)
Figure 2.
U-Pb age concordia plots (a-d) and cathodoluminescence (CL) images (e) of zircon grains from the Pinghe granitoids
图 3
平河花岗岩类的K2O-SiO2图解(实线据Peccerillo and Taylor, 1976; 虚线据Middlemost, 1985) 和A/CNK-A/NK图解(据Maniar and Piccoli, 1989)
Figure 3.
K2O-SiO2(solid line after Peccerillo and Taylor, 1976; dash line after Middlemost, 1985) and A/CNK-A/NK (after Maniar and Piccoli, 1989) plots for the Pinghe granitoids
图 4
平河花岗岩类的QAP图解(据Streckeisen, 1976)
Figure 4.
QAP classification diagram for the Pinghe granitoids (after Streckeisen, 1976)
图 5
平河花岗岩类的稀土元素球粒陨石标准化配分曲线图及微量元素原始地幔标准化蛛网图
Figure 5.
Chondrite-normalized REE patterns and primitive mantle-normalized trace element patterns for the Pinghe granitoids
图 6
平河花岗岩类的εHf(t) 值-U-Pb年龄图解
Figure 6.
εHf(t) values vs. U-Pb ages plot of the Pinghe granitoids
图 8
平河花岗岩类Rb/Ba-Rb/Sr (a) 和CaO/Na2O-Al2O3/TiO2(b) 图解(据Sylvester, 1998)
Figure 8.
Rb/Ba-Rb/Sr (a) and CaO/Na2O-Al2O3/TiO2(b) diagram of the Pinghe granitoids (after Sylvester, 1998)
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