Compositions and Formation History of the Basement Metamorphic Rocks in Northeastern Guangdong Province
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摘要: 基底变质岩的成分和形成时代对揭示地壳演化至关重要.利用锆石U-Pb-Hf研究和全岩成分分析, 发现粤东北及邻区的许多基底变质岩是晚新元古代形成的沉积岩, 它们具有高的SiO2、Rb、Zr、Y和过渡金属元素含量以及相对低的Al2O3、CaO、Na2O、Sr、Nb含量.它们沉积于活动大陆边缘环境.盆地的形成与Rodinia超大陆裂解时的张性背景相关.粤东北龙川地区新元古代沉积岩主要由新太古代和中元古代碎屑物质组成, 并含少量中太古代和新元古代物质, 明显不同于闽西南和粤北地区新元占代沉积岩.粤东北这些变质岩没有受到加里东运动的强烈影响, 而是在印支期发生变质-重熔作用.
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关键词:
- 锆石U-Pb-Hf研究 /
- 基底变质岩 /
- 新元古代沉积盆地构造 /
- 粤东北
Abstract: The compositions and formation history of basement metamorphic rocks are the key to understanding crust evolution. Based on zircon U-Pb-Hf isotopic studies and the compositions of basement metamorphic rocks in northeastern Guangdong Province and southern Jiangxi Province, this paper indicates that the protoliths of these metamorphic rocks are sedimentary rocks deposited in the Late Neoproterozoic. They have high SiO2, Rb, Zr, Y and transitional metal concentrations and relatively low Al2O3, CaO, Na2O, Sr and Nb contents. They were deposited in an active continental margin basin near a volcanic arc. The basin formation was closely related with the extensional regime during the breakup of the supercontinent Rodinia. Neoproterozoic sediments in the Longchuan area of northeastern Guangdong Province are mainly composed of Neo-archaean and Mesoproterozoic clastic constituents with small amounts of Mesoarchaean and Neoproterozoic fragments, quite distinct from those in southwestern Fujian Province and northern Guangdong Province, showing different provenances. The basement metamorphic rocks in northeastern Guangdong Province were not influenced by the Early Paleozoic Caledonian movement, but underwent strong Indosinian metamorphism and anatexis. -
图 1 粤赣闽交界地区地质简图及采样点(据程裕祺(1990)主编的1:500万地质图修改和简化)
Fig. 1. Schematic geologic map of the Yue-Gan-Min area and sampling locations
图 2 兴宁县茅塘附近混合岩的构造特征
样品XN-6-1采自基体, XN-6-2为花岗质脉体
Fig. 2. Photographs showing structure of migmatites near the Maotang of Xingning county
图 3 粤东北—赣南地区变质岩的原岩恢复和岩石类型确定(砂岩分类图据Pettijohn et al.(1988))
其他数据来源: 澜河据Xu et al.(2005);桃溪据于津海等(2005a); 古寨据丁兴等(2005); 白云-潘洞据陈忠权等(2001)
Fig. 3. (a) Protolith discrimination of metamorphic rocks in northeastern Guangdong and southern Jiangxi; (b) determina-tion of sandstone types
图 4 基底变质岩的稀土配分模式
Fig. 4. Chondrite-normalized REE patterns of basement metamorphic rocks
图 5 龙川片麻岩(XN-2)中锆石的BSE图像(小圈为U-Pb分析点; 大圈为Lu-Hf分析点; 左下线段都是100μm)
Fig. 5. BSE images of zircons from the Longchuan gneiss (XN-2)
图 7 (a) 龙川片麻岩中锆石的εHf(t)-t投影图; (b)由锆石Hf同位素计算的全岩εNd(t)与基底变质岩对比
a.桃溪群麻粒岩中锆石的Hf同位素据于津海等(2005a); b.1.周潭群; 2.麻源群; 3.桃溪群.它们的Nd同位素组成转引自于津海等(2005b).龙川片麻岩和澜河片麻岩εNd(t)范围的上界和下界是分别根据(176Lu/177Hf)crust=0.015和0.022计算获得
Fig. 7. (a)εHf (t)-t plot for zircons of the Longchuan gneiss; (b) comparison ofεNd(t) of Longchuan gneiss calculated fromεHf of zircons with the basement metamorphic rocks
图 8 变质砂岩沉积环境的构造判别图(据Bhatia(1983); Bhatia and Crook(1986))
Fig. 8. Tectonic setting discrimination for sandstones
表 1 粤东北和赣南部分基底变质岩的主量元素组成
Table 1. Major element compositions of basement metamorphic rocks in northeastern Guangdong and southern Jiangxi
表 2 粤东北和赣南部分基底变质岩的微量元素组成(10-6)
Table 2. Trace element contents of some basement metamorphic rocks in northeastern Guangdong and southern Jiangxi
表 3 龙川片麻岩(XN-2)中锆石的LA-ICPMS U-Pb定年结果
Table 3. LA-ICPMS U-Pb dating results of zircons from the Longchuan gneiss (XN-2)
表 4 龙川片麻岩中锆石的Lu-Hf同位素分析结果
Table 4. Lu-Hf isotopic compositions of zircons from the Longchuan gneiss (XN-2)
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