Geochronology of Serpentinized Harzburgite in Miaowan Ophiolite, Yangtze Block and Its Tectonic Implications
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摘要: 扬子陆核崆岭高级变质地体内出露一套强变形的基性-超基性岩岩石组合, 主要呈似层状、透镜状分布于崆岭群中, 该套变基性-超基性岩组合对扬子陆块早期构造演化过程具有重要意义.通过同位素稀释法(isotope dilution thermal ionization mass spectrometry, 简称ID-TIMS)获得该套岩石组合中蛇纹石化方辉橄榄岩全岩Sm-Nd等时线年龄为1063±12Ma, 说明Sm-Nd同位素体系可用于对超低含量、发生强蚀变作用的超基性岩样品进行定年.样品Nd同位素组成相对均一(εNd(t)值为6.90~7.32), 表明形成于封闭体系中, 其对应Nd同位素两阶段模式年龄为1.13~1.09Ga, 与形成年龄接近, 说明来自亏损软流圈地幔部分熔融.结合区域上已有的中元古代末期到新元古代早期构造岩浆事件研究, 认为在该时期扬子陆块可能由多个微陆块组成, 就扬子陆核而言, 其与扬子陆块西侧之间很可能存在分隔的大洋.Abstract: A series of strongly-deformed mafic-ultramafic rocks occur at the Kongling high-grade metamorphic terrain in Yangtze continental nucleus, scattering in the Miaowan Formation in layers and podiforms. This metamafic-ultramafic rock association is of great significance to the early tectonic evolution of Yantgze block. In this study, ID-TIMS whole-rock Sm-Nd isochron dating is carried out for the serpentinized harzburgite and amphibolite from this rock suite. The results indicate that the serpentinized harzburgite was formed at 1063±12Ma, showing that it is feasible to date tiny amount of heavily-altered ultramafic rocks by means of the Sm-Nd isotopic system. The comparatively uniform Nd isotopic composition (with εNd(t) value ranging from 6.90 to 7.32) indicates that they were formed in a closed system. Two-stage Nd modal age of 1.13-1.09Ga is close to their emplacement time, implying an origin of partial melting from depleted asthenospheric mantle. Combined with documented work on late Mesoproterozoic to early Neoproterozoic tectonic-magmatic events in the area, it is suggested that the Yangtze block may have been composed of several microcontinents at that time. There could have been an ocean separating the Yangtze continental nucleus from western Yangtze continental segment.
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Key words:
- Yangtze continental nucleus /
- Miaowan ophiolite /
- Sm-Nd isochron /
- isotope /
- geochronology /
- petrology /
- geochemistry
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图 1 崆岭高级变质地体南部庙湾地区地质简图
据彭松柏等(2010)修改
Fig. 1. Geological map of Miaowan area at the southern Kongling high-grade terrain
图 2 庙湾蛇绿岩蛇纹石化橄榄岩野外(a)及薄片照片(b)
Fig. 2. Field outcrop (a) and photomicrograph (b) of the serpentinized harzburgite in the Miaowan ophiolite
图 3 庙湾蛇绿岩蛇纹石化方辉橄榄岩-斜长角闪岩全岩Sm-Nd等时线
Fig. 3. Whole rock Sm-Nd isochron of the serpentinized harzburgites and amphibolites in the Miaowan ophiolite
表 1 庙湾蛇绿岩蛇纹石化方辉橄榄岩及斜长角闪岩Sm-Nd同位素组成
Table 1. Whole rock Sm-Nd isotopic composition of the Miaowan ophiolite
样号 样品 Sm(10-6) Nd(10-6) 147Sm/144Nd 143Nd/144Nd 2σ T2DM(Ga) εNd(t) zg94-1 蛇纹石化方辉橄榄岩 0.2821 0.7908 0.2157 0.513135 0.000009 1.11 7.13 zg94-2 蛇纹石化方辉橄榄岩 0.2042 0.5619 0.2197 0.513167 0.000010 1.10 7.20 zg94-3 蛇纹石化方辉橄榄岩 0.3915 1.0400 0.2276 0.513210 0.000012 1.12 6.97 zg94-4 蛇纹石化方辉橄榄岩 0.1798 0.5669 0.1918 0.512976 0.000015 1.10 7.29 zg94-5 蛇纹石化方辉橄榄岩 0.1038 0.3219 0.1950 0.513020 0.000013 1.06 7.71 zg94-6 蛇纹石化方辉橄榄岩 0.0679 0.2163 0.1899 0.512955 0.000021 1.11 7.13 zg94-7 蛇纹石化方辉橄榄岩 0.1671 0.5075 0.1991 0.513008 0.000013 1.13 6.91 zg94-8 斜长角闪岩 1.3965 4.6985 0.1797 0.512891 0.000010 1.10 7.27 zg94-9 蛇纹石化方辉橄榄岩 0.2081 0.6534 0.1926 0.512962 0.000016 1.13 6.90 zg94-10 蛇纹石化方辉橄榄岩 0.3005 0.8356 0.2174 0.513150 0.000017 1.10 7.18 zg94-11 蛇纹石化方辉橄榄岩 0.2576 0.8665 0.1797 0.512880 0.000011 1.12 7.05 zg95-1 蛇纹石化方辉橄榄岩 0.0688 0.3534 0.1177 0.512446 0.000018 1.12 7.03 zg95-2 蛇纹石化方辉橄榄岩 0.0832 0.4213 0.1194 0.512466 0.000014 1.10 7.19 zg95-3 蛇纹石化方辉橄榄岩 0.1244 0.6591 0.1141 0.512425 0.000012 1.11 7.10 zg95-4 蛇纹石化方辉橄榄岩 0.0502 0.2427 0.1249 0.512493 0.000020 1.12 6.96 zg95-5 蛇纹石化方辉橄榄岩 0.0855 0.4759 0.1086 0.512390 0.000010 1.11 7.17 注：Sm、Nd含量根据ID-TIMS测量结果计算获得，误差＜5‰；计算εNd(t)值和两阶段模式年龄时，年龄值t采用Sm-Nd等时线年龄1063Ma；T2DM计算过程中参数(147Sm/144Nd)DM=0.2137，(143Nd/144Nd)DM=0.513150，(147Sm/144Nd)CC=0.1180(DM、CC分别代表亏损地幔和大陆地壳)；εNd(t)值计算过程中参数(147Sm/144Nd)CHUR=0.1967，(143Nd/144Nd)CHUR=0.512638. 
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