青海省共和盆地周缘晚古生代镁铁质火山岩Sr-Nd-Pb同位素地球化学及其地质意义

Sr-Nd-Pb isotopic geochemistry of late-Paleozoic mafic volcanic rocks in the surrounding areas of the Gonghe basin,Qinghai province and geological implications

青海省共和盆地周缘晚古生代镁铁质火山岩分属阿尼玛卿蛇绿混杂带,宗务隆构造带和苦海-赛什塘带。阿尼玛卿带正常洋中脊玄武岩(N-MORB)样品具有较高的^(87)Sr/^(86)Sr(t)比值(0.7066～0.7084)、高的ε_(Nd)(t)(12.2～12.8)和较低的^(206)Pb/^(204)Pb初始值(17.72～17.79)。这些同位素特征类似于秦岭勉略蛇绿岩带的N-MORB以及印度洋低^(206)Pb/^(204)Pb高^(143)Nd/^(144)Nd N-MORB。该带中的洋岛玄武岩(OIB)的^(87)Sr/^(86)Sr比值为0.7036～0.7044,ε_(Nd)(t)=4.4～4.8,^(206)Pb/^(204)Pb=17.45～17.62。其Sr和Nd同位素比值可与印度洋代表热点构造的洋岛玄武岩对比,但^(206)Pb/^(204)Pb低于印度洋的热点构造玄武岩,因此,具有类似印度洋低^(143)Nd/^(144)Nd比值MORB同位素特征。宗务隆构造带的N-MORB的Sr同位素比值在0.7041～0.7058,ε_(Nd)(t)=6.1～8.4,^(206)Pb/^(204)Pb=17.51～17.90,划归高^(143)Nd/^(144)Nd比值的N-MORB。苦海大陆裂谷玄武岩显示了高的^(87)Sr/^(86)Sr同位素比值(0.7115和0.7104)和低的ε_(Nd)(t)值(-1.7和-2.5),其^(206)Pb/^(204)Pb(17.64和17.46)与上述大洋玄武质岩石无显著区别。上述各岩类的同位素特征反映了它们生成的构造环境和陆壳组分混染的程度。阿尼玛卿带的N-MORB代表了典型的来自亏损地幔源区的洋中脊产物。与勉略带同类岩石可能来自同一源区。OIB可能属于热点构造成因的洋岛产物并与MORB一起构成了阿尼玛卿洋洋壳。宗务隆带MORB同样代表了主要源自相对亏损地幔的洋脊产物并指示宗务隆带曾开裂成洋。苦海大陆裂谷玄武岩极高的Sr和低的Nd同位素比值是陆壳物质组分的强烈印记,这与该类火山岩发育在前寒武纪基底之上不无关系。结合本区大洋玄武岩普遍低的Nb/U和Ce/Pb比值,推测它们可能源自EMII与DMM物质的交代混合。按照习惯的想法,明显的Dupal异常(△^(208)Pb/^(204)Pb值=46～103和△^(207)Pb/^(204)Pb值=4～18;大多样品^(87)Sr/^(86)Sr>0.704)指示这些岩石在空间上代表了来自南半球印度洋位置的古洋壳残余。但是,北半球愈来愈多的Dupal异常的发现有可能指示它们是类似现今东南亚多洋岛构造历经"汇聚式(focused)俯冲"的产物。此外,宗务隆带MORB的Dupal异常指示本区古特提斯域的北界较先前所定还要北推200km。

The late-Paleozoic malic volcanics in the surrounding areas of the Gonghe basin are distributed in the Aneymaqen ophiolite zone, Zongwulong tectonic zone and Kuhai-Saishitang tectonic zone, respectively. N-MORB in the Aneymaqen ophiolite zone have higher ^87Sr/^86Sr（350Ma）ratios （0. 7066-0. 7084）, highεNd（350Ma） ratios （ 12. 2-12. 8 ）and low ^206Pb/^204 Pb initial ratios （ 17.72- 17.79）. These isotopic ratios are similar to those of the same type rocks in the Mianlue ophiolite zone and the low ^206Pb/^204Pb and high ^143Nd/^144Nd N-MORB in Indian Ocean. The Sr and Nd isotopic rations in OIB cropped out in the same zone can be compared with the hotspot OIB in the Indian Ocean but ^206Pb/^204Pb ratios are lower, similar to the low ^143Nd/^144Nd MORB in the ocean. N-MORB in the Zongwulong tectonic zone have ^87Sr/^86Sr（350Ma） ratios = 0. 7041-0. 7058, εNd（350Ma） = 6. 1 -8. 4 and ^206Pb/^204pb = 17.51 - 17.90, and belong to the high ^143Nd/^144 Nd N-MORB. The continental rift basalts in the Kuhai-Saishitang zone show high ^87Sr/^86Sr ratios （0.7115 -0.7104） and loWεNa（350Ma） （ -1.7和-2.5 ） , and have little difference from the above MORB in ^206Pb/^204Pb ratios （ 17.64和17.46）. The above isotopic characteristics imply tectonic settings and contamination degree from continental crust for the mafic rocks. The Aneymaqen N-MORB are products generated in the mid-ocean ridge. The isotopic similarity to the same type rocks in the Mianlue zone indicates that they could have come from the common source region, while the Aneymaqen OIB represent materials derived from the hotspot. The Zongwulong MORB similarly belong to the depleted mantle-originated products and indicate that the zone once rifted to an ocean. The high Sr and low Nd isotopes of the mafic orcks in the Kuhai-Siashitan zone reflect the strong continental crust imprint, relating to their eruption on the Precambrian basement. Combined with the low Nb/U and Ce/Pb ratios from the beth MORB and OIB, it can be inferred that the rocks could be derived from a source mixed between EMII and DMM materials through metasometism. Conventionally, obvious Dupal anomaly （△^208Pb/^204Pb =46 - 103 and △^207 Pb/^204pb =4-18 ;^87 Sr/^86Sr 〉 0. 704 for most samples） can mean that the MORB and OIB are remnants of oceanic crust spatially formed in the site of the present Indian Ocean. However, the more and more discoveries of the Dupal anomaly in the North hemisphere could be interpreted by ＂focused subduetion＂ in an arehipelagie setting like today＇s SE Asia.