北祁连西段双龙镁铁质-超镁铁质岩地球化学特征及构造意义

Geochemical characteristics and tectonic implication of the Shuanglong maficultramafic rocks in western section of the North Qilian

北祁连西段双龙一带出露的镁铁质-超镁铁质岩主要由蛇纹石化二辉橄榄岩、辉长岩、辉绿岩及玄武岩组成,这些岩石单元与构造卷入的前寒武纪变质岩系和深海沉积的硅质岩一起构成蛇绿混杂岩带。辉长岩LA-ICP-MS锆石U-Pb年龄为464±2 Ma(MSWD=1.3)。Zr/TiO_2-Nb/Y图解显示辉绿岩为拉斑玄武岩系列,玄武岩为碱性系列,两者均具有低的MgO和相容元素Cr、Ni含量,指示在岩浆作用过程中发生过橄榄石、辉石的分离结晶作用。球粒陨石标准化稀土元素配分模式图显示,辉绿岩呈平坦型,(La/Yb)_N=1.35~1.45,无Eu异常(δEu=1.0~1.1),以及Lu/Yb(~0.15)、Zr/Y(~2.5)、Y/Tb(~39)比值等均类似于N-MORB;但它们又相对富集Ba、Sr,亏损Nb、Ta、Pb,以及Nb/U(~21)、Zr/Nb(~16)、Th/Ta(~5.7)比值则显示了岛弧玄武岩或弧后盆地玄武岩的特征,推测辉绿岩应源于受俯冲流体交代的尖晶石二辉橄榄岩部分熔融的产物。玄武岩地球化学特征明显不同于辉绿岩,表现为相对富集LREE,~8.48,轻微负Eu异常(δEu=0.91),以及Zr/Y(~8)、Y/Tb(~30)、Ta/Yb(~0.6)、Th/Yb(~2.5)比值等类似于OIB,指示玄武岩应源于富集地幔部分熔融的产物,但岩石又表现出富集Rb、Ba、Th、U、Pb、Sr、Zr、Hf,亏损Nb、Ta、Ti、P,以及Th/Ta(~4.8)、Zr/Nb(~14)、Nb/Yb(~10)、Nb/U(~11)比值又具有岛弧或弧后盆地玄武岩的亲缘性,显示了玄武岩应源于地幔深部流体交代的石榴石二辉橄榄岩或尖晶石+石榴石二辉橄榄岩部分熔融的产物,表明早古生代北祁连古大洋在俯冲闭合过程中具有洋岛或海底高原的增生作用。结合区域地质背景,利用全球大数据建立的构造判别图解综合研究认为,北祁连西段熬油沟-玉石沟-双龙-小龙孔-卡瓦-东沟等镁铁质-超镁铁质岩共同构成一条重要的SSZ型蛇绿岩带,为北祁连古大洋向南俯冲形成的弧后盆地的残留体。

The mafic-ultramafic rocks exposed in Shuanglong region in western section of the North Qilian, composed of serpentinized lherzolite, gabbro, diabase, basalt, together with tectonically involved Precambrian metamorphic rocks and deep sea sedimentary siliceous rocks, are considered as a set of ophiolitic mélange. The LA-ICP-MS zircon U-Pb dating of the gabbros is 464±2Ma （MSWD=1.3）. In the diagram of Zr/TiO2-Nb/Y, the diabases in this ophiolite belt belong to the tholeiitic series, however, the basalts are plotted into the alkaline series. Both of them have low MgO and Cr, Ni contents, indicating the fractional crystallization of olivine and pyroxene in the evolution of their magma. In the chondrite-normalized REE patterns diagram, the diabases display a flat REE pattern with （La/Yb）N=1.35~1.45, no significant Eu anomalies （δEu=1.0~1.1）, and have ratios of Lu/Yb （~0.15）, Zr/Y （~2.5） and Y/Tb （~39）, which are similar to those of the normal mid-oceanic ridge basalts （N-MORB）, but relatively enriched in Ba and Sr and depleted in Nb, Ta, Pb, and ratios of Nb/U （~21）, Zr/Nb （~16） and Th/Ta （~5.7） display the affinities of island arc basalts （IAB） or back-arc basin basalts （BABB）, suggesting that the diabases were the products of partial melting of spinel lherzolite source metasomatismed by subducted fluids. The geochemical characteristics of the basalts are obviously different from diabases, relatively enriched in LREE, （La/Yb）N=~8.48, with slightly negative Eu anomalies. Ratios of Zr/Y （~8）, Y/Tb （~30）, Ta/Yb （~0.6） and Th/Yb （~2.5） for the basalts are similar to those of oceanic island basalts （OIB）, indicating that they are derived from the partial melting of deep enriched mantle （similar to OIB source）. However, the rocks are enriched in Rb, Ba, Th, U, Pb, Sr, Zr and Hf, depleted in Nb, Ta, Ti, P, and have ratios of Th/Ta （~4.8）, Zr/Nb （~14）, Nb/Yb （~10） and Nb/U （~11）,respectively, display the affinities of IAB or BABB, suggesting that the rocks should be the products of partial melting of garnet or spinel ＋ garnet lherzolite metasomatized by the deep-mantle fluid cycling, and further indicating that the accretion process of oceanic island could exist during the subduction of the oceanic lithosphere in the Early Paleozoic. Together with the analyses on regional geological settings, the tectonic discrimination diagrams based on the global basalt data suggest that the mafic-ultramafic rocks from Aoyougou, Yushigou, Shuanglong, Xiaolongkong, Kawa, Donggou constitute an important supra-subduction zone type （SSZ-type） ophiolite belt indicative of the relics of an back-arc basin formed by the southward subduction of North Qilian ancient ocean.