东天山四顶黑山层状杂岩体成因及其对与塔里木大火成岩省的关系

Several sizable magmatic sulfide deposits have been found in the Huangshan mafic-ultramafic rock belt, East Tianshan, Xinjiang. Up to now, three large-sized deposits (Huangshan, Huangshandong, Tulargen) have been proved; in addition, there are several medium- and small-sized deposits, as well as a number of copper-nickel mineralization points and mafic-ultramafic intrusions. The post-collisional environment and Large Igneous Provinces (LIGs) are the two backgrounds for these deposits. Many regional geological characters and A-type granites in Tianshan region suggest that the subduction-collision tectonics had been completed by the Late Carboniferous (~300 Ma), and was followed by a phase of intraplate tectonic processes, the widespread occurrence of mantle-derived mafic-ultramafic intrusions were formed in a post-collisional extensional environment, although geochemical suggesting island-arc magmas, may be due to the subduction-related material in the mantle source. On the contrary, some researchers to propose a single mantle plume for the Tarim alkaline basalts and the mafic-ultramafic intrusions in Tianshan-Beishan region because many mafic-ultramafic intrusions took place at a similar time, such as Huangshandong in Tianshan region (274Ma), Pobei in Beishan area (274Ma), Kalatongke intrusions in Altay (287Ma), and Tarim basalts (~280Ma). And recently, some researchers suggested that an overlapping model of early Permian plume to Central Asian Orogenic Belt responsible for the Ni-Cu-Co metallogeneses because the special geochemical characteristics. The Sidingheishan mafic-ultramafic intrusion is one of many important mafic-ultramafic intrusions in the Huangshan rock belts which is located in the eastern part, the large-scale (outcrop area of 35km2), layer complex, is composed by peridotite phase and gabbro phase. This study plan to carry out the Hf-Nd-Sr-Pb isotope test and single-grain zircon dating for the Sidingheishan layered complex, determination of the Hf-Nd-Sr-Pb isotopic compositions and the age of Sidingheishan intrusion, combined with the study of the mineralogy, petrology, and geochemical methods to address the nature of the mantle source and the geodynamic setting for Sidingheishan intrusion; contrast the characteristics of the mafic-ultramafic intrusions in the Tarim large igneous province and East Tianshan-Beishan area, explore the relationship between the Sidingheishan intrusion and the Tarim large igneous province, to constrain the influence range of large igneous province in Tarim.

新疆东天山黄山岩带发育众多铜镍硫化物矿床和镁铁质-超镁铁质岩体，备受国内外学者的关注。目前，对于这些镁铁质-超镁铁质岩体形成背景的认识主要存在三种观点：造山带后碰撞伸展、地幔柱、地幔柱与造山带的叠置。与黄山、黄山东、土墩、葫芦等小岩体大矿床明显不同的是，在黄山岩带的最东端还存在有四顶黑山这样的规模较大（出露面积>35km2）、具有层状特征的镁铁-超镁铁岩体，其岩石组合主要为橄榄岩相和辉长岩相岩石。本次拟在野外地质调研和岩石矿物学、矿床学研究基础上，对四顶黑山层状杂岩体开展Hf-Nd-Sr-Pb同位素测试和单颗粒锆石U-Pb定年，查明地质地球化学特征，示踪岩浆源区性质，厘定不同期次岩相的形成年龄，结合区域构造演化历史，论述岩体的形成背景及岩石成因；对比塔里木大火成岩省和东天山—北山地区镁铁质-超镁铁质岩体的异同点，探讨四顶黑山岩体与塔里木大火成岩省的关系，约束塔里木大火成岩省的影响范围。

四顶黑山镁铁质-超镁铁质岩体位于北天山东段、中亚造山带的南缘、中国新疆的北部地区。岩体主要由单辉橄榄岩、橄榄辉石岩、橄榄辉长岩、辉长岩、角闪辉长岩组成。岩体至少有两次岩浆的上侵到泥盆系花岗岩和花岗闪长岩中；第一次上侵的岩浆形成了不含橄榄石的岩石端元，第二次岩浆上侵形成了含有橄榄石的岩石端元。通过LA-ICP-MS方法获得了辉长岩的锆石U-Pb年龄为359.2 ± 6.4 Ma，利用SHRIMP方法测得了橄榄辉长岩的锆石U-Pb年龄为351.4 ± 5.8Ma（早石炭世）。四顶黑山岩体中的橄榄石最高Fo值为82.52，Ni含量最高为1414×10-6。利用橄榄石-熔体平衡原理，根据岩体中基性程度最高的单辉橄榄岩样品，模拟计算出四顶黑山母岩浆中MgO 和FeO的含量分别为10.43 % 和13.14 %。四顶黑山岩石样品中适度富集Th和Sm、轻度亏损轻稀土元素、亏损Nb、Ta、Zr和Hf元素。岩石的εNd(t)= +6.70～+9.64，(87Sr/86Sr)i=0.7035～0.7042，(206Pb/204Pb)i=17.23 ～17.91，(207Pb/204Pb)i=15.45～15.54，(208Pb/204Pb)i=37.54～38.09。这些特征与黑山岩体以及三塘湖盆地、北天山和北山地区早石炭世俯冲相关的火山岩的特征十分相似。岩石低的(La/Gd)PM值（0.26～1.77）暗示四顶黑山岩体极有可能来自亏损的尖晶石-橄榄岩地幔。(Th/Nb)PM比值（0.59～20.25）表明母岩浆发生了上地壳的混染作用。结晶模拟计算表明，四顶黑山的母岩浆是由软流圈物质与约10%的花岗质熔体混合发生部分熔融而形成的，在这之后又经历了约5%的上地壳物质的同化混染作用。所以，四顶黑山岩体的高镁玄武质母岩浆是在俯冲板片的折断过程中由软流圈物质的部分熔融形成的。