西昆仑塔什库尔干陆块赞坎铁矿赋矿地层形成时代及其地质意义

Formation age of ore-bearing strata of the Zankan iron deposit in Taxkorgan landmass of Western Kunlun Mountains and its geological significance

布伦阔勒岩群既是西昆仑塔什库尔干陆块的主体组成部分,也是该区域铁矿床的赋矿地层,在其中已经发现了赞坎、老并、莫喀尔和叶里克等大型磁铁矿床。文章通过对塔什库尔干陆块内赞坎铁矿区布伦阔勒岩群及侵入地层的岩浆岩进行精细的岩相学和SHRIMP年代学研究,确定赋矿地层的形成时代,进而探讨该矿床的形成时代。研究表明布伦阔勒岩群斜长角闪片岩中岩浆结晶锆石的的形成年龄为（1845.0±11.0）Ma,限定了地层的形成时代可能晚于（1845.0±11.0）Ma。再根据次斜长花岗斑岩侵入体的形成年龄（544.5±4.7）Ma,限定地层的形成时代要早于（544.5±4.7）Ma。因此推测赞坎铁矿区布伦阔勒岩群的形成年龄介于1845.0~544.5 Ma,为元古宙。而且在英安岩中的部分锆石核部还发现有3048.0~3054.0 Ma和2032.0 Ma的年龄信息,说明地层中还保留有更古老的基底物质（古元古代甚至中太古代基底岩石）,也进一步说明塔县—瓦恰隆起带所出露的布伦阔勒群岩是最古老的地层之一,为塔什库尔干古陆块的存在提供了依据。由于赞坎磁铁矿床的形成主要与沉积成矿作用密切相关,其主要矿体是与布伦阔勒岩群底部含铁岩系同生的,因此铁矿床的形成时代应与布伦阔勒岩群含铁岩系的形成时代一致,为元古宙全球性前寒武纪铁矿成矿事件的产物。

The Bulunkuole Group is one of the main components of Taxkorgan landmass in the Western Kunlun area, and is also important ore- bearing strata of the regional iron deposits. Recently, many large- sized iron deposits were found within the Bulunkuole Group, such as Zankan, Laobing, Mokaer and Yelike iron deposits. On the basis of detailed petrography features and SHRIMP U- Pb zircon dating of metamorphic rock and magmatic rock in the intrusion strata of the Bulunkuole Group from the Zankan iron deposit, the authors discussed the formation age of ore- bearing strata, and investigated the ore- forming age. The results show that the formation age of the representative magmatic zircon from plagioclase-hornblende schist in Bulunkuole Group is（1845.0±11.0） Ma, suggesting that ore-bearing strata are younger than（1845.0±11.0） Ma. Zircon SHRIMP dating of plagiogranite porphyry that intruded into the strata yielded a U-Pb age of（544.5±4.7） Ma, suggesting that the ore-bearing strata are older than（544.5±4.7） Ma. Thus the ore-bearing strata were formed during 1845.0-544.5 Ma. Moreover, SHRIMP dating of the zircon core from dacite yielded U-Pb ages of 3048.0-3054.0 Ma and 2032.0 Ma, indicating that there possibly exists an early Proterozoic or Archean basement in the surveyed area and adjacent areas, which is of important significance for recognizing Bulunkuole Group and the basement of Taxkorgan landmass. Bulunkuole Group outcropped in Taxian- Waqia uplift zone is one of the earliest strata in Western Kunlun Mountains, and this provides a basis for the existence of Taxkorgan paleo- landmass. Comparative researches on different types of deposits indicate that the Zankan iron deposit was mainly produced by the sedimentary mineralization process, and iron bodies with iron- bearing rocks of Bulunkuole Group were syngenetic, and hence the mineralization age of the Zankan iron deposit is Proterozoic, which is in accordance with the formation age of Bulunkuole Group and is also a very important period for the generation of global iron deposits.