塔里木南缘铁克里克构造带东段前寒武纪地层时代的新限定和新元古代地壳再造:锆石定年和Hf同位素的约束

Zircon U-Pb and Hf Isotopic from the East Segment of Tiekelike Tectonic Belt:Constrains on the Timing of Precambrian Basement at the Southwestern Margin of Tarim,China

铁克里克山位于塔里木地块西南缘,被认为是塔里木板块的褶皱基底。其前寒武纪地层主要由前人认为的"古元古代埃连卡特岩群"和"长城纪塞拉加兹塔格岩群"组成,保存有塔里木盆地前寒武纪地壳形成和演化的重要信息。LA-ICP-MS锆石微区原位定年结果显示,埃连卡特岩群绿泥方解石石英片岩中锆石的206Pb/238U年龄值主体集中于736～810Ma,780Ma年龄数据构成峰值;塞拉加兹塔格岩群变凝灰岩中锆石的206Pb/238U年龄值介于779～792Ma之间,其加权平均年龄为787±1Ma。这些测点相对应的锆石CL图像、U、Th含量和Th/U比值具有岩浆锆石特征,同时,这些峰值年龄具有较好的谐和性,可以作为相应地层的最大沉积时代,指示其形成于新元古代南华纪。通过对铁克里克山地区的埃连卡特岩群和塞拉加兹塔格岩群的物质组成、地球化学特征和新的精确年代学数据对比分析,可以推测二者地层可能为同一地层,其形成时代下限应为新元古代早期,为塔里木板块南缘前寒武纪基底存在Rodinia超大陆裂解事件的物质记录。塞拉加兹塔格岩群中锆石Hf同位素分析表明,787Ma左右的锆石颗粒具有负的εHf(t)值和约1900Ma的两阶段Hf模式年龄,说明塞拉加兹塔格岩群主体是由古元古代的地壳物质重熔后再沉积形成的,为古老地壳的再造。约1986Ma的捕获锆石εHf(t)值变化于-2.3～5.0之间,两阶段模式年龄介于2241～2813Ma,具有正和负的εHf(t)值,指示这些锆石的母岩中存在古元古代的新生地壳和太古宙地壳再造。

Tiekelike Mountain is located at the sourthwestern margin of the Tarim block and has been regarded as a folded basement. Precrambrain sequence consisting of Ailiankate Group and Sailajiazitage Group contains important signatures of the Preerambrain and evolution of Tarim basin, but were previously considered to be Paleoproterozoic-Mesoproterozoic. Zircon U Pb LA-ICP-MS dating of chlorite quartz schist and tuff from the Ailiankate Group and Sailajiazitage Group have yielded crystallisation age peaks at 780 and 787 Ma, respectively. These data indicate the Ailiankate and Sailajiazitage groups are Neoproterozoic succession deposited at ca. 780-800 Ma. Those strata are all temporally equivalent and are likely parts of a single widespread succession and reflect a rifting process related to break-up of Rodinia supercontinent. In situ zircon Hf isotopic data from the Sailajiazitage Group show that some zircons have Hf model ages up to 2241-2813 Ma, suggesting an ancient crustal growth and reworking. The Hf isotopic data also indicate that the -787 Ma zircons have -1900 Ma Hf model ages, suggesting derivations from recycled Paleoproterozoic material.