塔中地区上泥盆统东河砂岩和志留系砂岩的锆石特征、SHRIMP U-Pb年龄及地质意义

Characteristics of and U-Pb ages of zircons and its implication from zircon of Donghe sandstones of The upper Devonian and Silurian sandstones in Tazhong area,Tarim basin.

对塔中地区中1井等6口探井的东河砂岩和志留系砂岩中的25件重砂矿物中的锆石进行了一般特征分析,并选择了9件样品中的1666个单颗粒锆石进行了内部特征和72次微区U-Pb同位素测龄研究。研究表明:塔中东河砂岩中的锆石一般具有较高的磨圆度、相对高比例的岩浆岩来源和稳定的再循环沉积来源特征,反映了较高成熟度和较稳定构造-沉积环境;志留系下砂岩中的锆石来源差异较大,除了再循环沉积岩和岩浆岩来源外,还有变质岩锆石,但它又不具有典型变质锆石的Th/U含量的特征,有二种可能性:一是岩浆型锆石形成后经历了重结晶作用;二是原岩锆石受岩浆或岩浆期后热液影响。锆石的地质年代主要分为:(1)太古宙-古元古代(1.8Ga、2.2Ga和2.6Ga);(2)中新元古代(0.84Ga)和(3)早古生代(477～439.8Ma、431.6～421.1Ma)。上述锆石原岩形成时代及其环境可能是一系列的大陆构造动力学事件的时代响应。

72 zircons for U, Th and Pb contents and U-Pb zircon dating of 25 sandstone rocks from Donghe and Silurian in 6 wells were analyzed by using ion micro-probe technique with the use of cathodoluminescence for internal structures of 1666 zircon grains, the results shows the zircons from Donghe sandstones are of characteristic of high roundness with a rather high contents of magmatic rocks and re-sedimentary origin, indicting that it had been deposited in steady tectonic and sedimentary environment with a great content of the high maturity index of minerals, while the sources of zircons from the Lower Silurian sandstones varies in a great range, a few of them believed to come from metamorphic rocks, besides the magmatic and re-sedimentary origin, however, It have not the typical features of metamorphic origin with the lower Th/U ratios ( usually less than 0.1 for average), and it can be inferred that the primitive zircons either derived from magmatic rocks and undergone the annealing and re-crystallization or be affected by the magmation and magmatic hydrothermal replacement. the U-pb isotopic data of zircons can be divided into three groups, i. e. 1.8Ga, 2.2Ga and 2.6Ga from Meso-Proterozoic to Neo-Proterozoic; 0.84Ga in Cambrian; and 477 similar to 439.8Ma and 431.6 similar to 421.1Ma from Ordovician to The Lower Silurian, which have a concordance with the records of rocks derived from that of wells or outcrop of Tarim basin and its surrounding orogenic belts. the authors believe that the U-pb isotopic data of zircon from sensitive high resolution ion microprobe (SHRIMP) analyses may provide the accurate age constrains for record of events associated with the major geodynamic evolution of stage from Proterozoic to the Early Paleozoic in Tarim basin and its surrounding orogenic belts.