CHARACTERISTICS OF AND U-PB AGES OF ZIRCONS AND ITS IMPLICATION FROM ZIRCON OF DONGHE SANDSTONES OF THE UPPER DEVONIAN AND SILURIAN SANDSTONE IN TAZHONG AREA, TARIM BASIN

Abstract

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 477-439.8Ma and 431.6-421.1Ma 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.