SHRIMP zircon age of the high aeromagnetic anomaly zone in central Tarim Basin and its geological implications ()
1. INTRODUCTION
Tarim basin, located at northwest of china, is an ancient craton basin that has an uniform crystalline basement [1]. There exsits obviously high magnetic anomaly zones extending in EW for over 500 km at the basement of Tarim basin (Figure 1). The high magnetic anomaly zones devided the basin into south and north blocks which have different basement structures from each other. Above the basement, the huge thick strata had deposited from Nanhua period to Quaternary but only well TD2 had drilled Precambrian formations. Due to deficiency of intrabasinal chronology data and surrounding outrops data, the structure and forming age of the basement are not understood clearly [1-3]. Through zircon SHRIMP U-Pb dating of basement granites in well TD2, this article can provide constraints for deeply discussing the forming age and evolution of Precambrian basement of Tarim plate.
2. SAMPLE AND METHOD
2.1. Characteristics of Samples
TD2 well had drilled Precambrian basement formations in depth between 4996 to 5040 m and 0.97 meters long of core was acquired. The lithology is altered hornblende granite with light gray or light green-gray color. The rocks have granitic texture, vermicular structure and block structure. The mineral content of platy prismatic K-feldspar, hypautomorphic columnar albite, anhedral quartz, hornblende and pyrite are 49% - 54%, 5% - 30%, 1% - 20%, 10% - 20% and <1%. The hornblendes (only remains can be seen) are columnar and granular which metasomatized by byssolite and chlorite. Fine-grained pyrite can also be seen in hormblendes. Quartz (wavy extinction) and feldspar usually generate cracks resulting from structural compression.
2.2. Sample Preparation and Analysis
This article choose cores of Precambrian granite in well TD2 to analyse. Samples were prepared using facilities
Figure 1. The aeromagnetic ∆T contour diagram and sampling place in Tarim basin.
at the Department of Hebei Geological Survey in Langfang city, China. Rock samples were first disaggregated in a jaw crusher and powdered. The zircons were then separated using standard elutriation, heavy-liquid and magnetic mineral separation techniques. Zircons with different crystal shape and color were picked up through bioscopes to make zircon target. The zircon grains were then imaged by visible light and scanning electron microscope equipped with cathodoluminescence (CL) at Institute of Mineral Resources, Chinese Academy of Geological Sciences. The isotopic U-Pb-Th analyses were obtained using the SHRIMP II at Beijing Shrimp Center and the Pb, U and Th concentrations were referenced to the standard zircon TEMORA (417 Ma). The undetermined and standard zircons were first fixed together at epoxy target, then polished, washed and plated gold to measure. Cracks and inclusions would be avoid while measuring. Statistical treatment and plotting of concordia diagrams were achieved using the Squid 1.0 and Isoplot/Ex program of Ludwig (2001). The value of common Pb in zircons were normalized to the measured 208Pb. Individual corrected ratios and ages are reported with 1σ analytical errors. The weighted mean 206Pb/238U ages are presented at 95% confidence levels.
Most of select zircon particles with uniform size are colorless transparent euhedral crystals. Individual zircon crystals which have developed cylindrical and conical surface are columnar euhedral and on average 100 - 200 µm in longth and 1:1 - 3:1 in aspect ratio. Some zircon crystals contain cracks and cores. Most of zircons that have obviously zonal structures were all magmatic crystallization (Figure 2).
3. RESULTS AND DISCUSSION
3.1. Ages of Rocks
By analyzing U-Pb isotopic age of 16 measuring points from well TD2 samples (Figure 3, Table 1), the zircons had the average content of U and Th of 163 - 597 × 10–6 and 34 - 495 × 10–6. The value of Th/U between 0.06 to 2.07 indicates that the zircons may be influenced by metamorphism.
Zircon-SHRIMP dating demonstrates that the apparent age of 206Pb/238U is concentrated between 1755.3 - 1942.4 Ma, with average age of 1845.7 Ma. In 207Pb/235U - 206Pb/238U diagram, 16 measuring dates are crowded together on or near concordia and obtain an concordant age of 7908.2. ± 8.6 Ma (MSWD = 1.2), which may represent the crystallization age of granites.
Figure 2. Some of the CL images of zircon in well TD2，Tarim Basin.
Figure 3. Zircon 207Pb/238U - 206Pb/238U concordia diagram of granodiorite from Well TD2.