Abstract:

On the basis of the field geological survey, the author discovered the highalumina basalt in Tuotuo riverKaixingling area located in the central region of the QinghaiTibet Plateau. UPb isotopes of zircon from the basalt were determined by LAICPMS and the causes of formation of the zircon were studied by using CL images. The 206Pb238U dating from 12 zircon suggests an isotopic age of (380.1 ±5.6) Ma (MSWD=0.21), which indicates that the basalt was the product of the Late Devonian. The basalt shows the characteristics of low SiO2（average 48.32%）, Na2O（average 2.29%）, K2O（average 0.39%）, TiO2（average 0.754%）, Alk（average 2.68%）and high Al2O3（average 20.16%）. Its chemical composition has the characteristics from low K Tholeiitic series to calcalkiline series. The CIPW normative mineral of the basalt indicate that it belongs to quartz tholeiitic basalt（The contents of quartz was 2.77%9.32%）. The characteristics of high Al, low K and Alk were similar to the typical highalumina basalt. It had low total quantity of REE elements, ΣREE= 32.69×10-6-41.01×10-6. δEu was between 1.12 and 1.28 with the mean of 1.2; chondritenormalized REE patterns of basalt  approximate to the level, with positive anomaly for Eu and Ce. The rocks are enriched in large ion lithophile elements (LILE), depleted in high strength field elements (HFSE), with positive Sr, Ba and Ce but negative Nb anomalies. Based on these geochemical characteristics and combined with a series of rock geochemistry diagrams for the tectonic setting, the discovery suggested that the basalt was formed in island arc condition, determined that the volcano stratum was formed in the Eopaleozoic in the Northern Part of QiangtangSanjiang area, and provided the information for studying the formation and evolution of the PaleoTethys Ocean.

Key words: QinghaiTibet Plateau, the Northern Part of QiangtangSanjiang area, Late Devonian, highalumina basalt, LAICPMS, Islandarc setting