西藏普兰石英岩中发现41亿年碎屑锆石

Detrital Zircon of 4100 Ma in Quartzite in Burang,Tibet

在西藏普兰县石英岩中，利用SHRIMPU—Pb法测年发现4．1Ga碎屑锆石。该年龄为目前中国最老单颗粒碎屑年龄。〉4．0Ga锆石两测点的Th／U值介于0．76～0．86之间，属岩浆锆石。这一发现，为研究青藏高原的地质演化提供了重要的年龄数据。

High-Mg ultrapotassic rocks present in the NS-striking rift system of Lhasa Block in Tibetan Plateau. Geochemically these High-Mg ultrapotassic rocks are characterized by relatively high SiO2 （53 %- 50 % ）, and high K2O （7%-6%）, MgO （11%-8%）, Cr （500×10^-6-400×10^-6）,Ni （400×10^-6 - 260×10^-6） contents. Isotope data show these volcanic rocks have high ^87Sr/^86Sr （0. 7265 -0. 7199） and low 143Nd/144N （0. 511844 - 0. 511769） ratios, and a relatively broad oxygen isotope range （δ18OvsMow ： 10. 4‰-6. 4‰）. These geochemical and isotopic characteristics indicate these ultrapotassic rocks are derived from a source of enriched mantle, which were mixed a large quantity component of the subducted India crust. ^40Ar/^39Ar dating results show these rocks were erupted in 17 - 13 Ma. Combined with the relation between the normal faults and volcanic rocks, the normal faults were inferred retaining active intensively in 23- 13 Ma and lasting about 10 Ma, and spreading at a rate of 5.6 -3.0 mm/a. The high-Mg ultrapotassic rocks and the rift system are consistent temporally and overlapped spatially, which indicates the high-Mg ultrapotassic rocks are closely correlated with the form and evolution of the rift system in Tibetan Plateau. The break-off of the subducted India crust result in the extension of the lithosphere of Tibetan Plateau. The rifts in Lhasa Block were mainly active in two phases. Firstly, going with the uplift of the Tibetan Plateau （23-13 Ma）, and then occurring gravitational collapse with decrease in elevation of the Tibetan Plateau （13 Ma-present）.