台湾玉里带变质岩LA-ICP-MS锆石U-Pb年龄及其地质意义

台湾玉里带位于台湾东部,其主体为变质砂岩、千枚状泥岩夹绿帘片岩、阳起片岩、浅变质枕状玄武岩等(原岩为海相火山-陆源沉积岩),其次为高压相的蓝闪片岩。火山岩属钙碱性系列,具低Ba、Nb、Sr和高Pb丰度,稀土元素总量普遍较低,具有轻稀土元素弱富集、Eu无亏损的稀土元素配分模式;(含火山碎屑)变质砂岩的LA-ICP-MS锆石U-Pb年龄表明,~(206)Pb/~(238)U表面年龄有6个峰值:最新年龄23.3Ma(古近纪)为火山碎屑锆石年龄,也是成岩年龄;中生代—古生代年龄有96.7Ma(晚白垩世)、124.2Ma及130.6Ma(早白垩世)、214.1~228Ma(晚三叠世)、283.7Ma和289Ma(早二叠世)等,锆石为次棱角状或次圆状,代表了源岩的岩浆年龄;~(207)Pb/~(206)Pb表面年龄出现古元古代峰值(1769~1852Ma)和新太古代峰值(2506Ma和2530Ma),均为磨圆的碎屑变质或岩浆锆石,代表了不明古元古代变质基底的年龄。玉里带变质岩原岩可能形成于古近纪(23.3Ma),与南海扩张的白云运动同时,属白云运动有关的火山作用的产物。

U-Pb isotopic ages of zircon from Yuli Belt in Taiwan and its geological significance

Located in the east of Taiwan, the Yuli belt is composed of volcano-sedimentary rocks and metamorphic rocks character-ized by high pressure and low temperature. The volcanic rocks belong to the calc alkaline series, with low Ba, Nb, Sr and high Pb; the total rare earth is generally low, with the patterns of less enrichment of light rare earth Eu and complete preservation of rare earth elements. LA-ICP-MS zircon U-Pb ages indicate that the apparent ²⁰⁶Pb/²³⁸U ages have 6 peaks:23.3Ma (Paleogene), the upper rock age; 96.7Ma (Late Cretaceous), 124.2Ma and 130.5Ma (Early Cretaceous), 214.1~228Ma (Late Triassic), 283.7Ma and 289Ma (Early Permian). Zircons with the sub-angular or sub-rounded shape represent the magmatic zircon and indicate magmatic source rock age. Zircons with apparent ²⁰⁷Pb/²⁰⁶Pb ages of 2 peaks 1769~1852Ma (Paleoproterozoic), 2506Ma and 2530Ma (Neoarchaean) are rounded or clastic metamorphic magmatic zircons, which represent Palaeoproterozoic ages of metamorphic basement. The meta-morphic rocks in the Yuli belt might have been formed after 23.3Ma, with the expansion of the South China Sea, at the same time as the expansion of the South China Sea during the formation of the Eurasian continental block edge rift tectonics.