南海新生代碱性玄武岩中钛闪石的地球化学特征

Geochemistry of Kaersutites in Cenozoic Alkali Basalts from the South China Sea

南海新生代碱性玄武岩中发育有角闪石捕虏晶。本文以南海新生代碱性玄武岩中的钛闪石为研究对象,对钛闪石的地球化学特征以及形成条件进行了研究,并探讨其地质意义。研究表明钛闪石是岩浆早期高压结晶的产物,其高Ti富Al,Mg#平均为0.68,形成于幔源岩浆中,成分相对于寄主岩为捕虏晶,其主微量元素的成分范围以及微量元素的配分形式与碱性玄武岩中的巨晶角闪石以及捕虏体中的脉状角闪石一致。根据其主量元素的含量,我们计算出了钛闪石形成的温压条件以及结晶深度。按结晶深度钛闪石可分为两期,一期结晶深度较大(35.09~43.74 km),位于岩石圈底部,形成压力在1019~1305 MPa,温度在1049~1066℃;另一期结晶深度相对较小(27.20~27.29 km),位于岩石圈中下部,形成压力在759~771MPa,温度在1031~1078℃。钛闪石是岩浆形成早期在不同深度发生分离结晶的结果,可能记录了导致南海新生代玄武岩由拉斑玄武岩向碱性玄武岩转变的岩石圈变厚的趋势。

There are amphibole xenocrysts in Cenozoic basalts from the South China Sea. We have investigated the geochemical characteristics of the kaersutites in these alkali basalts from the South China Sea in this study,and calculated their crystallization pressure and temperature. Results show that the kaersutites crystallized from the magmas of a mantle source. They are rich in Ti and Al with average Mg # at 0. 68. The abundances of major elements,trace elements as well as REE and multi-trace element patterns of kaersutites are all similar to that of amphibole megacrysts in alkali basalts and amphibole veins in mantle xenolithos. The crystallization pressure and temperature of kaersutite have been calculated by using geothermobarometer,and the depth they crystallized is estimated. The kaersutites could be divided into two groups by depths. One group crystallized at depth ~ 35. 09 ~43. 74 km,which formed at the bottom of the lithosphere and has pressure ~ 1019 MPa ~ 1305 MPa and temperature~ 1049 ~ 1066℃. Another group crystallized at shallow depth ~ 27. 20 ~ 27. 29 km and formed at lower part of the lithosphere ~ 759 ~ 771 Mpa and ~ 1031 ~ 1078℃. The kaersutites in this study are possible the products of fractional crystallization of alkali magmas at different depths,recording the progressive lithosphere thickening of the South China Sea which leads to the tholeiites transforming to alkali basalts.