摘要
榍石在各类岩石中普遍存在,其稳定性受全岩成分、氧逸度和水活度以及温度和压力等因素影响。它在岩浆岩中主要存在于高Ca/Al比值的岩石中,在变质岩中常见于绿片岩相、蓝片岩相和角闪岩相岩石,在钙质变质岩中其稳定范围可达榴辉岩相或高压麻粒岩相。一般榍石结构中U含量较高,且具有高达高角闪岩相上限的U-Pb同位素体系封闭温度,是理想的U-Pb定年矿物。由于榍石的组成元素均为岩石中的主要元素,很容易与其它矿物、熔体及流体发生反应,所以榍石的U-Pb年龄记录的更可能是结晶年龄,而不是简单的扩散重置年龄;也因为它容易反应,变质榍石复杂的U-Pb体系可能记录了岩石的整个变质历史信息。通过与榍石平衡共生的矿物组合或利用榍石Zr温压计可确定岩石的P-T条件,结合相关的榍石年龄信息即可建立变质过程的P-T-t轨迹。利用SHRIMP、LA-MC-ICP-MS以及LA-ICP-MS方法可对不均一榍石颗粒内部进行原位微区分析得到有意义的U-Pb年龄;利用榍石中Zr含量对温度,尤其是对压力比较敏感,可建立榍石Zr含量温压计。
The stability of titanite is controlled by pressure, temperature, compositions of the bulk rock and the coexisting fluid, activity of H2O, fugacity of O2. Titanite is a common accessory mineral in a large range of rocks. It is widespread in magmatic rocks with high Ca/Al ratio. It appears in different metamorphic rocks ranging from greenschist facies, blueschist facies and amphibolite facies. Its stability may extend into eclogite-facies and high-pressure granulite-facies in calcic rocks. Because of its high U in its structure and high closure temperature （ upper limit of amphibolite-faces）, titanite is an ideal U-Pb dating mineral. In contrast to zircon, titanite （ composed mainly of Ca and Ti） is quite reactive in most rocks contain Ca and Ti at different conditions. Therefore, its U-Pb ages are likely to yield the age of metamorphic crystallization rather than resetting by simple diffusion, such that metamorphic titanite may preserve complicated U-Pb system that contains information of the whole metamorphic history of the rock, which can be revealed by in situ U-Pb dating, e. g. , SHRIMP, LA-MC-ICP-MS and LA-ICP- MS. Importantly, Zr concentration in titanite is sensitive to temperature and pressure, and accordingly Zr concentration in titanite is a good thermobarometer. Both mineral assemblages in equilibrium with titanite and titanite-Zr thermobarometer can be used to determine pressure and temperature. Consequently, P-T-t paths of metamorphism can be defined in combination with U-Pb titanite ages.
出处
《地球科学进展》
CAS
CSCD
北大核心
2007年第12期1258-1267,共10页
Advances in Earth Science
基金
国家重点基础研究发展计划项目"华北大陆边缘造山过程与成矿"(编号:2006CB403502)
西北大学大陆动力学国家重点实验室开放课题基金项目"浙闽地区华夏地块前寒武纪变质基底榍石等变质矿物定年研究"(编号:06LCD12)
浙江省国土资源厅地质勘查资金项目"庆元-遂昌地区金银铅锌钼资源潜力调查评价"(编号:2004005)
中国地质大学研究生学术探索与创新基金项目"榍石U-Pb年代学及P-T-t轨迹建立"(编号:CUGYS0701)资助
