摘要
对北桐柏地区镁铁质麻粒岩变质锆石进行LA-ICP-MS U-Pb定年,获得了418.7±3.2Ma的谐和年龄。详细的岩石学、矿物学研究表明,桐柏地区镁铁质麻粒岩遭受过较强的部份熔融,其变质峰期温压条件为～845℃,～0.85GPa。根据zr的溶解度模型进行部分熔融模拟计算,发现在峰期变质条件下,熔体中zr不饱和,在冷却到750～800℃时zr才达到饱和;同时锆石Ti含量温度计也给出了750℃左右的结晶温度。这些表明418.7Ma记录的并不是麻粒岩峰期变质年龄,而是峰期后冷却到750～800℃左右的年龄,麻粒岩的峰期变质年龄可能为430～445Ma左右。部分锆石边部具明显增生边,相对核部的变质锆石区域CL发光性强,且普通Pb含量高,U-Pb定年结果为404.7±6.5Ma,该年龄反映了404Ma左右的一次退变质作用。大量研究表明,沿中央造山带存在一次十分强烈的加里东期板块构造事件,期间的大陆俯冲碰撞或岛弧-大陆碰撞导致大陆壳充分加厚,北桐柏地区的高温麻粒岩相变质作用很可能发生在碰撞加厚的地壳基底。
LA-ICP-MS U-Pb dating of metamorphic zircons from mafic granulite in northern Tongbai area yielded weighted mean (206)Pb/(238)U concordia age of 418.7 +/- 3.2Ma. Detailed studies on petrology and mineralogy of the mafic granulites indicate that the peak metamorphic condition was at ca. 845 degrees C and ca. 0.85GPa. According to the Zr saturation model, Zr was not saturated in melts at peak metamorphic condition and would be saturated when cooling to 750 similar to 800 degrees C, calculated by partial melting modeling. Additionally, Ti-in-zircon geothermometer also gives the crystallization temperature of ca. 750 degrees C. These are suggested that the age of 418. 7Ma is not the age of peak metamorphic condition, and probably is a record for the time when cooling to 750 similar to 800 degrees C. The age of peak metamorphism could be around 430 similar to 445Ma. Some zircon grains have obvious overgrown rims, with stronger cathodoluminescence and higher common Pb than the core. The rims and high common Pb zircons domain yield a age of 404.7 +/- 6.5Ma, indicating that the mafic granulite suffered a retrograde metamorphism at about 404Ma. We suggest that the high-temperature mafic granulites from north Tongbai were probably formed in residual overthickened crustal basement in continental subduction collision zones or arc continent collision belts.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第2期348-358,共11页
Acta Petrologica Sinica
基金
本研究得到国家自然科学基金(40873044)
国家重点基础研究发展规划(2006CB403502)
西北大学大陆动力学国家重点实验室开放课题基金(06LCD12)资助
