广东龙窝和白石冈岩体锆石U-Pb年代学、黑云母矿物化学及其成岩指示意义

Zircon U-Pb geochronology,biotite mineral chemistry and their petrogenetic implications of the Longwo and Baishigang plutons in Guangdong province.

龙窝和白石冈岩体位于近东西向展市的佛冈花岗岩带的东端,锆石 LA-ICP-MS U-Pb 定年结果表明,二者的形成年龄分别为(169.1±2.5)Ma 和(157.8±2.3)Ma,它们是南岭地区燕山早期由壳幔相互作用形成的花岗质岩石的典型代表。本文在对岩体形成年龄精确测定的基础上,着重运用电子探针和原位激光探针技术,系统测定了二岩体中黑云母的主量和微量元素组成,并讨论了它们的成岩指示意义。电子探针主量元素测定结果表明,龙寓岩体中的黑云母较白石冈岩体中的黑云母富钛和镁,贫铁和铝,F、Cl 等挥发组分含量低,黑云母主量元素组成指示龙窝岩体形成于相对高温氧化的个质环境。激光探针 ICP-MS 微量元素测定结果表明,黑云母是花岗质熔体中R11)'Ba、Nb、11d及s(·、V、C r、Co、Ni等元素的主要载体,而I|、Th、Ph、S r、zr、Hf、Y 等元素的含量则显著低于寄主岩花岗岩中黑云母的稀土元素含量极低,不是影响全岩稀土特征的主要矿物相黑云母的成分特征可以有效地示踪岩浆的分异演化程度及成岩物理化学条件,也可在一定程度上反演宕浆源区性质,但后者往往要结合其它资料(特别是同位素组成),才能得出更准确的信息。

The Longwo and Baishigang plutons, locating in the eastern end of the EW-trending Fogang granite belt, are typical examples of granitic rocks formed by crust-mantle interaction in Nanling region. Zircon LA-ICP-MS U-Pb dating yields an age of （ 169. 1 ＋ 2.5 ） Ma for the Longwo pluton and an age of （ 157.8 ＋ 2. 3 ） Ma for the Baishigang pluton, indicating that both of them were formed in early Yanshanian. The major and trace element contents of biotites are measured by employing EMPA and LA-ICP-MS techniques, and their petrogenetic significances are discussed. The EMPA analyses of major elements demonstrate that biotites of the Longwo pluton are distinguished from that of the Baishigang pluton by higher abundances of TiO2 , MgO, and lower concentrations of FeO, Al2O3 and volatile constituents （e. g. , F and Cl） , indicating that the Longwo pluton was likely to have formed under a relative high temperature and a more oxidated environment. Trace elements analyses by LA-ICP-MS technique illustrate that biotite is an important carrier of Rb, Ba, Nb, Ta, Sc, V, Cr, Co, Ni in granitic melts, but has less ability in hosting U, Th, Pb, Sr, Zr, Hf, Y, and thus the concentrations of these elements in biotite are much lower than that in the host rocks. Biotite in granitic rocks also displays quite low REE concentrations, suggesting that it is not the main mineral phase affecting the REE features of the host rocks. The mineral chemistry of biotite can be used as a powerful tool in identifying the differentiation degree of the host magmas and in evaluating the rockforming physical-chemical conditions, and can also be employed in tracing the nature of the magma source. But in the latter aspect, other data （especially isotope compositions） should be coupled with in order to obtain more accurate information.