鲁西碳酸岩中磷灰石的原位激光探针分析及其成岩意义

以鲁西雪野和八陡碳酸岩中的磷灰石为对象,运用EMPA和LA-ICP-MS技术,分别测定了它们的主量与微量元素组成,并据此讨论了它们的成岩意义.研究结果表明,这些磷灰石富F(=1.07%～2.74%)贫Cl(＜0.04%),种属为羟氟磷灰石或氟羟磷灰石.微量元素组成上富Sr、Th、U、Pb和轻稀土,是全岩中上述元素的主要载体之一.磷灰石的Sr、F含量与∑REE及LREE/HREE比值均表现出较明显的正相关性,其富Sr、贫Y和富轻稀土等特点与世界典型碳酸岩中的磷灰石相似,但它们具有更高的Sr/Y和Th/U比值,Sr、Ce、Th、Y含量接近地幔中由交代作用形成的磷灰石,说明其寄主碳酸岩岩浆源区应为遭受过流体交代作用的富集地幔.这些磷灰石的(La/Nd)_N比值＞1,(La/Yb)_N比值多数在100以上,与世界其他地区典型碳酸岩中的磷灰石相比铕负异常相对更明显,表明其寄主碳酸岩浆经历一定程度的分异演化.雪野较八陡碳酸岩中磷灰石含更高的F、Sr和∑REE含量及(La/Yb)_N比值,说明其寄主岩浆的演化程度更高.

In situ LA-ICP-MS analyses of apatites from carbonatites in western Shandong

In situ major and trace element analyses for apatites in earbonatites from Xueye and Badou of western Shandong Province have been undertaken by using EMPA and LA-ICP-MS techniques,respectively,and their petrogenetie significance are accordingly discussed.Investigation results have shown that the studied apatites are enriched in fluorine(= 1.07%～2.74%)and depleted in chlorine(＜0.04%),and can be categorized to hydroxyl-fluorapatites or fluor-hydroxyapatites.Trace elements analyses by LA-ICPMS technique demonstrate that the apatites concentrate high contents of Sr,Th,U,Pb and LREEs,and served as one of the main carriers of these elements for whole-rock.The Sr and F contents of apatites are positively correlated with both ∑ REE values and LREE/HREE ratios.The apatites in western Shandong earbonatites share some common trace element characteristics with those hosted in typical global carbonatites,such as rich in Sr and LREEs,and poor in Y,but display much high Sr/Y and Th/U ratios,and their Sr,Ce,Th and Y concentrations are close to that of the metasomatised apatites in mantle.The above features indicate that the source region of the host carbonatites should be an enriched mantle induced by fluid metasomatism.The apatites also have(La/Nd)_N ＞ 1,(La/Yb)_N generally above 100 for most samples,and display more pronounced negative europium anomalies relative to those in other typical global carbonatites,suggesting that they were crystallized from an evolved carbonatite melt.Apatites in carbonatites at Xueye contain higher contents of F,Sr and ∑ REE,and higher(La/Yb)_N ratios in comparison with those in Badou,indicating the host magma has experienced higher degree of fractionation.