五台山地区1.76Ga辉石闪长岩的成因及其地质意义

Petrogenesis of 1.76 Ga pyroxene diorite in Wutai Mountains area of North China Craton and its geological significance

五台山地区的黄金山辉石闪长岩侵入早元古代滹沱群中,同时又被早元古代末期辉绿岩脉穿切。辉石闪长岩锆石SHRIMP U-Pb定年结果为1 755±12 Ma,与华北克拉通广泛分布的基性岩墙群的时代相近。辉石闪长岩高TiO2、Fe2O3、P2O5、K2O和Na2O,低MgO(Mg#值为36~39)。稀土元素总量较高(365.75×10-6~411.97×10-6),轻重稀土元素分异强烈,其中轻稀土元素分异较明显,重稀土元素分异较弱,具有微弱的负铕异常。微量元素中,富集Rb、Ba、Sr、Zr和Hf,Cr、Ni含量低。(87Sr/86Sr)i和(143Nd/144Nd)i变化范围小,εNd(t=1.76 Ga)值为-6.08^-4.81。综合分析认为,黄金山辉石闪长岩为富集岩石圈地幔部分熔融后发生分离结晶形成的,与华北克拉通广泛分布的基性岩墙群具有相同的源区,可能代表华北南缘地幔柱活动时同一岩浆旋回不同阶段的岩浆侵位记录。辉石闪长岩中锆石具有岩浆成因锆石特征,但锆石的Th/U比值普遍较低,与变质锆石类似。辉石闪长岩中Th/U比值高,同时Th、U在斜长石、单斜辉石和角闪石中配分系数低。因此推测,低Th/U比值锆石的形成与原始岩浆和主要成岩矿物结晶无关,很可能与富含Th的副矿物结晶有关。辉石闪长岩中低Th/U比值岩浆锆石的出现表明,利用Th/U比值解释锆石成因和年龄结果意义时应慎重。

The Huangjinshan pyroxene diorite intruded Paleoproterozoic Hutuo Group,and was cut and penetrated by late Paleoproterozoic dolerite in Wutai Mountains area.SHRIMP zircon dating of pyroxene diorite yielded a 207Pb/206Pb age of 1 755±12 Ma,which is similar to ages of the widely distributed Paleoproterozoic mafic dyke swarms in the North China Craton.The pyroxene diorite sample contains high TiO2,Fe2O3,P2O5,K2O and Na2O,and low MgO（Mg#,36~39）.The sample has high concentration of REE（365.75×10-6~411.97×10-6）,with strong LREE fractionation and slight fractionation of HREE,and slightly negative Eu anomaly.The rock samples are enriched in Rb,Ba,Sr,Zr and Hf,and are depleted in Cr and Ni.The variation of（87Sr/86Sr）i and（143Nd/144Nd）i ratios is small,and εNd（t=1.76 Ga）values vary from-6.08 to-4.81.Based on a comprehensive analysis of all evidence,the authors hold that Huangjinshan pyroxene diorite was partially melted from enriched upper lithosphere mantle related to the mantle plume and was fractionally crystallized with the same magmatic source of mafic dyke swarms widely distributed in North China Craton,thus representing a different stage of the same magmatic cycle.Zircons in pyroxene diorite samples show the same characteristics as basic-intermediate rocks,but their Th/U ratios are very low,similar to features of the metamorphic zircon.Generally speaking,Th/U ratios are high in pyroxene diorite whereas the partition coefficients of Th and U are very low in plagioclase,clinopyroxene and hornblende.It is thus inferred that the Th/U ratios in zircon are independent of the source magma and major diagenetic minerals,and are probably related to crystallization of Th-rich accessory minerals.The zircons with very low Th/U ratios in pyroxene diorite indicate that we should be very careful in explanation of the genesis of zircon by Th/U ratios.