华北克拉通南缘大庄铌-稀土矿床碱性岩中榍石的地球化学、U-Pb年龄和Nd同位素特征

位于华北克拉通南缘河南省方城县的大庄铌-稀土矿是近年来新发现的一例具有中型规模的铌-稀土矿床.该矿床赋存于新元古代双山碱性侵入岩中，黑云母正长岩和角闪霞石正长岩是铌-稀土矿的主要赋矿岩石.通过电子探针与LA-ICP-MS等方法对大庄铌-稀土矿碱性岩中的榍石开展了主量、微量元素、U-Pb年代学和微区Nd同位素研究.大庄铌-稀土矿碱性岩中榍石的SiO₂（28.597%~31.169%）、CaO（25.492%~28.949%）、TiO₂（34.126%~38.940%）平均含量明显低于理论值，含有相对较高的F（0.141%~1.086%）、Al₂O₃（1.573%~2.817%）和FeO（0.446%~2.147%）.榍石具有极高的稀土元素总量（∑REE=781.47×10-6~31 766.59×10-6），球粒陨石标准化稀土配分曲线呈明显的右倾型，轻重稀土分馏明显，样品具有明显的负δEu异常（0.27~0.86）和不明显的δCe异常（0.97~1.22），Th/U值介于1.40~11.86，暗示大庄铌-稀土矿碱性岩中榍石为岩浆成因榍石，榍石Zr温度计获得大庄碱性岩的形成温度为667~964 ℃，平均为933 ℃.大庄铌-稀土矿碱性岩中的榍石具有极高的Nb和REE含量，表明榍石是大庄铌-稀土矿的重要成矿矿物，铌、稀土等元素是伴随岩浆演化逐步富集的结果.富铌榍石样品获得848±35 Ma、849±22 Ma、843±30 Ma、845±20 Ma、845±24 Ma和849±40 Ma的形成年龄，限定大庄铌-稀土矿化富集的时代为新元古代，与Rodinia超大陆裂解事件相关.榍石具有较为均一的Nd同位素值（ε_Nd（t）=-3.25~-0.66），表明方城地区霞石正长岩的原始岩浆形成于富金云母的地幔源区. 

Geochemical,U-Pb Age and Nd-Isotopic Characteristics of Titanite in Alkaline Rocks from Dazhuang Nb-REE Deposit in Southern Margin of North China Craton

The Dazhuang deposit is a newly discovered medium-scale Nb-REE deposit which is located at the Fangcheng County of Henan Province on the southern margin of the North China craton. The Nb-REE orebodies are hosted in biotite syenite and hornblende nepheline syenite of Neoproterozoic Shuangshan alkaline pluton. In this paper, major and trace elements, U-Pb geochronology and in-situ Nd isotope studies were carried out on the titanite from the alkaline rocks by EPMA and LA-ICP-MS. The average contents of SiO₂ (28.597%-31.169%), CaO (25.492%-28.949%) and TiO₂ (34.126%-38.940%) in titanite are significantly lower than the theoretical values, whereas it contains relatively high contents of F (0.141%-1.086%), Al₂O₃ (1.573%-2.817%) and FeO (0.446%-2.147%). The titanite in Dazhuang deposit has a very high total amount of REE (∑REE=781.47×10-6-31 766.59×10-6). In the chondrite-normalized REE patterns, the rare earth distribution curve is obviously right-leaning, and the light and heavy rare earth fractionation is obvious. There are obvious negative δEu anomalies (0.27-0.86) and inconspicuous δCe anomalies (0.97-1.22), with Th/U values ranging from 1.40 to 11.86, suggesting that the titanite is magmatic origin. The formation temperature of Dazhuang alkaline rocks obtained by the titanite Zr thermometer is 667-964 ℃, with an average of 933 ℃. The titanite in Dazhuang deposit has very high Nb and REE contents, indicating that the titanite is an important ore-bearing mineral, and the Nb and REE are the result of the gradual enrichment of magma evolution. Formation ages of 848±35 Ma, 849±22 Ma, 843±30 Ma, 845±20 Ma, 845±24 Ma, and 849±40 Ma were obtained from the Nb-rich titanite samples, constraining that the mineralization age of Dazhuang Nb-REE deposit is Neoproterozoic, of which related to the breakup of the Rodinia supercontinent. Titanite has a relatively uniform Nd isotope (ε_Nd(t)=-3.25 to -0.66) value, indicating that the original magma was derived from mantle where rich in phlogopite.