冀北沽源铀矿田粗面岩的年代学、地球化学特征及岩石成因

发布时间：2018-02-13 01:49

  本文关键词： 粗面岩 SHRIMP锆石U-Pb年龄 Sr-Nd-Pb-O同位素 年轻下地壳 早白垩世早期 沽源铀矿田　出处：《地球化学》2017年02期 　论文类型：期刊论文

【摘要】：沽源铀矿田位于开源-赤峰断裂带以南的华北古板块北缘,燕辽Mo-U-Ag-Pb-Zn多金属成矿带西段和沽源-红山子铀成矿带西南段,铀矿赋存在粗面岩-流纹岩组合中。2个粗面岩样品(样号分别为GY101和GY108)的锆石均具有明显的环带结构,Th/U比值高,属典型的岩浆成因锆石。SHRIMP锆石U-Pb测年结果表明:GY101粗面岩13个分析点的206Pb/238U年龄范围为142~136 Ma,加权平均值为(138.4±1.3)Ma,MSWD=0.47;GY108粗面岩12个分析点的~(206)Pb/~(238)U年龄范围为143~137 Ma,加权平均值为(139.5±1.3)Ma,MSWD=0.49,指示地质时代属早白垩世早期。粗面岩具有较高的SiO_2、K_2O+Na_2O含量和K_2O/Na_2O、Fe_2O_3/FeO比值,在SiO_2-(K_2O+Na_2O)图解中落在碱性系列粗面岩或粗面英安岩范围,在SiO_2-K_2O图解中落在橄榄玄粗岩系列范围,结合标准矿物Q含量小于20%,属典型的高钾碱性粗面岩。稀土元素含量高,富集轻稀土,Eu负异常不明显,稀土分布模式为右倾轻稀土富集型,具高压型粗面岩的稀土元素特征;富集大离子亲石元素K、Rb、Ba、Pb和高场强元素Zr、Y,亏损Nb、Ta、Sr、Ti元素,具高Sr-Ba英安岩-流纹岩的微量元素特征。较低的Ti/Zr、Nb/Ta比值和较高的Rb/Sr比值,具有壳源火山岩的特征;具有较高的(~(87)Sr/~(86)Sr)_i(0.707781~0.708156)、εNd(t)( 5.31~ 5.83)和δ~(18)OVSMOW(7.08‰~8.05‰),较小的tDM2(1361~1376 Ma),较低的(~(206)Pb/~(204)Pb)_i(16.97~17.00)、(~(207)Pb/~(204)Pb)_i(15.06~15.62)和(~(208)Pb/~(204)Pb)_i(36.73~36.80),在ε_(Nd)(t)-(~(87)Sr/~(86)Sr)_i图解中落在EMⅠ富集地幔与亏损地幔的联线附近,在(~(208)Pb/~(204)Pb)t-(~(206)Pb/~(204)Pb)_t图解中落在靠近EMⅠ富集地幔的下地壳和地幔之间,在(~(87)Sr/~(86)Sr)_i-(~(206)Pb/~(204)Pb)_t和(~(143)Nd/~(144)Nd)_i-(~(206)Pb/~(204)Pb)_t图解中均落在EMⅠ富集地幔端元附近,在Zr/Al_2O_3-TiO_2/Al_2O_3和Y-Zr判别图解中都落在板内构造环境内。可见,沽源铀矿田粗面岩的成因可用两阶段模式进行解释:中元古代源于EMⅠ富集地幔和亏损地幔的岩浆混合并在底侵于下地壳的过程中受到少量古老下地壳物质混染后形成年轻下地壳;中侏罗世晚期-晚侏罗世发生的挤压构造事件导致地壳加厚,早白垩世早期在拉张构造环境下加厚地壳的年轻下地壳发生部分熔融形成的粗面质岩浆喷出地表形成粗面岩。
[Abstract]:The Guyuan uranium ore field is located in the northern margin of the North China paleo-plate south of the open-source Chifeng fault zone, the western section of the Yanliao Mo-U-Ag-Pb-Zn polymetallic metallogenic belt and the southwestern section of the Guyuan-Hongshanzi uranium metallogenic belt. The uranium deposits occur in the trachyte-rhyolite assemblage. The zircon samples of two trachyte samples (GY101 and GY108, respectively) have obvious zonal structure and high Th / U ratio. Typical magmatic zircon SHRIMP zircon U-Pb dating results show that the 206Pb / 238U age range of 13 analysis points of the: GY101 trachyte is 142136Ma. the weighted average is 138.4 卤1.3MMSWDU 0.47GY108 trachyte with a 206PbP / 238U age range of 143137Ma.The weighted mean value is 139.5 卤1.3MMSWDU 0.49. It is shown that the geological age belongs to the early Cretaceous. The coarse rock has a high SiO2S / K _ 2O Na_2O content and a K _ 2O / Na _ 2O / Na _ 2O / T _ 2O _ 3 / Feo ratio. It falls in the range of alkaline series trachyte or trachyte, in SiO_2-K_2O diagram, in the range of olivine trachyte series, and in combination with the standard mineral Q content less than 20%, it is a typical high potassium alkaline coarse rock with high REE content. The negative anomalies of EU enriched with LREE are not obvious, and the REE distribution pattern is right LREE enrichment type, which is characterized by REE of high pressure coarse-faced rocks, and is rich in large ion lipophilic elements K ~ (+) Rb ~ (+) Ba ~ (+) Pb and high field strength elements Zr ~ (2 +) Y, and is depleted of NB ~ (2 +) Taer Sr ~ (2 +) Ti. It has the characteristics of trace elements of high Sr-Ba dolomite and rhyolite. The lower Ti / ZrN NB / Ta ratio and the higher Rb/Sr ratio have the characteristics of crust-source volcanic rocks. The smaller tDM2(1361~1376 Mai, the smaller tDM2(1361~1376, the lower 20204Pbt rbt rsrrrsrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrnnbrnrrrrrrnnbrrrnrr.204pbr-nbrnbr-nbrrrrrr-204pbr-nbr-nbrr-nbrr-nbr-nbrrr-208pbnr-208Pbnr-208Pb / m is close to 6.736.80, and is close to the upper mantle at the bottom of the map of the earth's crust, near the bottom of the map of the earth's crust, near the bottom of the earth's crust, and near the bottom of the Pb6EM map, where the upper mantle is located near the bottom of the earth's crust, and in the lower part of the earth's crust, there is a loss in the mantle, and in the lower part of the earth's crust, there is a loss in the mantle, and in the upper mantle, there is a loss in the earth's crust, and in the upper mantle, there is no difference between the two groups. The smaller tDM2(1361~1376 is 8.05 鈥，

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