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

Geochronology,geochemical characteristics and petrogenesis of trachytes in the Guyuan uranium ore field,northern Hebei Province

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。

The Guyuan uranium ore field, located in the northern part of the ancient North China plate in the south of the Kaiyuan-Chifeng Fault zone, is an integral part of the Yan-Liao polymetallic （Mo-U-Ag-Pb-Zn） and Guyuan- Hongshanzi uranium belts. The rhyolite-trachyte rock assemblage is the main wall rock of hydrothermal-type uranium deposits. Zircons in two trachyte samples （samples GY101 and GY108） from the Guyuan uranium ore field have obvious girdle structure, and high Th/U ratios, indicating that the zircons belong to the typical magmatic zircons. The results of SHRIMP zircon U-Pb dating indicate that the 206pb/238U ages of 13 analysis points of sample GY101 are 142-136 Ma, with the weighted average of （138.4±1.3） Ma and MSWD = 0.47; the 206pb/238U ages of 12 analysis points of sample GY203 are within the range of 143-137 Ma, with the weighted average of （139.5±1.3） Ma and MSWD = 0.49. Obviously, two age data of trachyte from the Guyuan uranium ore field are 140-138 Ma, so the trachyte should be a product of early Early Cretaceous volcanic activity. The trachyte has high contents of SiO2 and K2O＋Na2O and high K2O/Na2O and Fe2O3/FeO ratios, just falling within the alkaline-series trachyte or dacite range as shown in the TAS diagram, or in the shoshonite series in the SiO2-K2O diagram. The CIPW results showed that Q is less than 20%, indicating that the trachyte belongs to the typical one. Trachyte has a high content of rare-earth elements, an enrichment in light rate-earth elements, and unobvious negative Eu anomaly, showing the right-leaning enrichment type of light rare-earth elements, with a high pressure-type trachyte characteristics of rare-earth elements; enrichment in large ion lithophile elements （K, Rb, Ba, Pb） and high field-strength elements （Zr, Y）, loss of high field- strength elements （Nb, Ta, Ti ）, with the characteristics of trace elements in Sr-Ba-high dacite-rhyolite; low Ti/Zr, Nb/Ta ratios, high Rb/Sr ratio, higher （87Sr/86Sr）i （0.707781-0.708156）, εNd（t） （-5.31- -5.83） and δ18 OVSMOW （7.08‰-8.05‰）, smaller tDM2 （1361-1376 Ma）, lower （206pb/204pb）i （16.97-17.00）, （207pb/204pb）i （15.06-15.62）, （208Pb/204Pb）i （36.73-36.80）, the projection points located between DMM and EM I in the diagram of εNa（t）-（87Sr/86Sr）i, between the lower crust and mantle in the diagram of （208pb/204pb）t-（206pb/204pb）t, close EM I in the diagrams of （87Sr/86Sr）i-（206pb/204pb）t and （143Nd/144Nd）i- （206pb/204Pb）t, within the plate tectonic setting in the diagram of Zr/Al2O3-TiO2/Al2O3 and Y-Zr. The genesis of trachyte may be explained b.y the two-stage model： mid-Mesoproterozoic mixed magma was derived from the EM I -enriched mantle and DMM mantle and a small amount of ancient lower crust material came from a young lower crust, early Early Cretaceous magma from partial melting of the young lower crust formed trachyte magma in the background thickened crust.