北祁连构造带冷龙岭地区火山岩地球化学特征及年代学

Geochemical Characters and LA-ICP-MS Zircon U-Pb Dating of the Lenglongling Volcanic Rocks,Tectonic Belt of the North Qilian

门源冷龙岭地区位于北祁连造山带的东段，该区火山岩以中基性为主，有少量酸性火山岩；对安山岩进行单颗粒锆石微区U—PbLA—ICP—MS同位素测定，获得（460．17±0．92）Ma加权平均年龄，代表岩石的形成年龄。主量元素中w（SiO2）=48．94％-60．97%，w（K2O）／w（Na2O）〈1，w（TFeO）-w（K2O＋Na2O）-w（MgO）图解显示样品全部属于拉斑玄武岩系列，低钛、高铝铁等显示具岛弧火山岩的特．苣。岩石富集LILE元素（Rb、Ba、Th、K等），亏损HFSE元素（Ta、Nb、Ti等）；稀土元素总量较低[w（∑REE）为9．88×10^-5～42．21×10^-6），轻重稀土元素分馏不明显[w（∑LREE）／w（∑HREE）为0．81～5．70]，稀土元素分配模式为轻稀土平坦型，具弱Eu负异常[δ（Eu）=0．59～1．10]。w（Zr）／w（Y）-w（Zr）、w（Ti）-w（Zr）、W（Y）／15-w（La）／10-w（Nb）／8和w（Hf）／3-w（Th）-w（Ta）等火山岩构造环境判别图解均显示该套火山岩形成于岛弧环境，形成时代为中奥陶世。

The Qilian orogen involves North, Centrer and South Qilian. Lenglongling area of Menyuan, located in the eastern segment of north Qilian, is composed of volcanic rocks and few acid volcanic rocks. LA-ICP-MS U-Pb zircon weighted ^238U/^204Pb ages of （460.17 ±0.92）Ma dated for the ceratophyre can be interpreted as its emplacement age. These rocks have SiO2 ranging from 48.94 to 60.97 wt %, K2O/Na2O less than 1, and TFeO-K2O＋Na2O-MgO diagram shows these samples belong entirely to the tholeiitic series. While the low Ti and high Al and Fe indicate that they are typically characterized as island arc volcanic rocks. These monzogranites are depleted in HFSE, such as Ta, Nb and Ti, and enriched in LILE, such as K, Th, Rb and Ba, with total REE contents ranging from 9. 88 to 42. 21 ppm and ∑ LREE/ HREE ratios of 0.81- 5. 70. The chondrite-normalized REE patterns of these volcanic rocks display weak negative Eu anomalies （δEu=0. 59-1.10）. Zr/Y-Zr,Ti-Zr,Y/15-La/10-Nb/8, and Hf/3-Th -Ta illustrates that these volcanic rocks could represent typical island arc volcanic rocks.