南秦岭胭脂坝花岗岩成因:锆石U-Pb年龄、地球化学和Sr-Nd-Pb同位素的制约

Petrogenesis of Yanzhiba Granite in South Qinling:Constraints from Zircon U-Pb Ages,Geochemistry and Sr-Nd-Pb Isotope

秦岭造山带广泛发育新元古代、古生代、早中生代和晚中生代岩浆作用,是长期复杂造山作用的结果。胭脂坝花岗岩岩体出露于南秦岭宁陕地区五龙岩体群的最东部,是南秦岭早中生代花岗岩的重要组成部分。5个花岗岩样品的锆石U-Pb定年结果分别为(202.9±3.5)、(201.0±3.0)、(202.1±2.6)、(200.4±5.4)、(^(205).5±3.3)Ma,平均年龄为(201.9±1.5)Ma,属于早中生代五龙岩体群岩浆作用中晚期的产物。岩石地球化学成分显示该岩体的主体为黑云母花岗岩,具有高硅、富碱的地球化学特征,富集大离子亲石元素,亏损高场强元素,属于弱过铝质、高钾钙碱性岩石系列。初始Sr同位素组成为0.704 31~0.706 78,ε_(Nd)_(t)值为-5.2^-2.0,初始Pb同位素组成相对高,(N(^(206) Pb)/N(^(204) Pb))_(t)=17.737~18.191,(N(^(207) Pb)/N(^(204) Pb))_(t)=15.511~15.574,(N(^(208)Pb)/N(^(204)Pb))_(t)=37.439~38.071。较高的放射性成因Pb同位素组成说明胭脂坝岩浆源区具有扬子地块的属性;初始Sr同位素组成和ε_(Nd)_(t)值随着SiO2含量的升高呈现出线性变化趋势,说明胭脂坝花岗质岩浆在上升侵位过程中存在围岩物质的同化作用;结合与新元古代火山沉积岩(如耀岭河群、武当群、碧口群)相似的初始Sr-Nd同位素组成,推测胭脂坝岩体的岩浆源区主要是具有扬子地块属性的新元古代地壳物质。

Qinling orogenic belt is produced by long-time complicated orogenic processes with widely distributed Neoproterozoic to Mesozoic magmatic activities. Yanzhiba granitic pluton, which is located in the easternmost part of Wulong granitic plutons, is one of the major components of Early Mesozoic magmatism in Ningshan area of South Qinling. Zircon U-Pb dating results and geochemical characteristics of Yanzhiba pluton were reported. Zircon U-Pb dating results of five samples give （202.9±3.5）, （201.0±3.0）, （202. 1±2.6）, （200.4±5.4）, （205. 5± 3.3） Ma, respeetively. A mean U-Pb age of （201.9 ±1.5）Ma suggests that Yanzhiba pluton is formed in the middle and late stage of Early Mesozoic Wulong granitic plutons. Yanzhiba granitic pluton is characterized by high silica, alkali-rich, enrichment of large-ion lithophile elements,depletion in high field-strength elements, belonging to weakly peraluminous and high-potassic calc-alkaline series. Initial Sr isotopic composition ranges from 0. 704 31 to 0. 706 78, εNd （t） values are -5. 2--2. 0, and initial Pb isotopic composition is relatively high, including that （N（206 Pb）/N（204pb））ω =17. 737-18. 191, （N（207pb）/N（204pb））（,）= 15. 511-15.574, and （N（208Pb）/N （208Pb））（1） =37. 439-38. 071. The high radioactive Pb isotopic composition implies that Yanzhiba granite has the affinity of Yangtze Block. Linear variations of initial Sr isotopic composition and end （t） values with the contents of SiO2 suggest that there is assimilation of the surrounding rocks during the rising and emplacement of Yanzhiba granitic magma. The initial Sr-Nd isotopic compositions are similar to those of Neoproterozoic volcanic sedimentary sequences in South Qinling （such as Yaolinghe, Wudang and Bikou Groups）. Sr-Nd-Pb isotopic compositions indicate that the magma source of Yanzhiba granite originates from Neoproterozoic crustal materials with an affinity of Yangtze Block.