豫南伏牛山余脉铜山岩体锆石U-Pb定年、地球化学特征及其地质意义

LA-ICP-MS Zircon U-Pb Dating, Geochemical Features and Geological Implications of Tongshan Rock Mass in Stretching Branch of Funiu Mountain, Southern Henan Province

铜山岩体位于华北克拉通南缘伏牛山余脉南端,南侧紧邻北秦岭构造带,对其开展锆石定年和岩石成因研究,有助于丰富华北克拉通南缘燕山期深部构造演化的认识。铜山岩体的LA-ICP-MS锆石U-Pb年龄为(147.0±4.3)~(144.1±4.4)Ma(n=5)和(133.0±3.9)~(127.4±3.6)Ma(n=21),形成了双峰式单颗粒锆石年龄谱,加权平均年龄分别为(145.5±4.5)Ma和(130.6±1.8)Ma,其中前者暗示铜山岩体周缘存在约145 Ma的隐伏岩浆活动,后者代表了铜山岩体的形成时代。铜山岩体样品具有富硅、碱和贫钙、镁的特征,属于高钾钙碱性系列。在微量元素蛛网图中,显示了明显的Rb、Th、Zr及Hf的异常峰和Sr、P、Ti的异常谷。稀土总量介于43.07×10-6~173.14×10-6之间,(La/Yb)N值为1.97~19.58,稀土配分模式具有右倾、轻稀土富集、重稀土亏损和负Eu异常(δEu值介于0.33~0.73之间)的特征。其(La/Yb)N-w(LaN)投点显示了线性正相关关系,表明部分熔融作用控制了铜山岩体的成分变异。铜山岩体形成于约40 km的正常厚度地壳内,部分熔融源区位于角闪岩相,源区残余相主要矿物为角闪石和斜长石。早白垩世桐柏山三合店—鸡公山区域岩石圈拆沉作用导致深部流体快速释放,注入到未完全固结岩浆/流体库(已晶出约145 Ma的锆石),混合活化之后上侵固结形成具有双峰式锆石年龄谱的铜山岩体。铜山岩体是对桐柏山三合店—鸡公山区域岩石圈拆沉作用近同时的岩浆响应,与之相关的岩浆效应没有局限在桐柏—大别造山带内,在北秦岭构造带和华北克拉通南缘均有显示。

The Tongshan rock mass is located at the south of stretching branch of Funiu Mountain in the southern margin of the North China craton, which is composed of medium-grained biotite syenogranite and monzonitic granite. The study on the zircon U-Pb dating and petro-genesis of the Tongshan rock mass is significant for understanding the Yanshanian deep tectonic evolution of the southern margin of the North China craton. The LA-ICP-MS zircon U-Pb dating results of the Tongshan rock mass fall into two groups of(147.0±4.3)-(144.1±4.4) Ma(n = 5) and(133.0±3.9)-(127.4±3.6) Ma(n = 21), characterized by a bimodal zircon age spectrum with weighted averages of(145.5±4.5) Ma and(130.6±1.8) Ma, respectively. The age of(145.5±4.5) Ma suggests the magmatism concealed around the Tongshan rock mass, and the age of(130.6±1.8) Ma represents its formation of the Tongshan rock mass in the Early Cretaceous. The Tongshan rock mass is characterized by higher SiO2 and alkali, lower MgO and CaO, and its compositional spots fall into the high-K calc-alkaline series in the w(K2O)-w(SiO2) diagram. The peaks of Rb, Th, Zr and Hf and troughs of Sr, P and Ti are shown in the spider diagram. The total rare earth elements are from 43.07×10-6 to 173.14×10-6 with the(La/Yb)N ratios of 1.97-19.58. In the chondrite-normalized REE patterns, the Tongshan rock mass has the characteristics of LREE-enrichment and HREE depletion with Eu negative anomalies ranging from 0.33 to 0.73. It is suggested that the compositional change of the Tongshan rock mass was resulted mainly from the partial melting of source rocks as the compositional trend shown in the(La/Yb)N-w(LaN) diagram. The partial melting source of the Tongshan rock mass is located in the amphibolite-facies with a crust thickness of 40 km, and its residual phases of the partial melting source includes mainly hornblende and plagioclase. Due to the lithosphere delamination of the Sanhedian-Jigongshan area in the Tongbai Mountain at about 131 Ma in the Early Cretaceous, the deep fluids/magmas discharged rapidly, intruded into the incompletely consolidated magma/fluid reservoir, crystallized the 145 Ma zircon grains, and finally formed the Tongshan granite with the mixed materials in the activation reservoir as shown on the bimodal zircon age spectrum. The Tongshan rock mass was almost the simultaneous magmatic response to the Sanhedian-Jigongshan lithosphere delamination, and the related magma activities occurred not only on the Tongbai-Dabie orogenic belt, but also on the northern Qinling orogenic belt and the southern margin of the North China craton.