下扬子地区冶山埃达克质岩的年代学、地球化学特征及其岩石成因

Geochronology,Geochemical Characteristics and Petrogenesis of Adakite in Yeshan,Lower Yangtze Region

冶山岩体位于苏皖交界处江苏境内,下扬子地块东北部,为一大致呈“V”字形的岩株。岩体主体岩性为花岗闪长岩,西北边缘见石英二长岩、闪长岩。锆石LA-ICPMS U-Pb定年结果表明,冶山侵入体的形成年龄为128.0±1.5 Ma,属于早白垩世早期。冶山侵入岩为一套富硅(SiO2为63.34%~66.44%)、富镁(Mg^#为50.6-57.6)、钙碱性(σ为2.54~2.69)、准铝质(A/CNK为0.867~0.946)岩石;具较强的轻重稀土元素分异、无明显-弱正Eu异常、富集Ba、K、Sr等大离子亲石元素,而亏损Ta、Nb、Ti等高强场元素、高Sr/Y和(La/Yb)N比值等特点,与C型埃达克岩具有相似的地球化学特征,属于C型埃达克岩。锆石Lu-Hf同位素证据显示,冶山埃达克质岩具有较低且均一的εHf(t)值(-15.76^-18.54)和较古老的地壳模式年龄t(DM)^C(2170~2340 Ma)。结合前人Sr-Nd-Pb同位素、地球物理学等资料,认为冶山侵入体可能来源于拆沉下地壳的部分熔融,并在源区混入地幔组分,形成于郯庐断裂带早白垩世活化阶段。

Yeshan pluton is located within Jiangsu Province at the borders of Jiangsu and Anhui Provinces,northeastern part of the Lower Yangtze Block,which appears as V-shaped rock stock.The main lithology of the rock mass is granodiorite.Quartz monzonite and diorite are found in the northwest margin.In-suit zircon U-Pb dating and Hf isotopic compositions for the Yeshan intrusive rocks.zircon U-Pb dating indicates that its emplacement took place at 128.0±1.5 Ma,suggesting a product of Early Cretaceous magmatism.the Yeshan intrusive rocks belong to Calc-alkaline(σ:2.54~2.69),metaluminous rocks(A/CNK values ranging from 0.867~0.946)which are characterized by high SiO2(63.34~66.44 wt%)and Mg^# value(50.6-57.6),enriched in LREE and LILE(Ba、K、Sr),depleted in HREE and HFSE(Ta、Nb、Ti),with non-slightly positive Eu anomalies and high Sr/Y,La/Yb ratios.Geochemical characteristics are similar to adakite,It belongs to C-type adakite.The Yeshan intrusive rocks have homogeneous zircon Hf isotopic composition,with relatively lowεHf(t)values(-15.76^-18.54)and old Hf isotopic crustal model ages(2170~2340 Ma).Combined with previous Sr-Nd-Pb isotope and geophysics data,these geochemical features imply that the Yeshan intrusive rocks were derived from partial melting of thickened lower crust,with small amount of mantle components may be involved during the process of ascent,related to the reworking of Tanlu fault zone in Early Cretaceous.