江西宁冈岩体的形成时代、地球化学特征及其构造意义

Formation age,geochemical characteristics of the Ninggang granite body in Jiangxi Province and its tectonic significance

宁岗岩体是湘赣交界部位呈南北向展布的加里东花岗岩带的重要组成部分,它主要由二长花岗岩组成。锆石LA-ICPMS U-Ph年龄为433.8±2.2Ma,属于加里东期岩浆活动产物。虽然宁冈岩体的SiO_2含量变化明显(67.46%～74.85%),但它们的ACNK值都大于1.1,钾大于钠(K_2O/Na_2O=1.36～2.37),CaO/Na_2O比值大于0.3,富集Rb、Th、Cs,亏损Nb、Ta、Ba、Sr,LREE富集(LREE/HREE=5.8～12.2)和Eu亏损相对明显(δEu=0.28～0.61)。同时,它们还具有较低的ε_(Nd)(t)值(-9.3～-8.6)、较高的(^(87)Sr/^(86)Sr)1值(0.71172～0.71937)和古老的Nd模式年龄(1838～1909Ma)。这些特征表明,宁冈岩体属于典型的壳源型花岗岩,很可能是在华夏古陆残块与扬子地块之间发生陆-陆碰撞拼贴而引发的地壳伸展-减薄的构造背景下,通过砂质岩石部分熔融的方式形成。

The Ninggang granite body, located at the juncture of Hunan and Jiangxi provinces, is an important member of the Caledonian granite zone with south-north trend. It is mainly composed of monzogranite. LA-ICPMS U-Pb dating of zircons yields an age of 433.8 ± 2. 2Ma, suggesting that it was formed in the Caledonian magmatism event. Although Ninggang granite body exhibit obvious variation of SiO2 contents （67.46% - 74. 85% ）, their ACNK values are all bigger than 1.1, potassium larger than sodium（ K2O/Na2O = 1.36 ±2. 37）, CaO/Na2O ratios larger than 0. 3 and enrichment of Rb, Tb and Cs, depletion of Nb, Ta, Ba and St, strong REE fractionation （ LREE/HREE = 5.8 - 12. 2 ） and relative obvious Eu depletion （ 8Eu = 0. 28 - 0. 61 ）. They also have lower εNd（t） values （ -9. 3 - - 8. 6） , higher （^87St/^86 Sr）i ratios （0. 71172 - 0. 71937 ） and old Nd model ages （ 1838 - 1909Ma）. These geochemical characteristics indicate that the Ninggang granite body belonged to crust sourced type of granite and were derived from the partial melting of psammitic rock under an extensional tectonic setting which is caused by continent-continent collision between remnant Cathaysia fragments and Yangtze block.