安徽滁州地区闪长质岩锆石U-Pb年龄与地球化学:岩石成因和动力学意义

Zircon U-Pb age and geochemistry of the dioritic rocks in Chuzhou area in Anhui Province:Petrogenesis and dynamics significance

用LA-ICP-MS测得安徽滁州2个闪长玢岩样品中锆石^(206)Pb/^(238)U年龄为126.19±0.44Ma和126.4±0.7Ma,结合前人研究,得出滁州地区岩体的侵位时代应为120~130Ma之间,为早白垩世。岩石地球化学研究显示,Si O_2含量变化范围为56.75%~60.90%,具有高Al_2O_3(14.82%~15.77%)、Mg O(>4%)、Sr(>750×10^(-6))、Sr/Y(62~110)、La/Yb(20~36),低Y、Yb的特征,同时富集轻稀土元素和大离子亲石元素,亏损高场强元素,Eu异常不明显,属于典型的埃达克质岩。Mg~#值为39~45,K_2O/Na_2O值为0.57~0.96,平均值为0.75,明显低于大别造山带加厚下地壳埃达克岩,Ce/Pb值较低,大多集中在3~5之间,类似于陆壳而明显低于洋壳。研究认为,安徽滁州地区埃达克质岩由拆沉下地壳部分熔融形成,埃达克质岩浆在上升过程中与地幔橄榄岩发生反应,导致熔体Mg O、Cr、Ni等含量增加。早白垩世中国东部地壳伸展减薄导致下地壳拆沉,地幔物质的参与带来铜、金等成矿物质,埃达克质岩可作为该地区重要的找矿标志。

Abstract： LA-ICP-MS zircon 206 Pb /238 U ages of two diorite porphyrite samples from Chuzhou area of A nhui Province are 126.19± 0.44Ma and 126.4±0.7Ma, respectively. Previous study shows that the emplacenient age of dioritic rocks should be 120-130Ma, sug-gesting early Cretaceous. Geochemical study shows the values of SiOi vary from 56.75% to 60.90%, w i th the characteristics of high ALO 3 （14.82%-15.77%）, MgO（〉4%）, Sr（〉750x10-6 ）, Sr/Y（62？110）, and La/Yb（20？ 36） but low Y , Yb, LREE as we l l as enrichment of LREE and LILE but depletion of HREE, wi th indistinct Eu anomaly, indicating typical adakitic rocks. Mg # values range from 39 to 45, K iO/Na iO vary from 0.57 to 0.96, 0.75 on average, significantly lower than the values of the adakitic rocks in the thickened crustal of Dabie orogenic belt; Ce/Pb ratios are low, mostly concentrated in the range of 3.5, similar to the values of the continental crust but significantly lower than those of the oceanic crust. The authors hold that the adakitic rocks in Chuzhou area of Anhui was formed by partial melting of delaminated lower crust, and the reaction of upward migrating adakitic magmas and mantle peridotite led to the increase of MgO, Cr and Ni melt content. In Early Cretaceous, eastern China crustal extension thinning resulted in lower crust delamination, and mantle material was involved in bringing Cu, Au and other ore- forming elements; hence adakitic rocks can be used as an important prospecting indicator in this area