滇西漕涧地区两类晚奥陶世强过铝花岗岩锆石U-Pb年龄、地球化学和Hf同位素特征及对泛非运动的指示

Zircon U-Pb age,geochemistry and Hf isotope of two types of Late Ordovician super-aluminum granites in the Caojian area of western Yunnan,and its indication to Pan-African movement

滇西云龙县漕涧复式花岗岩体出露于保山地块北端东缘,由早奥陶世、晚奥陶世、晚白垩世、新近纪4期花岗岩组成。以晚奥陶世花岗质岩体为主体,主要岩性为中粒黑云二长花岗岩、似斑状中粗粒黑云二长花岗岩,岩石普遍发育片麻状构造。二者的LA-ICP-MS锆石U-Pb年龄分别为453.9±3.3 Ma和456.8±3.5 Ma。二者均属后碰撞环境的高钾钙碱性强过铝花岗岩,富集K、Rb等大离子亲石元素,亏损Zr、Ta、Nb、Th等高场强元素。在Sr-Yb图解上,前者属喜马拉雅型花岗岩,为下地壳环境较高压力条件下的部分熔融物;而后者属南岭型花岗岩,指示了中-上地壳环境较低压力条件下的部分熔融作用;在B-A图解上,前者显示白云母过铝花岗岩的趋势,岩浆的形成主要与隆升减压作用相关;而后者显示了含堇青石富黑云母过铝花岗岩的特点,岩浆的形成主要与深部地幔物质上涌导致的增温加热作用有关。锆石Hf同位素分析表明,前者ε_(Hf)(t)=-11.65~0.28,平均为-6.36;后者ε_(Hf)(t)=-11.61~-1.57,平均为-6.78,二者具有大致相当的源区,均为古老的地壳物质,但中粒黑云二长花岗岩中可能有少量地幔物质的加入。综合研究表明,在泛非运动的后碰撞阶段背景下,深部壳-幔物质相互作用的过程可能首先是隆升减压导致下地壳物质的低度部分熔融,随后地幔物质上涌,导致中-上地壳物质的增温加热的较高程度部分熔融。该两类晚奥陶世花岗岩属冈瓦纳大陆北缘的小地块之间相互挤压、碰撞的岩浆作用响应,也表明保山地块存在泛非运动。

The Caojian complex granite located in Yunlong County of western Yunnan is exposed on the eastern edge of the northern end of the Baoshan block,which is composed of four period granites of Early Ordovician,Late Ordovician,Late Cretaceous and Neogene.The main Late Ordovician intrusive is lithologically made of medium grained biotite monzogranite and the porphyritic medium-coarse grained biotite monzogranite,in which gneissic structure is well developed.Zircon LA-ICP-MS U-Pb ages are 453.9±3.3 Ma and 456.8±3.5 Ma,respectively.Both belong to post-orogenic strongly-peraluminous high-K calcalkaline peraluminous granite with enrichment of K、Rb and other large ions lithophile element,but depletion of the high field strength element such as Zr,Ta,Nb,Th etc.According to the Sr-Yb diagram,the former belongs to the Himalayan granite,which is a partial melt under higher pressure in lower crust environment.The latter belongs to the Nanling granite,which indicates partial melting under the condition of lower pressure in the middle-upper crust environment.In the light of A-B diagram,the former shows the trend to muscovite peraluminous granite,and its magma is mainly related to uplift and decompression.The latter shows the characteristics of cordierite-bearing biotite-rich peraluminous granite,and its magma is mainly related to the warming and heating caused by upwelling of the deep mantle materials.Zircon Hf isotope analysis shows that the former yieldsε_(Hf)(t)value of 11.65~0.28,averaging-6.36.The latter givesε_(Hf)(t)value of 11.61~-1.57,averaging-6.78.Both share the roughly equivalent source regions,and are ancient crustal materials,with a small amount of mantle material added to the medium grained biotite monzogranite.It shows that the process of deep crust-mantle interaction may be firstly uplift and decompression leading to lower crust materials low-degree partial melting,not far behind,because to upwelling of mantle materials,warmed and heated medium-upper crust materials happened to higher-degree partial melting,in the background of the post-collision phase of thePan-African movement.The two types of the Late Ordovician granites are the magmatism responses of mutual extrusion and collision between small blocks on the northern edge of Gondwana continent,it shows that there is the Pan-African movement in Baoshan block.