会东菜园子花岗岩的年龄、地球化学——扬子地台西缘格林威尔造山运动的机制探讨

Age and Geochemistry of the Caiyuanzi Granite in Sichuan,SW China:Mechanism of the Grenvillian Orogenic Movement in the Western Margin of Yangtze Block

本文报道了康滇地区中部,会东县菜园子地区Ⅰ、Ⅲ号岩花岗岩中锆石的SHRIMP U-Pb年龄,分别为1040.4±6.1Ma(MSWD=0.53,n=18)和1063.2±6.9Ma(MSWD=1.15,n=11),与全球格林威尔造山运动的时间高度一致。花岗岩的常量元素表现为高SiO2(SiO2平均为74.96%)、过铝质(绝大多数样品A/CNK值大于1.1)、碱含量中等(Na2O+K2O平均为6.4%)、低MgO(平均为0.17%)、TiO2(平均为0.28%)、CaO(平均为0.09%)含量。稀土元素总量低(14个样品的平均∑REE为36.99×10-6),LREE/HREE平均值为3.34,(La/Yb)N平均为8.57,轻重稀土元素分异较弱,δEu平均为0.41,球粒陨石标准化配分模式为较平缓、右倾的海鸥型;大离子亲石元素(LILE),如Rb、Th、U以及La相对富集,Ba、Eu和高场强元素(HFSE),如Nb、Zr、Ti等亏损,上述特征说明,菜园子为典型的S型重熔花岗岩。地球化学表明,菜园子花岗岩的源区为以泥质岩(主)和杂砂岩(辅)为源岩的副片麻岩组成。计算显示,熔体形成于深约25km、温度约850℃的中上地壳内块体碰撞后的板内伸展环境,经历过黑云母和斜长石的结晶分异。中元古代末期,东川群、汤丹群沿菜子园-踩马水-麻塘断裂向北侧的会理群俯冲碰撞,形成菜子园蛇绿岩和菜园子花岗岩的侵位、天宝山组中酸性火山岩的喷发事件,此类碰撞、拼贴造山过程有可能在滇中地区的昆阳群和东川群之间同时发生。扬子板块西缘的基底可能是由不同时代的小陆块碰撞、拼贴导致陆壳的增生扩大,并经历多次拉张、碰撞过程。

This study conducted SHRIMP U-Pb age analyses for zircons from Caiyuanzi granites of two granitic bodies in the central Kangdian area,Sichuan,SW China,yielding the two ages of 1040.4±6.1 Ma（MSWD=0.53,n=18） and 1063.2±6.9 Ma（MSWD=1.15,n=11）,which are highly in accordance with the time of the global Grenvillian orogenic movement.The Caiyuanzi granites are characterized by high SiO2（ave.74.96%）,peraluminous（most A/CNK values higher than 1.1）,medium alkali contents（ave.Na2O＋K2O content of 6.5%）,and low MgO（ave.0.17%）,low TiO2（ave.0.28%）,low CaO（ave.0.09%）.They are also low in ∑REE contents with an average ∑REE of 36.99×10-6（14 samples）,an average LREE/HREE value of 3.34,a（La/Yb）N value of 8.57 and a δEu value of 0.41.The chondrite-normalized REE pattern displays flat right dipping curves.The large ion lithophile elements,such as Rb,Th,U,and La are relatively enriched,whereas Ba,Eu and high HFSE,such as Nb,Zr and Ti,are relatively depleted.These characteristics show that the Caiyuanzi granites are typical S-type remelt granite.The geochemical features of the Caiyuanzi granites imply that material source is the paragneiss consisting mainly of pelite with minor sandstone.Calculation also shows that the melt of Caiyuanzi granites formed in an intraplate extension environment with a depth of 25 km and a temperature of 850℃ after the collision of blocks in the upper-middle crust and experienced strong differentiation of biotite and plagioclase.At the end of the mid-Proterozoic,the Dongchuan and Tangdan groups subducted northward along with Caiziyuan-Caimashui-Matang fault belt and collided with the Huili Group,resulting in the emplacement of Caiziyuan ophiolite and Caiyuanzi granite,and the eruption of the Tianbaoshan Formation intermediate-acidic volcanic rocks.The collision and collage mountain-building process maybe take place contemporaneously in Dongchuan and Kunyang groups in central Yunan province.The basement of the western margin of the Yangtze craton may be the result of accretion and expansion of the continental crust due to collision and collage of small blocks in different epochs and experienced a process of multiple extensions and collisions.