扬子西南缘易门地区~1.4 Ga岩浆事件的厘定及其对菜子园—易门洋演化的指示

Determination of~1.4 Ga magmatic event in the Yimen area on the southwestern margin of the Yangtzeblock and its indication to the evolution of Caiziyuan-Yimen Ocean

通过会理—东川—易门地区1∶50000区域地质调查,在滇中易门六街一带“昆阳群”黑山头组底部发现一套流纹质凝灰岩夹层,厚约20~45 m,采用锆石激光剥蚀法(LA-ICP-MS)对流纹质凝灰岩进行年代测定,获得锆石U-Pb年龄加权平均值为1412±20 Ma(MSWD=0.72,n=15),代表了该凝灰岩的形成时代,说明黑山头组沉积时代为中元古代中期,是滇中地区首次发现~1.4 Ga岩浆记录,进而限定了昆阳群沉积时代为中元古代中期—新元古代早期(1.5~0.9 Ga)。岩石地球化学及Hf同位素研究表明,流纹质凝灰岩富集K、Rb、Th等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,显示出与岛弧钙碱性火山岩的特点,属于典型的岛弧型岩浆岩;锆石ε_(Hf)(t)值兼具正值和负值,指示岩浆源区主要与古元古代(2438~2030 Ma)俯冲洋壳板片的熔融有关,并伴有地壳物质的混入。本年龄数据与中元古代菜子园蛇绿混杂岩端元辉长岩(~1.4 Ga)年龄相当,可能为菜子园—易门洋开始削减的产物,该时期是菜子园—易门洋裂解—俯冲转换的重要时期。结合河口、新平、撮科一带陆续发现高压变质岩及蛇绿混杂岩,绿汁江断裂在绿汁镇—元谋一带存在构造混杂现象,推断菜子园—易门洋在河口金沙江一带由西转向南沿着绿汁江断裂带由通安—河口—元谋—绿汁—新平一带展布。

Objectives:Through the successive 1∶50000 regional geological survey in Huili—Dongchuan—Yimen area,a large set of rhyolitic tuff interlayers with a width of about 20~45 m at the base of the Heishantou Formation of the"Kunyang Group"in the Liujie area of Yimen,central Yunnan.Results:Zircon laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)was employed to date the rhyolitic tuff,with a weighted average zircon U-Pb age of 1412±20 Ma(MSWD=0.72,n=15),representing the formation age of the tuff,This result shows that the sedimentary age of the Heishantou Formation is in the middle Proterozoic,which is the first 1.4 Ga magmatic record found in central Yunnan,and defines the sedimentary age of the Kunyang Group as the middle Proterozoic to the early Neoproterozoic(1.5~0.9 Ga).Conclusions:The study of rock geochemistry and Hf isotopes shows that the rhyolitic tuff is rich in large ion lithophile elements such as K,Rb,and Th,and deficient in high field strength elements such as Nb,Ta,and Ti,displaying characteristics similar to island arc calc-alkaline volcanic rocks,zirconε_(Hf)(t)value has both positive and negative values,indicating that the magma source area was mainly formed by the melting of subducted oceanic crust sheets in the Paleoproterozoic(2438~2030 Ma),accompanied by the mixing of crustal materials.This age data is similar to that the age of the important end-member gabbro(~1.4 Ga)in the Mesoproterozoic Caiziyuan ophiolite melange belt,and may be the product of the beginning of the reduction of Caiziyuan—Yimen Ocean,which is an important period of the splitting and subduction transformation of Caiziyuan—Yimen Ocean.Combined with the high pressure metamorphic rocks and ophiolite melanges successively discovered in Hekou,Xinping and Cuoke,the tectonic melange phenomenon of the Lu¨zhijiang fault is found in the Lu¨zhizhen—Yuanmou area,which infers that Caiziyuan—Yimen Ocean is likely to be distributed along the Lu¨zhijiang fault zone from the west to the south along the Jinsha River in the Hekou from Tong’an—Hekou—Yuanmou—Lu¨zhi—Xinping.