东北延边地区晚中生代柳洞岩体的成因:锆石U-Pb年代学和地球化学证据

Petrogenesis of the Late Mesozoic Liudong Pluton in Yanbian Area NE China:Evidence from Zircon U-Pb Geochronology and Geochemistry

柳洞岩体位于延边地区和龙市南部，大地构造位置上处于古洞河断裂西南侧，龙岗地块北缘，其岩性为二长花岗岩。为确定其侵位时代和大地构造环境，对柳洞岩体进行了系统的岩相学和年代学研究。N 6和N 7样品LA ICP MS锆石U Pb测年结果显示，其加权平均年龄分别为（118．63＆#177;0．69）Ma和（118．58＆#177;0．87）Ma。岩体具有高硅（w（SiO2）＝70．30％～76．30％）、富碱（w（Na2O＋ K2O）＝7．82％～8．52％）、富铝（w（Al2 O3）＝12．30％～15．50％）和低钛（w（TiO2）＝0．16％～0．29％）的特点，属于高钾钙碱性系列。A/CNK值为1．03～1．11，为弱过铝质岩石。分异指数（D I）为87．99～94．89，固结指数（S I）为2．39～5．31，反映了该岩体经历了高程度分异演化作用。δEu为0．63～0．92，具有中等铕负异常，LREE/HREE和（La/Yb）N 值分别为3．46～8．09和10．59～33．94，显示轻稀土元素相对富集、重稀土元素亏损的特点。柳洞岩体明显富集Cs、Rb、Ba、K等大离子亲石元素（LILE），同时亏损 Nb、Ta、Ti等高场强元素（HFSE）以及P元素。综合岩相学、年代学、岩石地球化学特征及区域地质资料，柳洞岩体具有高分异的高钾钙碱性I型花岗岩的特点，形成于后造山向伸展转换的体制环境下。

The Liudong pluton is located in the southern Helong area of Yanbian area ,the southwest segment of Gudonghe fault and north margin of Longgang block .It’s composed of monzonitic granite . Systematic researches on petrography and geochronology have been carried out to determine the emplacement age and tectonic environment of the pluton .The LA ICP MS zircon U Pb dating yields the weighted mean age of （118 .63 ＆#177; 0 .69） Ma and （118 .58 ＆#177; 0 .87） Ma for N 6 and N 7 sample , respectively .The rock is weak peraluminous and belongs to the high potassium calc-alkaline series , characterized by high Si （70 .30% 76 .30% ） ,high alkali （7 .82% 8 .52% ） ,high aluminum （12 .30%15 .50% ） and low titanium （0 .16% 0 .29% ） ,with A/CNK value of 1 .03 1 .11 .Such geochemical＆amp;nbsp;characteristics as DI （87 .99 94 .89）and SI （2 .39 5 .31） indicate the rock experienced a highly fractionated evolving stage .The rock is of LREE enrichment （LREE/HREE=3 .46 8 .09） and HREE depletion （（La/Yb）N =10 .59 33 .94） .The Liudong pluton is characterized by the strong enrichment in large ion lithophile elements （LILE） ,such as Cs ,Pb ,Ba ,K ,depletion in high field-strength elements （HESE） ,such as Nb , Ta , Ti ,and P ,and middle negative Eu anomalies （δEu = 0 .63 ~ 0 .92） . Combining petrography , geochronology , rock geochemical characteristics , and regional geological material ,it can be concluded that the Liudong pluton belongs to the highly fractionated high-potassium calc-alkaline I-type granite and emplaced in the transferring stage from a post-orogenic to an extensional tectonic setting .