内蒙古喀喇沁堆晶岩捕虏体和寄主闪长岩的同位素年龄  

Geochronology of cumulate xenoliths and their host diorites from Harqin, eastern Nei Mongol

作　　者：邵济安[1] 张任祜[2] 韩庆军[2] 张履桥 乔广生[2] 桑海清[2] 

机构地区：[1]北京大学地质学系,北京100871 [2]中国科学院地质与地球物理研究所,北京100029 [3]内蒙古地质矿产局地质研究所,内蒙古呼和浩特010020

出　　处：《地球化学》2000年第4期331-336,共6页Geochimica

基　　金：国家自然科学基金资助项目!(49272156)

摘　　要：喀喇沁地区位于大兴安岭中生代构造岩浆岩带与华北克拉通交汇部位。研究区的闪长岩侵入到太古宙基底中。闪长岩中含有二辉岩、辉长岩、苏长岩、角闪岩、斜长岩等不同组分的镁铁质 超镁铁质堆晶岩捕虏体和基性麻粒岩捕虏体 ,它们是幔源玄武质岩浆底侵作用的产物。分别用Rb Sr法、K Ar法和Ar Ar法 ,对堆晶岩捕虏体及其寄主岩全岩或单矿物进行了详细的同位素地质年代学研究 ,共获得各种年龄数据8件。橄榄辉石岩的全岩Rb Sr等时线年龄为(229±23)Ma ,87Sr/86Sr初始比值为0 70554±0 00012。斜长岩捕虏体中斜长石40Ar 39Ar坪年龄为(228±1)Ma ,相应的等时线年龄为(240±5)Ma ;苏长岩中斜长石的40Ar 39Ar坪年龄为(230±2)Ma ,相应的等时线年龄为(232±60)Ma ,两个样品的40Ar/36Ar初始比值分别为290和296 ,接近于大气Ar组成 ,说明年龄是真实可信的。斜长岩捕虏体中斜长石和角闪岩中角闪石的K Ar年龄分别为(237±8)Ma和(220 9±4 1)Ma ,暗示尽管堆晶岩捕虏体都是早中生代形成的 ,但并非是同一批次岩浆分异结晶的产物。捕虏体的寄主岩石———闪长岩的全岩K Ar年龄为(218 5±4 5)Ma和(221 3±4 3)Ma ,角闪石单矿物K Ar年龄为(223 3±3 3)Ma ,角闪石和全岩的K Ar年龄之间几乎没有差别 ,表明闪长岩体从(530±40)Harqin area is located in a joint place of Mesozoic tectono magmatic belt in the Da Hinggan Mts. and North China craton. Mafic ultramafic cumulate xenoliths with various lithological population including pyroxenite, gabbro,norite, hornblendite and anorthorsite as well as basic granulite xenoliths occurred in a series of diorite which intruded into Archean metamorphic rocks in thearea, and these xenoliths are interpreted as products of mantle derived basaltic underplating. Isotopic ages of the cumulate xenoliths and their host dioritehave been systematically measured by means of whole rock Rb Sr isochron, K Ar and 40Ar 39Ar techniques and 8 ages are obtained. Two whole rock Rb Sr isochron ages for olivine pyroxenite are (229±23) Ma with initial 87Sr/86Sr ratios of 0 705 54±0 000 12, respectively. 40Ar 39Ar dating of plagioclase from anorthorsite yields a plateau age of (228±1) Ma and an isochron age of (240±5) Ma with 40Ar/36Ar initial ratio of 290; 40Ar 39Ar dating of plagioclasefrom norite yields a plateau age of (230±2) Ma and an isochron age of (232±6) Ma with 40Ar/36Ar initial ratio of 296 very near to that of atmospheric argon(Neir value = 295 5), indicating these ages are reasonable. K Ar ages of plagioclase from anorthorsite and hornblende from hornblendite are (237 4±8) Ma and (220 9±4 1) Ma, respectively, suggesting that they are not products of fractionational crystalline from the same patch of magma. K Ar ages of whole rockand hornblende single mineral from diorites, the host of cumulate xenoliths, range from (218 5±4 5) Ma to (223 3±3 3) Ma, indicating that the diorite cooled at a high rate from (530±40) ℃(closure temperature of K Ar systematics for hornblende) down to about 200 ℃(closure temperature of K Ar systematicsfor whole rock), inferring a tectonic environment of uplifting at a relatively high rate for North China craton in the Early Mesozoic.

关 键 词：堆晶岩捕虏体 同位素年代学 闪长岩 岩浆岩 

分 类 号：P597[天文地球—地球化学] P588.1[天文地球—地质学]