胶南桃行超高压变质岩的氧同位素地球化学及其年代学制约  

Oxygen isotope geochemistry of UHP metamorphic rocks at Taohang in the Sulu terrane and its constraints on radiometric dating.

作　　者：龚冰[1] 郑永飞[1] 吴元保[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《岩石学报》2004年第5期1097-1115,共19页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(40334036)资助成果

摘　　要：对苏鲁超高压变质带内诸城桃行地区榴辉岩及其花岗片麻岩围岩进行了单矿物氧同位素组成分析和锆石U-Pb定年。氧同位素组成显示出不均一亏损^(18)O的特征。石英-石榴石等高温矿物对的氧同位素温度为600～950℃,指示它们在榴辉岩相变质条件下达到并保存了氧同位素平衡。而部分石英-长石和白云母-金红石等矿物对的氧同位素温度为350～570℃,指示它们在峰期变质之后的角闪岩相退变质过程中达到并保存了同位素退化交换再平衡。锆石氧同位素组成低达-1.3‰～4.2‰,对这种低δ^(18)O值进行锆石U-Pb定年,分别得到762～834Ma的原岩年龄和202～249Ma的变质年龄。因此,桃行低δ^(18)O值锆石形成于新元古代(700～800Ma)的低δ^(18)O值岩浆。这种低δ^(18)O值岩浆是由于变质岩原岩经历新元古代高温大气降水热液蚀变后再部分熔融所形成。对于在角闪岩相退变质之后保存了封闭体系的花岗片麻岩样品(石英-长石矿物对温度为355～405℃),石榴石在榴辉岩相变质温度下达到并保存了氧同位素平衡(石英-石榴石矿物对温度为685℃),指示石榴石中Sm-Nd体系在同样的变质务件下也达到了平衡。因此,花岗片麻岩中石榴石-斜长石-全岩的Sm-Nd等时线年龄215±11Ma与锆石变质边的三叠纪年龄(202～249Ma)一样,代表了榴辉岩相峰期变质后的冷却年龄。而花岗片麻岩中石英-钾长石和石英-斜长石矿物对处于氧同位素不平衡状态,同时钾长石和斜长石相对于样品中其它矿物异常亏损^(18)O,指示在角闪岩相退变质之后体系曾经开放,岩石受到低^(18)O流体在低温和中温下(200～400℃)的热液蚀变。这种奈件下矿物氧同位素的退化交换是由表面反应机制控制,与Nd的扩散机制不同,因此氧同位素平衡无法制约Sm-Nd矿物等时线的有效性。Zircon U-Pb dating and mineral oxygen isotope analysis were carried out for eclogite and granitic gneiss at Taohang in the Su-Lu terrane. Beside heterogeneous (18)O depletion, oxygen isotope temperatures show that some mineral pairs (quartz-garnet) achieved and preserved equilibrium during the eclogite-facies metamorphism (600 to 950degreesC), and the others (quartz-feldspar, dolomite-rutile) achieved oxygen isotope reequilibration during amphibolite-facies retrograde metamorphism (350 to 570degreesC) subsequent to the eclogite-facies metamorphism. All zircons from the UHP metamorphic rocks give low delta(18)O values of -1.3parts per thousand to 4.2parts per thousand, which are lower than the delta(18)O values of 5.3 +/- 0.2parts per thousand for the normal mantle zircon. U-Pb discordia dating for the (18)O-depleted zircons yields Neoproterozoic ages of 762 to 834 Ma for protoliths and recrystallization ages of 202 to 249 Ma for metamorphism. Therefore, the meteoric-hydrothermal alteration is directly dated to occur at mid-Neoproterozoic. Oxygen isotope temperatures for one granitic gneiss sample show that the quartz-granet pair achieved and preserved oxygen isotope equilibrium during the eclogite-facies metamorphism (685degreesC), and the quartz-feldspar pair achieved oxygen isotope reequilibration during retrograde metamorphism (355 to 405degreesC). This indicates that the Sm-Nd system in the minerals was closed during retrogression. Therefore, Sm-Nd isochron age of 215 +/- 11 Ma for this sample represents the cooling age during exhumation. This cooling age is consistent with the metamorphic ages of zircons. Both the oxygen isotope disequilibrium in minerals quartz-feldspar pair and the anormaly low delta(18)O values for K-feldspar and plagioclase occur in the other granitic gneiss sample from Taohang. It is likely that the systems were opened and disturbed by exotic fluid at lower retrogression temperature (200 to 400degreesC). The Oxygen isotope exchange for this sample was controlled by surface reaction. Thu

关 键 词：氧同位素 锆石 U-Pb SM-ND 榴辉岩 花岗片麻岩 苏鲁造山带 

分 类 号：P588.346[天文地球—岩石学] P597.2[天文地球—地质学]