大别山北部榴辉岩的Sm-Nd年龄测定及其对麻粒岩相退变质时间的制约  

Sm-Nd dating of eclogites from Northern Dabie Mountains and its constraints on the timing of granulite- facies retrogression

作　　者：刘贻灿[1,2] 李曙光[1] 徐树桐[2] 江博明[3] 郑永飞[1] 张宗清[4] 江来利[2] 陈冠宝[2] 吴维平[2] 

机构地区：[1]中国科学技术大学地球和空间科学系,安徽合肥230026 [2]安徽省地质科学研究所,安徽合肥230001 [3]法国雷恩大学 [4]中国地质科学院地质研究所,北京100037

出　　处：《地球化学》2001年第1期79-87,共9页Geochimica

基　　金：国家自然科学基金项目! (49794041和 40033010);国家重点基础研究发展规划项目! (G1999075503);安徽省地质矿产局资助项目

摘　　要：报道了大别山北部三个榴辉岩样品的矿物 Sm- Nd等时线年龄，它们分别为 (210± 6) Ma或 (214± 6) Ma、 (208± 38) Ma和 (208± 4) Ma。氧同位素研究表明，这些样品中的石榴子石与绿辉石之间处于氧同位素平衡状态，因此，该 Sm- Nd等时线定年结果可靠。本区榴辉岩的高压麻粒岩相退变质阶段的冷却年龄为 210 Ma左右；榴辉岩的钕同位素初始比ε Nd(t)(两个样品一个为－ 10左右，另一个为－ 2)基本上表现为陆壳岩石特征，可能类似于南部超高压带中的榴辉岩，为印支期扬子陆壳俯冲变质成因。它们的全岩δ 18O值较低，为＋ 2.4‰～＋ 3.6‰，可能指示其原岩同大别山南部超高压带中榴辉岩一样，在板块俯冲之前，经受过高温地表水热液蚀变。年代学结果表明，大别山北部榴辉岩在 230～ 210 Ma期间经历的是一等温或升温过程，这与大别山南部含柯石英榴辉岩在这一时期的快速冷却过程形成强烈对比，这对理解俯冲陆壳中不同构造岩片折返过程的差异有重要意义。Sm- Nd and oxygen isotope ratios of garnet and omphacite from eclogites in North Dabie Mountains were measured along with their petrological features. Unlike coesite- bearing eclogites in South Dabie Mountains which underwent amphibolite- facies retrogression, the eclogites in North Dabie Mountains suffered granulite- facies overprinting. Relationship between theδ 18O values of the eclogite minerals suggests oxygen isotope equilibrium fractionation, corresponding to the Sm- Nd isotope equilibrium between the minerals according to comparability between O and Sm- Nd diffusivities. Theδ 18O values of whole- rocks for the eclogites are＋ 2.4‰ to＋ 3.6‰ , indicating that the eclogites underwent the hydrothermal alteration of high- temperature surface water prior to the subduction of the Yangtze continental plate. The Sm- Nd isotope ages for the three samples of eclogites are identical with 210 Ma and they are interpreted to represent the cooling ages in the retrogressive stage of granulite- facies metamorphism. The initial ratios of Nd isotope for the eclogites are negative withε Nd(t)=－ 10 (two samples) and－ 2 (one sample), implying that they were derived from the subducted Yangtze continental crust like the eclogites in South Dabie. There is the isothermal or temperature rise history from 230 to 210 Ma for the eclogites in North Dabie, which is in contrast to that of the eclogites in South Dabie. This places important constraint on exhumation processes of the eclogites in northern and southern Dabie blocks.

关 键 词：榴辉岩 麻粒岩 变质作用 扬子陆壳俯冲 大别山 钐-钕同位素年龄 氧同位素 

分 类 号：P588.34[天文地球—岩石学] P597[天文地球—地质学]