大别造山带碧溪岭橄榄岩中锆石U-Pb年龄和Hf同位素研究  

Study on U-Pb age and Hf isotope of zircons in the Bixiling peridotites,the Dabie ultrahigh-pressure metamorphic belt

作　　者：郑建平[1] 孙敏 Griffin WL 钟增球[1] 汤华云[1] 张志海[1] 

机构地区：[1]中国地质大学地球科学学院地质过程与矿产资源国家重点实验室,武汉430074 [2]Department of Earth Sciences,The University of Hong Kong,Pokfulam Road,Hong Kong,China [3]GEMOC ARC National Key Centre,Department of Earth and Planetary Sciences,Macquarie University,NSW 2109,Australia

出　　处：《岩石学报》2007年第2期343-350,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金面上项目(40673002);杰出青年科学基金项目(40425002);创新群体项目(40521001)资助

摘　　要：在详细形态和内部结构研究基础上,对碧溪岭橄榄岩锆石进行了配套的U-Pb年龄和Hf同位素分析。碧溪岭橄榄岩锆石主体是变质成因的,但有少量岩浆锆石残留。这些锆石主体给出了220～210Ma的变质生长/重结晶作用年龄;一颗具较高Th/U比值的锆石给出646Ma的近协和年龄,限定了早期岩浆结晶作用的最小年龄。真正的侵位年龄可以由745Ma的上交点年龄来限定;210～222Ma则记录着深俯冲陆壳折返过程中的锆石生长。除少数颗粒的ε_(Hf)为负值外(-2.9),大部分锆石的ε_(Hf)是正的(高达+8.1)并具新元古代的亏损地幔模式年龄(0.6～1.0Ga,平均0.8Ga)。这些结果说明碧溪岭橄榄岩的初始物质是扬子大陆岩石圈内新元古代岩浆堆积作用产物。这样的岩浆堆积物来源于亏损的软流圈地幔,侵入于扬子的深部地壳之中(如底侵)并部分混染了古老地壳组分。它们随扬子向华北之下深俯冲碰撞和造山带折返过程中经历着复杂的变质改造作用。Abstract On the basis of the form and internal structure of the Bixiling peridotitic zircons from the southern Dabie Orogen, in situ U-Pb ages and Hf isotope of them were analyzed. The zircons are mainly recrystalizated/metamorphic growth （210 ~ 220Ma） with minor relics of magmatism. One grain with high Th/U ratio gives a nearly concordant age of 646Ma, representing the minimum age estimate of magmatism. The intrusion age can be obtained by the upper intercept of 745 Ma. The ages of 210 ~ 220Ma record the growth of zircons during exhumation of the Dabie ultrahigh-pressure belt. The Bixiling peridotitic zircons, except few grains with negative БHf （ e.g. - 2.9 ）, have positive БHf （ high up to ＋ 8.1 ） and Neoproterozoic depleted mantle model ages （ mean 0.8Ga）. The data suggest that the initial materials of the Bixiling peridotites are Neoproterozoic cumulates beneath the northern margin of the Yangtze craton. The magma derived from the depleted asthenospheric mantle, erupted at the lower part of the continental crust （e. g. underplating） and assimilated a slight proportions of pre-existing crust. The protolith of the Bixiling peridotites experienced complex processes accompanying with subduction of the Yangtze lithosphere, collision with the North China craton, and exhumation of the Dabie-Sulu orogen. The metasomatic growth or recrystalization age of zircons （ 〈 230Ma） in the Bixiling peridotites may mainly record the exhumation event of the Dabie orogen following the collision.

关 键 词：橄榄岩锆石 年龄和Hf同位素 新元古代底侵 扬子地块 早中生代折返 大别造山带 碧溪岭 

分 类 号：P597.3[天文地球—地球化学] P588.125[天文地球—地质学]