康定杂岩锆石SHRIMP U-Pb定年及其地质意义

康定杂岩在扬子地块西缘呈南北向带状分布，这套岩石岩性变化很大(从基性到中性、酸性)，中性、酸性岩是组成该杂岩的主体。岩石类型有变辉长岩、闪长质片麻岩、石英闪长质片麻岩和花岗质片麻岩。康定杂岩以往被认为是扬子地块太古宇的基底，但近年来的同位素年龄测定结果表明其并不是形成于太古宙。本研究样品采自康定-泸定地区，挑选有代表性的两个样(样号71704.2、71501.1)，对其进行岩石化学与微量元素分析及SHRIMP U-Pb测年。分析得出样品为钙碱性；原始地幔标准化微量元素分布型式显示相对富集大离子亲石元素K、Rb等，亏损高场强元素Nb、Ta、Ti、P等；稀土元素具有略向右倾的配分型式，且铕异常不明显，显示其具有与消减作用有关的岛弧火成岩的特征。锆石U-Pb分析给出的结晶年龄为765～771 Ma，w(Th)/w(U)大于0.1，具示岩浆成因特征。个别年轻的锆石颗粒，如71704.2中的12.1测点年龄为(430±8)Ma，可能是在后期构造、岩浆、变质作用中新形成的锆石，该年龄可解释为后期变质热事件的年龄。这些充分说明康定杂岩形成于新元古代，而非太古宙。此外，康定杂岩可能形成于岛弧环境，其岩浆主要起源于俯冲洋壳的熔融，结合其形成年龄说明扬子地块西缘新元古代岩浆活动可能是Rodinia超大陆裂解后的产物。

Zircon U-Pb SHRIMP Dating for the Kangding Complex and Its Geological Significance

The Kangding complex is a S-N trending zone in the western margin of the Yangtze Block.It is composed of basaltic,intermediate and felsic rocks,with the essential components of intermediate and felsic rocks. It contains meta-gabbro,dioritic-gneiss,quartz-dioritic gneiss and granitic gneiss.The complex was traditionally considered as the Archean basement of the Yangtze Block,but recent isotopic dating results indicate that it was not formed in Archean.The samples in this paper are collected from the Kangding-Luding region,two representative ones(71704-2,71501-1)are taken for chemical analyses and SHRIMP U-Pb dating.The results show that the samples are of calc-alkaline series,the primitive mantle normalized trace element patterns are characterized by relative enrichment in large ion lithophile elements(K,Rb etc.)and depletion in high field strength(elements)(Nb,Ta,Ti,P etc.).REE patterns are of slightly right inclined type with unobvious Eu anomaly.These are typical features of subduction-related islandarc igneous rocks.SHRIMP U-Pb zircon analyses give crystallization age of zircons from the complex ranging from 765 to 771 Ma,whose Th/U ratios are greater than 0.1,indicating its igneous genesis.There are a few younger zircon grains,such as the grain 12.1 of 71704-2 with the age of(430±8)Ma,interpreted as the age of post event when it was newly formed in the post tectonic,magmatic and metamorphic activities.The above data indicate that the Kangding complex was formed at the Late Neoproterozoic.Above mentioned geochemical characteristics suggest that the Kangding complex was probably formed in islandarc setting.The magma source originated from subducted oceanic crust.Late Neoproterozoic igneous activity was related to dispersion of the Rodinia supercontinent in the western Yangtze Block.