辽宁南芬变质核杂岩核部花岗岩地球化学特征及构造意义

Geochemical Characteristics and Tectonic Significance of Granite from Nanfen Metamorphic Core Complexes in Liaoning

弓长岭花岗岩是南芬变质核杂岩的组成部分,同时也是鞍山-本溪地区前寒武基底的重要组成部分,对其进行探讨对于了解华北克拉通地区在2.50 Ga时期的大地构造背景具有重要意义.选取鞍山-本溪地区的南芬变质核杂岩核部弓长岭花岗片麻岩为研究对象,挑选2件样品中的锆石进行了LA-ICP-MS U-Pb年龄测试,获得不一致线上交点年龄分别为2505±14 Ma(MSWD=1.30,n=70)和2507±11 Ma(MSWD=1.16,n=80),代表岩浆结晶的年龄.岩石富SiO2为70.14%~75.36%、K2O为3.12%~6.84%和Al2O3为11.78%~13.75%、全碱(Na2O+K2O)含量在7.8%~9.64%,里特曼指数为1.98~2.94,AR值3.6~9.1显示高硅、富碱特征,A/CNK在0.95~1.23,大部分大于1.11,为强过铝质岩石.较高的稀土含量(ΣREE=177.15×10-6~505.64×10^-6),轻、重稀土分异明显,LaN/YbN=8.14~29.92,显著富集Rb、Th、Zr、Hf等元素,亏损Nb、Ti等元素,这些特征表明弓长岭花岗岩是在陆块发生碰撞的大地构造背景下由地壳的泥质岩类提供的物源发生部分熔融经分异结晶形成.表明在2.50 Ga时期鞍山-本溪地区处在由陆块发生拼合且向稳定的克拉通化演化的后碰撞阶段.

The Gongchangling granite is the part of Nanfen metamorphic core complex and is also an important part of the Precambrian basement of Anshan-Benxi area.In order to investigate the tectonic significance of North China craton in Neoproterozoic to Paleoproterozoic,the Gongchangling granite in the Nanfen metamorphic core complex was selected as the research object.LA-ICP-MS U-Pb analyses of 2 zircon samples yield the upper intercept ages of 2505±14 Ma(MSWD=1.30,n=70)and 2507±11 Ma(MSWD=1.16,n=80),which represents the crystallization time of the magma.The studied samples have high SiO2 of 70.14%to 75.36%,K2O of 3.12%to 6.84%,Al2O3 of 11.78%to 13.75%and total alkali(Na2O+K2O)of 7.8%to 9.64%.The samples show Rittmann index(δ)from 1.98 to 2.94,and AR from 3.6 to 9.1,which suggests that the granite is enriched in silicon and alkali.A/CNK=0.95-1.23,with the majority of more than 1.11,indicate that it is peraluminous rock.Results showΣREE=177.15×10^-6-505.64×10^-6,LaN/YbN=8.14-29.92,obvious fractionation of LREE to HREE,enriched LILE,such as Rb,Th,Zr,Hf,depleted of Nb,Ti and other elements.The geochemical characteristics of rocks indicate that the Gongchangling granite was formed by partial melting of the crustal argillite in the collision tectonic setting of the continental block through differentiation and crystallization.It is indicated that the Anshan-Benxi area was in the transition period in 2.50 Ga from post-collision to stable stage after assemblage of the continental blocks.