内蒙古北山造山带百合山地区350Ma石英闪长岩的成因及对红石山-百合山洋俯冲时限的制约

The petrogenesis of quartz diorite(350 Ma)from the Baiheshan area of the Beishan orogenic belt,and its chronological constraint on Hongshishan-Baiheshan Ocean’s subduction initiation

对内蒙古白山岩浆弧内百合山石英闪长岩进行了LA-ICP-MS锆石U-Pb测年、Hf同位素及岩石地球化学分析。结果显示,该岩体的形成年龄为350.8±1.5 Ma,岩浆锆石的εHf(t)值在5.28~9.88之间,模式年龄为724~1018 Ma。地球化学数据显示,百合山石英闪长岩为一套I型钙碱性系列岩石,富碱(Na 2 O+K 2 O=6.03%~6.96%),富钠(Na 2 O/K 2 O>2),较低的Mg#(35.48~39.78)、Ni(2.06×10-6~5.54×10-6)和Cr(2.11×10-6~7.93×10-6)含量。相对富集轻稀土元素,重稀土分馏不明显,较低的(La/Yb)N值(4.28~5.60),无明显Eu异常,相对富集Rb、K、U等大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素等,地球化学成分表明,石英闪长岩具有俯冲带成因的特征。石英闪长岩的锆石U-Pb年龄和岩石成因证明了350 Ma石英闪长岩是晚古生代白山岩浆弧内首次发现的可以明确与红石山-百合山洋俯冲消减作用有关的最早的弧岩浆记录。表明红石山-百合山洋于约350 Ma之前就已经开始发生俯冲消减,比前人一般认为的俯冲开始的时间提前了22 Ma。

In this paper,the precise dating of LA-ICP-MS zircon U-Pb and Hf isotope analysis and geochemical analysis were performed on Baiheshan quartz diorite from Baishan magmatic arc,Inner Mongolia.The results show that the formation age of the rock is 350.8±1.5 Ma and theεHf(t)values of magmatic zircon are between 5.28 and 9.88 with crustal model ages ranging from 724 Ma to 1018 Ma.Geochemical data show that Baiheshan quartz diorite is a set of I type calc-alkaline rocks.All the samples have rich alkali(Na 2 O+K 2 O=6.03%~6.96%)and sodium(Na 2 O/K 2 O>2),low values of Mg#(35.48~39.78)and low content of Ni(2.06×10-6~5.54×10-6)and Cr(2.11×10-6~7.93×10-6).Light rare earth elements(LREEs)are enriched in the chondrite-normalized REE patterns while heavy rare earth elements(HREEs)fractionation and negative Eu anomalies are not obvious.They are enriched in large ion lithophile elements such as Rb,K and U and depleted in high field strength elements(HFSEs)such as Nb,Ta,P and Ti.The geochemical composition indicates that the rock has the genetic characteristics of the subduction zone.The zircon U-Pb age and petrogenesis of the quartz diorite prove that it is the earliest arc magmatic record that was first discovered in the Late Paleozoic Baishan magmatic arc,which is clearly related to the subduction of the Hongshishan-Baiheshan Ocean.It is shown that the subduction of Hongshishan and Baiheshan Ocean occurred before^350 Ma,which is 22 Ma earlier than the previous conclusion about the start of the ocean subduction.