辽北开原地区房木花岗斑岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

LA-ICP-MS Zircon U-Pb Dating and Geochemical Characteristics of Granitic Porphyry in Kaiyuan Area of Northern Liaoning Province

研究区位于华北克拉通北缘东段、华北克拉通与兴蒙造山带的交接部位。本文通过对辽北开原地区房木花岗斑岩的岩石学、年代学和地球化学研究,探讨了其形成时代、岩石成因和构造环境,并进一步探讨了华北克拉通东部岩石圈减薄的起始时间。用LA-ICP-MS技术测定了花岗斑岩的锆石U-Pb同位素,获得其锆石206Pb/238U加权平均年龄为(117±1)Ma,表明花岗斑岩形成于早白垩世。花岗斑岩具有高硅、富碱、贫镁和钙的特征,富集大离子亲石元素(K、Rb)和高场强元素(Th、Ta、Nb),亏损Sr、Ba、P、Ti元素,稀土配分曲线均呈现右倾海鸥型,具有明显的负Eu异常;显示研究区花岗斑岩为A型(A1亚型)花岗岩,形成于古太平洋俯冲的后造山伸展环境,其岩浆源于地幔物质上涌而导致的地壳物质重熔。研究区在(156±1)Ma之前,属于挤压造山环境,表明研究区岩石圈减薄时间应处于156~117 Ma之间。

The study area is located in the eastern part of the north margin of the North China craton, which is the intersection of the North China craton and the Xingmeng orogenic belt. The author studied the detailed petrology, geochronology and geochemistry of the Fangmu granitic porphyry in Kaiyuan area of northern Liaoning Province, discussed their formation age, petrogenesis, tectonic settings, and the thinning time of North China craton. The LA-ICP-MS zircon U-Pb dating was tested for the granitic porphyry. The 206Pb/238U age is(117±1) Ma, which indicates that the granitic porphyry was formed in the Early Cretaceous. The granitic porphyry is characterized by high Si, rich in alkali,poor in Mg and Ca, rich in large ion lithophile elements(e.g. K and Rb),and high field strength elements(e.g. Th, Ta, and Nb), while deplete in Sr, Ba, P, and Ti. Besides, the REE pattern shows a typical right dipping seagull shape. There is a negative Eu anomaly, which indicates that the granitic porphyry is A-type granite(A1 subtype),and formed in the post orogenic extensional environment during paleo Pacific subduction. The magma was from the re-melting of the crustal material caused by the mantle material upwelling. In addition, the study area was a compressional orogenic environment before(156±1) Ma. Thus, we believe that the crustal thinning began in 156-117 Ma.