秦皇岛茹各庄火山碎屑岩地球化学、锆石U-Pb定年、Hf同位素组成及其地质意义

冀东秦皇岛地区位于华北板块东部，广泛发育中生代火山岩，为进一步研究该地区中生代岩浆作用的构造背景及其运动机制，本文对秦皇岛茹各庄流纹质晶屑凝灰岩开展了LA-ICP-MS锆石U-Pb年代学,Hf同位素组成以及全岩主量、微量元素测试分析。研究结果显示：锆石具有清晰的环带结构，Th/U值为0.37~0.83，为岩浆锆石，测年结果为（113±1）Ma；岩石具高硅（平均质量分数为75.95%）、富碱（平均质量分数为8.47%）、明显贫钙（平均质量分数为0.39%）的特点；岩石中富集高场强元素（Th、U、Ce、Nd、Zr、Hf），亏损大离子亲石元素（Ba、Sr、P、Ti），稀土元素配分型式为微弱右倾型，δEu强负异常（0.02~0.30），暗示斜长石大量晶出；岩浆在演化过程中伴随磷灰石、钛铁矿等矿物晶出，Nb/Ta值接近陆壳，暗示岩石具地壳岩石特征；锆石Hf同位素组成和演化说明该岩石的岩浆源区为古元古代晚期—中元古代中期华北板块古老下地壳幔源岩浆底侵热源引起部分熔融的产物。研究结果表明茹各庄流纹质晶屑凝灰岩为早白垩世岩浆活动的产物，形成于古太平洋板块俯冲后撤而驱动的板内拉张环境。 

Geochemistry,Zircon U-Pb Geochronology,Hf Isotopic Compositions and Tectonic Significance of Pyroclastic Rocks in Rugezhuang,Qinhuangdao

The Qinhuangdao area of eastern Hebei is located in the eastern part of the North China plate, where Mesozoic volcanic rocks are widely developed. In order to further study the tectonic setting and movement mechanism of Mesozoic magmatism in this area, the LA-ICP-MS zircon U-Pb geochronology, Hf isotopic compositions, major and trace elements of the rhyolitic crystal tuffs in Rugezhuang were analyzed. The results show that zircons have a clear oscillatory zoning, and the Th/U value of zircons is 0.37-0.83, indicating they are magmatic zircons with the weighted average age of (113±1) Ma. The rocks are characterized by high silicon (75.95%), high alkali (8.47%) and low calcium (0.39%). HFSE (Th, U, Ce, Nd, Zr, Hf) are enriched and LILE (Ba, Sr, P, Ti) are depleted in analyzed samples. The REE distribution patterns dip slightly to the right, andδEu (0.02-0.30) is obviously negative anomaly, suggesting that a large amount of plagioclase was crystallized. During the evolution of the magma, along with the crystallization of apatite, ilmenite and other minerals, the value of Nb/Ta is close to that of the continental crust, suggesting that analyzed rocks have the characteristics of crustal rocks. Zircon Hf isotopic compositions and temporal variations show that the magma source of the rocks is the product of partial melting of the ancient lower crust of the North China plate in the Late Early Proterozoic to Middle Mesoproterozoic due to the heating from mantle-derived magma. The results show that the rhyolitic crystal tuffs in Rugezhuang are the products of magmatic activities in the Early Cretaceous, and was formed in the intraplate extensional environment driven by the subduction and roll-back of the Paleo-Pacific plate.