辽西凌源地区义县组火山岩锆石U-Pb年代学和地球化学特征

孟凡雪; 高山; 柳小明

辽西凌源地区义县组火山岩锆石U-Pb年代学和地球化学特征

U-Pb zircon geochronology and geochemistry of volcanic rocks of the Yixian Formation in the Lingyuan area, western Liaoning

摘要

摘要: 对辽西凌源地区义县组的1件流纹岩样品进行锆石U-Pb年代学研究，获得的年龄为(124.4±1.4)Ma，与前人对辽西北票、义县、凌源等地区通过K-Ar、Ar-Ar、U-Pb等定年方法获得的义县组年龄基本一致，时代为早白垩世。7件典型火山岩样品的地球化学研究结果表明，该火山岩的SiO2＝65.6%～73.4％，MgO＝0.35%～1.23％，Mg#＝16～41，Al2O3=13.40%～16.79%，Na2O＝3.54%～4.26％。Sr＝(445～774)×10-6,Yb＝(0.51～0.97)×10-6,Y＝(7～14)×10-6,Sr/Y＝49～66，LaN/YbN＝33～48，高Sr和Sr/Y比值，低Y和Yb，轻重稀土元素分异明显，显示出埃达克岩的地球化学特征，但其Mg＃较低。它们可能是加厚的基性下地壳部分熔融的产物。

Abstract: Zircon U-Pb dating of a rhyolite sample from the Yixian Formation in the Lingyuan area, western Liaoning, yields an age of 124.4±1.4 Ma, indicating an Early Cretaceous age. The age agrees with the previous results obtained using K-Ar, Ar-Ar and U-Pb methods for the Yixian volcanic rocks in the Beipiao and Lingyuan areas. Geochemical compositions of 7 samples of typical Yixian volcanic rocks from the Lingyuan area show that they consist of trachyte and rhyolite, with SiO2＝65.6-73.4％, MgO＝0.35-1.23％, Mg#＝16-41, Al2O3=13.40-16.79% and Na2O＝3.54-4.26％. In addition, the volcanic rocks are characterized by enrichment in LREE and depletion in HREE, with high Sr (445-774)×10-6 and Sr/Y (49-66) and low Y (7-14)×10-6 and Yb (0.51-0.97)×10-6 and LaN/YbN＝33-48, showing the geochemical characteristics of adakite. However, they are low in Mg#. The authors suggest that they may have originated by partial melting of the thickened mafic lower crust.

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