华北克拉通北缘承德下板城盆地三叠系二马营组凝灰岩夹层SHRIMP锆石U-Pb定年及源区分析

SHRIMP zircon U-Pb dating and source analysis of tuff interlayer of Triassic Ermaying Formation in Xiabancheng Basin of Chengde area,northern margin of North China Craton

鉴于目前对华北克拉通北缘承德下板城盆地三叠系二马营组的时代归属存在较大的分歧,本文以该盆地二马营组上段一厚层火山凝灰岩为研究对象,对其进行高精度的SHRIMP锆石U-Pb定年及原位Lu-Hf同位素分析,以准确厘定其沉积时代与来源区.定年结果显示,该凝灰岩层的沉积年龄为(243.5±3.3)Ma.依据定年结果及前人研究成果,认为盆地二马营组的沉积时代应为中三叠世早期,即中三叠世安尼期,而不是一些学者认为的晚三叠世或中-晚三叠世.对年龄较为集中的锆石进行了原位Lu-Hf同位素分析,表明其具有较低的εHf(t)值(-7.7^-6.1)和较老的二阶段模式年龄(1.66~1.76Ga).结合凝灰岩定年结果中所携带的继承锆石信息,以及前人研究成果,表明火山凝灰岩应来源于华北克拉通北缘的内蒙古隆起,而非其北邻的兴蒙造山带.

In view of the great differences in the age of the Triassic Ermaying Formation in the Xiabancheng Basin of Chengde area on the northern margin of the North China Craton,this paper takes a thick layer of volcanic tuff in the upper part of the Ermaying Formation of the Basin as the research object.Highprecision SHRIMP zircon U-Pb dating and in-situ Lu-Hf isotope analysis to accurately determine the deposition age and source area.The dating results show that the deposition age of the tuff interlayer is(243.5±3.3)Ma.Based on the results of dating and previous research results,we believe that the sedimentary age of the Ermaying Formation in the basin should be the early Middle Triassic,that is,the Middle Triassic Anisian period»rather than the Late Triassic or Middle-Late Triassic considered by some scholars,ln-situ Lu-Hf isotopic analysis of zircon with concentrated age shows that it has a lowerεHf(t) value(-7.7 to -6.1)and an older two-stage model age(1.66 to 1.76 Ga).Combined with the inherited zircon information carried in the tuff dating results and previous research results,it is indicated that the volcanic tuff should originate from the Inner Mongolia uplift on the northern margin of the North China Craton,not the Xing-Meng orogenic belt adjacent to the north.