Geochemistry and SHRIMP U-Pb Zircon Dating of Mafic Rocks North of Zunhua City, Eastern Hebei, North China Craton: Paleoproterozoic Gabbro rather than Neoarchean Ophiolite

摘要

Abundant mafic-ultramafic blocks and dikes occur in the area north of Zunhua City,eastern Hebei Province,and were previously suggested to be part of a late Archean ophiolitic assemblage.We employed SHRIMP zircon dating and a geochemical study on these mafic and surrounding rocks to test the ophiolite hypothesis.The SHRIMP data suggest that three metagabbro samples were metamorphosed at ～1.8 Ga.Numerous ～2.5 Ga zircons display strong oscillatory zoning,characteristic of zircons from granitoid rocks but not from gabbro,so we suggest that these are xenocrystic grains.The age of these xenocrystic zircons and their metamorphic rims suggests that these mafic blocks formed in Paleoproterozoic.The surrounding gneiss of intermediate composition also contains 2.5 Ga zircons with oscillatory zoning and 1.8 Ga metamorphic rims.Fractionated REE patterns and Nb,Ta,Zr,Hf negative anomalies to variable extent were observed in the mafic blocks and surrounding rocks,also supporting a significant difference in the chemistry of ophiolitic rocks.Our data suggest that many mafic blocks in northern Zunhua are not part of a late Archean ophiolite complex but part of a tectonically dismembered Paleoproterozoic intrusive gabbro complex.This study shows that late Paleoproterozoic metamorphism occurred in the western part of eastern Hebei Province.