Petrology,Zircon Chronology and Geochemistry of the Late Silurian Ophiolitic Mélanges and the Baiyunshan Forearc Complex in the Central Beishan Orogenic Belt,NE China

The WNW-ESE-trending Baiyunshan ophiolitic mélange is exposed in the central Beishan orogenic belt(BOB),between the Hongliuhe-Niujuanzi and Xichangjing ophiolitic mélanges in the west and east,respectively.Here we present new petrological,geochemical,and zircon U-Pb age data for the Baiyunshan ophiolitic mélange and metamorphosed lithic sandstones of“the Baiyunshan Formation”.The Baiyunshan ophiolitic mélange is characterized by a block-in-matrix structure,and the blocks of ultramafic-mafic rocks are mainly pyroxenites,dunites,gabbros,and minor hornblendites.Zircon U-Pb dating yielded an emplacement age of 424.2±3.7 Ma for the gabbros.The ultramaficmafic rocks have geochemical features of supra-subduction zone-type ophiolites,including low Ti contents(TiO2=0.01 wt.%–0.35 wt.%),negative Nb-Ta anomalies,and U-shaped rare earth element patterns with positive Eu anomalies(δEu=0.99–3.25).“The Baiyunshan Formation”is exposed to the north of the ophiolitic mélange,and comprises mainly meta-cherts in its lower part and metasandstones and marbles in its upper part.Blocks of basalt are locally dispersed in the sand-slate matrix.The ages of detrital zircons in the metamorphosed lithic sandstones range from 521 to 464 Ma,with an age peak of 500 Ma,which is similar to those of the Gongpoquan arc in the north.The central Beishan orogenic belt experienced continuous subduction of the Hongliuhe-Xichangjing oceanic lithosphere in the Late Silurian,which produced the Baiyunshan forearc complex and supra-subduction zone-type ophiolite.