Petrogenesis And The Tectonic Attribute Of The Mafic-Ultramafic Rocks From Zhenghe-Dapu Fault Zone

Mafic-ultramafic rocks are extensively exposed in the NE-SW trending Zhenghe-Dapu fault zone of the Cathaysia block.The petrogenesis of these rocks has been a controversial issue for many years.In this study,geochronological and geochemical studies have been carried out on the Mafic-ultramafic rocks,in order to constrain their petrogenesis and the tectonic setting,and also to explore the tectonic evolution of the Cathaysia Block during Neoproterozoic and Paleozoic period.Our studies have made the following progresses:The protolith of the hornblende plagioclase schist in Shuiji Town and that of plagioclase amphibole schist in Zhenghe County erupted at 807 ± 4 Ma and 725 ± 22 Ma,respectively,suggesting that these mafic rocks mainly formed during Neoproterozoic.The geochemical data show that the magma of the amphibole schists from Dikou Town derived from enriched mantle source whereas the magma of the plagioclase amphibolite schists from Zhenghe Town originated from relatively depleted mantle source.The amphibole schists of Dikou Town plot in the intraplate basalt field whereas the plagioclase amphibolite schists from Zhenghe Town exhibit the characteristic of N-MORB on the tectonic discrimination diagrams.Electron microprobe data show that hornblendites are mainly composed of edenite component.Sr-Nd isotope and Re-Os isotope characteristics indicate that the magma of the hornblendites in Yanghou Village originated from the enriched mantle.In contrast,Re-Os isotopic data show both the serpentine from Mamianshan Village and serpentinized peridotite from Beili Village are relics of the continental mantle peridotite.In conclusion,the Neoproterozoic mafic rocks of the Zhenghe Dapu fault zone formed at extensional setting.In contrast,some ultramafic rocks are magmatic origin and some of them are residual mantle peridotite.Theather these ultramafic rock are part of ophiolite or not,further study are needed to identify.Some mafic rocks with Mesozoic age also exposed along the Zhenghe Dapu fault zone.The protolith of plagioclase amphibole schists in Dongyou Town erupted at 221 ± 1 Ma.Sr-Nd isotope characteristics indicate the magma of the rocks derived from enriched mantle sources.These rocks plot in the intraplate basalt field on the tectonic discrimination diagrams.