Assemblage Of Andesitic-Mafic-Ultramafic Volcanic And Intrusive Rocks And Its Tectonic Environment In Baotan,Northern Guangxi

The Baotan area,northern Guangxi province,is the key area to research the closure of the Huanan ocean.Mafic-ultramafic complexes?volcanic rocks and granites are widely exposed in the Sibao Group.Some mafic-ultramafic complexes are related to copper nickel ore.Based on investigation and observation under microscope,we carried out the research on mafic-ultramafic complexes,volcanic rocks and gabbro-diorite without deformation.Determining the type of mafic-ultramafic complexes and builting volcanic rocks sequence is a key question in this area.We took the field investigation,rock-mineral determination,EMPA,zircon chronology,geochemistry and so on.1.Based on cutting relationship,intrusive rocks or blocks,degree of deformation and zircon U-Pb chronology,we find the magmatic rocks in Baotan area are not emplaced at same time.Basaltic ignimbrite\andesitic-rhyolitic volcanic rocks(about 851Ma)>ultramafic-mafic complexes(about 850 Ma)>granodiorites(825?835Ma)>granites(800?835Ma)>lenticular quartz diorite(about 766Ma)?pillow basalt>gabbros in Jinzhuao?diorite in Baotandaiqiao(about 134Ma).2.The mafic-ultramafic complexes intruded into the Sibao Group.They consist of serpentinized peridotite,olivine pyroxenite,pyroxenite gradually and some complexes contain gabbro.Hornblende is discovered in each unit of mafic-ultramafic complexes,demonstrating the fluid-rich feature in the mafic-ultramafic complexes.Cr-spinels are characterized by high Fe?Cr?Ti and low Al,which is similar to the Alaskan-type mafic-ultramafic complexes.The country rocks are turbidite intercalated island arc volcanic rocks.Therefore,the mafic-ultramafic complexes in the Baotan area may be formed in a subduction-related environment where Alaska complexes occurred.The case is also for copper nickel ore.The gabbro within complexes was emplaced at 850.3±4.3Ma.3.There are a large number of volcanic rocks in the Sibao Group.The volcanic rocks consist of basalt-basaltic tuff,andesite,and rhyolite-rhyolitic tuff.Based on investigation,the volcanic rocks show multiple eruptions.The earliest magmatic activity is characterized by the rhyolitic volcanic rocks,which was followed by andesite.Then the basaltic volcanic rocks were emplaced but large in quantity.Andesites were volumetrically much more abundant than rhyolitic volcanic rocks in the Baotan area.The volcanic magma varied in composition from acid to basic.The andesites were erupted at 851.5±4.6Ma.4.Meanwhile,we discovered some new age data of diorite and pillow lava.A high-Mg diorite-porphyrite occurred near the Yuxi village.Its geochemical characteristics are similar to sanukites in Japan.It may be formed in the subduction environment.The diorite was emplaced at 776±18Ma.Near the high-Mg porphyritic diorite,a series of pillow basalts existed.We discovered the fossils in the chert,the cover of pillow basalts.It may be erupted at early Ordovician.Its geochemical characteristics are like both back-arc basin basalts and island arc basalts.Combined with regional geological data,the subduction still existed in northern Guangxi,during 770-780Ma.5.Late mafic rocks are distinguished.In Baotan area,there was a diabase with brittle deformation which was intruded into the Sibao Group.It may be crystalized within plate at Late Permian.There were some gabbro-diorites without deformation intruding into mafic-ultramafic complexes,granites and layers,showing they were the youngest magmatism in the Baotan area.They were crystalized at Cretaceous.