Studies Of Mafic And Ultramafic Rocks Along The Shang-Dan Suture From Qinling Orogenic Belt And Their Implications

The Shang-Dan fault was suggested to be a main suture of the Qinling orogenic belt.It's of critical importance for resuming the formation and evolution of the Qinling orogenic belt and understanding the interaction between the North china and Yangtze blocks.On the basis of field investigations,three mafic ang ultramafic rocks, such as the Songshugou ultramafic massif,the Lajimiao gabbros and the Sifangtai complex,were selected for comprehensive studies on petrology,geochemistry,PGE, LA-ICP-MS zircon dating and ⁴⁰Ar-³⁹Ar dating.These studies play important roles in discussing petrogenesis,evolution and geochronology,moreover,speculating on evolution of the Qinling orogenic belt and interacting model of the North China and Yangtze blocks.Some achieves are as following:1.Characteristics of geochemistry and PGE(platinum group element)of Songhugou ultramafic massif show it forms through melt percolate flow at the crust-mantle transition zone,and is oceanic lithospheric fragment.Main evidences such as:(1)The whole rock concentrations of trace elements are depleted,and highly incompatible elements(Rb to Nb)and LREE relative to HREE are enrichment.(2) In-situ analyses for olivine grains of a transition from dunite to harzburgite in the Songshugou ultramafic massif through LA-ICPMS,show compositional variations. REE are higher in dunite than in harzburgite.In comparison with Oman ophiolite,we propose that the Songshugou dunite is a product of reaction of harzburgite with melt through porous percolation flow.Songshugou Ulatramafic massif is a part of ophiolite.2.Studies of contact metamorphic zone(mainly are garnet amphibolite)and rock wall(mainly are amphibolite),confirmed they are not parts of ophiolite.Zircon from garnet amphibolite yielded a metamporphic age of 506±7.4Ma(LA-ICP-MS),and inclusions in the zircon are plagioclase,these studies show the amphibolite formed earlier than the intruding of the Songshugou ultramafic massif.At the same time, analysis of REE of zircon for dating show,most zircon with lower REE and consistent REE patterns,without Eu anomaly,one zircon with higher REE and Eu negative anomaly,these indicate that the zircon formed earlier than garnet,while others formed posterior to garnet.Songshugou altramafic massif compared with rock wall is simpler in metamorphism,rock wall is similar with the Qinling group.3.Analysis foe major and trace elements of 14 garnet amphibolite and amphibolite across the contact metamorphic zone.Major elements are steady.REE are gradually increase from inner to outer,indicate mantle fluid lead to losses of REE during intrusions.The observations shows that significant REE depletions in heavily altered rocks in ODP(Sparks et al.,1995;Bach et al.,1996,1998).4.4 amphibole samples(the most outboard is amphibolite,others are garnet amphibolite)collected across the contact metamorphic zone were studied using ⁴⁰Ar-³⁹Ar dating.Two of the samples that were collected close to the ultramafic massif yield plateau ages of 372±15 Ma,464±12 Ma,the other two samples yield disturbed plateau ages of 474±8 Ma,781±146 Ma.There is a～130Ma gap between the most inner age(～370Ma)and the Songshugou ultramafic emplacement age(～500Ma), suggested the Songshugou ultramafic massif cooling is slow after intruding.5.Combining petrological and geochemical characteristics of Lajimiao gabbros, we speculated the Lajimiao gabbros forms melt of lower crust and asthenosphere.The Lajimiao gabbros were affected by plagioclase accumulation as indicated by Eu positive anomaly.Using CL and LA-ICP-MS techniques,this study obtains a magmatic age 422±6.8Ma,indicate close of the Shang-Dan fault.And early captured zircon age 973±60Ma(898Ma-1083Ma),show early tectonic event which are consistent with Grenvillian(～1.3-1.0Ga)(Dalziel,1991;Hoffman,1991;Moores 1991; Keppie,2001).6.The SiO₂ content of Sifangtai complex ranges from 45%-54%.These rocks belong to two series,tholeiitic and talc-alkaline series.Compared to common mantle rocks,they have high REE concentrations,especially LREE,with Rb,Th,Nb,P and Zr depletions and Ba,Pb,K and Sm enrichments.Gabbro is the main component of the complex,with gray-black colour and medium granularity.Its geochemical characteristics indicate that it is likely to be a product of crust-mantle interaction.In comparison with typical arc-related gabbros,it has Rb and Th negative anomalies. Considering that it is located near the Shang-dan zone,we propose that the formation of the Sifangtai mafic-ultramafic complex was due to break-off of subducted slabs or delamination of the mountain root along the Shang-dan fault,which disturbed the asthenosphere,resulting in partial melting of lower crust as well as mantle-crust interaction.The Sifangtai complex body was affected by plagioclase accumulation as indicated by Eu positive anomaly.Using LA-ICP-MS method,we obtain magmatic age 460±29Ma of Sifangtai mafic-ultramafic complex.All 32 data can be divided into 4 phases:(1)400-500Ma(n=4),(2)700-900Ma(n=2),(3)1800-2600Ma(n=20),(4) 2700-2918Ma(n=4).7.Three mafic and ultramafic rocks along the Shang-Dan suture from the North Qinling are studied.Songshugou ultramafic massif is oceanic lithospheric fragment,it intruded the Qinling group at～500-510Ma.There is a 20-30°cornu between the Shang-Dan suture and axis of Songshugou ultramafic massif,indicate that Qinling ocean diving towards NNE.Lajimiao gabbro formed～422Ma,Sifangtai complex formed～460Ma,they show the time that close of Shang-Dan ocean respectively. Recently,studies of Guanzizhen maric and ultramafic rocks which located to the west part of the Shang-Dan suture,obtained gabbro which formed island arc setting zircon magmatic age 489±10Ma(SHRIMP)(Li et al.,2006)and another gabbro zircon magmatic age 475±4.6Ma(LA-ICP-MS)(Dong et al.,2006),suggested that the Guanzizhen ocean dived in lower Ordovician.All these mafic-ultramafic rocks are formed in Paleozoic during Qinling ocean diving and later collisions of continents. Ages of mafic-ultramafic rocks from west to east along the Shang-Dan suture, indicate(North China block)continent-continent(Yangtze block)collisions leaded to by closure of the Qinling ocean,occurred earlier in the west than in the east along the Qinling-Dabie orogen.The idea explained logically why HP-UHP metamorphism is～400-500Ma in Qinling orogen,while is～230Ma in Dabie orogen.