Geochemical Study Of Early Paleozoic Mafic Magmatic Rocks From The Shangdan Suture Zone

The Qinling orogen is a typical composite orogenic belt,which records the tectonic evolution processes of oceanic subduction,arc-continent collision,continental subduction and continent-continent collision.The Shangdan suture zone,which is spread within the Qinling orogen along the east-west direction,represents an oceanic relic formed during the subduction and closure of the Proto-Tethys Shangdan ocean.Many early Paleozoic mafic magmatic rocks with different geochemical features are exposed in the Shangdan suture zone,they are closely related to the oceanic subduction process,and thus provide a good object to study the tectonic evolution of the fossil oceanic subduction zone,crust-mantle interaction and the evolution of slab fluid composition.This dissertation has performed a systematic study of petrology,zircon U-Pb geochronology and isotopic geochemistry,whole-rock elemental and isotopic geochemistry for various types of early Paleozoic mafic magmatic rocks exposed in the western,middle and eastern parts of the Shangdan suture zone.The results qualitatively and quantitatively constrain the nature of slab fluids and their proportions invloved in the mantle sources,and eventually broaden our knowledges of different types of crust-mantle interaction,the petrogenesis of mafic arc rocks,and the tectonic evolution of the oceanic subduction zone.Subduction zone is an important place for energy and material exchange between the earth’s crust and mantle,determining the initiation time of subduction zone has great significance for understanding its tectonic evolution.Although a lot of studies has been done for the Qinling orogen,it remains unclear about when the subduction of the Proto-Tethys Shangdan ocean initiated and the evolution process during its early stage.Here,we have collecteted the zircon U-Pb geochronological data of mafic magmatic rocks exposed within the Shangdan suture zone.These geochronological data display a trend of older in the west and younger in the east,suggesting that the subduction initiation time of the Proto-Tethys Shangdan ocean is likely to be diachronous from the west to the east.The Guanzizhen area located in the western part of the Shangdan suture zone,mafic magmatic rocks with forearc basalt-like(G-1),boninite-like(G-2)and arc-like(G-3 and G-4)geochemical features were identified in this area.A combined study of zircon U-Pb geochronology,whole-rock elemental and Sr-Nd isotopic geochemistry have been carried out for these rocks,the results indicate that the subduction initiation time is at the early Cambrian(ca.517 Ma)for the western part of the Proto-Tethys Shangdan ocean.The Guanzizhen area is close to the Kunlun orogen and the Qilian orogen,and located in the intersection region of the Proto-Tethys Kunlun ocean,the Qilian ocean and the Shangdan ocean during the Paleozoic,suggesting that the subduction initiation of the weastern part of the Proto-Tethys Shangdan ocean might be influenced by the tectonic evolution of the Proto-Tethys Kunlun ocean and Qilian ocean.The subduction initation time of the eastern part of the Proto-Tethys Shangdan ocean is at the late Cambrian(about 490 Ma),which is most likely triggered by the northward subduction and collision of the Qinling microcontinent beneath the Erlangping unit.In addition,the Guanzizhen mafic magmatic rocks exhibit characteristics of large ion lithophile element(LILE)enrichment,and depleted to enriched SrNd isotopic compositions,indicating that some subducting slab fluids contributed into their mantle source.Therefore,the recycling of subducted crustal materials occurred even in the early stage of the oceanic subduction zone.Overall,the mafic magmatic rocks of G-1,G-2 and G-3(517-502 Ma)display low Th/Yb ratios,high Ba/La and Ba/Th ratios as well as weakly depleted Sr-Nd isotopes;the G-4 mafic magmatic rocks(500-489 Ma)display high Th/Yb,Ba/La and Ba/Th ratios and most enriched Sr-Nd isotopes.This suggests that the compositions of slab fluids are mainly composed of aqueous solutions plus minor contribution of sediment melts for the G-1,G-2 and G-3,while the contribution of sediment melts is high in G-4.After the formation of the ocean subduction zone,the recycling of subducted crust material into the deep mantle becomes more common,usually in the form of slab fluids to metasomatize the overlying mantle wedge peridotite.However,little is know about the nature and evolution of the slab-derived fluids produced under various depths during oceanic subduction.The Yanwan-Yinggezui area is located in the middle part of the Shangdan suture zone.Here,we have carried out a study of zircon U-Pb geochronology on the mafic magmatic rocks from the Yanwan-Yinggezui area,and obtaind their formation ages of ca.489-454 Ma,correspodding to the stage of the subduction of the Proto-Tethys Shangdan ocean.These mafic magmatic rocks can be divided into three types:Group A(ca.488-489 Ma)exhibit enrichment in fluid-mobile elements(e.g.,Rb,Ba,K and Pb)and weak depletion in Sr-Nd isotopes,and have FAB-like low Ti/Vand Yb/V ratios;Group B(ca.461 Ma)also exhibit enrichment in fluid-mobile elements and negative Nb-Ta anomalies as well as less depleted Sr-Nd isotope compositions;Group C(ca.454 Ma)exhibit enrichment in light rare earth element(LREE)and LILE but depletion in high field strength element(HFSE),and have enriched Sr-Nd isotope compositions.These geochemical characteristics suggest that they are products of metasomatized mantle sources.They display systematic geochemical differences in the compositions of whole-rock major-trace elements(e.g.,SiO2,K2O,LREE and LILE)and whole-rocks Sr-Nd isotopes as well as zircon Hf-O isotopes,indicating that the different types and proportions of subducted slab fluids have contributed to their mantle sources.Geochemical studies and model calculation results show that from Group A to Group B to Group C,the releasing depths of slab fluids increase from 3.0 GPa to 4.7 GPa,and the proportion of sediment-derived melts in these slab fluids increases gradually from 5%to 40%.The high proportions of sediment-derived melts for Group C mafic magmatic rocks could be responsible for their most high contents of incompatible elements and most enriched radiogenic isotope compositions.Therefore,the Yanwan-Yinggezui early Paleozoic mafic magmatic rocks record the crust-mantle interaction at different depths and the evolution of slab fluids during the subduction of the Proto-Tethys Shangdan ocean,which provide a good example to deepen our understanding of crust-mantle interaction in oceanic subduction zones.Mo and Fe in mafic magmatic rocks usually exist as trace and major elements,respectively.Studying the mafic magmatic rocks in subduction zones using Mo and Fe isotopes is of great significance for understanding the geochemical behavior and recycling mechanism of Mo and Fe isotopes.The Lajimiao pluton in the eastern part of the Shangdan suture zone is the products formed during the late stage subduction of the Proto-Tethys Shangdan ocean(ca.412-419 Ma),which mainly consist of gabbro and gabbroic diorite.These mafic magmatic rocks display enrichment in LREE and LILE,depletion in HFSE,and have high zircon O isotopes as well as weakly depleted whole-rock Sr-Nd-Hf isotopic compositions,indicate that their mantle source was modified by subducted slab fluids.The Lajimiao gabbro has δ98 Mo values of-0.111‰ to 0.225‰ with a weighted average value of 0.019‰±0.064‰(2SE),which are obviously higher than the mantle value(δ98Mo=-0.204‰ ± 0.008‰).After excluding the effects of fractional crystallization and partial melting,we suggest that their high Mo isotopes mainly inherit from oceanic crust-derived fluids(δ98Mo ≈0.25‰).Combine their high Li/Y,Rb/Y,Th/Zr and low Th/Nb,Nb/Y,Nb/Zr ratios as well as the quantitative modelling results of trace elements and Mo-Sr-Nd-Hf isotopes,we concluded that the slab fluids added into the mantle source of the Lajimiao gabbro are mainly basaltic oceanic crust-derived aqueous solutions(2%-10%)with minor sediment-derived melts(<0.5%).The low Mo content and Mo/Ce ratio in the Lajimiao gabbro are mainly attributed to the contribution of sediment-derived melts.The Lajimiao gabbros have δ56Fe values of 0.069‰ to 0.175‰ with a weighted average value of 0.123‰± 0.023‰(2SE).The corrected δ56Fe values for their primary magma are0.058‰ to 0.105‰ with a weighted average value(δ56Fe=0.008‰±0.032‰,2SE),which is slightly lower than that of the mantle(δ56Fe=0.02‰±0.03‰),suggesting that the mantle source of Lajimiao gabbro has received the contribution of slab derived Fe2+-riched fluids and/or experienced the prior melt extraction-induced depletion.The binary mixing model indicates that the slab fluids have insignificant influence on the Fe isotopes of the Lajimiao gabbros,and the early depletion process of their mantle source would mainly account for their light Fe isotopic compositions.Although the subducted slab fluids did not significantly affect the Fe isotope compositions of the Lajimiao gabbros,but changed their Fe3+/ΣFe ratios,causing decoupling between δ56Fe and Fe3+/ΣFe.Therefore,Mo stable isotopes are more sensitive and powerful than Fe stable isotopes in tracing slab fluid activity in subduction zones.This doctoral thesis shows that the mafic magmatic rocks exposed in the suture zones within orogenic belts provide a good chance for exploring the formation and evolution of ancient oceanic subduction zones,the nature and evolution of subducted oceanic crust-derived fluids,and the crust-mantle interaction.This study provides a precise constraint on the timing of subduction initiation of the Proto-Tethys Shangdan ocean,and deciphers the evolution of slab fluids and Mo-Fe isotope geochemical behaviors in oceanic subduction zones.