Different Types Of Crust-mantle Interaction In Subduction Zones

Subduction zone mafic igneous rocks generally record the recycling of crustal components at convergent plate boundaries.These mafic rocks are the ideal targets of investigating the different types of curst-mantle interaction in subduction zones,the tectonic evolution of oceanic-continental subduction zones,as well as the nature of orogenic lithospheric mantle.The North China Block(NCB)was influenced by southward subduction of the Paleo-Asian oceanic slab,northward subduction of the Paleo-Tethyan oceanic slab and the South China Block(SCB),and westward subduction of the Paleo-Pacific oceanic slab since the Paleozoic.This makes the NCB an ideal target for tracing the different types of crust-mantle interaction.Mesozoic-Cenozoic mafic igneous rocks with different ages and properties were emplaced in the NCB,and they can be used to identify the recycled crustal components during the subduction and orogeny at the convergent plate boundary.This dissertation provides a comprehensive study of petrology and geochemistry for MesozoicCenozoic mafic igneous rocks from the NCB.The results provide insights into the recycling of crustal components into the mantle sources of these mafic rocks,with implications for reconstruction of the plate subduction history and related crust-mantle interaction,the tectonic evolution of oceanic-continental subduction zones,and the different types of geochemical transfer processes in oceanic-continental subduction zones.Identifying and investigating syn-subduction mafic magmatism in continental collision zones have great bearing on the recycling of crustal components in continental subduction zones and the evolution of collisional orogeny.This dissertation presents a new dataset of zircon U-Pb ages and Hf-O isotopes,whole-rock major and trace elements,and Sr-Nd-Hf isotopes in mafic dikes from the Dalian area in the southeastern NCB.Zircon U-Pb dating yields Early-Middle Triassic ages of 247-244Ma for emplacement of the mafic dikes,coeval with the initial collision of the SCB with the NCB to build the Dabie-Sulu orogenic belt.Thus,the Early-Middle Triassic mafic rocks are the product of syn-subduction magmatism above the continental subduction zone.The mafic rocks show OIB-like trace element distribution patterns,with enrichment in large ion lithophile elements(LILE)and light rare earth elements(LREE),but no depletion in high field strength elements(HFSE).They also exhibit depletedweakly enriched Sr-Nd-Hf isotope compositions,with(87Sr/86Sr)i ratios of 0.7057 to 0.7091,?Nd(t)values of-3.8 to-1.2,and ?Hf(t)values of-3.2 to-1.3.These elemental and isotopic compositions indicate that the Early-Middle Triassic mafic rocks were originated from mantle sources that were redily enriched by subducting oceanic crust-derived melts.The Hf-O isotopes in syn-magmatic zircons from the mafic rocks can be divided into two groups.One group exhibits depleted Hf isotopes and high ?18O values,whereas the other group has enriched Hf isotopes and high ?18O values,corresponding to the low-temperature altered oceanic basalt and its overlying sediment,respectively.Meanwhile,relict zircons in the mafic rocks show Paleozoic U-Pb ages of 429-422 Ma and Meso-Neoproterozoic ages of 1059-786 Ma,which are respectively comparable with metamorphic ages of the subducted PaleoTethyan oceanic crust and magmatic ages of the SCB.This suggests that the subducted PaleoTethyan oceanic crust and its overlying terrigenous sediment were involved in the mantle metasomatism.Considering the history of continental convergence between the SCB and the NCB in the Paleozoic,it is proposed that the subducting Paleo-Tethyan oceanic crust underwent dehydration melting at postarc depths of>200 km,giving rise to hydrous melts to metasomatize the overlying mantle wedge peridotite during the Late Paleozoic,generating fertile and enriched mantle domains.The continental collision between the SCB and the NCB would have started in the Early-Middle Triassic,causing rollback of the subducting PaleoTethyan oceanic slab and thus partial melting of the metasomatic mantle domains to induce the syn-subduction magmatism.In this regard,the Early-Middle Triassic mafic rocks from the Dalian area have signified the syn-subduction magmatism above the continental subduction zone.These mafic rocks have provided the geochemical evidence for recycling of the ancient oceanic crust in the continental collision zone,and have recorded the tectonic transition from oceanic subduction to continental collision.There are also minor mafic dikes of Late Triassic in the Dalian area in the southeastern NCB,showing alkaline and subalkaline compositions,respectively,in lithochemistry.Zircon U-Pb dating obtains that the two types of mafic rocks have consistent emplacement ages of 219-218 Ma,coeval with exhumation of the deeply subducted continental crust in the DabieSulu orogenic belt.Thus,the Late Triassic mafic rocks are the product of syn-exhumation magmatism above the continental subduction zone.All of the mafic rocks show arc-like trace element distribution patterns,with enrichment in LILE and LREE,but depletion in HFSE.They also exhibit significantly enriched Sr-Nd-Hf isotopes,with(87Sr/86Sr)i ratios of 0.7057 to 0.7079,?Nd(t)values of-14.2 to-7.1,and ?Hf(t)values of-11.4 to-4.7.These geochemical features indicate that the Late Triassic mafic rocks were originated from mantle sources that were readily enriched by subducting continental crust-derived melts.These mafic rocks have zircon Hf model ages of Paleoproterozoic,comparable with those of ultrahigh-pressure metamorphic rocks in the Dabie-Sulu orogenic belt,verifying that the subducted SCB were involved in the mantle metasomatism.The alkaline series rocks have higher incompatible element contents and more enriched radiogenic isotopes than those of the sub-alkaline series rocks,suggesting more crustal components in their mantle source.Meanwhile,the two series of mafic rocks show systematic differences in Fe/Mn,Zn/Fe,and Nb/Ta ratios,suggesting that they were derived from partial melting of pyroxenite-rich and hornblendite-rich lithologies,respectively.Thus,the subducted SCB continent-derived melts would have metasomatized the lithospheric mantle wedge beneath the NCB in the Triassic,generating the enriched mantle metasomatites.Then the metasomatites were partially melted to form the syn-exhumation mafic rocks in the hangingwall of the continental collision zone.In this regard,the Late Triassic mafic rocks from the Dalian area have signified the syn-exhumation magmatism above the continental subduction zone.These mafic rocks have recorded the crust-mantle interaction during the continental subduction,and thus have revealed the tectonic evolution during the collisional orogeny.Subduction of the Paleo-Pacific plate is generally considered to have exerted a substantial influence on the evolution of lithospheric mantle beneath eastern Asia in the Mesozoic.Identifying the Mesozoic mafic arc magmatism related to the Paleo-Pacific oceanic subduction have great bearing on the history of Paleo-Pacific oceanic subduction and the evolution of the lithospheric mantle beneath the NCB.This dissertation provides a comprehensive study of geochronology and geochemistry for lamprophyres from the Huaziyu area in the eastern part of the NCB.Two types lamprophyres(spessartites and kersantites)were distinguished and both have consistent zircon U-Pb ages of?155 Ma.All of the lamprophyres have arc-like trace element distribution patterns,with enrichment in LILE and LREE but depletion in HFSE.In comparison to the lithospheric mantle beneath the NCC,the spessartites have relatively depleted Sr-Nd-Hf isotopes with(87Sr/86Sr)i ratios of 0.7075 to 0.7085,?Nd(t)values of-3.9 to-1.3,and ?Hf(t)values of-5.4 to-0.3.In contrast,the kersantites have weakly enriched Sr-Nd-Hf isotopes with(87Sr/86Sr)i ratios of 0.7096 to 0.7117,?Nd(t)values of-12.2 to-7.6,and ?Hf(t)values of-12.8 to-4.7.These geochemical compositions indicate that the mantle sources of the Huaziyu lamprophyres contain subducted oceanic crustal components.Subducted oceanic crust-derived fluids and seafloor sedimentderived melts were respectively incorporated into the lithospheric mantle in different proportions,and their interaction with the mantle wedge peridotite would have generated the mantle sources for the lamprophyres.In particular,the kersantites have higher incompatible element contents,more enriched radiogenic isotopes,more enriched zircon Hf isotope,and higher Ba/Th,Sr/Nd,Th/Yb,and La/Sm ratios than those of the spessartites,suggesting more incorporation of the sediment-derived melts into their mantle source.The two types of lamprophyres have Fe/Mn and Zn/Fe ratios different from normal mantle peridotite,suggesting that they were originated from fertile metasomatic pyroxenite sources.Thus,the Huaziyu lamprophyres record the history of the Paleo-Pacific subduction in the Jurassic,and reveal that the Paleo-Pacific slab-derived fluids have exerted a substantial influence on decratonization of the NCB.In combination with the spatial and temporal distributions of Mesozoic mafic magmatism in the NCB,there is a magmatic interval between?200 Ma and?144 Ma,and the Huaziyu lamprophyres serve as one of the rare material records of the Paleo-Pacific subduction in the Early Mesozoic.In this regard,the Paleo-Pacific slab was subducted in low angle beneath the NCB in the Jurassic,and slab-derived fluids would have metasomatized and weaken the cratonic lithospheric mantle.Then the subducted Paleo-Pacific slab would have started to roll back on a large scale in the Early Cretaceous.For this reason,the cratonic mantle was thinned and decstructed,resulting in intensive crust-derived and mantle-derived magmatism in this period.This dissertation provides a study of whole-rock geochemistry and phenocryst water contents and O isotopes for Cenozoic OIB-like basalts from the Fushun area in the eastern NCB.These Cenozoic basalts have OIB-like trace element distribution patterns,with enrichment in LILE and LREE,and no depletion in HFSE.They also show depleted Sr-Nd-Hf isotopes,with(87Sr/86Sr)i ratios of 0.7038 to 0.7047,?Nd(t)values of 3.0 to 5.6,and ?Hf(t)values of 6.0 to 9.0.The analyses for clinopyroxene phenocrysts yield water contents of 6-72 ppm,suggesting the primary magmas of the basalts are characterized by low water contents.Furthermore,olivine and clinopyroxene phenocrysts are characterized by low ?18O values of 3.9-5.0‰.These observations reveal that the mantle source of the Fushun basalts contains the dehydrated lower oceanic crust-derived component.The H2O/Ce ratio thermometry gives 1055-1267? for dehydration of the subducting oceanic slab to produce fluids in the mantle source of the basalts.This temperature range is significantly higher than that of subduction zone fluids responsible for metasomatism of the mantle wedge at subarc depths of 80-160 km for the formation of oceanic arc basalts.Thus,partial melting of the dehydrated oceanic crust with the rutile breakdown would have occurred in the postarc depths,generating felsic melts that were enriched in LILE and LREE but no depleted in HFSE.These melts would have metasomatized the postarc mantle wedge peridotite to generate the mantle sources for the Fushun basalts,In addition,mantle melting temperatures for the basalts are calculated to be 1456-1522?,which are much higher than the slab dehydration temperatures.This temperature difference leads to the conclusion that the formation of the Fushun basalts involves at least two-stage processes.The first is the chemical metasomatism of the mantle wedge peridotite by subducting oceanic crust-derived melts at the postarc depths to generate fertile and enriched mantle domains,and the second is the partial melting of the metasomatites to produce basaltic magmas upon heating by the asthenospheric mantle.In this regard,the water contents and O isotopes in the Fushun basalts record the geochemical transfer in the oceanic subduction zone at greater depths.Unlike arc-like mafic rocks,the origin of OIB-like mafic rocks generally involves the dehydration melting of subducting oceanic crust at postarc depths.The Mo isotope study of mafic rocks in convergent plate boundaries has great bearing on understaning the Mo isotope recycling in subduction zones and the recycling of crustal components during continental subduction.This dissertation provides a Mo isotope case study for Cretaceous basalts from the Xinbin area in the eastern NCB,the hangingwall of the continental subduction zone.The Xinbin basalts have ?98Mo values of-0.98 to-0.16‰,significantly lower than those of 0.20±0.01‰ for the depleted MORB mantle.Furthermore,these basalts show arc-like trace element distribution patterns and significantly enriched SrNd-Hf isotopes,with(87Sr/86Sr)i ratios of 0.7050 to 0.7058,?Nd(t)values of-15.2 to-10.4,and?Hf(t)values of-18.7 to-7.9.These geochemical features indicate that subducted continental crustal components were incorporated into the mantle source of the basalts.In comparison with post-collisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent southeastern margin of the NCB,they show similar geochronological and geochemical features to the Xinbin basalts.Thus,the light Mo isotopes in the Early Cretaceous Xinbin basalts were mainly inherited from the subducted continental crust of the SCB.The Xinbin basalts have low Mo isotope ratios,enriched Sr-Nd-Hf isotope compositions,and high meltmobile element contents.These features indicate that the subducted SCB continental crustderived melts would have light Mo isotopic compositions,and they would have metasomatized the lithospheric mantle beneath the NCB to generate the mantle source of the basalts.Because the continental crust generally has Mo isotopes heavier than the depleted MORB mantle,it is proposed that the dehydration process during subduction decrease the Mo isotope ratios in the residual continental crust.Model calculation results suggest that partial melting of the dehydrated continental crust can produce low ?98Mo melts to metasomatize the overlying mantle wedge,then partial melting of such mantle sources can produce mafic magmas with low Mo isotope ratios and enriched radiogenic Sr-Nd-Hf isotopic compositions.Thus,the low Mo isotope ratios of the Early Cretaceous Xinbin basalts record the recycling of the subducted continental crust.The Mo isotopes in mafic rocks from continental collision zones can provide new insight into the geochemical transfer in the continental subduction zones.This dissertation provides a systematic comparison of Mo isotopes in Mesozoic and Cenozoic mafic igneous rocks with respect to subduction of the Paleo-Pacific slab in the eastern China.The results reveal that Mesozoic-Cenozoic mafic rocks show systematic differences in Mo isotopic compositions,Mo/Ce ratios,and melt/fluid mobile element contents.Among them,the?155 Ma Huaziyu lamprophyres are characterized by high ?98Mo values of-0.15 to 0.09‰ and low Mo/Ce ratios of 0.013-0.023,as well as arc-like trace element compositions,indicating that these mafic rocks were originated from mantle sources metasomatized by subducting Pacific slab-derived fluids produced at subarc depths.In contrast,the?60 Ma Fushun basalts and the?19 Ma Linqu basalts are characterized by low?98Mo values of-0.53 to-0.19‰ and high Mo/Ce ratios of 0.030-0.050,as well as OIB-like trace element compositions,indicating that these mafic rocks were originated from mantle sources metasomatized by subducting Pacific slab-derived melts at postarc depths.In combination with the systematic differences in trace element compositions,Nb/U and TiO2/Al2O3 ratios for the Mesozoic-Cenozoic basalts,it is proposed that accessory mineral rutile within the subducting slab would have played a key role in the Mo element and isotope fractionation between liquid and solid phases.Dehydration of the subducting oceanic crust at subarc depths would give rise to fluids that are enriched in heavy Mo isotopes and fluidmobile elements,whereas partial melting of the dehydrated oceanic crust with rutile breakdown at postarc depths would produce melts that are enriched in light Mo isotopes and melt-mobile elements.These metasomatic agents would react with the overlying mantle wedge peridotite at subarc and postarc depths,respectively,generating the mantle sources of Jurassic arc magmatism and Cenozoic OIB-like magmatism.In this regard,the Mo isotopic compositions of mafic rocks in oceanic subduction zones can efficiently record the geochemical transfer at different depths.