Geochronology,petrogenesis And Tectonic Significance Of The Laiyuan Magmatic Complex In The Central North China Craton

The igneous complex incorporating variable magmatic suites is the perfect object to investigate the magma differentiation,tectonic setting,crustal structure and lithospheric evolution.In the North China Craton(NCC),during the late Mesozoic,the extensive magmatism occurred and generated plenty of magmatic complexes with complicated compositional variations offering the opportunity to study the NCC destruction.The focus of this study is the craton destruction and lithospheric evolution in the central NCC compared with the eastern NCC by reconstructing the Laiyuan magmatic complex.Here,we present new petrological,mineral chemical,geochemical,geochronological and isotopic data to reveal the spatial,temporal,petrogenetic and tectonic linkages between different magmatic suites and to establish an integrated petrogenetic model for the Laiyuan complex,and furthermore to assess the craton destruction mechanism and lithospheric evolution in the central NCC.Various magmatic units with different compositions constitute the Laiyuan complex including the volcanic rocks(andesite-dacite),intermediate-felsic granitoids(syenogranite,monzogranite,quartz monzonite,and monzonite),mafic-ultramafic intrusions and some dyke suites(felsic dykes,dolerites and lamprophyres).The volcanic rocks,granitoids,felsic dykes,dolerites and lamprophyres were formed at 131-127 Ma,137-128 Ma,131-127 Ma,125-117 Ma and 115-110 Ma,with zircon?Ht(t)values ranging from-23.5 to-19.4,-21.8 to-16.8,-22.3 to-17.2,-23.3 to-14.2,and-17.2 to-3.7,respectively.The studied Early Cretaceous volcanic suite represented by rocks of andesitic-dacitic composition shows enrichment in light rare earth elements(LREE)and large ion lithophile elements(LILE),and depletion in high field strength elements(HFSE)with no obvious Eu anomalies.They also show typical adakitic features such as high Sr/Y and La/Yb ratios,as well as high Ba-Sr concentrations.The geochemical and isotopic data show that the enriched lithospheric mantle source experienced fluid-related subduction metasomatism.The distinct geochemical features of these identified in this study including high Ba-Sr concentrations and adakitic affinities were not only inherited from their magma source,but are also a result of fractional crystallization during magma evolution including amphibole-dominated fractional crystallization at depth and limited plagioclase removal.The granitoids display variable compositions and can be classified as two groups.The parental mafic magma for the Group I rocks(monzonite,quartz monzonite and MMEs)were derived from the enriched lithospheric mantle,and subsequently experienced the hornblende-dominated fractional crystallization to form the monzonitic/dioritic magmas,then the magma mixing and mingling with the crustal melts/magmas generated the hybrid magmas to form the MMEs and variable intermediate suites.Group II rocks are classified as high-K calc-alkaline I-type suite including monzogranites and syenogranites.These granitic rocks were most likely generated by partial melting of the thickened mafic lower crust at high pressure,with some addition of the mafic magma from an enriched mantle,and followed by intense plagioclase-dominated fractional crystallization to form the highly-fractionated syenogranites.The compositional heterogeneities of the igneous complex resulted from the intense crust-mantle interaction which involved multiple mantle and crustal sources,chaotic mixing and mingling and complicated fractional crystallization.The mafic dyke samples exhibit enrichment in LILE and no obvious Eu anomalies,and the dolerites show strong depletion in HFSE whereas the Th-U and Ta-Nb depletions in lamprophyres are not obvious similar to OIB-type.The new presented geochemical data suggest that the mafic dykes experienced limited crustal contamination and were dominated by olivine and clinopyroxene fractional crystallization.The mafic dykes were derived through partial melting of mantle previously enriched by subduction-related fluids,within amphibole-and garnet-stability field(80-100 km)with increasing input of asthenospheric material through time.The felsic dykes show adakitic feature sharing similar petrogenesis with the granitoids which were formed through crust-mantle interaction.Subsequently,the enriched mantle-derived magmas migrated through lithospheric faults and were emplaced as dolerite dykes at 125-117 Ma.The asthenosphere upwelling contributed to the thermo-mechanical erosion along weak zones,and the limited lithosphere-asthenosphere interaction generated the lamprophyres with transitional geochemical features during?115-110 Ma.The intense magmatism to form the Laiyuan complex was under the extension tectonic setting triggered by the subduction of Paleo-Pacific Plate.The subduction,retreat,roll back and stagnation of the Paleo-Pacific slab result in the heterogeneous lithosphere and various evolutional processes in the deep beneath the NCC.The slow and gradual thermal-mechanical erosion occurred at the central NCC whereas the rapid and intense lithospheric delamination occurred at the eastern NCC contributing to different lithospheric evolution.Both of the two mechanisms combined with the Paleo-Pacific slab played a significant role in the NCC destruction process to form variable magmatic rocks.An integrated formation model has been proposed to describe the formation mechanism of the Laiyuan magmatic complex.During Early Jurassic(-200-150 Ma),the subduction of the Paleo-Pacific Plate has reached the position beneath the central NCC.During this period,the central NCC was under compression which thickened the lithosphere.The Late Jurassic volcanic rocks(?146 Ma)were formed during this period whose petrogenesis could be attributed to the.partial melting of thickened lower crust and the deep hornblende-dominated fractional crystallization process.During 145-140 Ma,the fast slab rollback occurred leading to hot asthenosphere upwelling and extensional setting in the central NCC.Induced by the upwelling of hot asthenosphere,the extensive crust-mantle interaction occurred accounting for the petrogenesis for the formation of granitoids(137-126 Ma),MMEs(129-126 Ma),volcanic rocks(131-127 Ma),and felsic dykes(131-127 Ma).Through time,the lithosphere became substantially thin that lithospheric mantle-derived magmas could migrate through the lithospheric faults and intruded the plutons or country rocks leading to the formation of dolerite dykes at 125-117 Ma.The continuous lithospheric thinning eventually resulted in the upwelling asthenosphere reached the depth where it could melt(80 km).The partial melting of asthenospheric mantle and its interaction with the eroded lithospheric mantle material produced the lamprophyre from?115-110 Ma representing the end of Early Cretaceous magmatism in the central NCC.