Zircon U-Pb Ages, Geochemistry Of The Rushan Gabbro-diorites And Their Mafic Xenoliths From The Northern Sulu Orogen, Shandong Province, Eastern China

The nature and evolution of the North China lithosphere have attracted many attentions in the past few years,because of considerable losing of the Archean keels during the Phanerozoic. Recent published data indicated that the Dabie-Sulu orogenic belt has become one of the most ideal regions for the studies on deep subduction of continental lithosphere,and its affections to the destruction of North China Craton.Mesozoic igneous rocks from the Dabie-Sulu orogenic belt are considered to be the products of subduction-induced and post-collisional extension-related magmatism and thus can provide constraints on the processes involved in the lithosphere evolution.In this paper,we present geochronological and geochemical investigations for the Gonajia and Shangkou intermediate-mafic intrusive complex,and their mafic xenoliths, as well as the coeval Sanfoshan granites from the northern Sulu orogen,with the aim of constraining the petrogenesis and origin of these rocks and mantle processes involved in their generation,and will further discuss the crust-mantle interaction that occurred underneath the collision zone and its implication for the lithosphere evolution.In addition,the evolution crust-mantle interaction occurred underneath the southern wedge of the North China lithosphere have also been discussed.1.The Gongjia and Shangkou complexes are two independent small intermediate-mafic plutons that located at the southeastern and northeastern part of the Rushan city,respectively. The Gonajia complex comprises of four phases including(1)medium- to fine-grained biotite gabbro;(2)medium-grained biotite gabbro;(3)medium- to coarse-grained biotite gabbro;(4) fine-grained pyroxene-beafing diorite.The pyroxene-bearing diorite predominates in the outermost part of the Gongjia complex.The Shangkou pluton has relative poor occurrence and only the intermediate phase been found.Mafic xenoliths captured in the Gongjia complex include olivine-pyroxenite,plagioclase-bearing clinopyroxenite,clinopyroxenite and biotite-bearing gabbro,all of which mainly distributed in the third phase of the host rocks.2.The Gongjia complex has rather uniform zircon U-Pb ages of 114±1Ma-111±1Ma, whereas the Shangkou pyroxene-bearing diorite has an emplacement age of 110.5±1Ma, indicating that they were both intruded in the late of the Cretaceous.Zircons extracted from an olivine-pyroxenite xenoliths give a weighted mean ²⁰⁶pb/²³⁸U age of 111.5±1 Ma.3.The Gongjia and Shangkou complexes have high Al₂O₃ and low MgO,and show high-K calc-alkaline to shoshonitic affinities.They are highly enriched in large ion lithophile elements (LILE)and light rare earth elements(LREE),depleted in high field strength elements(HFSE) and Pb,and exhibit EMII-like Sr and Nd isotopes.These complex are interpreted to originate from decompression melting of spinel-facies cratonic mantle that metasomatized by the subducted Yangtze crustal materials and the melts experienced fractionation of olivine, clinopyroxene,apatite and Fe-Ti oxides.Such mantle is not only different from the Palaeozoic refractory lithosphere,but also contrasts with the fertile one in Cenozoic.These complexes are similar in composition to other Early Cretaceous mafic-intermediate complexes from the southern margin of the North China Craton,implying the mantle source of the Rushan intrusion is tectonically affiliated to the southern wedge of the North China Craton.We infer that the Gongjia and Shangkou complexes formed in an extensional regime corresponding to the asthenosphere upwelling during gradual erosion and replacement of the cratonic mantle by the fertile lithosphere beneath the eastern North China Craton.4.The petrographic and rock assemblages of the mafic xenoliths are not only different from that of the Archean or Proterozoic ultramafic bodies from the adjacent regions,but also contrast with the deep-seated xenoliths entrained in the Mesozoic diorites from the eastern North China Craton.On the contrary,the magmatic textures and similar elemental and isotopeic compositions to the host rocks demonstrate that the xenoliths were most probably represented high pressure fractionational products of the host magmas.The different major and trace element compositions of the four kinds of xenoliths indicate that they might form during the different evolutional periods of the magma intrusion.5.For the Sanfoshan granites,we mainly focused on the Wangjiakou and Panjia monzonite granites,and Tongling alkali-granite.They were both emplaced in late Cretaceous(～110Ma),and display high-K calc-alkaline affinities,have high SiO₂,low FeO_t and MgO,with Na₂O/K₂O ratios lower than 1.They are enriched in LILE(such as Sr and Ba)and LREE,and with Sr and Nd isotopic compositions very closed to the nearest mantle derived igneous rocks such as the Gonajia complex and lamprophyres.These Cretaceous granitic rocks were formed by crustal-derived acid magma mixed with mantle originated basic magma,and the mixed magmas experienced minerals fractionation during their followed evolution.The concave-up chondrite-normalized REE patterns suggest that the precursor magma from a garnet-free source and the depletion of the MREE may be an indication of amphibole as residual in the melting source of the acid magma.These imply that the crust was thinned to a normal thickness and the lithospheric mantle thinning peaked around 110Ma underneath the studied regions.6.The lithosphere evolution beneath the southern wedge of the eastern North China Craton during the Mesozoic to Cenozoic may have achieved in the following processes:The Early Mesozoic deep subduction of the Yangtze block at the southern part of the North China Craton destroyed the physical integrity of the North China lithosphere,and then triggered the thin of the North China lithosphere through thermo-mechanical erosion and chemical erosion caused by the asthenosphere upwelling.Those fluids and melts released from the subducted Yangtze slab would progressively migrate up,perturbate and metasomatize the overlying lithospheric mantle wedge and finally account for the formation of the highly enriched Mesozoic lithospheric mantle beneath the North China Craton.During the middle-late Jurassic,the pre-metasomatized enriched lithospheric mantle would begin to melt due to the upwelling of the hot asthenosphere,these mantle-derived magmas would underplate at the base of the lower crust or crust-mantle transition zone,causing the partial melting of the subducted Yangtze crust and giving the formation of the granitic rocks.The left-laeral strike-slip fault of the Tanlu fault in the Early Cretaceous triggered the strong upwelling of the asthenosphere,and simultaneously caused the intensive extension of the lithospheric mantle and crust,accompanied by splited apart of the lithosphere;the majority of the enriched lithospheric mantle was then considerably thinned by partial melting,these mantle-derived magmas may mixed with the crustal acid magmas and formed granitic rocks and certain environments.From late Cretaceous to Cenozoic,the oblique subduction of the pacific plate caused the oceanic crustal materials adequately mingled with the asthenosphere,and then the progresive asthenospheric cooling lowered the boundary between the lithosphere and the asthenosphere and created new oceanic-type lithospheric mantle.