Petrogenesis Of Ocean Crust And Its Significance In Continental Crust Growth

As important part of oceanic crust and continental crust respectively,Oceanic basalts and continental granitoids are windows by which we can study the petrogenesis and evolution of the Earth Crust.Detailed Studies on the oceanic basalts and continental granitoids can improve our understanding on some significant geodynamic problems like mantle heterogeneity and continental crust growth.In this work,we first study the Hf isotopes of seamount basalts near the East Pacific Rise(EPR)between 5° and 15°N and present a combined studies of detailed zircon U-Pb geochronology,whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional granitoids in the West Kunlun Orogen,and also discuss the relationship between the ocean crust and the continental crust growth.We obtain the following new conclusions by integrated detailed studies:1.New Hf isotopic data on basaltic glasses from seamounts flanking the EPR between 5° and 15°N demonstrate that the Hf and Nd isotopes of these samples are correlated well and form a trend parallel to the mantle array approximately.This correlated Hf-Nd isotopic variations together with the Sr-Nd-Pb-Hf isotopic data correlate with each other as well as with abundances of major elements and ratios of incompatible elements suggests that the geochemistry of the near-EPR seamount lavas is most consistent with melting-induced mixing of a two-component mantle(i.e.,an easily-melted enriched component dispersed in the more depleted refractory matrix).2.We propose that the recycled ancient metasomatized basal portions of ocean lithospheric mantle(ROLM)are the best candidate for the enriched component.The melt of low-degree melting that enriched in volatiles and incompatible elements is collecting atop the seismic low velocity zone(LVZ),percolating through and metasomatizing the growing lithosphere by mineral vein via basal accretion;Such metasomatized deep portions of the oceanic lithosphere melt by compression during subduction with the enriched melt retained in the upper mantle polluting the depleted mantle through mantle circulation supplying MORB mantle(i.e.,ROLM model);The melting-induced mixing between depleted MORB mantle and the enriched ROLM component can generate the off axis seamount basalts and the ridge axis MORB;The mixing curves of the two-component mantle mixing model can be consistent with the EPR seamount lavas and MORB.Moreover,it's implicit that the ROLM components can go into the deep mantle and contribution to be an OIB source.The ROLM model can also explain the Hf-Nd isotopic variations in different ridges on a global scale.So the ROLM model must have extensive geodynamic significance.3.The mafic magmatic enclaves(MMEs)and granitic host rocks of the Arkarz Shan(AKAZ)pluton have coincident zircon U-Pb age(225 Ma),same mineralogy,approximately similar REE patterns and trace element characteristics,and indistinguishable Sr-Nd-Hf isotope compositions,all of which indicate that the MMEs represent disturbed cumulate of common source with the host felsic magmas;The petrogenesis of MMEs from the Taer pluton is consistent with those from the AKAZ pluton.The parental magma of Taer pluton was affected by the AFC(assimilation-fractional crystallization)process during ascending intrusion and the MMEs were less affected as early accumulation.These two plutons are best explained by melting of amphibolite of MORB protolith(the Paleo-Tethys ocean crust)during continental collision,which produces andesitic melts with a remarkable compositional similarity to the bulk continental crust(BCC)and the inherited mantle like isotopic compositions,and the involvement of terrigenous sediments is necessary to explain the crustal signature.Simple Sr-Nd-Hf isotopic mixing calculations suggest that even more than 80% and 85% Paleo-Tethys ocean crust materials contribute to the source of the AKAZ and Taer plutons respectively.The AFC model indicates that the Taer pluton generated by the fractional crystallization of hornblende,plagioclase and biotite dominated coupled with 40% crustal contamination.The AKAZ and Taer plutons can represent the net continental crustal growth because of the significant contribution from mantle materials and their BCC-like trace element characteristics.We also preliminarily discussed the petrogenesis of the early Paleozoic West Datong pluton.Together with the findings of the research in the Southern Tibet,East Kunlun,North Qilian and West Qinling Orogen,we find that continental crust growth is do exist in the continental collision zones and the hypothesis “continental collision zones as primary sites for net continental crust growth” have a universal significance.4.This study prove that the continental collision zones really is primary sites for net continental crust growth and voluminous granitoid intrusions in the West Kunlun Orogen originated from the melting of upper ocean crust(oceanic basalt and MORB in particular)during continental collision,and moreover,the oceanic basalt is the important part of ocean crust.So there is closely relationship between the ocean crust and the continental crust growth.There is important geodynamic significance that ocean crust could contribute materials to the net continental crust growth directly.