A Geochemical Study Of Cenozoic Basalts From The Southeastern Part Of The North China Craton

It is well known that the lithosphere of the North China Craton (NCC) has been widely thinned since the Mesozoic. In response to this tectonism, intensive magmatism occurred during the Mesozoic and Cenozoic in the eastern NCC, among which the Cenozoic igneous rocks are mainly basalts. These continental basalts are generally characterized by OIB-like trace element distribution patterns and depleted Sr-Nd isotope compositions. There are different hypotheses with respect to the nature of their mantle source:(1) upwelled asthenospheric mantle;(2) metasomatism of the asthenospheric mantle by carbonatic fluid;(3) mechanical mixture of the asthenospheric mantle with continental eclogitic lower crust;(4) juvenile lithospheric mantle formed by reaction of mantle-wedge peridotite with felsic melts derived from subducting oceanic slab. Most of these hypotheses are primarily based on either radiogenic isotopes or trace elements without integration of all geocehmical data including major elements and stable isotopes. For example, partial melting of the normal asthenospheric mantle can generate basalts with depleted Sr-Nd isotope compositions, which such basalts also have MORB-like trace element distribution patterns. However, this is not the case for the Cenozoic basalts in eastern China. Therefore, this PhD dissertation presents a combined study of whole-rock major-trace elements and Sr-Nd-Pb isotopes, phenocryst mineral rare gas isotopes as well as water contents and H-O isotopes in the Cenozoic basalts from the southeastern NCC. The results are used to constrain the nature of their mantle source and to explore the process and mechanism for the thinning of the NCC lithospheric mantle.The Cenozoic basalts from the Changle-Linqu area in the southeastern NCC can be divided into two subtypes of basanite and alkali basalt according to their major element compositions. They exhibit high whole-rock Fe/Mn ratios but low Fe/Zn ratios, and their olivine phenocrysts exhibit different chemical compositions from olivines crystallized from peridotite-derived melts, indicating involvement of pyroxenite in mantle melting. The alkali basalts with relatively enriched Sr-Nd isotope compositions do not have MgO contents higher than the basanites with relatively depleted Sr-Nd isotope compositions. Thus, the relatively enriched components are unlikely derived from ancient cratonic lithospheric mantle but derived from crustal materials that metasomatized the depleted mantle. Considering the OIB-like trace element distribution patterns for these basalts, the enriched components were introduced into their mantle source by subducting oceanic crust and its overlying sediments rather than the continental crust with arc-like trace element distribution patterns. The youngest Nd model age of basanites with relatively depleted Sr-Nd isotope compositions is334Ma, suggesting involvement of a juvenile lithospheric mantle in their source. Most of the phenocryst minerals have δ18O values than those of the normal mantle, indicating18O-depletion for their mantle source. This can be ascribed to crustal metasomatism by melts derived from low δ18O oceanic crust. The preservation of low δ18O mantle source appeals that the juvenile lithospheric mantle source was separated from the convective asthenospheric mantle after its formation. The Sr-Nd-Pb isotope compositions of the two subtypes of basalts are different from each other but correlated with major element compositions, indicating that their mantle source is heterogeneous. The geochemical compositions of these basalts suggest that their mantle source would be generated by reaction of depleted mantle peridotite with adakitic and felsic melts derived from subducting oceanic crust and overlying sediments, respectively.The olivine and pyroxene phenocrysts in Cenozoic basalts from the Changle-Linqu, Nushan and Hefei areas exhibit large variations in noble gas isotope compositions. The Changle-Linqu and Nushan olivines exhibit similar3He/4He ratios to MORB, indicating their depleted mantle origin. There is a ositive correlation between He/4He and4He/Ar*ratios, suggesting diffusion-controlled degassing during magma ascent. In contrast, the Hefei basalt exhibits very low olivine and pyroxene He/4He ratios, suggesting the predominance of crustal He in its mantle source. The phenocryst Ne and Ar isotope compositions are significantly different from MORB but similar to the atmosphere, suggesting their derivation from subducting slab. It is the subduction of alternated oceanic crust and overlying sediments that transferred such crustal He and atmospheric Ne-Ar isotope signatures to the mantle source of these intraplate basalts. This involves a series of isotope exchange and transfer processes from surface to mantle depth, which can be outlined as dissolution-exchange-equilibration-digestion (DEED).Based on the structural water contents of Cpx in the Changle-Linqu basalts, the calculated water content for their mantle source is about450ppm. It is higher than that of MORB-type mantle but similar to OIB-type mantle, suggesting relative enrichment of water in their mantle source. The phenocryst minerals of the Changle-Linqu and Hefei basalts have5D values of-82%o to-145%o, which are significantly lower than those of fresh MORB. These low8D values might result from the involvement of dehydrated subducting oceanic components in their mantle source, because dehydrated oceanic slab would have δD values significantly lower than those of fresh MORB. Moreover, the involvement of subducting slab can also make their mantle source enrich water. Therefore, slab dehydration and slab-mantle interaction in oceanic subduction channel are the key factors that lead to the enrichment of water and low8D values for the mantle source of these Cenozoic basalts.The studies of seismic tomography have revealed a gradual decrease in the NCC lithosphere thickness from west to east, suggesting the effect of western subduction of the Pacific plate on the thinning of the NCC lithosphere. It is the subducting Pacific plate that provided the enriched components for the mantel source of Cenozoic basalts in eastern China. By integrating all available geochemical observations, a hypothesis is presented for petrogenesis of the basalts. During the Mesozoic, the overlying depleted mantle wedge was metasomatized by melts derived from subducting oceanic crust and sediments, generating the juvenile lithospheric mantle with the presence of both silica-excess and silica-deficient pyroxenites. Partial melting of the juvenile lithospheric mantle brought about the continental basalts due to extension in the Cenozoic. The western subduction of the Pacific slab led to delamination of the lower part of the NCC lithospheric mantle, and the lateral flow of the asthenospheric mantle filled the gap left by the delaminated cratonic mantle. Melts derived from the subducting oceanic crust and overlying sediments metasomatized the juvenile lithospheric mantle, generating fertile and enriched mantle sources for the continental basalts. The mantle sources were separated from the convective asthenospheric mantle in order to preserve the18O-depleted crustal sugnature. In this regard, the Cenozoic continental basalts in eastern China are a snapshot of the chemical geodynamics for the thinning of the NCC mantle triggered by the subduction of the Pacific plate during the Mesozoic.