A Geochemical Study Of Cenozoic Continental Basalts From The Subei-Hefei Areas In East-central China

Cenozoic continental basalts widely occur in eastern China, and a great number of geological and geochemical studies were made for them in the past three decades. In view of radiogenic isotope compositions, these studies have identified the existence of distinct mantle components (including both enriched and depleted mantle). However, mantle processes responsible for the Cenozoic basalts are still issues for debate. The dominant viewpoints on their petrogenesis are: derivation from convective asthenosphere or from lithosphere-asthenosphere interaction, inetraction between mantle and eclogite-derived melt/residue formed by delamination/subducted mafic lower continental crust, or reaction between the subducted oceanic crust-derived melt and the mantle wedge peridotite.In order to constrain the petrogenesis of Cenozoic continental basalts in eastern China, this PhD thesis has made a combined study of whole-rock Ar-Ar dating, whole-rock major and trace elements, radiogenic isotopes Sr-Nd-Hf-Pb, whole-rock and mineral O isotopes, micro-amount carbonate C-O isotopes as well as phenocryst olivine elements for the Cenozoic basalts from eastern Anhui (Hefei) and northern Jiangsu (Subei) in east-central China. In interpreting the geochemical data, the constraints from major and trace elements are integrated with the constraints from stable and radiogenic isotopes. In this regard, the results suggest that these basalts were derived from pyroxenite of juvenile subcontinental lithospheric mantle (SCLM) that was generated by the interaction between mantle peridotite and adakitic melts derived from the dehydrated oceanic crust.Whole-rock Ar-Ar dating for basalts from Xiaoshushan and Jimingshan in the Hefei basin yields Eocene ages of 37.8±0.4 to 38.3±0.3 Ma. The Subei-Hefei basalts have low SiO₂ contents of 41.61 to 48.57 wt.% and Mg# of 52.9 to 68.7 wt.%, high Fe/Mn ratios of 60.9 to 74.2 and alkali contents of 4.05 to 10.10 wt.%. They have relatively narrow range of Al₂O₃ (11.86 to 15.55 wt.%), Fe₂O₃t (11.78 to 15.45 wt.%), and CaO (7.56 to 10.63 wt.%) contents. All these samples are classified into alkali basalts with the occurrence of CIPW normative mineral nepheline (Ne). The basalts are characterized by OIB-like patterns of trace element distribution such as enrichment in light rare earth elements (LREE), depletion in heavy rare earth elements (HREE), no depletion in Nb and Ta, but with negative Rb and Pb anomalies on primitive mantle-normalized spidergrams. The contents of incompatible trace elements and the differentiation extents between LREE and HREE are positively correlated with the alkali contents.The Subei-Hefei basalts show considerable variations in initial 143Nd/144Nd, Sr⁸⁷/Sr⁸⁶ and 176Hf/177Hf ratios, being generally depleted in these radiogenic isotopic compositions. They have initial Sr⁸⁷/Sr⁸⁶ ratios of 0.7033 to 0.7042,ε_Nd(t) values of 1.8 to 7.3 with single-stage Nd model ages of 237 to 874 Ma, andε_Hf(t) values of 6.7 to 13.1 with single-stage Hf model ages of 177 to 545 Ma. They have Pb²⁰⁶/Pb²⁰⁴ ratios of 17.414 to 18.815, Pb²⁰⁷/Pb²⁰⁴ ratios of 15.355 to 15.473, and Pb²⁰⁸/Pb²⁰⁴ ratios of 37.365 to 38.041.Olivine phenocrysts haveδO¹⁸ values of 4.9 to 5.9‰, basically similar to normal mantle values of 5.2±0.2‰. TheδO¹⁸ values of clinopyroxene phenocrysts are 4.1 to 6.2‰, most of which are lower than normal mantle values of 5.6±0.2‰. Most plagioclase phenocrysts haveδO¹⁸ values of 4.4 to 7.1‰, and several samples have very highδO¹⁸ values of 9.6 to 15.9‰. Most of the basalts have low carbonate contents (<0.3 wt.%), and some samples have carbonate contents higher than 0.5 wt.%. The minor-amount carbonates haveδC¹³ values of -15.6 to -4.7‰andδO¹⁸ values of 14.2 to 21.3‰. The presence of lowδC¹³ and highδO¹⁸ values for the carbonates as well as the highδO¹⁸ values for some plagioclase phenocrysts suggest the effect of CO2 degassing during magma eruption, which would decrease the minor carbonate contents andδC¹³ values, and the low-T hydrothermal alteration, which would increase theδO¹⁸ values of some plagioclase phenocrysts and carbonate. However, these basalts haveδO¹⁸ values of 4.1 to 6.0‰for whole-rock, most of them are close to or slightly lower than the normal mantle values of 5.7±0.5‰. This suggests that theδO¹⁸ values of these basalts have not been significanty affected by the low-T hydrothermal alteration.The Subei-Hefei basalts have OIB-like patterns of trace element distribution. They have Nb/U ratios of 18.9 to 50.1 andΔNb values of 0.11 to 0.41, which are significantly different from those of MORB. This suggests that these basalts were unlikely derived from the asthenospheric mantle, the source of MORB. The high Fe/Mn ratios of 60.9 to 74.2, relatively low Mg# of 52.9 to 68.7 and high Ni contents for olivine phenocrysts suggest the existence of pyroxenite in their mantle source. They have relatively depleted but variable radiogenic isotope compositions, suggesting their derivation from a mixed mantle source between depleted and enriched components. The juvenile SCLM is capable of generating basaltic melts with radiogenic isotopic compositions similar to those derived from the asthenospheric mantle. The low-δO¹⁸ phenocrysts would have crystallized from lowδO¹⁸ melts and thus the mantle source appears to have been depleted in O¹⁸ before partial melting. The O¹⁸ depletion of mantle source would be caused by reaction of mantle peridotites with lowδO¹⁸ melts that would be derived from partial melting of the dehydrated oceanic crust that experienced high-T seawater-rock interaction during the magma emplacement. Finally, this study proposes a model for petrogenesis of the Subei-Hefei basalts. It assumes low-angle subduction of the Pacific Plate beneath the Eurasian continent during the Early Mesozoic, leading to dehydration of the high-T altered oceanic crust below the ancient SCLM in east-central China. The low-angle subduction would delaminate the lower part of the ancient SCLM into the asthenosphere, and transformed the mantle wedge overlying the subducting oceanic crust to the juvenile SCLM. Dehydrated oceanic basalt would initially become eclogite during subduction and then undergo partial melting to generate the adakitic melt. Subsequently, the adakitic melts would ascend rapidly and react with the overlying juvenile SCLM peridotite to form silica-deficient pyroxenites. Thus the juvenile SCLM source is composed of pyroxenite-peridotite mixtures, whose partial melting at the lithosphere-asthenosphere thermal boundary due to continental rifting in the Cenozoic gave rise to the alkalic basaltic magmas. Therefore, the interaction between the oceanic crustal-derived melt and the mantle-wedge peridotite is suggested as a key to formation of the mantle source, and the composition of continental basalts provides a snapshot of SCLM with respect to the regime of plate tectonics.