The Influence Of Subducted Pacific Plate On The Petrogenesis Of Cenozoic Volcanism In South China

We analyzed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the Cenozoic basalts in South China to investigate their mantle source characteristics,melting process and evolution during ascent.Our samples are from two areas,including those from SE China and those from Leizhou Peninsula.The basalts from SE China are spatially associated with three extensional fault systems parallel to the coast line.After correction for the effect of olivine microlites on bulk-rock compositions and the effect of crystal fractionation,we obtained primitive melt compositions for these samples.These primitive melts show increasing SiO2,Al2O3 but decreasing FeO,MgO,TiO2,P2O5,CaO and CaO/Al2O3 from the interior to the coast.Such spatial variations of major element abundances and ratios are consistent with a combined effect of fertile source compositional variation and increasing extent and decreasing pressure of decompression melting from beneath the thick lithosphere in the interior to beneath the thin lithosphere in the coast.These basalts are characterized by incompatible element enrichment but varying extent of isotopic depletion.This element-isotope decoupling is most consistent with recent mantle source enrichment by means of low-degree melt metasomatism that elevated incompatible element abundances without yet having adequate time for isotopic ingrowth in the mantle source regions.Furthermore,Sr and Nd isotope ratios show significant correlations with Nb/Th,Nb/La,Sr/Sr* and Eu/Eu*,which substantiates the presence of recycled upper continental crustal material(UCC)in the mantle sources of these basalts.Pb isotope ratios also exhibit spatial variation,increasing from the interior to the coastal area.The significant correlations of major element abundances with Pb isotope ratios indicate that the Pb isotope variations also result from varied extent and pressure of decompression melting.We conclude that the elevated Pb isotope ratios from the interior to coast are consistent with increasing extent of decompression melting of the incompatible element depleted mantle matrix,which hosts enriched Pb isotope compositions.Studies have shown that mantle xenolith-bearing magmas must ascend rapidly to carry mantle xenoliths to the surface.It has thus been inferred inadvertently that such rapid ascending melt must have undergone little crystallization or evolution.However,this inference is apparently inconsistent with the observation that the Cenozoic mantle xenolith-bearing alkali basalts from several locations in SE China have experienced varying degrees of fractional crystallization(Mg# = ~ 48-67).The variably evolved composition of host alkali basalts is not in contradiction with rapid ascent,but rather reflects inevitability of crystallization during ascent.Calculations for equilibrium temperatures and pressures of clinopyroxene(Cpx)megacrysts show an ascent path of the host melt with dT/dP ? 7°C/kbar.This path is steeper than the expected adiabat of molten mantle(4-6°C/kbar),which is consistent with the effect of conductive cooling and melt crystallization during ascent because TMelt > TLithosphere.The equilibrium pressure of 18-27 kbar for Cpx megacrysts suggests that the crystallization takes place under lithospheric mantle conditions with limited low-pressure residence in the shallower crust levels.This is in fact consistent with the rapid ascend of the host melt to bring mantle xenoliths to the surface.The late Cenozoic volcanic rocks from Leizhou Peninsula have geochemical characteristics generally similar to the Cenozoic basalts in SE China,i.e.,they are all enriched in incompatible elements but variably depleted in Sr-Nd-Pb-Hf isotopes.However,the volcanic rocks in Leizhou Peninsula show variable trace element patterns.The samples from Huoju have prominent negative Nb-Ta anomalies,positive Pb anomaly and more enriched Sr-Nd-Pb-Hf isotope compositions.In contrast,the samples from Jiujiang and Tianyang show positive Nb-Ta anomaly,moderate positive Pb anomaly,prominent positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions.The geochemistry characteristrics of these rocks are generally consistent with mixing of two components with different elemental and isotopic characteristics in the mantle source region.The volcanic rocks from Jiujiang and Tianyang largely originated from a mantle source region that experienced a low-degree melt metasomatism.Such melt is enriched in incompatible elements and volatile components,which can re-firtilize the asthenospheric mantle and the overlying lithospheric mantle,cystallize hydrous minerals(e.g.amphibole)and form metasomatic veinlets in the ambient lithospheric mantle.The volcanic rocks from Huoju are derived from a mantle source region that experienced metasomtism of recycled UCC materials.Such UCC material modified mantle source can explain the negative Nb-Ta anomaly,positive Pb anomaly and enriched Sr-Nd-Pb-Hf isotope compositions of Huoju samples.