Early Cretaceous Magmatism Of Gan-Hang Belt And The Subduction Of Paleo-pacific Plate

Increasing studies have revealed that mantle-derived mafic rocks can be more isotopically enriched than co-existing crustal felsic rocks,exhibiting “reversed isotope”,especially within some accretionary orogens(the Central Asian Orogenic Belt,Gangdese Belt,Jiangnan Orogenic Belt and Appalachian orogenic Belt).However,its origin remains to be elucidated.Here,we report on some Early Cretaceous(ca.130 Ma)mafic rocks in Gan-Hang Belt(South China)that are more isotopically enriched than the temporally and spatially associated granitoids.New and published whole-rock Sr-Nd-Hf-Pb-O isotopic data [87Sr/86Sr)i = 0.70788-0.70833;?Nd(t)=-7.6 ~-3.5;?Hf(t)=-6.8 ~-1.9;(206Pb/204Pb)i = 18.028-18.108,(207Pb/204Pb)i = 15.565-15.577,(208Pb/204Pb)i =38.198-38.341;δ18OSMOW = 4.5-5.5 ‰] highlight that the isotopically enriched Siling hornblende gabbros were predominantly sourced from an older metasomatically enriched SCLM of immediately adjacent Yangtze blockic affinity.Yet relatively depleted Nd-Hf isotopic signatures[εNd(t)=-0.4 to +0.6,εHf(t)= 3.4 to 4.8,εHf(t)zircon =-2.3 to +9.3],coupled with low zircon δ18O values(5.33-6.84 ‰)and geographical proximity to the exposed island arc rocks,indicate that the isotopically depleted Daixi alkali feldspar granites were dominantly derived from the relatively young Neoproterozoic accreted Shuangxiwu island arc sequences.Compared with the Daixi granite,the other coeval granitoids(e.g.,Lingshan and Siling)have lower whole-rock ?Nd(t),εHf(t)values and zircon εHf(t)value,but elevated zircon δ18O value,Dy/Yb and Sm/Yb ratios,possibly implying ca.5-20 wt.% input of ancient Yangtze basement materials.The arc crustal allochthon is assumed to have been tectonically emplaced onto the southeast Yangtze foreland,resulting in isotope inversion with younger crust on top of the older lithospheric mantle.Our results reveal a juxtaposition geometry of continental material after collision and accretion of island arcs by using isotope methods,which is independently supported by tectonic geological and geophysical studies.Magma mixing commonly takes place between isotopically depleted mafic and enriched felsic magmas.Here we present isotopic evidence exhibiting the opposite behavior in the Early Cretaceous Siling complex(South China),which comprises gabbro,quartz diorite,granodiorite,and alkali feldspar granite with locally many mafic microgranular enclaves.Field observations and zircon U-Pbdating indicate that all of the rock units crystallized contemporaneously at ca 127-129 Ma.Mineralogical and petrochemical analyses indicate that the Siling quartz diorite and granodiorite crystallized from hybrid magmas of temporally and spatially coexisting gabbroic and granitic melts.“Reversed isotope” feature of the Siling hornblende gabbro and alkali feldspar granite means that the quartz diorite and granodiorite recorded “reversed isotope” magma mixing between isotopically enriched mafic and relatively depleted felsic magmas.The results indicate that the injection of mantle-derived mafic magma does not necessarily imprint relatively depleted isotopic signatures on the host felsic melt.Oceanic crust is progressively recycled back into the mantle through subduction and can subsequently contribute to the mantle source of many OIB-type rocks.The paleo-Pacific plate has been subducted into the mantle beneath the eastern China continent during the Mesozoic era,but less is known when such recycled component had initially ‘returned’ through magmatism.Here we report an early Cretaceous(~123 Ma)mafic dyke in the eastern China,the Daixi diabase,which is characterized by OIB-type elemental affinities and depleted Nd-Hf isotope signatures [?Nd(t)= 5.4-6.9;?Hf(t)= 9.6-12.1] that require an asthenospheric source.Their exceptionally high Nb/Nb*,Nb/Th,and Ti/Ti* ratios are well correlated with the elevated Eu/Eu*,Sr/Sr*,and Ba/Th ratios,indicating that both recycled upper basaltic and lower gabbroic oceanic crusts were involved as source components of the Daixi diabase.Additionally,the systematic increase in Hf/Sm and Th/LREE(e.g.,Th/La-Nd)ratios with decreasing Ce/Pb and ?Hf(t)values of the diabase specifically requires recycled terrigenous sediments as another source component.The recycled oceanic crustal materials may have been converted into pyroxenite component resulted in the lithologically heterogeneous mantle source for the Daixi diabase.This is supported by the elevated,highly variableδ56FeIRMM-014 values(0.21 to 0.35 ‰)of the diabase and their good correlations with Fe/Mn,FC3MS(Fe O/Ca O-3*Mg O/Si O2),Nb/Nb*,Sr/Sr*,?Hf(t),and(206Pb/204Pb)i values,typical of pyroxenitederived melts.The most striking feature of the Daixi diabase is their extraordinarily high time-integrated U/Pb(μ = 25.0-33.4)and Th/Pb(ω = 85.6-118)ratios but absence of characteristically high radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 17.863-18.084,(207Pb/204Pb)i = 15.499-15.517,(208Pb/204Pb)i= 37.605-37.904] as expected.This discrepancy between parent-daughter isotope ratios and decay products obviously requires the relatively short storage of the recycled oceanic crust in the mantle source.Three stage Pb isotopic growth modeling indicates that average Pb isotopic ratios of the Daixi diabase can be reproduced using the recycling age of ~180-250 Ma.Thus,we propose that the young recycled oceanic crustal components were most likely derived from the extinct paleoPacific oceanic crust that had subducted beneath the eastern China continent during the Mesozoic.Our study thus provides first compelling evidence that the ‘return’ of subducted paleo-Pacific oceanic slab,as ingredients of OIB-type magmatism in eastern China,can be traced back to the early Cretaceous(~123 Ma),significantly earlier than previously suggested(~110 Ma).