A Geochemical Study Of Cretaceous Igneous Rocks From The Dabie-Sulu Orogenic Belt And The Southeastern Margin Of The North China Block

The Dabie-Sulu orogenic belt and the southeastern margin of the North China Block were modified by the Triassic subduction of the South China Block and the subduction of the Pacific plate,and thus are the ideal target regions to study reworking and recycling of subducted crustal materials as well as subsequent crust-mantle interaction.The widely exposed postcollisional igneous rocks in this region provide potential objects for such study.This dissertation has carried out systematical petrological and geochemical studies for Early Cretaceous andesitic and alkaline volcaincs from the Dabie orogen,the results have indentified the diagnositic geochronological and geochemical evidence for recycling and reworking of the subducted continental crust of the South China Block.Whole-rock and minreal geochemical study has been performed for Early Cretaceous and Late Cretaceous mafic igneous rocks from the Sulu orogen and the southeastern margin of the North China Block,the results indicate that they are the products of slab-mantle interaction in continental and oceanic subduction channels,respectively.A combined study of whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopesas well as zircon U-Pb ages and Hf-O isotopes,has been carried out for postcollisional intermediate volcanic rocks from the Dabie orogen,China,the results provide not only constraints on the origin of andesite in continental collision zone,but also insights into slab-mantle interaction in continental subduction channel.The volcanic rocks exhibit variable contents of SiO2(50.28-63.86 wt.%),MgO(1.18-4.65 wt.%),(Fe2O3)T(3.60-8.53 wt.%),Al2O3(12.92-18.95 wt.%),Na20(2.08-6.30 wt.%)and K20(0.73-5.25 wt.%).They are mainly trachyandesite,with small amounts of basaltic trachyandesite,andesite,dacite and trachyte.They exhibit arc-like trace element distribution patterns,with enrichment in large ion lithophile elements(LILE)and light rare earth elements(LREE)but depletion in high field strength elements(HFSE).The volcanic rocks have enriched Sr-Nd-Pb-Hf isotope compositions,with high initial 87Sr/86Sr ratios of 0.7075-0.7110,highly negative ?Nd(t)values of-23.1 to-15.0 and ?Hf(t)values of-29.8 to-18.3 and elevated 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios.Zircon Hf-O isotope analyses yield negative ?Hf(t)values of-31.0 to-17.8 and ?18O values of 4.4-6.8 for syn-magmatic domains.Zircon U-Pb dating yields consistent Early Cretaceous ages of 124 ± 3 to 130 ± 2 Ma for magma emplacement.Residual zircon cores yield Triassic,Neoproterozoic and Paleoproterozoic U-Pb ages,consistent with the ages of tectonothennal events involving ultrahigh-pressure metamorphism and protolith formation in the Dabie orogen.The zircon cores also yield a few low to negative ?18O values.An integrated interpretation of these geochemical characteristics is that the andesitic magmas were derived by partial melting of metasomatized zones in the orogenic lithospheric mantle.The metasomatites were generated by reaction of subcontinental lithospheric mantle wedge peridotite with felsic melts that originated from deeply subducted continental crust during continental collision in the Triassic.Melt-peridotite reaction in a subduction channel is therefore a key to the origin of the mantle sources of andesitic magmas in collisional orogens.Alkaline igneous rocks are a minor component in common associations of felsic to mafic rocks.Although they are usually regarded as the product of mantle-derived magmatism,they are possibly produced by partial melting of the overthickened orogenic crust.This is illustrated by post-collisional alkaline volcanics from the Dabie orogen in China.A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes,and zircon U-Pb ages and Hf-O isotopes for these volcanics provides geochemical constraints on the nature of their magma sources and thus insights into reworking of the lower continental crust in a collisional orogen.SIMS zircon U-Pb dating yields consistent Early Cretaceous ages of 136 ± 2 Ma and 133 ± 2 Ma for magma emplacement,contemporaneous with early-stage adakitic rocks in the Dabie orogen.Relict zircons record Neoproterozoic and Triassic U-Pb ages,respectively,in agreement with ages for protolith formation and ultrahigh-pressure metamorphism in the orogen.The alkaline volcanics are composed of phonolite and trachyte,with variable contents of SiO2(55.94-64.10 wt.%),MgO(0.16-0.95 wt.%),Al2O3(16.80-21.32 wt.%),K2O(4.65-14.55 wt.%)and Na20 + K20(11.51-15.01 wt.%).They show arc-like trace element distribution patterns,with enrichment in LILE and LREE but depletion in HFSE.They exhibit enriched whole-rock Sr-Nd-Hf isotope compositions,with generally high(87Sr/86Sr)i ratios of 0.7074 to 0.7104,but negative ?Nd(t)values of-16.3 to-15.1 and negative ?Hf(t)values of-18.4 to-16.3.Zircon Hf-O isotope analyses yield less variable ?Ht{t)values of-27.8 to-23.0 and ?18O values of 4.9 to 5.9‰ for magmatic zircons,but more variable ?Hf(t)values of-32.7 to-6.5 and ?18O values of 3.0 to 10.2‰ for Neoproterozoic and Triassic relict zircons.Such geochemical features indicate that these alkaline volcanics were derived from partial melting of the subducted continental crust of the South China Block in the Dabie orogen.The magma sources would be dominated by the mafic igneous rocks of middle Neoproterozoic age.Additional contributions were from supracrustal rocks with high ?18O and variable ?Hf(t)values,which were incorporated into the overthickened orogenic crust during the Triassic continental collision.The orogenic crust underwent low-degree partial melting during its thinning at the post-collisional stage,giving rise to the alkaline volcanics in the Early Cretaceous.As typical mantle-derived rocks,basalts record the geochemical and lithological nature of their mantle sources.Olivine is the firstly crystallized mineral from basaltic magmas,and can provid important constraints on primary magma compositions and source lithologies.A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes as well as olivine compositions and mineral O isotopes has been carried out for Cretaceous basalts in the Sulu orogen and the southeastern margin of the North China Block,the results provide constraints on the petrogenesis of continental basalts and the lithological nature of their mantle sources.They are mainly basanite and basalt,with minor trachybasalt.Early Cretaceous Fangcheng basalts have higher contents of SiO2(46.84-47.87 wt.%)and Mg#(66.3-72.2),lower contents of(Fe2O3)T(8.60-9.05 wt.%)and TiO2(1.07-1.13 wt.%);arc-like trace element distribution patterns;enriched whole-rock Sr-Nd-Hf isotope compositions,with high(87Sr/86Sr)i ratios of 0.7095-0.7098,but negative?Nd(t)values of-13.8 to-12.8 and negative ?Hf(t)values of-17.6 to-16.5;high olivine ?18O values of 6.0-6.5‰;most high Fo olivine phenocrysts exhibit Ni and Mn contents and Fe/Mn ratio similar to those of MORB olivine.Late Cretaceous alkaline basalts exhibit lower contents of SiO2(38.51-44.17 wt.%)and Mg#(50.4-71.1),but higher contents of(Fe2O3)T(10.62-15.12 wt.%)and TiO2(2.02-3.17 wt.%);OIB-like trace element distribution patterns;depleted whole-rock Sr-Nd-Hf isotope compositions,with low(87Sr/86Sr);ratios of 0.7036-0.7063.positive ?Nd(t)values of 1.0 to 5.9 and positivee ?Hf(t)values of 4.6 to 9.3;relative low olivine ?18O values of 4.4-5.4‰;most high Fo olivine phenocrysts exhibit higher Ni contents and Fe/Mn ratios,but lower Mn contents than those of MORB olivine.These geochemical features indicate that Fangcheng basalts were derived from partial melting of the enriched subcontinental lithospheric mantle(SCLM)of the North China Block,which was metasomatized by felsic melts derived from the subducted continental crust of the South China Block during the Triassic continental collision,the source lithology is mainly peridotite with minor pyroxenite.Whereas Late Cretaceous basalts from Junan,Qiangdao and Jiaozhou were derived from partial melting of an juvenile subcontinental lithospheric mantle,which would be generated by reaction of overlying asthenospheric mantle with felsic melts derived from the subducting Pacific oceanic crust,the source lithology is mainly pyroxenite with minor peridotite.Thus,these Cretaceous basalts in the Sulu orogen and the southeastern margin of the North China Block were formed by the mixing with different proportions of peridotite-pyroxenite derived melts.Slab-mantle interaction in subduction channels during continental and oceanic crust subduction is a key to the formation of mantle sources for continental basalts.