Geochemical Study Of The Neoproterozoic Igneous Rocks From The Northwestern Margin Of The South China Block

The Neoproterozoic Era is a crutial period for the evolution of the South China Block and the supercontinent Rodinia.Hot issues include,1)when and how the South China amalgamated into Rodinia;2)the accretionary history and tectonic evolution of South China during the early Neoproterozoic;3)when and how the South China Block separated from the supercontinent;4)the relationship between the rift-magmatism and the Neoproterozoic glaciation and paleoclimite.In this doctoral dissertation,Neoproterozoic magmatic rocks from the northwestern margin of the South China Block are selected for systematic petrogeochemical study.The results provide better understanding of the early Neoproterozoic accretion process,and gain insights into the Neoproterozoic rift magmatism.The relationship between the evolution of the South China Block and the configuration of Rodinia supercontinent from early to meso-Neoproterozoic was established.In-situ zircon U-Pb dating yields 206Pb/238U ages of 905±8 Ma for plagiogranite,936±6 Ma for andesite and 950:L±5 Ma for gabbro-diorite rom the Mianlue ophiolite in the northern margin of the South China Block.The plagiogranite is characterized by high Na2O contents of 6.6-7.0 wt%,low TiO2 contents of 0.2-0.4 wt%,and enrichment in LREE and LILE but depletion in Nb,Ta and Ti.The zircons have positive ?Hf(t)values of 8.9 to 14.9 and slightly elevated 8180 values of 4.6 to 6.8‰.The whole-rock yields low(87Sr/86Sr)i ratios of 0.7016-0.7031,positive ?Nd(t)values of 5.9 to 6.9 and positive ?Hf(t)values of 11.1 to 11.2.This indicates that the plagiogranite was produced by partial melting of juvenile oceanic crust that experienced seawater-hydrothermal alteration.The age of?905 Ma for the plagiogranite marks the obduction of the ophiolite on continental margin during Rodinia assembly.The andesite and gabbro-diorite are enriched in LILE and LREE but depleted in HFSE.The andesite shows highly positive zircon ?Hf(t)values of 8.6 to 15.8 and mantle-like zircon ?18O values of 4.1 to 5.9‰.In addition,both andesite and gabbric diorite whole-rocks are depleted in Sr-Nd-Hf isotopes,with positive ?Nd(t)values of 4.1 to 5.5 and highly positive ?Hf(t)values of 12.1 to 16.0.Such geochemical characteristics indicate that these mafic igneous rocks were derived from partial melting of arc-type mantle sources.In this regard,the ages of 936±6 Ma and 950±5 Ma for the andesite and gabbroic diorite date mafic arc magmatism above a Grenvillian oceanic subduction zone.Taken altogether,the subduction of the Grenvillian oceanic crust was continuous till the obduction of ophiolite at ca.900 Ma.Therefore,the Grenvillian oceanic subduction system was developed along the northern margin of the Yangtze Craton in the Early Neoproterozoic.This implies that South China was assembled into Rodinia through a Grenvillian accretionary orogeny.Low-?18O igneous rocks from the Tiewadian area,NW South China,have been reported.Zircon U-Pb dating suggests that they were formed at ca.750-780 Ma,consistent with the breakup of Rodinia.Zircon 8180 values range from-7.7 to 4.1 ‰with a major peak at ca.3.5‰.The 18O-depletion signature indicates the involvement of high temperature meteoric water-rock reaction.A rift extensional tectonism at middle Neoproterozoic resulting from Rodinia breakup is also required,as has previously been proposed.Additionally,they have positive ?Hf(t)values of 4.6-11.2 with Hf model age of ca.1.0-1.4 Ga,suggesting that they were derived from reworking of juvenile crust that formed during the late Meso-to early Neoproterozoic,at which remarkable of juvenile crust were produced by arc-magmatism at the north margin of the Yangtze Block during the assembly of Rodinia.The identification of low-?18O magmatism in the northwestern margin of the South China Block suggests that a large scale low-?18O magmatic belt was developed along the north margin of the South China Block.Furthermore,compilation of low-?18O data from the NW India,Madagascar and South China suggests close connections between these blocks in Rodinia configuration.Systematic geochronology and geochemistry have been studied for the mafic rocks from the north margin of Yangtze and South Qinling region.It exsits temporal and spatial variations,i.e.,mafic rocks with early Neoproterozoic age(>850 Ma)display clear arc-affinities with depleted zircon Hf and the whole-rock Nd isotopic compositions.While after 840 Ma,the arc-affinities of mafic rocks are weaken or disappear,and become more MORB-like and OIB-like.Zircon Hf and whole-rock Nd isotopes also become more enriched,indicating the involvement crustal materials into the mantle source.The change geochemical characteristics indicated the tectonic evolution the of the north margin of the South China Block.Oceanic subduction system was developed from 950 to 900 Ma,followed by accretion of oceanic arc and ocean crust subducted beneath the north margin of the Yangtze craton(900-850 Ma).Then arc-continent collision occurred at ca.840-830 Ma,and followed by lithospheric extension due to the collapse of post-orogeny.A number of magmatism developed in the extensional setting represented by reworking of juvenile cust and ancient materials.Rift magmatism was mainly developed after 780 Ma due to breakup of Rodinia.Associated mafic rocks during this period display MORB-and OIB-like elements features with depleted and enriched zircon Hf and whole-rock Nd isotopes,suggesting the heterogeneity of mantle source or the assimilation of crustal materials during the emplacement of mafic magma.