Neoproterozoic (900–780 Ma) Magmatism During The Evolution Of The Suiying Terrane In The Northern Margin Of The Yangtze Block

South China Craton was involved in the assembly and breakup of the Supercontinent Rodinia.However,the formation time,position and geological feature of the South China in the Supercontinent Rodinia are still unclear,as well as the geodynamic mechanism for the Neoproterozoic evolution for the South China.Firstly,whether the South China has recorded the Grenville orogenic event(~1300–l000 Ma)? Secondly,how did the uniformed Precambrian basement of the South China form? Thirdly,what is the time and geological record of the tectonic transition from convergence to extension of the South China in the Neoproterozoic? All these questions are important in understanding the crustal evolution history of the South China and reconstruction of the Supercontinent Rodinia.The Neoproterozoic magmatisms were extensively active along the North margin of the Yangtze Block,which recorded its whole transition process from the orogenic to anorogenic setting.Due to the eruptions by the latter tectonics and magmatisms and covers by the sediments,it is rare to massively discover the outcrops of early Neoproterozoic intrusions in the North margin of the Yangtze Block.The Sanligang pluton,the Daleishan pluton and their associated igneous rocks had been the precious early to middle Neoproterozoic geologic bodies for the further research and examination on the tectonic evolution and dynamic mechanics of the North margin of Yangtze Block.Thus,those igneous rocks are chosen as the research objects in this contribution with the approach of regional field geologic survey in large scale and detailed isotopic geochronology and geochemistry work to reveal the 860–780 Ma magmatisms and crustal-mantle evolutions in the North margin of Yangtze Block,and to discuss the geologic features and dynamic mechanics of South China in the assembly and break-up transition in the Rodinia supercontinent.Scondary Ion Mass Spectrometry(SIMS)zircon U–Pb dating indicates that these mafic instrusive rocks were emplaced at 871 Ma.The mafic rocks samples have high FeO(9.27–12.17 wt.%),MgO(6.25–9.85 wt.%),and CaO(8.83–10.57 wt.%),as well as low SiO2(45.37–49.73 wt.%),Al2O3(13.29–16.13 wt.%),K2O(0.32–1.13 wt.%)and Na2O+K2O(2.47–3.67 wt.%).Thus,the mafic rocks belong to a low-to medium-K subalkaline series.They demonstrate a slight right-sloping chondrite-normalized REE pattern with no significant Eu anomalies and subparallel PM-normalized patterns with enrichment in LILEs,depletion in HFSEs and with Ti negative anomalies,similar to those of arc volcanic rocks.Rocks from the mafic rocks have high whole-rock ?Nd(t)values(+4.0 – +7.1)and zircon ?Hf(t)values(+10.5 – +12.9),but relatively low zircon ?18O values(+5.25 ‰ – +6.03 ‰).Such arc-like geochemical characteristics might have been inherited from a sub-arc source modified by fluid released from the subduction slab in an arc setting.The Sanligang dioritic to granitic pluton is spatially associated with the Sanligang mafic instrusive rocks in the Sangligang–Sanyang fault belt(SSFB).SIMS and La-ICP MS zircon U–Pb dating from three representative samples indicates that the Sanligang pluton were emplaced at ~860 Ma,slightly later than the mafic rocks occurring at the SSFB.Rocks from the Sanligang pluton have intermediate to high SiO2(60.35–71.38 wt.%),intermediate to low K2O(1.38–3.67 wt.%)and Na2O(3.97–5.33 wt.%),and high MgO(1.03–3.16 wt.%),TiO2(0.44–0.87 wt.%)and TFe O(2.03–5.91 wt.%).The mineralogical and geochemical characteristics of the Sanligang pluton indicate that they are similar to the amphibole-rich calc-alkalinegranitoids in accordance with Barbarin's(1999)classification.They show LREE-enriched REE patterns(La/YbN=7.2–12.3)with no or minor negative Eu anomalies.Their primitive mantle-normalized trace element patterns are characterized by enrichment of LILEs and depletion of HFSEs.The Sanligang granitoids share similar zircon ?Hf(t)values(+7.9 to +11.7)and ?18O(+5.83 to +6.20 ‰)ratios,as well as whole-rock initial 87Sr/86 Sr ratios(0.7033 to 0.7041)and ?Nd(t)values(+3.4 to +4.9)with the mafic rocks,suggesting that the granitoids were generated by partial melting of juvenile basaltic crust.High Mg# values(49–58)in the granitoids may have resulted from assimilation of residual mafic miner-als in their source region.Based on its arc-related geochemical affinity and contemporaneous arc-related magmatism,the Sanligang pluton is proposed to be generated in a Neoproterozoic arc setting during crustal growth and reworking.The early Neoproterozoic assemblage from the Sangligang–Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.There are different magmatic assemblages during the middle and later Neoproterozoic at the Daleishan area in the northern margin of the Yangtze Block,and which can be divided into two different groups by their mineralogical and geochemical characteristics.The first group emplaced as leucogranite(Daleishan plutin)or erupted as calc alkaline volcanic sequences in the Daleishan area,and the second group emplaced as mafic dikes or erupted as alkaline bimodal volcanic rock in the Hong'An Complex.Three representative samples form Daleishan pluton yielded an accordant zircon U–Pb ages of ~800 Ma,rather than in the previously-thought Archean.They have relatively high SiO2(71.53–77.05 wt.%),K2O(3.93–5.00 wt.%)and Na2O(3.15–4.23 wt.%),but relatively low MgO(0.13–0.66 wt.%),TiO2(0.11–0.31 wt.%)?TFeO(0.83–2.30 wt.%)and P2O5(0.02–0.09 wt.%).The rock from Daleishan pluton have high ASI(1.08 – 1.27)and CIPW-normative corundum contents(1.2–3.2 vol.%),generally similar to the S-granitic rocks,or similar to the Muscovite-bearing Peraluminous Granitoids(MPGs)in Barbarin's(1999)classification.The samples display enrichments in LREEs with obvious negative Eu anomalies.Their primitive mantle-normalized trace element patterns are characterized by enrichment of Rb,Th and Pb,but depletion of Ba,Ta,Nb,Sr,Zr and Ti.Their whole-rock ?Nd(t)values(-9.5 –-7.9)are very similar to their zircon ?Hf(t)values(-12.0–-7.7).The Daleishan pluton is proposed to be the product of dehydration melting of muscovite in the relatively low temperature,and they were derived form anatexis of the mudstones from the ancient continental crust in the continental subduction zone.The Daleishan meta-basaltic andesite,spatially associated the Daleishan pluton,has the zircon U–Pb age of 787±7 Ma,which is slightly younger than the granitic pluton.The samples from the Daleishan meta-basaltic andesite has SiO2 ranging from 53.42 wt.% to 56.21 wt.%,MgO ranging from 0.78 wt.% to 3.57 wt.%,and K2 O ranging from 0.43 wt.% to 1.25 wt.%.They have high relatively TiO2(1.52–1.75 wt.%)and TFeO(9.03–11.73 wt.%),and hence can be classified as tholeiitic basalticandesite.They demonstrate a right-sloping chondrite-normalized REE pattern with no significant Eu anomalies,and subparallel spiky PM-normalized patterns with enrichment in LILEs,depletion in HFSEs and with Ti negative anomalies,similar to those of arc volcanic rocks.The basaltic magmas underwent partially-fractional crystallization during ascent,but were not intensely influenced by crustal contamination.Their zircon ?Hf(t)values are varying form to-18.0 to-3.6.The basaltic magmas underwent fractional crystallization during ascent,but were not intensely influenced by crustal contamination.Such are-like geochemical characteristics might have been inherited from a sub-arc source modified by fluid/melt released from the subduction sedimentary component in the continental subduction zone.The Early Neoproterozoic Dahongshan subducting accretion consists of two magmatic suites including an older MORB-type ophiolitic suite in the south belt and a younger arc-related magmatic suite in the north belt.The early Neoproterozoic assemblage(~860 Ma)from the Sangligang–Sanyang fault belt provides an important record of oceanic slab subduction in the northern margin of the Yangtze Block.Considering the very close temporal and spatial relationships,we propose that the Miaowan-Shennongjia accretionary wedge-arc system extended eastward to the Sanligang pluton.The arc-continent collision along the Miaowan–Dahongshan northern oceanic basin in the north margin of the Yangtze Block did not happened until ca.870–850 Ma.The S-type granites(~800 Ma)and basalticandesite(~787 Ma)in the Daleishan area are both related to the melting of recycled ancient sediments in their petrogenesis,which is obviously different from the Early Neoproterozoic(>850 Ma)magmatic rocks in an oceanic subduction setting.The geological and geochemical data from magmatic assemblage in the Daleishan and its surrounding areas suggest that the Daleishan pluton derived from the partial melting of mudstones in the upper crust which had be subducted into the middle-lower crust depth,while the basalticandesite formed by partial melting of enriched continental lithospheric mantle in post-collision setting.More and more old crust components were involved in the magmatic source regions in the northern margin of the Yangtze Block after ~800–780 Ma,which may indicated the tectonic setting transition from orogenic to anorogenic setting,and the tectonic mechanism transition from convergence to extension in the middle Neoproterozoic.