Petrogenesis And Tectonic Implications Of The Neoproterozoic Mafic Dikes In The Yangtze Block,South China

South China was a key member of the Rodinia supercontinent.Neoproterozoic magmatic and sedimentary rocks are widespread along the margin of the Yangtze Block,their petrogenesis and tectonic setting have been a matter of hot debate.Mafic dikes which are generally formed in extensional settings can provide important information for understanding the nature of mantle sources and geodynamic processes.We investigated mafic dikes with various ages and lithologies in the different locations of the Yangtze Block.These data are used to understand their petrogenesis,and to decipher the nature and thermal state of the lithospheric mantle,as well as to constrain the tectonic evolution and to infer the position of South China in the Rodinia supercontinent configuration during the Neoproterozoic.In this study,we present petrography,geochronology,major and trace elements,Sr-Nd-Pb-Fe-Hf-O isotopes for the Tongde and Dengxiangying mafic dikes in the west and Wudang mafic dikes in the north of the Yangtze Block.(1)Numerous ~830 Ma dikes intruded the Neoproterozoic Tongde intrusion at the western margin of the Yangtze Block.Rocks from the dikes are composed of fine to medium grained diorites which have high Mg#(40 to 69),low Si O2(50.98 to 61.01 wt.%),and variable Na2 O contents(1.94 to 5.50 wt.%).They show right inclined chondritenormalized REE patterns with slightly negative to positive Eu anomalies(Eu/Eu* = 0.90-1.35).Their primitive mantle-normalized trace element patterns are characterized by enrichment of Ba and Pb and depletion of Nb,Ta and Ti.They also show high and variable Sr/Y(12-115)and(La/Yb)N ratios(3-21).These high-Mg diorites have relatively constant initial 87Sr/86 Sr ratios(0.704862-0.706320)and ?Nd values(+0.6 to +2.0).These geochemical features reveal that the Tongde high-Mg dioritic dikes were not produced by partial melting of subducted oceanic slab or enriched mantle source.Rather,they were more likely formed by fractional crystallization of garnet and hornblende-bearing mineral assemblages from hydrous mafic magmas under high pressure conditions.The dikes intruded along the fractures that have resulted from magma thermal contraction after emplacement of the Tongde intrusion.The new model proposed for the petrogenesis of the high-Mg dioritic dikes suggest that the western margin of the Yangtze Block was in an active continental arc setting during the early Neoproterozoic.(2)Several mafic dikes of ~800 Ma,intruded the Neoproterozoic Dengxiangying Group.They consist of fine to medium grained diabase and have variable Si O2(42.80-57.20 wt.%),Mg O(5.16-13.15 wt.%)and total alkalis(0.05-4.41 wt.%).They show LREE-enriched chondrite-normalized REE patterns with negative to slightly positive Eu anomalies(Eu/Eu* = 0.67-1.09),and display enrichment of Pb and depletion of Nb,Ta,Sr in the primitive mantle-normalized trace element diagram.Rocks from the Dengxiangying mafic dikes have variable initial 87Sr/86 Sr ratios(0.652431 to 0.730039),but relatively constant ?Nd values(-2.5 to +2.6)and Pb isotope ratios(206Pb/204 Pb = 17.53-17.88,207Pb/204 Pb = 15.60-15.63,208Pb/204 Pb = 37.66-38.38).These geochemical features suggest that the mafic dikes were derived from a lithospheric mantle enriched by slab-derived materials.PRIMELT3 modeling for the Neoproterozoic mafic rocks in the western margin of the Yangtze Block reveals that their primary magmas were in equilibrium with harzburgite and spinel peridotite with potential temperatures of 1377-1498 °C.They have high primary magmatic water contents(1900-72000 ppm)that are similar to those of the Phanerozoic arc-basalts in the world.These lines of evidences confirm that the Neoproterozoic mafic and ultramafic rocks were formed in an active continental setting.(3)Another younger(700-650-Ma)group of mafic dikes were found intruding the Neoproterozoic Wudang and Yaolinghe Group in the northern Yangtze Block,South China.These dikes are composed of medium-grained gabbros,which have a range of Si O2 from 44.56 to 51.01 wt.%,Mg O from 4.84 to 8.49 wt.% and Na2O+K2O from 1.49 to 4.73 wt.%.They show slightly LREE-enriched chondrite-normalized REE patterns with positive Eu anomalies(Eu/Eu* = 1.00-1.51).Their primitive mantle-normalized trace element patterns are characterized by enrichment of large-ion lithophile elements(e.g.Rb,Ba,U)and depletion of high-field-strength elements(e.g.Th,Zr and Hf)with positive Pb and Sr anomalies.Rocks from the mafic dikes have variable initial 87Sr/86Sr(0.702691 to 0.706402),?Nd(+0.4 to +7.2)and Pb isotopic ratios(206Pb/204 Pb = 17.16-17.43,207Pb/204 Pb = 15.43-15.47,208Pb/204 Pb = 37.33-37.48).They also show large variation of zircon ?Hf values(-10.0 to +4.3),but relatively low ?18O values(+3.08‰ to +5.98‰).These geochemical features reveal that their parental magmas were formed by 10-20% partial melting of a spinel lherzolite mantle source that was modified by subduction components.The magmas experienced fractional crystallization of olivine and plagioclase and underwent crustal contamination during the magma ascent.AFC calculations show that 5-20% crustal components were involved in their petrogenesis.The Wudang mafic dikes were probably formed in an extensional setting,marking the tectonic transition from subduction to rifting in South China during the Neoproterozoic.(4)Neoproterozoic mafic dikes in the Yangtze Block are characterized by variable Fe isotope compositions.Rocks from the Tongde and Dengxiangying dikes have relatively light ?57Fe values(-0.08~+0.17‰ and-0.14~+0.14‰,respectively).By contrast,rocks from the Wudang dikes have heavy ?57Fe values(+0.05~+0.29‰).Compared with the MORB(?57Fe = +0.07~+0.22‰),OIB(?57Fe = +0.11~+0.27‰)and IAB(?57Fe =-0.04~+0.23‰),we suggest that mafic igneous rocks in the western margin of the Yangtze Block are consistent with the typical arc magmas,indicating melting of mantle wedge in the subduction zone.Metasomatism and extraction of melts formed heterogeneous Fe isotope compositions in the mantle source during the early Neoproterozoic.However,the mafic igneous rocks in the northerm margin of the Yangtze Block have been formed by mixing of arc and OIB melts.It was in rifting setting in South China during the late Neoproterozoic.(5)The 830-700 Ma mafic dikes in the Yangtze Block record ocean-continent subduction in the early Neoproterozoic and intracontinent rifting in the late Neoproterozoic in South China.These processes modified the subcontinental lithospheric mantle and charactered the mafic-ultramafic rocks enriched Sr-Nd-Pb and light Fe isotopes.With the rifting after 750-700 Ma,mixing of melts from upwelling asthenospheric mantle and metasomatic lithospheric mantle formed the mafic-ultramafic dikes with depleted Sr-Nd-Pb and heavy Fe isotopes.