Recognition Of ~865 Ma And ~815 Ma Mantle Magmatism And Its Implication For The Neoproterozoic Tectonic Evolution Of The South China Block

During the Neoproterozoic,evolutionary character of the Earth profoundly changed including the assembly-breakup of Rodinia supercontinent,global snowball and anoxic events as well as the subsequent Cambrian Explosion.In this global context,the unified South China block(SCB)formed by an amalgamation between the Yangtze and Cathaysia blocks,and this process was coupled with intensive and extensive Neoproterozoic magmatism.Although long-term and fruitful works have been made,sharp divergence of views on the nature and evolution history of magmatic activities has long remained,causing controversial interpretations on the unifying process of the SCB and its relationship with the Rodinia.Compared with the continental margins of the Yangtze block,its interior terrains show advantages in studies of Neoproterozoic igneous suites including avoiding superposition of Phanerozoic orogenesis and exploring relationship between the continental core and its surrounding microblocks/terranes,and thus may provide a more comprehensive understanding of the nature and evolution of Neoproterozoic tectono-magmatic events in the SCB.Neoproterozoic intrusions,known as the Huangling intrusive complex and emplaced in the Yangtze continental nucleus,the Kongling terrane,are selected as the research objects of this thesis.Based on detailed field geological survey and a comprehensive U-Pb geochronological and isotope-element geochemical study,mantle input related magmatic events of early-and mid Neoproterozoic are recognized.With discussions of the nature of their source rocks,petrogenesis and tectonic setting,Neoproterozoic tectonic evolution of the Yangtze continental interior is explored.Combined with recent progress in other studies of the Neoproterozoic events in the SCB,a new model for the continental unification process is proposed.The early Neoproterozoic intrusions of this thesis comprise the Gongjiaba gabbro,Yinxingtuo issite and diorite of the Sandouping unit and rapakivi-textured granites of the Daolaoling unit,whereas the mid Neoproterozoic ones consist of the Yezhuchi gabbro,diorite and granite of the Daolaoling unit.The Yezhuchi gabbro is dated at 813±3 Ma.It shows porphyritic texture with clinopyroxene and plagioclase phenocrysts and matrix minerals of clinopyroxene,plagioclase and olivine.Metasomatic hornblende and phlogopite variedly replaced the accumulated primary minerals or newly formed interstitial to them,indicative of a relatively anhydrous nature of the magma parental to the Yezhuchi gabbro.The gabbroic samples are tholeiitic with high MgO and Fe2O3 t,low TiO2 and alkali metal contents,and LREE-enriched patterns(LaN/YbN: 7.03-11.2)with variable negative Eu anomalies(Eu/Eu*=0.79-0.92).Coupled with a positive correlation between their TiO2 and Fe2O3t(r=0.94),it is indicated that the melt parental to the gabbro was moderately evolved.Both the clinopyroxene and hornblende are also characterized by LREE-enriched patterns,which suggest that the melt parental to the Yezhuchi gabbro had experienced certain extent of garnet fractionation when the primary magma was ponded at the crust-mantle boundary before its ascent or clinopyroxene fractionation at relatively shallow magmatic chamber.The gabbroic samples are depletd in HFSE,and their small negative ?Nd(t)values(-0.1 to-1.99)for the whole rocks are basically coupled with the ?Hf(t)values(-2.99 to-7.05)for the zircons.These indicate slightly enriched mantle source or crustal contamination during the mafic melt ascent.However,no detectable correlations of Mg# or MgO with ?Nd(t)or Nb/Ta are found,this suggests that the crustal contributions mainly occurred at the mantle source,pointing to a recycling process of crustal components into the mantle.A low slope trend of Ti-Al co-variation for the clinopyroxene suggests an affinity of extention-related gabbro.Along with a nature of dry melt,the gabbro is suggested to have formed in an anhydrous and rifted-related setting.The Yezhuchi dioritic and granitic intrusions associated with the gabbro are dated at 817±6 Ma and 801±6 Ma,respectively.The diorite is calc-alkaline with high Mg#,Al2O3,Fe2O3 t and Na2 O,and featured by LREE-enriched patterns with negligible Eu anomaly and variable depletion in HFSE.It displays Sr-Nd isotope compositions distinctly different from the gabbro with evidently lower initial Sr ratios,further indicative of a non-comagmatic relationship between them.Instead,the Sr-Nd isotopic data of the Yezhuchi diorite resemble those of the mafic dykes from the Xiaofeng unit.It is thus suggested that the lithospheric mantle sourced the Xiaofeng mafic dykes had been previously partial melted during ~860-850 Ma and formed substantial mafic plutonic suites;Anatexis of these suites at ~815 Ma was induced by underplating of mantle-derived magma ponded at the crust-mantle boundary and caused the Yezhuchi diorite.The Yezhuchi granite is also calc-alkaline and featured by high SiO2?Al2O3,Na2 O and K2 O,and LREE-enriched patterns with positive to negative Eu anomalies.It is also depleted in Nb-Ta,Ti and P but enriched in Zr-Hf.Proofs of inherited Archean zircons,Archean to Paleoproterozoic Nd model ages and slightly peraluminous composition suggest a prominent anatectic derivation for the Yezhuchi monzonitic granite from the Kongling crystalline basement by variably admixing with mantle-derived melt.The Gongjiaba gabbro is dated at 845±8 Ma.It is typically gabbroic textured,tholeiitic in composition and characterized by high MgO,moderate Fe2O3 t and CaO and low TiO2 and Na2 O.It displays LREE-enriched patterns with weak negative to positive Eu anomalies(Eu/Eu*: 0.82-1.10)and HFSE depletions,indicative of insignificant plagioclase fractionation or accumulation and crustal signatures in the mantle source rocks.The ?Nd(t)values(-7.3 to-6.1)for the whole rocks are basically coupled with ?Hf(t)values(-2.09 to-5.93)for the zircons.Along with its E-MORB-like composition,the enriched signatures for the source of the Gongjiaba gabbro is suggested to indicate a previous metasomatism by recycled crustal components.The Yinxingtuo issite is dated at 864±7 Ma.It shows medium-fine grained texture and a calc-alkaline trend.It is featured by high Al2O3 and Na2 O,low SiO2 and K2 O,and moderate TiO2,Fe2O3 t and CaO contents with a higher alkali content than the Gongjiaba gabbro.It displays LREE enriched patterns with weak negative Eu anomaly,and HFSE,Th and U depletions.The ?Nd(t)values(-6.8 to-4.6)for the whole rocks are basically coupled with the ?Hf(t)values(-4.47 to +2.65)for the zircons.Along with no detectable correlations of these values with magmatic evolution,MgO for instance,an E-MORB-like nature is suggested,which agrees with its enriched mantle source rocks inferred by the Hf-Nd isotope and(Nb/La)PM ratio(0.34-0.49)signatures.The Yinxingtuo dioritic and granitic intrusions are dated at 859±4 Ma and 813±9 Ma,respectively,agreeing well with their field contact.Both lithologies are calc-alkaline,and the diorite is limited in SiO2 range with high Al2O3,Fe2O3 t and Na2 O,low K2 O,and featured by LREE-enriched patterns,depletion in HFSE and moderate positive Pb-Sr anomalies.The diorite has ?Nd(t)and(87Sr/86Sr)i values ranging from-14.3 to-7.6 and 0.7041~0.7051,respectively,indicative of mafic lower crust derivation.The granite,like other granites by the 820-790 Ma peak magmatism of the complex,is featured by high SiO2,moderate Al2O3 and Na2 O and low K2 O contents.It has LREE-enriched patterns typical of TTG rocks and,relative to the diorite,is more depleted in HFSE,more positive K and Pb anomalies and moderate enriched in Zr and Hf.Ranges of the ?Nd(t)and(87Sr/86Sr)i values vary from-14.3 to-7.6 and 0.7041~0.7051,respectively.These suggest a prominent anatectic derivation for the Yinxingtuo granite from the Kongling crystalline basement admixed variably with mantle input.Two samples from the Daolaoling rapakivi-textured granite are dated at 857±5 Ma and 865±7 Ma,respectively.The granite is featured by rapakivi texture,and the plagioclases rimed the K-feldspar phenocryst are albite(An: 5-11),whereas those from the matrix and clusters within the phenocryst are oligoclase to andesine with An ranges of 14-49 and 18-60,respectively.These indicate that the albitic rim of the phenocrysts formed at subsolidus condition,whereas the plagioclases within the matrix and in form of cluster in phenocryst are magmatic.The rapakivi texture then is genetically explained by rapid decompression and cooling during the melt ascent of a H2 O unsaturated magmatic system.The features of relatively dry melt and rapid melt ascending mechanism due to decompression point to an regional extensional or rift-related tectonic setting during ~860-850 Ma.The granite possesses high SiO2,Na2 O and K2 O and moderate to low Al2O3 contents,and shows an affinity of high-K calc-alkaline series and chemical compositions typical of TTG rocks.It has LREE-enriched,HREE depleted patterns(LaN/YbN: 24.4-35.3)with positive to negative Eu anomalies(Eu/Eu*: 0.82-1.19)and pronounced HFSE depletion with positive K,Pb anomalies.Combined with values of ?Nd(850Ma)(-17.1 to-8.5)and zircon ?Hf(t)(-7.09 to-15.45),the grantic pluton is then suggested to have been derived from partial melts of mafic rocks of the lower crust at depth >40 km.The new findings about the ~865-845 Ma mantle-input related magmatism mentioned above suggest an extensional nature of regional tectonic setting,and the mafic intrusions were derived via low-degree partial melting from slightly enriched mantle caused by previous metasomatism of subducted hydrous liquids,whereas the diorite and rapakivi-textured granite from the meta-igneous rocks of thickened Paleoproterozoic lower crust,which were heated by coeval mantle-derived melts.Along with our progress on the ~820-790 Ma magmatism,it is inferred that the magmatism successively developed in the continental interior of the SCB is essentially featured by mantle input of the lithosphere.The mantle rocks were previously metasomated by the recycled crust due to slab subduction,and the enriched components were consumed continuously by the multi-stage partial melting and gradually evolved into a volatile-depleted dry system.This process was accompanied by an enhancement of mantle contribution,typifying an extensional nature of regional tectonic evolution.Integrated with progresses of our team and other workers on the Neoproterozoic magmatism of the Kongling and adjacent terranes as well as other regions of the SCB,we propose a new model for the Neoproterozoic development of the SCB.The amalgamation between the Yangtze and the Cathaysian blocks actually comprises multiple mass-limited collisional events related to subordinate continental blocks or terranes in them.Before the unification of the SCB,the Yangtze block comprised terranes or massifs of the Kongling(Archean to Paleoproterozoic basement)in the western Hubei,the Houhe(Paleoproterozoic)in the southern Shanxi,the Shengnongjia(Mesoproterozoic)in the western Hubei and the Kangdian in the central Yunnan(Paleoproterozoic).Of them,the Kongling-Shengnongjia collision occurred at ~950 Ma and successively developed into a post-collisional stage since ~865 Ma and caused the Huangling intrusive complex;the Houhe terrane is featured by arc-and back-arc related magtism during ~950-900 Ma and 965-850 Ma,respectively,and collied with the Shengnongjia at ~815 Ma.A welding between the Kangdian and the Kongling-Shengnongjia terranes is likely to have occurred at the end Mesoproterozoic or the initial Neoproterozoic,and then successively evolved to a post-collision setting since ~860 Ma.Accordingly,the unification process of the SCB is inferred to a regime of accretionary orogenesis,rather than that of collisional orogenesis.