Genesis And Tectonic Significance Of The Paleoproterozoic Ophiolitic Mélange In The Huangling Dome,Yangtze Craton

The Yangtze craton is one of the most important cratons of China which has been an important frontier and controversial scientific issue for the composition and structure and tectonic evolution of its Precambrian basement in recent years.The Huangling Dome in the northern Yangtze craton exposes the oldest Archean-Paleoproterozoic rocks of South China.It has a complex litho-tectonic sequence,and is one of the key areas to study the formation,evolution history of the basement of the Yangtze craton,providing us an excellent opportunity for understanding early Precambrian tectonic evolution of the craton.Mélange is one of the notable litho-tectonic units within subduction-accretion complexes that record the accretion process at active convergent plate margins and related structural,magmatic and metamorphic process during later emplacement.Mélange,especially the early Precambrian mélange,as one of the most important geological units in convergent plate boundaries?suture zones?,it is the key research objects and notable geological evidence for important frontier scientific issues in geoscience,such as the early tectonic evolution process of the Earth,as well as the operation and dynamic mechanics of plate tectonics.Based on detailed field investigation,we choose the metamorphosed mafic–ultramafic rocks,high-Mg basalts,andesites and related granitoids in the Paleoproterozoic Shuiyuesi Group in the northern Huangling Dome as our main research object.A systematic and detailed research including structural geology,petrographic,mineralogical,and petrology,mineralogy,geochemistry,isotopic geochemistry and geochronological studies has been operated to provide their tectonic affinity,petrogenesis,formation ages and tectonic setting.The main results are as follows:?1?For the first time,we demonstrate that the main part of metasedimentary rocks and mafic–ultramafic blocks in the Paleoproterozoic Shuiyuesi Group in the northern Huangling Dome is a Paleoproterozoic ophiolitic mélange belt.And correspondingly,the Archean–Paleoproterozoic metamorphic basement of the northern Huangling Dome can be divided into three different litho-tectonic units:the Mesoarchean Dongchonghe TTG gneiss in the western,Neoarchean Bashansi granitic gneiss complex in the eastern parts,as well as a mélange belt mainly composed of strongly deformed meta-sedimentary rocks with mafic–ultramafic lenses/blocks that are tectonically mixed in the central belt?Shuiyuesi Group?.?2?The mélange generally displays northwest-verging nappe structures,and underwent amphibolite to granulite facies metamorphism.The metasedimentary rocks are composed mainly of sillimanite-garnet-biotite-plagioclase gneiss,mica-schist,mica-graphite schist,marble,quartzite,and banded iron formation?BIFs?.The mafic-ultramafic tectonic blocks,including serpentinized harzburgite,meta-olivine pyroxenite,meta-pyroxenite,meta-diabase,meta-gabbro and meta-basalt,dispersed in the strongly sheared metasedimentary matrix.?3?The mafic–ultramafic rock assemblages shown as tectonic blocks in the Paleoproterozoic mélange shares the lithological and geochemical characteristics of supra-subduction zone ophiolites.Geochemical characteristics indicate that serpentinized harzburgite are LREE-enriched metasomatic abyssal residual mantle peridotite,the meta-olivine pyroxenites,metapyroxenites are cumulates at supra-subduction zone influenced by subduction fluid.The amphibolites?meta-gabbro,meta-diabase and meta-basalts?are tholeiitic rocks,and characterized by flat to light enriched LREE patterns,enriched LILE patterns?e.g.K,Rb,Ba,Th and Cs?,and negative anomalies of HFSE?e.g.,Nb,Ti?,suggesting a supra-subduction zone?SSZ?formation environment.LA-ICP-MS U–Pb dating of zircons from the Jiaozhanya meta-diabase yield ages between 2142 and 2148 Ma for the magmatic cores and between 2042 and 2048 Ma for the metamorphic rims.Zircon cores have?_Hf?t?values ranging from+5.4 to+10.3?average=+7.2?and T_DM1=2.24 Ga,indicating that these rocks are remnants of juvenile crust derived from the depleted mantle at ca.2.2 Ga.The serpentinized harzburgite yilde metamorphic ages of 2025–2028 Ma and several zircons show inherited cores with older ages from 2094–2188 Ma.Most zircons from the serpentinized harzburgite have negative?_Hf?t?values and Archean model age,which suggests recycle of the ancient sediments and a significant mantle wedge metasomatism by slab-derived melts/fluids during subduction process.All rock types in the mélange underwent Paleoproterozoic?2.0–1.95 Ga?amphibolite-granulite facies metamorphism and were intruded by syn-metamorphic granite?².0 Ga?,post-orogenic?ca.1.85 Ga?granite and mafic dykes.Intrusion age of one quartz monzonitic dyke crosscutting the serpentinized harzburgite is 1999 Ma,constraining the minimum emplacement age of the mafic–ultramafic complex.The mafic–ultramafic rocks within the metasedimentary rocks in the northern Huangling Dome are fragments of a ca.2150 Ma supra-subduction zone ophiolite incorporated into the suture zone during the accretionary-collisional process.?4?We report for the first time the presence of Paleoproterozoic?2.12 Ga?metamorphosed arc-related basalts and andesites in the Huangling dome,which provide new important evidence for sucductin-collision tectonic evolution in the Yangtze craton during the Paleoproterozoic era.The basalts and andesites occur as deformed layers within the metasedimentary rocks?Shuiyuesi Group?in the northern Huangling dome.They underwent deformation and metamorphism,and are mineralogically and texturally amphibolites and quartz-bearing amphibolites,respectively.The igneous zircons from the andesites yield a weighted age of ca.2.12 Ga that is interpreted to be the formation age of the magmatic protolith.The basalts and andesites are characterized by high MgO?6.59–14.6 wt.%?and high Mg-numbers?58–76?,and enriched in LILE and LREE,but depleted in Nb,Ta,and Ti,which is similar to typical high-Mg basalt and andesite.Zircons in the high-Mg andesites have?_Hf?t?values between–0.2 and+3.3?average is+1.42?and corresponding Hf isotopic model age(T_DM1)of 2.4 Ga,reflecting various degrees of crustal input.They have negative?_Nd?t?values,ranging from–4.4 to–2.7.The Nd and Hf isotope compositions of the high-Mg rocks suggest that they were derived from partial melting of the enriched subarc lithospheric mantle above a subducting oceanic slab.Geochemical characteristics of the basalts and andesites indicate that they are equivalent to high-Mg basalts and andesites,respectively,occurring in Phanerozoic supra-subduction zones.The age and general geochemical characteristics of the high-Mg arc-related rocks indicate the presence of a Paleoproterozoic?2.1–2.2 Ga?Andean-type continental margin developed on metasomatized lithospheric mantle in the northern Huangling dome.The geochemical and geochronological data presented in this study provide important insights into Paleoproterozoic oceanic subduction within the Yangtze craton.?5?Two stages granitoids related to the Paleoproterozoic collisional orogeny and post-orogenic extension events were discovered in Huangling Dome,and provide important evidence for the Paleoproterozoic subduction-collisional orogeny.The early group granites were formed at ca.2.0–1.95 Ga,and mainly composed of deformed or weak deformed monzogranite,syenite,and quartz monzonite occurred as small intrusions and partial melting veins/dikes.They are of high-K calc-alkaline to shoshonite series and have low crystal temperature ranging from 700 to 800?.They also show potassic adakite-like elevated Sr/Y?60–71?and?La/Yb?_cn?32–464?ratios,equilibrated with garnet-rich residues,which are inferred to be formed by melting of thickened lower crust in root of collisional orogen.The latter group granitoids mainly formed at¹.85 Ga,are represented by K-feldspar granitic-rhyolitic rocks with no or weak deformation and gneissic granodiorite.They belong to high-K calc-alkaline to shoshonite series,and display strongly negative Eu and Sr anomalies and depletion in HFSE?such as Nb,Ta and Ti?.They show metaluminous to weakly peraluminous A-type affinity with high 10000*Ga/Al ratios,and high original magmatic temperature of 850–950?.They formed in a post-orogenic tectonic setting as a result of partial melting at a thinner crust condition?<35 km?with residues rich in plagioclase,ilmenite and poor in garnet,which mark the lithosphere extension in late Paleoproterozoic era?ca.1.85 Ga?.Both the earlier and later granitic groups have a negative?_Hf?t?,and the two-stage Hf model age(T_DM2)range from 3.2 to 3.6 Ga,indicate that they are product of reworking of ancient Archean crust.?6?Based on the petrological,geochemical,and geochronological data presented in this study,we propose a tectonic model showing Paleoproterozoic subduction-accretion-collision orogeny in the Huangling Dome,Yangtze craton:?a?².15 Ga,oceanic crust keep a southeast-dipping subduction between the eastern and western micro-continents in Huangling dome,and an Andean-type active continental margin was built on the eastern micro-continent at ca.2.12 Ga,marked by the high-Mg basalts and andesites.Meanwhile a suite of terrigenous clastic,carbonate sedimentary and quartzite formation developed on the active continental margin.?b?The oceanic basin closed at 2.0–1.95 Ga,oceanic crust and the metasedimentary rocks between the eastern and western micro-continents were incorporated into the orogenic belt.They underwent amphibolite-granulite facies metamorphism and constituted a suite of subduction-accretion complex,named Shuiyuesi ophiolitic mélange.The micro-continents collided together and result in crustal thickening.And a suit of homochromous anatexis granites formed by partial melting of the TTG rocks in lower crust triggered by heat and metamorphic fluids.?c?Finally,lithosphere operated thinning at¹.85 Ga,showing as orogenic belts collapsing and upwelling of the ashospheric mantle.The A-type granites and mafic dykes generated in an extensional tectonic setting,mark the end of the Paleoproterozoic subduction-collision orogenic cycle.?7?The research on the Paleoproterozoic Shuiyuesi ophiolitic mélange in the northern Huangling Dome indicates that the Yangtze craton is constituted of different micro-continents.The Yangtze craton has experienced a multiple stage evolution history composed of at least two period?Paleoproterozoic and Neoproterozoic?subduction-collision orogenic events in Proterozoic era.Of which,the Paleoproterozoic subduction-collision orogeny is inferred to be related to the amalgamation and break-up of the proposed Columbia/Nuna supercontinent.Additionally,our research also suggests that the onset of modern style plate tectonics is no later than Paleoproterozoic.