Genesis And Tectonic Implications Of Ultramafic Rocks In The Paleoproterozoic Shuiyuesi MéLange Belt,Huangling Dome,Yangtze Craton

Metamorphic ultramafic massifs within orogenic belts are generally derived from the upper mantle and are later tectonically exhumed and emplaced in orogenic belt during plate convergence.These metamorphic ultramafic massifs recorded crucial information on the mantle properties,and subduction initiation-accretion-collision during plate convergence.Therefore,they are of great importance for understanding the geodynamic evolution of orogens,the crustal-mantle interaction,and the operation timing of plate tectonics.The Huangling Dome is located in the north-central part of the Yangtze Craton,which is an important area to understand the early Precambrian tectonic evolution of the Yangtze Craton.This dissertation focuses on the serpentinized harzburgites and olivine pyroxenites in the Paleoproterozoic Shuiyuesi mélange belt,northern Huangling Dome.By applying multi-disciplinary study methods including detailed field investigation,petrology,mineralogy,geochronology,geochemistry and isotopes,in order to determine their petrogenesis and tectonic significance.This study provides new constraints on the Paleoproterozoic tectonic evolution of the Huangling Dome and even the Yangtze Craton,and new insights into the petrogenesis and evolution of heterogeneous ultramafic rocks in the orogenic belt.The main results are as follows:（1）The primary minerals and whole-rock Re-Os isotope compositions from the Jiaozhanya serpentinized harzburgites in the eastern margin of the Paleoproterozoic Shuiyuesi mélange belt,preserved highly depleted primary compositions,including high Mg^#in the olivines,high Mg^#and low contents of heavy rare earth elements（HREEs）in the orthopyroxenes,extremely low contents of Re（<0.09 ppb）and¹⁸⁷Re/¹⁸⁸Os ratios（0.00455—0.01029）in the whole-rocks.The harzburgites have negativeγ_Os（t）（-5.1～-4.0）and unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios（0.10927—0.11072）,and yield an oldest Re-depletion model age(T_RD)of～2.82 Ga.The above results indicate that the protolith of the harzburgite originated from the highly depleted late Mesoarchean（～2.82 Ga）subcontinental lithospheric mantle（SCLM）.The Jiaozhanya serpentinized harzburgite contains hydrous minerals such as amphibole and chlorite.They have low whole-rock Mg^#（87.6—89.1）and are characterized by enrichment of light rare earth elements（LREEs）and large ion lithophile elements（LILEs）,and negativeε_Nd（t）values（-4.0～-3.7）.The metasomatic zircons and whole-rock Sm-Nd isotope record the 2.04 Ga metasomatic event.The above results indicate that the protolith of the harzburgite experienced refertilization and metasomatism during Paleoproterozoic（～2.04 Ga）subduction.The metasomatic agents are hydrous fluids and Fe-enriched silicate melts derived from the Paleoproterozoic metasedimentary rocks.We propose that～2.82 Ga is an important period for the formation of the Archean continental nucleus lithospheric mantle in the Yangtze Craton,represented by the Kongling Complex.During the assembly of the Paleoproterozoic Columbia supercontinent,the Archean refractory lithospheric mantle beneath the Yangtze Craton was metasomatized and refertilized by subducting slab-derived melts/fluids.（2）The Zhangjiawan and Zhaojiayangpo olivine pyroxenites have high contents of Mg O（25.0—26.9 wt%）,Cr（1942—3080 ppm）,Ni（1008—1312 ppm）and high Mg^#（84.4—85.6）,suggesting that they are cumulates,formed by fractional crystallization of primary hydrous mafic magmas which are analogue to the products of fractional crystallization of hydrous basaltic or basaltic-andesitic magmas in the subduction zone setting.The olivine pyroxenites have fractionated with depleted to enriched LREE patterns,depletion in high field strength elements（HFSEs）,and positiveε_Nd（t）（+1.9～+5.8）.Their Sm-Nd isotopes yield an errorchron age of～2.2 Ga and Two-stage depleted mantle Nd model ages(T_DM2)of 2.18—2.49 Ga,indicating the olivine pyroxenites formed at～2.2 Ga,and their parent magma derived from metasomatized Paleoproterozoic depleted mantle source.During this period,their parent magmas formed by partial melting of the asthenospheric mantle wedge induced by subduction initiation,and the olivine pyroxenites formed by fractional crystallization as the initial oceanic arc root.We propose that the 2.2—2.05 Ga rocks in the Shuiyuesi mélange belt formed in a similar supra-subduction zone setting,and constituted a part of the Paleoproterozoic intra-oceanic arc-forearc accretionary complex,and collectively record a Paleoproterozoic geodynamic process that from the subduction initiation to mature arc formation.（3）According to the genesis and evolution of the heterogeneous ultramafic rocks in the Shuiyuesi mélange belt,we propose that the late Mesoarchean SCLM peridotites and Paleoproterozoic oceanic lithospheric pyroxenites were simultaneously accreted during a Paleoproterozoic orogenic event.During the tectonic exhumation,the harzburgites with minor metasedimentary rocks emplaced into the eastern microcontinent by vertical diapir,while the pyroxenites together with Paleoproterozoic mafic blocks and metasedimentary rocks continued to subduct and later exhumated to crustal level along the subduction channel.We further propose that the subduction channels include not only oceanic lithospheric materials,but also SCLM wedge materials accreted during plate subduction,and emphasize that tectonic mixing during plate convergence is an important mechanism for the heterogeneous ultramafic rocks in orogenic belts.In addition,this study also provides a crucial geological instance for the numerical modeling result that the materials in the subduction channel can not only emplaced along the subduction channel,but also emplaced by the vertical diaper.（4）Based on the new achievement from the harzburgite and olivine pyroxenite in this study,a model including 2.2—2.0 Ga subduction initiation-accretion and 2.0—1.95Ga collision is proposed to explain the Paleoproterozoic tectonic geodynamic evolution of the North Kongling Complex,Huangling Dome.A southeast-dipping intra-oceanic subduction between the eastern and western microcontinents in the North Kongling Complex was started at～2.2 Ga.In the subduction initiation stage（2.2—2.15 Ga）,the parent magmas of the～2.2 Ga olivine pyroxenites and the 2.15 Ga proto-arc magmas were generated by partial melting of the metasomatized Paleoproterozoic depleted mantle.With the subduction continued,2.15—2.05 Ga diorite-high-Mg basalt-high Mg andesite-Nb-enriched basalts arc-forearc assemblage were produced by partial melting of mantle wedge metasomatized mainly by sediment-derived melts.At～2.04 Ga,the SCLM wedge experienced metasomatism by the melts/fluids released from the subducting slab.Consequently,the SCLM became weakened and was scraped off into the subduction channel.The fragments of the SCLM then emplaced into the overlying continental lithosphere by the vertical diaper.The ocean basin closed at 2.0—1.95 Ga,and the rocks in the subduction channel underwent 2.0—1.95 Ga high-pressure granulite-eclogite facies metamorphism and then emplaced into the crustal level along the subduction channel.（5）Based on the above comprehensive study,it is proposed that the tectonic evolution of the northern basement of the Yangtze Craton was accreted by different micro-blocks during the Paleoproterozoic,which can be divided into three stages:the2.2—2.0 Ga subduction initiation-accretion;the 2.0—1.95 Ga collision;～1.85 Ga post-orogenic extension.The basements on the eastern of the Shuiyuesi mélange belt formed as a whole block before the Paleoproterozoic.While different micro-blocks on the western of the Shuiyuesi mélange belt continued to accrete to the east during the Paleoproterozoic（2.2—1.85 Ga）orogeny.This Paleoproterozoic orogenic event is consistent with the worldwide assembly of the Columbia supercontinent during 2.1—1.8 Ga.