Partial Melting Of UHP Metamorphic Rocks In The Sulu Orogen And Its Geochemical Effects

The study of continental deep subduction and ultrahigh-pressure(UHP)metamorphism has been a forefront area in the field of Earth science and has contributed much to the development of plate tectonics.Studies of fluid action in continental subduction zones,especially the partial melting of UHP metamorphic rocks,are important to understand the processes and mechanism for exhumation of of deeply subducted continental crust,crust-mantle interaction and the recycling of crustal material at convergent plate boundaries.The Dabie-Sulu orogenic belt is one of the largest UHP metamorphic belts in the world.It was built by the Triassic subduction of the South China Block beneath the North China Block in eastern China.It provides us an ideal natural laboratory to study the partial melting of UHP metamorphic rocks and its geochemical effects in continental subduction zones.This PhD thesis has chosen UHP metamorphic rocks from the Weihai and Yangkou areas in the Sulu orogen as the target.The study focused on geochronology,petrology and geochemistry of granitic gneisses,migmatites,eclogites and other partial melting related rocks.The results provide not only constraints on the timing,processes and reaction mechanism of partial melting in the continental subduction zone,but also on the geochemical compositions of HP to UHP anatectic melts and the genetic links to the origin of granites.This provides insights into the details of element mobility and the behaviors of accessory minerals during the crustal anatexis,and deepens our understanding of the chemical geodynamics of subduction zones.The study of granitic gneisses from the Yangkou area suggests that their protolith contains Eoarchean crustal components in the Yangtze craton,and the ancient crustal basement experienced multistage reworking in the Paleoproterozoic and Neoproterozoic.Eoarchean magmatic zircons(?3.7 Ga)are reported for the first time from the granitic gneisses,indicating the possible occurrence of the Eoarchean basement of Yangtze craton in the Sulu orogen.In addition,the other three groups of zircon domains show younger U-Pb ages in Paleoproterozoic(2.1 Ga)and Neoproterozoic(790 Ma,720 Ma),respectively.The Eoarchean zircons have negative ?Hf(t)values(-2.8 to-0.9)with chondrite Hf model ages of 3.74-3.96 Ga,indicating growth of the juvenile crust as early as the early Eoarchean in the Yangtze craton.They have low Al and P contents,high(Y+REE)/P molar ratios,suggesting that these zircons grew from the partial melting of metaluminous I-type granites.The Paleoproterozoic and Neoproterozoic zircons have similar two-stage depleted mantle Hf model ages of 2.6-3.0 Ga,suggesting the episodic reworking of the Archean crust from the Paleoproterozoic to Neoproterozoic.The Paleoproterozoic zircons may record magmatism in response to the assembly of Columbia supercontinent,whereas the Neoproterozoic zircons record magmatic events in response to continental rifting during the breakup of Rodinia supercontinent.This study demonstrates the presence of Eoarchean crustal remnants in the Yangtze craton and multiphase reworking of the Archean crust during the Paleoproterozoic and Neoproterozoic,respectively.The study of migmatites from the Weihai area indicates multiple episodes of partial melting of the deeply subducted continental crust in the stages of exhumation and post-collision.Peak UHP metamorphism occurred at ca.230±1 Ma,followed by four episodes of partial melting at 222±2 Ma,215±1 Ma,177±2 Ma and 152±2 Ma,respectively.Metamorphic zircon domains containing coesite inclusions exhibit relatively low Th/U ratios(<0.1),flat REE patterns with no Eu anomalies.In addition,anatectic zircon domains show very high contents of U and are enriched in Th,Nb,Y,and REE,with low Th/U ratios(<0.1),steep HREE patterns and negative Eu anomalies.Both metamorphic and anatectic zircon domains have elevated ?Hf(t)values relative to relict magmatic cores,suggesting contributions from the breakdown of minerals with more radiogenic Hf to zircon growth.The leucosome and melanosome pairs in each sample locality have homogeneous Sr-Nd-O isotopic compositions,suggesting that they were derived from partial melting of the same source rocks.Significant geochemical differentiation occurred on various scales during crustal anatexis and subsequent melt segregation.The leucosomes and leucocratic veins with more felsic components contain low contents of REE,Th,and Y with positive Eu anomalies,but have elevated ratios of Ba/Th and Sr/Y relative to the associated melanosomes.The Dabie and Sulu orogens show considerable differences in the timing of post-collisional crustal anatexis.The Jurassic anatexis is prominent in the Sulu orogen but it is absent in the Dabie orogen,suggesting that these two orogens underwent differential reworking and evolution.The study of phengite-bearing granites from the Yangkou area provides constrains on their petrogenetic relationship to the host UHP eclogites.The granites show different Sr-Nd-O isotope compositions from the associated eclogites,indicating that these granites did not derive from partial melting of the eclogites.Instead,they would derive from partial melting of regional granitic gneisses,which have comparable isotopic compositions with them.Zircon U-Pb dating results suggest that the granites formed at 216-222 Ma,slightly younger than the UHP metamorphism for zircon growth in the eclogites,indicating that the partial melting of granitic gneisses occurred at the early exhumation stage of the deeply subducted continental crust.Si-in-phengite pressures in these granites indicate that they formed under high pressure(HP)to UHP conditions(1.9-3.0 GPa).This is supported by the occurrence of large amounts of rutile and high Ca garnets in the granites.These granites were crystallized from HP-UHP granitic melts and enriched in large ion lithophile elements(LILE)and LREE with negative Eu anomalies,depleted in high field strength elements(HFSE)with remarkable negative anomalies of Nb,Ta,and Ti.Their geochemical compositions,especially the trace element contents,are different from migmatite leucosomes from the Weihai area and the other areas.This difference may be attributed to their higher formation pressures and the dissolution of more accessory minerals like apatite and epidote into the melts.These granites with low Sr/Y and(La/Yb)N ratios are also different from adakitic rocks which are usually considered to form under HP and UHP conditions.Geochemical parameters such as K2O/Na2O,A/CNK,P2O5,LREE,Li,Ba and V show positive correlations with pressure,indicating the significant pressure effects on melt compositions.It is consistent with melt composition features from previous high pressure melting experiments.Under HP to UHP conditions,the stabilities of phengite,clinopyroxene and alkaline feldspar dominate melt composiiton such as LILE,K/Na and A/CNK.Peraluminous melts under higher pressures can further facilitate the dissolution of accessory minerals like apatite,resulting in higher LREE and P contents and elevated ?Nd(t)values in the granitic melts.The elevated Hf isotope ratios for anatectic zircons also indicate the breakdown of radiogenic Hf-bearing minerals like garnet and apatite with high Lu/Hf ratios during partial melting.Therefore,by using the natural samples from the typical collisional orogen,this study provides further constraints not only on the effects of pressure on melt compositions under HP to UHP conditions,but also on the behavior of minerals that can account for the geochemical features of anatectic melts.