Petrogenesis Of The Mafic-ultramafic Rocks In Sulu Ultrahigh-pressure Metamorphic Belt

The Dabie–Sulu orogenic belt formed during the collision of the North ChinaCraton (NCC) with the Yangtze Craton (YZC) followed by the subduction of the latterin Mesozoic time. The terrane contains volumetrically minor mafic-ultramafic rocksthat form massifs and large lenticular bodies closely associated with UHP rocks. Onthe one hand, these mafic-ultramafic rocks preserve abundant information about thenature and structure of the lithospheric mantle; on the other hand, thesemafic-ultramafic rocks could help to tracer dynamic processes about hydrous fluidactivity, crust-mantle interaction, mechanism of island arc magma, UHPmetamorphism and exhumation in the subduction zone. The study on thesemafic-ultramafic rocks is an effective and direct approach way to explore thecompositions and dynamics of the deep Earth.This thesis selects the representative areas of mafic-ultramafic rocks distributingin the central region of Sulu UHP metamorphism belt, including Yangkou area,Suoluoshu area and Hujialing area. The studied mafic-ultramafic rocks includeserpentinites (Yangkou area and Suoluoshu area), dunite and garnet-bearingclinopyroxenite (Hujialing area).Mineralogy and mineral chemistry studies confirmed that the protoliths for YKBand SLS serpentinites are probably harzburgite and dunite, respectively. Theultramafic rocks in YKB and SLS are hydrated to form serpentinites and they containlow concentrations of moderately incompatible elements (Al，Ti，V), high contents ofcompatible elements (Ni, Cr, Co) and IPGE (IPGE; Ir, Os, and Ru), and high ratios ofIPGE to PPGE in bulk rocks. Spinel contains high Cr (Cr#=Cr/(Cr+Al);0.57–0.79). The data suggest that they likely represent hydrated forearc mantle peridotitesunderlying the margin of the NCC. Dunite samples from HJL are not fully hydratedwith loss on ignition values ranging from6.6to13.2wt.%, and contain olivine grainswith high Fo (91.7–92.4) and NiO contents (0.36–0.41wt.%). Spinel grains showhigh Cr#(0.68–0.76). The bulk rock contains high IPGE and high ratios of IPGE toPPGE. The data suggests that they are also residual mantle peridotites after highdegrees of influx partial melting in the subduction setting, and they also representhydrated forearc mantle peridotites underlying the margin of the NCC. The protolithof Suoluoshu serpentinites are similar to Hujialing dunites before their hydration, thedifference between them is that one is totally hydrated and the other is partiallyhydrated. The studied ultramafic rocks likely represent the relic of old refractorylithospheric mantle peridotites that underwent partial melting in the margin of theNCC. Different from abundant garnt peridotites and garnet pyroxenite in same terrane,the studied ultramafic rocks didn t undergo deep subduction and UHP metamorphism.These ultramafic rocks just have a short loop shallow subduction after incorporationinto the subduction channel, then they exhume together with granitic gneiss in thereturn flow of the subduction channel.The studied serpentinites and dunites show a significant enrichment offluid-mobile elements (Pb, Sb, Sr). These fluid-mobile elements and water werereleased from the subducted slabs at a certain depth range, and these componentsmigrated upward and hydrated the mantle peridotites to form a serpentinite layer inthe mantle wedge. The serpentinites in other subduction zones worldwide, showsimilar characteristics of enriched in these fluid-mobile elements. Comparison withserpentinites in other subduction zones, the degrees of enrichment in fluid-mobileelements are greater in continental subduction zones than oceanic subduction zones.The difference likely reflects the compositions of subducted material and shallowwater sediments on the margins of continents are commonly enriched in fluid–mobileelements and readily release them to the overlying mantle. Continuous hydration inthe mantle wedge often forms different scales of serpentinite layer the subducted slabsand mantle wedge. The serpentinite layers are dragged into the deep mantle by mantleflow, and the followed breakdown of serpentinites releases a large amount of water as well as fluid-mobile elements. These components migrate upward and areincorporated by the partial melt that generates arc magmas. Although the decouplingof the continental slabs plays a leading role in the exhumation of HP-UHPmetamorphic material, the serpentinite layers in subduction zone often form lowviscosity channe, which efficiently facilitated subduction and exhumation of thesubducted rocks by acting as a lubricant.Layers of garnet-bearing clinopyroxenite are enclosed in a serpentinite body atHujialing (HJL), and the genesis of these garnet-bearing clinopyroxenites isinterpreted as ultramafic cumulates formed by mantle-derived melts. The petrologyand geochemistry evidences as followed: a. low contents of IPGE and low ratios ofIPGE to PGE (PPGE) in bulk rock compositions suggest that the protolith is formedby crystal accumulations of ultramafic magma, not the product of the partial meltingof mantle peridotities. b. The major element and trace element compotisions in bulkrocks are similar to the supra-subduction zone ophiolites, bulk rock compositions aregenerally enrichment in compatible elements relative to MORB, and theirchondrite-normalized REE patterns are convex upward and no Eu anomaly,confirming that studied garnet-bearing clinopyroxenites likely formed by crystalaccumulations of arc magma derived from mantle melts, not gabbro. c. The studiedsamples preserve igneous textures, relict olivine show low Fo (76.6～76.8) and NiOcontents (0.25%～0.26%), clinopyroxene compositions are highly similar to those ofisland-arc ultramafic cumulates, suggesting that the studied samples are island-arcultramafic cumulates, and exclude the possibility of mantle peridotites. The highdegree of enrichment of fluid-mobile elements (Pb and Ba) in and significantly varycontents of compatible elements and incompatible element compositions in bulk rocks,indicating that the studied samples were cumulated not from primary magmas inequilibrium with mantle peridoties, but from evolved magmas after variable amountsof differentiation, and the primary magmas likely mixed with hydrous fluids or meltsderived from subducted continental slabs. The protoliths of garnet-bearingclinopyroxenite are dragged into subduction channel by the mantle flow, and subjectto the deep subduction process and UHP metamorphism (P≥5.0GPa; T≥750℃;according to the previous results). The garnet-bearing clinopyroxenite finally exhume together with the buoyant granitic metamorphic rocks in the Sulu belt and undergo anapparent retrograde metamorphism.The genesis of mafic-ultramafic rocks in Sulu UHP metamorphism belt includesfour types: a. Garnet phase peridotites originally derived from ancient lithosphericmantle and subjected to UHP metamorphism. b. Spinel-stable peridotites derived fromancient lithospheric mantle, commomly experienced various degrees of metasomatism,such as serpentinization. c. Ultramafic cumulates formed the accumulation of arcmagmas originally derived from mantle melts. d. Mafic-ultramafic rocks emplacedinto Yangtze continental lithosphere prior to the subduction, and commonly subjectedto UHP metamorphism.The abundant mafic-ultramafic rocks derived from ancient lithospheric mantle ofthe NCC and two distinctly nature (ancient and cenozoic lithospheric mantle) ofmantle xenoliths in Cenozoic basaltic magma indicate that lithospheric mantle blowthe Sulu terrane were destructed during the dynamics of collision and subductionbetween Yangtze continent and the NCC. The author attempts to correct dynamicsmodel of the collision and subduction between Yangtze continent and the NCC basedon the summary of the petrogenesis and tectonic evolution of mafic-ultramafic rocksin Sulu UHP metamorphism belt, and the relationship between forearc mantleserpentinites and arc magmatism, exhumation of HP-UHP metamorphic material.