Geochemistry Of Deeply Subducted Continental Crust And Fluid Activity During Its Exhumation: Insights From Studies Of The Chinese Continental Scientific Drilling (CCSD) Main-hole Core Samples

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonic theory in the twenty-first century.Along the Dabie-Sulu orogenic belt in east-central China crops out the largest lithotectonic unit containing ultrahigh-pressure (UHP) metamorphic rocks in the world.Much of our understanding of the world's most enigmatic processes in continental deep-subduction zones has been deduced from various records in this belt.By taking advantage of having depth profiles from core samples of the Chinese Continental Scientific Drilling(CCSD) project in the Sulu orogen,a series of combined studies,including petrography,whole-rock major and trace elements,whole-rock Sr-Nd-Hf isotopes,mineral O and H isotopes and water content(structural hydroxyl content and total water content),and zircon trace elements,U-Pb and Lu-Hf isotopes,were carried out for UHP metamorphic rocks from the CCSD main hole(MH) at continuous depths of 200 to 4000 m.The results provide insights into the chemical geodynamics of continental subduction-zone metamorphism,especially on the issues that are not able to be resolved from discrete surface outcrops.Simultaneous in-situ analyses of trace elements,U-Th-Pb and Lu-Hf isotopes were carried out for distinct domains of zircons from the CCSD-MH core samples.For the first time,trace elements are directly linked to Lu-Hf isotopes in metamorphic zircons with reference to their U-Pb dates.This enables methodological integration to distinguish solid-state,replacement and dissolution recrystallizations of protolith zircons from new growth during continental subduction-zone metamorphism.Metamorphically grown zircons under eclogite-facies metamorphism are characterized by concordant U-Pb ages for the metamorphism,flat HREE patterns typical of the garnet effect,low contents of REE(especially HREE),Y,Nb+Ta and Th+U,high contents of Hf,low(Lu/Gd)_N,Lu/Hf and Th/U(＜0.1) ratios,and elevated ¹⁷⁶Hf/¹⁷⁷Hf ratios relative to solid-state recrystallized zircons.This suggests the effects of both garnet and fluid on the growth of metamorphic zircons.In contrast,metamorphic recrystallization has reset the U-Th-Pb isotope system of protolith zircons to different extents, depending on the extents of fluid action during metamorphism.Solid-state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons,which are characterized by discordant U-Pb ages close to or below the protolith age,steep MREE-HREE patterns typical of magmatic origin,high contents of trace elements and their ratios,and low ¹⁷⁶Hf/¹⁷⁷Hf ratios.On the other hand,dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant U-Pb ages for the metamorphism,steep MREE-HREE patterns,lowered contents of trace elements such as REE,Th,U,Y,Nb,Ta and Ti relative to the protolith zircons,and almost unchanged Hf isotope ratios.Replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes inbetween.While the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of HREE with flat pattern and the low contents of trace elements,the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of MREE to HREE without the flat HREE pattern.Therefore,the simultaneous in-situ analyses of metamorphic zircons have the advantage over single-term analyses in making distinction between the new growth and the different types of recrystallization.Petrography,mineral O and H isotopes,mineral water contents(total water and structural hydroxyl) were systematically investigated for UHP metamorphic rocks in a depth of 200-4000 m from the CCSD-MH.Premetamorphic protolith is deduced to have underwent high-T meteoric hydrothermal alteration and even lowδ¹⁸O magmatism before the continental subduction.Minimum depth of ¹⁸O depletion is up to 3300 m,together with areal ¹⁸O depletion of over 30000 km² in surface outcropped rocks along the Dabie-Sulu orogenic belt, the three-dimensional ¹⁸O depletion of over 100000 km³ occurs along the northern margin of the Yangtze Block.The UHP metamorphic rocks show large variations in H and O isotopes. Both equilibrium and disequilibrium H and O isotope fractionations are observed between coexisting minerals.The same minerals show different H and O isotope behaviors in different lithologies.All these indicate different retrograde effects on the UHP minerals and isotopes during the exhumation of deeply subducted continental crust.Inspection of the relationship between the distance,petrography and ¹⁸O values of adjacent samples shows significant O isotope heterogeneities between the different and same lithologies on scales of 20 to 50 cm, corresponding to the maximum scales of fluid flow during the continental collision.Both TC/EA-MS and FTIR analyses show that nominally anhydrous minerals contain significant amounts of water in the form of structural hydroxyl and molecular water.Despite the widespread retrogression,retrograde fluid was internally buffered in the stable isotope compositions.The retrograde fluid is of deuteric origin and thus was derived from the decompression exsolution of structural hydroxyl and molecular water as well as decomposition of hydrous minerals.Changes in mineral O isotope,H isotope and water content occur in eclogite-gneiss contacts,concordant with petrographic changes.The contacts between the different lithologies are thus the most favorable place for fluid action.Fluid for retrogression of the eclogites away from the eclogite-gneiss contact was derived from the decompression exsolution of internal water.For the eclogites adjacent to gneiss,in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss that is relatively rich in water.A quantitative estimate suggests that a 1 m³ volume of bimineral eclogite composed of garnet and omphacite can release 3.07-3.44 kg of water by decompression exsolution of structural hydroxyl,which can in turn form 139.58-156.28 kg of amphibole during exhumation.This can provide a sufficient amount of water for amphibolitization of eclogite.Systematic measurement and calculation of mineral water contents suggest that the gneiss is capable of storing more water than the eclogite under the same UHP conditions.Thus,the UHP gneiss can release more water than the UHP eclogite during the initial exhumation of deeply subducted slabs.By decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and thus result in remarkable hydration of the eclogite adjacent to the gneiss.Whole-rock major-trace elements and Sr-Nd-Hf isotopes were systematically investigated for continuous core segments from the CCSD-MH to decipher element mobility in UHP eclogite-facies metamorphic rocks during subduction and exhumation of continental crust. The UHP metamorphic rocks from continuous core segments exhibits a large variation in major and some trace elements such as LILE(e.g.,K,Rb,Ba,Th and U) and LREE,but a relatively limited range in HFSE and HREE.This suggests high mobility of LILE and LREE but immobility of HFSE and HREE during continental subduction-zone metamorphism. Though aqueous fluid can result in large variation of LILE,large variation of SiO₂ and LREE in some eclogite suggests metasomatism of felsic melt produced by partial melting of the associated gneisses during the exhumation.The petrographic observation also demonstrates the presence of partial melting.Concordant variations of element and petrography at the contact between the different lithologies imply the occurrence of aqueous fluid or hydrous melt and its associated element mobility between the different slab components during exhumation.Partial melting locally occurs in the same lithologies.While the partial melting petrographically occurs in some samples,there is no corresponding change in lithochemistry. This suggests that the partial melts did not escape from the host rocks,resulting in a kind of metatexite migmatites.Therefore,partial melting of continental crust took place locally during "hot" exhumation of the deeply subducted continental crust,causing significant mass transfer within the slab.Nevertheless,the efficient transport of elements only occurs on small scales and is thus limited in local open-systems at the lithological contrasts and fractures. Zircon U-Pb and Lu-Hf isotope studies indicate that the UHP rocks from the CCSD-MH have protolith of contrasting origins.Mid-Neoproterozoic protoliths of bimodal UHP metaigneous rocks formed during supercontinental rifting at the northern margin of the Yangtze Block.The first type of bimodal magmatism formed by reworking of juvenile Late Mesoproterozoic crust,whereas the second type of bimodal magmatism was principally generated by rifting anatexis of ancient Middle Paleoproterozoic crust.Melting of orogenic lithosphere has potential to bring about bimodal magmatism with contrasting origins.Because arc-continent collision zones are the best place to accumulate both juvenile and ancient crusts, the contrasting types of bimodal magmatism are proposed to occur in an arc-continent collision orogen during the supercontinental rifting,in response to the attempted breakup of the supercontinent Rodinia at ca.780 Ma.As weak zones in the lithosphere,they would be further developed by the supercontinental rifting,finally transformed to the supercontinental breakup at ca.750 Ma.Underplating of mafic magma during the tectonic advance from supercontiental rifting to breakup results in repetitive reworking of both the meteoric-hydrothermally altered juvenile crust and the Paleoproterozoic ancient crust.In this regard,the dual-bimodal compositions for the CCSD-MH metaigneous protoliths lent support to the plate-rift model for petrogenesis of mid-Neoproterozoic igneous rocks in South China.