The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very import...The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.展开更多
In this study,we report systematically field observations,geochronology,whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area.Chemically,the diabas...In this study,we report systematically field observations,geochronology,whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area.Chemically,the diabase and gabbro have a low-K calc-alkaline affinity,whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics.In addition,the basalts are highly enriched in light rare earth elements(LREE)and large ion lithophile elements(LILE),but strongly depleted in high strength field elements(HFSE),indicating that their magma source probably was derived from a subduction-or arc-related setting.In contrast,both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt(N-MORB)origin.The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca.158–161 Ma(Oxfordian stage),but the gabbro was crystallized at ca.131 Ma(Hauterivian stage of Early Cretaceous).The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt.Furthermore,the plagiogranite has a weighted mean age of ca.160 Ma(MSWD=2.1)consistent with the basalt within the uncertainty.The basalt and the plagiogranite have significantly positiveεHf(t)values(+5.8 to+15.6 and+8.6 to+16.1,respectively),suggesting that they were originated from partial melting of a depleted source.However,basalt and plagiogranite are characterized by the wide variations ofεHf(t)values indicating minor amounts of exotic crustal material input during the later magma evolution.Additionally,the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt,corresponding to the supra-subduction zone-(SSZ)type affinity.To sum up,two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic,but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase.Integrated with regional background and different rock types,as well as geochemical features,we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area,southern Tibet.展开更多
Voluminous platinum-group mineral(PGM)inclusions including erlichmanite(Os,Ru)S2,laurite(Ru,Os)S2,and irarsite(Ir,Os,Ru,Rh)AsS,as well as native osmium Os(Ir)and inclusions of base metal sulphides(BMS),including mille...Voluminous platinum-group mineral(PGM)inclusions including erlichmanite(Os,Ru)S2,laurite(Ru,Os)S2,and irarsite(Ir,Os,Ru,Rh)AsS,as well as native osmium Os(Ir)and inclusions of base metal sulphides(BMS),including millerite(NiS),heazlewoodite(NiaS2),covellite(CuS)and digenite(Cu3S2),accompanied by native iron,have been identified in chromitites of the Zedang ophiolite,Xizang.The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them;most are less than 10~m in size and vary in shape from euhedral to anhedral.They occur either as single or composite(biphase or polyphase)grains composed solely of PGM,or PGM associated with silicate grains.Os-,Ir-,and Ru-rich PGMs are the common species and Pt-,Pd-,and Rh-rich varieties have not been identified.Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle.If the activity of chalcogenides(such as S,and As)is low,PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys.Under appropriate conditions of fS2 and fO2,PGE alloys might react with the melt to form sulfides-sulfarsenides.Thus,we suggest that the Os,Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS2,followed by crystallization of sulphides of the laurite-erlichmanite,solid-solution series as the magma cooled and fS2 increased.The abundance of primary BMS in the chromitites suggests that fS2 reached relatively high values during the final stages of magnesiochromite crystallization.The diversity of the PGE minerals,in combination with differences in the petrological characteristics of the magnesiochromites,suggest different degrees of partial melting,perhaps at different depths in the mantle.The estimated parental magma composition suggests formation in a suprasubduction zone environment,perhaps in a forearc.展开更多
Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic m...Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.展开更多
We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better ...We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better constrain the petrogenesis of the Zedang and Luobusa ophiolites and the tectonic evolution of the Neo-Tethys. Plots of chondrite-normalized PGE, PGE vs. Mg#, and PGE vs. Al_2O_3 suggest that the lherzolite and harzburgite from Zedang and Luobusa have similar PGE characteristics. The Zedang and Luobusa peridotites display U-shaped REE patterns and are enriched in some incompatible elements, indicative of melt-rock interaction. The PGE characteristics may be attributed to partial melting and heterogeneous melt-rock interaction. Mineral chemistry and whole rock major and trace elements data suggest that lherzolite and harzburgite from Zedang and Luobusa have similar geochemical properties. On the spinel Mg# vs. Cr# plot, the composition of the Zedang and Luobusa peridotites is consistent with both abyssal and subduction-zone peridotites. This study indicates that the Zedang and Luobusa peridotites have a similar origin and evolution path： they could have originated from a normal mid-ocean ridge environment and got refertilization in a supra-subduction zone setting.展开更多
Magnetic signature of serpentinized mantle peridotite has crucial importance in understanding the serpentinization process and interpreting the origin of strong magnetization anomalies at ultramafic-hosted hydrotherma...Magnetic signature of serpentinized mantle peridotite has crucial importance in understanding the serpentinization process and interpreting the origin of strong magnetization anomalies at ultramafic-hosted hydrothermal settings. However, different groups of serpentinized peridotites from both ocean drillings and ophiolite complexes have shown considerable variations in the abundance of magnetite(Oufi et al., 2002;Bonnemains et al., 2016;Li et al., 2017). We examined the magnetic properties, petrography and mineral chemistry of variably serpentinized peridotites from Zedang ophiolite in the eastern Yarlung-Zangbo suture in south Tibet to evaluate the conditions of serpentinization and magnetite formation as well as magnetic sources in suture zones. The studied samples were 0–90% serpentinized with densities from 3.316 to 2.593 g cm–3 and show typical mesh textures of olivine replaced by serpentine on thin sections of core specimen. Serpentines were divided into type-1 Fe-poor serpentine mesh(1.84–2.88 wt% FeO) associated with magnetite in the early stage and type-2 Fe-rich serpentine cores(3.92–5.12 wt% FeO) with no formation of magnetite in the late serpentinization. Brucite vein appeared in central serpentine veins and show Mg/(Mg+Fe) values of 0.74–0.87 at ~50–70% of serpentinization. Pure magnetite was identified as the main magnetic carrier by thermomagnetic analyses, but minor Cr-magnetite(~0.8 mole fractions of Fe3O4) was also detected due to oxidation of early spinel. All the peridotite samples show a rapid increase of magnetic susceptibility from ~0.001 to ~0.03 SI before 40–50% of serpentinization and a following flat trend in values 0.02–0.03 SI at > 50% of serpentinization. This density-susceptibility relationship differs from the rapid production of magnetite above 60-70% of serpentinization for many abyssal peridotites(Oufi et al., 2002;Bach et al., 2006) and suggests that magnetite formation was coupled with hydration of olivine in the early serpentinization but the two decoupled at ~ 40–50% of serpentinization. This transition is consistent with the petrographic observation that magnetite-free serpentinization was developed in higher degrees(> 50%) of serpentinization. Prior studies suggested that serpentinization of < 200℃ would generate Fe-rich brucite, serpentine and little magnetite, whereas magnetite-rich serpentinization was associated with Fe-poor brucite and occurred at higher temperatures of 200–300℃(Klein et al., 2014). The petromagnetic features of serpentinized peridotites from the Zedang ophiolite indicate that the serpentinization process took place initially above 250℃(estimate from brucite composition) and continued to lower temperatures of < 200℃, probably during the mantle lithosphere cooling down in forearc settings(Xiong et al., 2017). These serpentinized peridotites have higher magnetization intensities(average 2.26 Am-1) than mafic dolerite dykes and basaltic volcanic rocks(mostly < 1 Am-1) and should be significant sources of aeromagnetic highs in the Yarlung-Zangbo suture.展开更多
基金jointly supported by the Geological Survey Project of Chinese (Grant No.1212010911070 and No.12120113093900)National Science Foundation of China (Grant No. 41072167)Institute of Geology, Chinese Academy of Geological Sciences (Grant No.J1309)
文摘The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.
基金financially co-supported by the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (Nos. J1901-7,J1901-16)the State Scholarship Fund (No. 201904180031)+4 种基金the National Key Research and Development Project of China (No. 2016YFC0600310)the 2nd Tibetan Plateau Scientific Expedition (No. 2019QZKK0802)the National Natural Science Foundation of China (Nos. 41672046,41641015,41762005,41720104009,41703036)the China Geological Survey (No. DD201190060)the International Geological Correlation Project (No. IGCP-649)
文摘In this study,we report systematically field observations,geochronology,whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area.Chemically,the diabase and gabbro have a low-K calc-alkaline affinity,whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics.In addition,the basalts are highly enriched in light rare earth elements(LREE)and large ion lithophile elements(LILE),but strongly depleted in high strength field elements(HFSE),indicating that their magma source probably was derived from a subduction-or arc-related setting.In contrast,both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt(N-MORB)origin.The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca.158–161 Ma(Oxfordian stage),but the gabbro was crystallized at ca.131 Ma(Hauterivian stage of Early Cretaceous).The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt.Furthermore,the plagiogranite has a weighted mean age of ca.160 Ma(MSWD=2.1)consistent with the basalt within the uncertainty.The basalt and the plagiogranite have significantly positiveεHf(t)values(+5.8 to+15.6 and+8.6 to+16.1,respectively),suggesting that they were originated from partial melting of a depleted source.However,basalt and plagiogranite are characterized by the wide variations ofεHf(t)values indicating minor amounts of exotic crustal material input during the later magma evolution.Additionally,the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt,corresponding to the supra-subduction zone-(SSZ)type affinity.To sum up,two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic,but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase.Integrated with regional background and different rock types,as well as geochemical features,we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area,southern Tibet.
基金financially supported by the National Natural Science Foundation of China(Grant No.41262002,40930313,41302052,41502062)National Industry Special Projects(Sino Probe-05-02)+2 种基金China Bureau of Geological Survey Projects(DD20160023-01)IGCP-649Institute of Geology,Chinese Academy of Geological Science(J1526)
文摘Voluminous platinum-group mineral(PGM)inclusions including erlichmanite(Os,Ru)S2,laurite(Ru,Os)S2,and irarsite(Ir,Os,Ru,Rh)AsS,as well as native osmium Os(Ir)and inclusions of base metal sulphides(BMS),including millerite(NiS),heazlewoodite(NiaS2),covellite(CuS)and digenite(Cu3S2),accompanied by native iron,have been identified in chromitites of the Zedang ophiolite,Xizang.The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them;most are less than 10~m in size and vary in shape from euhedral to anhedral.They occur either as single or composite(biphase or polyphase)grains composed solely of PGM,or PGM associated with silicate grains.Os-,Ir-,and Ru-rich PGMs are the common species and Pt-,Pd-,and Rh-rich varieties have not been identified.Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle.If the activity of chalcogenides(such as S,and As)is low,PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys.Under appropriate conditions of fS2 and fO2,PGE alloys might react with the melt to form sulfides-sulfarsenides.Thus,we suggest that the Os,Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low fS2,followed by crystallization of sulphides of the laurite-erlichmanite,solid-solution series as the magma cooled and fS2 increased.The abundance of primary BMS in the chromitites suggests that fS2 reached relatively high values during the final stages of magnesiochromite crystallization.The diversity of the PGE minerals,in combination with differences in the petrological characteristics of the magnesiochromites,suggest different degrees of partial melting,perhaps at different depths in the mantle.The estimated parental magma composition suggests formation in a suprasubduction zone environment,perhaps in a forearc.
基金co-supported by Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou)(No.GML2019ZD0201)the Second Tibetan Plateau Scientific Expedition and Research Program(No.2019QZKK0801)+3 种基金the National Natural Science Foundation of China(NNSFCProject Nos.42272048,41720104009,42172069,92062215)the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources Fund(No.J1901-28)the China Geological Survey(CGS,Project Nos.DD20230340,DD20221630)。
文摘Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.
基金supported by the Marine Geological Survey of the 1 : 250 000 Rizhao Sheet and Lianyungang Sheet (No. GZH201400206)
文摘We present a new dataset on platinum group elements（PGEs）, whole-rock major and trace elements, and mineral chemistry for the peridotites from the Zedang and Luobusa ophiolite suites, Tibet, in an attempt to better constrain the petrogenesis of the Zedang and Luobusa ophiolites and the tectonic evolution of the Neo-Tethys. Plots of chondrite-normalized PGE, PGE vs. Mg#, and PGE vs. Al_2O_3 suggest that the lherzolite and harzburgite from Zedang and Luobusa have similar PGE characteristics. The Zedang and Luobusa peridotites display U-shaped REE patterns and are enriched in some incompatible elements, indicative of melt-rock interaction. The PGE characteristics may be attributed to partial melting and heterogeneous melt-rock interaction. Mineral chemistry and whole rock major and trace elements data suggest that lherzolite and harzburgite from Zedang and Luobusa have similar geochemical properties. On the spinel Mg# vs. Cr# plot, the composition of the Zedang and Luobusa peridotites is consistent with both abyssal and subduction-zone peridotites. This study indicates that the Zedang and Luobusa peridotites have a similar origin and evolution path： they could have originated from a normal mid-ocean ridge environment and got refertilization in a supra-subduction zone setting.
基金granted by the Fundamental Research Funds for the Central Universities (Grant No. CUG180620)the NSFC project (Grant No. 41520104003)
文摘Magnetic signature of serpentinized mantle peridotite has crucial importance in understanding the serpentinization process and interpreting the origin of strong magnetization anomalies at ultramafic-hosted hydrothermal settings. However, different groups of serpentinized peridotites from both ocean drillings and ophiolite complexes have shown considerable variations in the abundance of magnetite(Oufi et al., 2002;Bonnemains et al., 2016;Li et al., 2017). We examined the magnetic properties, petrography and mineral chemistry of variably serpentinized peridotites from Zedang ophiolite in the eastern Yarlung-Zangbo suture in south Tibet to evaluate the conditions of serpentinization and magnetite formation as well as magnetic sources in suture zones. The studied samples were 0–90% serpentinized with densities from 3.316 to 2.593 g cm–3 and show typical mesh textures of olivine replaced by serpentine on thin sections of core specimen. Serpentines were divided into type-1 Fe-poor serpentine mesh(1.84–2.88 wt% FeO) associated with magnetite in the early stage and type-2 Fe-rich serpentine cores(3.92–5.12 wt% FeO) with no formation of magnetite in the late serpentinization. Brucite vein appeared in central serpentine veins and show Mg/(Mg+Fe) values of 0.74–0.87 at ~50–70% of serpentinization. Pure magnetite was identified as the main magnetic carrier by thermomagnetic analyses, but minor Cr-magnetite(~0.8 mole fractions of Fe3O4) was also detected due to oxidation of early spinel. All the peridotite samples show a rapid increase of magnetic susceptibility from ~0.001 to ~0.03 SI before 40–50% of serpentinization and a following flat trend in values 0.02–0.03 SI at > 50% of serpentinization. This density-susceptibility relationship differs from the rapid production of magnetite above 60-70% of serpentinization for many abyssal peridotites(Oufi et al., 2002;Bach et al., 2006) and suggests that magnetite formation was coupled with hydration of olivine in the early serpentinization but the two decoupled at ~ 40–50% of serpentinization. This transition is consistent with the petrographic observation that magnetite-free serpentinization was developed in higher degrees(> 50%) of serpentinization. Prior studies suggested that serpentinization of < 200℃ would generate Fe-rich brucite, serpentine and little magnetite, whereas magnetite-rich serpentinization was associated with Fe-poor brucite and occurred at higher temperatures of 200–300℃(Klein et al., 2014). The petromagnetic features of serpentinized peridotites from the Zedang ophiolite indicate that the serpentinization process took place initially above 250℃(estimate from brucite composition) and continued to lower temperatures of < 200℃, probably during the mantle lithosphere cooling down in forearc settings(Xiong et al., 2017). These serpentinized peridotites have higher magnetization intensities(average 2.26 Am-1) than mafic dolerite dykes and basaltic volcanic rocks(mostly < 1 Am-1) and should be significant sources of aeromagnetic highs in the Yarlung-Zangbo suture.
