Evidence for UHP metamorphism of eclogites from the Altun Mountains

Ultrahigh pressure (UHP) metamorphism refers to metamorphism that has occurred at pressures for the stability of coesite. The polycrystalline quartz inclusions showing the characteristic texture within garnets of eclogites indicates the pre-existence of coesites under the peak metamorphic condition. The unusual exsolution textures in ompacites and apatites, and the pressure estimations of phengite-bearing eciogites have been taken to provide further proof of eclogite formation under the UHP conditions.Combined with the fact that coesites have been observed in country rocks of eclogites in North Qaidam Mountains, another UHP metamorphic belt cut by the large-scale strikeslip fault in the AItun-North Qaidam area of China is confirmed.``     

Carbon isotope anomaly in marbles associated with eclogites from the Dabie Mountains

This is a report of carbon isotope anomaly in marbles associated with the UHP eclogites from the Dabie Mountains. The results place constraints on the degree of crust-mantle interaction during the UHP metamorphism.     

冀北退变榴辉岩的Pb同位素特征

冀北退变榴辉岩的全岩Pb同位素比值(206Pb/204Pb)i,(207Pb/204Pb)i和(208Pb/204Pb)i分别为17.324～17.460,15.412～15.464和37.216～37.380;△(207Pb/204Pb)i和△(208Pb/204Pb)i相应为4.026～8.034和61.285～69.928,表明岩浆源区为具DUPAL异常的富集地幔,可能为亏损的软流圈地幔(DMM)与第一类富集地幔端元(EMI)相混合的异常地幔,指示该区早先存在有一次板块消减事件.这对正确认识华北古陆块北缘的大地构造格局和演化具有十分重要的意义.     

Partial melting of deeply subducted continental crust and the formation of quartzofeldspathic polyphase inclusions in the Sulu UHP eclogites

Two types of quartzofeldspathic inclusions hosted by omphacite and garnet were identified in the Sulu UHP eclogites. The first consists of albite, quartz, and various amounts of K-feldspar. In contrast, the second consists predominantly of K-feldspar and quartz without any albite. The presence of quartzofeldspathic inclusions within the UHP mafic eclogites indicates that partial melting occurred in deeply subducted continental crust via mica dehydration melting reactions at an early stage of rapid exhumation. Such a melting event generated hydrous Na-K-Al-Si melts. These melts infiltrated into the mafic eclogite and were captured by recrystallizing garnet or omphacite, which together followed by dehydration and crystallization to form feldspar-bearing polyphase inclusions. Formation of silicate melts within the deeply subducted continental slab not only provides an excellent medium to transport both mobile （LILE） and immobile （HFSE） elements over a large distance, but also induces effective changes in the physical properties of the UHP slab. This process could be a major factor that enhances rapid exhumation of a deeply subducted continental slab.     

Determination and geological significance of the eclogites from the northern Dabie Mountains, central China

Numerous eclogite-amphibolite blocks have been found along the Mayanling fault in the northern Dabie complex. Most of the eclogites are strongly retrograded and contain garnet, rutile and the amphibole-plagioclase symplectite. The occurrence of these eclogites may have something to do with the inverted movement of the Mayanling thrust fault.     

New finding of micro-diamonds in eclogites from Dabie-Sulu region in central-eastern China

Micro-diamonds were only found ten years ago in eclogite associated with marble at Xindian in the Dabie Mountains. This paper reports our new finding of micro-diamonds not only in eclogites at Maobei in the Sulu region and at Xindian and Laoyoufang in the south part of the Dabie Mountains (South Dabie), but also in eclogites at Baizhangya and Huangweihe in the northern part of the Dabie Mountains (North Dabie) that has usually been considered not to experience ultrahigh pressure metamorphism.Except the micro-diamond at Huangweihe that was found from the artificial heavy sands of zircons used for isotopic dating, the micro-diamonds from other localities were identified in thin sections of the eclogites. Besides a few interstitial grains, most of the micro-diamond grains in thin sections occur as inclusion in garnet. Three crystals of microdiamond at Maobei in the Sulu region are sized in 120, 60 and 30 lira, respectively. Crystal forms look like octahedron and the composite of octahedron and hexahedron. The largest micro-diamond crystal comes from Xindian, which is measured to be 180 lira in diameter with distinct zonal structure and inclusions. The zonal structure occurs as an inclined octahedron inside rounded by an incomplete hexagonal girdle. A smaller micro-diamond inclusion occurs inside the central octahedron, and a larger graphite inclusion is within the outer zone. The Laoyoufang micro-diamond is partially retrograded to graphite. Micro-diamond from the Baizhangya eclogite in the ultramafic rock belt of North Dabie is an aggregate of 70 lira X 90 lira in size. All the micro-diamonds are confirmed by the Raman spectrum analysis. The occurrence of the micro-diamonds from the eclogites in the ultramafic rock belt of North Dabie demonstrates that this region was also subjected to ultrahigh pressure metamorphism as well as the South Dabie did.     

Hydroxyl in continental deep subduction zone: Evidence from UHP eclogites of the Dabie Mountains

Infrared spectra in the region of 3 000 to 4 000 cm⁻¹ have been measured by an FTIR spectrometer on minerals in UHP eclogites from the Bixiling and Shuanghe areas, the Dabie Mountains. It is shown that omphacite is the most important hydrous mineral (100–200 μg/g) in UHP eclogites. Garnet and quartz contain virtually no H₂O or OH. Accessory mineral rutile contains a significant amount of OH (>1 000 μg/g). Coesite gives no indication of the presence of OH but apparent H₂O bands. The results have considerable implications for existing forms and contents of hydroxyl in the continental deep subduction zone, the eastern Dabie Mountains.     

辽宁二道沟金矿床稳定同位素地球化学研究

通过对石英中流体包裹体的氢、氧同位素研究,揭示二道沟金矿床含矿热水溶液的δＤ为－７１‰～－１０２．５‰、δ１８ＯＨ２Ｏ为１．５３‰～６．７‰;在δ１８Ｏ－δ１８Ｄ图解中它们位于２００℃时大气降水与火山岩发生水岩交换的演化线与３００℃时大气降水与火山岩发生水岩交换的演化线所夹的区域内,表明成矿流体主要为大气降水来源的地下水,而不是岩浆热液;说明该矿床产于火山岩中的大气降水参与的浅成低温热液型金矿床．     

Zircon U-Pb SHRIMP ages of eclogites from the North Qilian Mountains in NW China and their tectonic implication

TheNorthQilianMountainsattheboundarybe-tweenGansuandQinghaiProvincesareafamousCaledo-nianplatesuturezoneinthenorthwesternChina.Thissuturezoneis80to100kmwideandextendsintheNWdirectionparalleltotheNorthQaidamUHPbeltinthesouth[1,2].Itconsistspredominantlyofophiolite,island-arccomplexandhigh-presuremetamorphicrocks[38],andshowstypicalfeaturesofPacific-type[9](orB-type)sub-ductionzone.Spatially,thehigh-pressurerockscanbesubdividedintohigh-gradeblueschistbeltandlow-gradeblueschistbelt[5,6].Th…     

Helium isotope compositions of Machangqing copper deposit in western Yunnan, China

Helium isotopic composition of inclusion fluid in pyrites from Machangqing copper deposit, China, has been measured. The measurements, with³He/⁴He ratios ranging from 0.46–2.08 Ra, indicate that the helium in ore-forming fluid is a mixture of crust and mantle, and are consistent with the fact that the parental magma responsible for the formation of ore-forming fluid of the deposit is of crust-mantle provenance.     

Isotopic compositions of small shelly fossil Anabarites from Lower Cambrian in Yangtze Platform of South China: Implications for palaeocean temperature

Anabarites belong to small shelly fossils (SSF), which occur widely in the Lower Cambrian strata of Yangtze Platform in South China. They are phosphate shell in composition and represent the earliest stage of the Cambrian bioradiation of Bilateria, the so-called “Cambrian Explosion”. In this study, we attempted to separate Anabarites fossils from Lower Cambrian dolostones, and we obtained samples of both the fossils (SSF) and the granular phosphates (GP). Isotopic analyses were performed on samples of SSF, GP, and matrix dolostone (DH-23). The results showed that the δ³⁰Si values of the quartz filling in fossils celoms, and the siliceous materials in granular phosphates are -0.6‰ and -0.7‰, which is different from normal sedimentary siliceous rocks from the Lower Cambrian strata (0—0.7‰) as reported by Li et al., but is consistent with the data for siliceous rocks and cherts of submarine hydrothermal origin. It is likely that a later hydrothermal replacement could have taken place in the SSF-bearing sedimentary rocks. The oxygen isotope values of the phosphate of SSF and GP are 16.8‰ and 17.0‰, respectively. These are significantly higher than the Neoproterozoic phosphate ores (10.9‰—13.9‰) as reported by Ling et al.， hence, late diagenesis and hydrothermal replacement may not have caused a significant change in the oxygen isotope compositions of the small shelly fossils, and the calculated temperatures (25.4—26.3℃) for palaeo-seawater using a SSF phosphate oxygen isotope thermometer are therefore considered here as the upper limit of seawater temperature in the Early Cambrian ocean of the Yangtze Platform.     

萨拉乌苏河流域钙结核中碳氧同位素特征及古环境意义

通过对萨拉乌苏河流域酒坊台剖面中普遍发育的钙结核进行碳、氧同位素分析,结果显示各岩性组中同位素分布特征差异较大,无明显线性关系。利用钙结核中碳同位素值推算了不同岩性组中C_3和C_4植物的相对贡献量,结合孢粉数据,探讨了不同气候条件下植被演变情况,这与邻区的黄土高原研究成果基本吻合。同时,利用氧同位素和温度的经验公式,定量恢复了倒数第二次冰期-末次冰期的温度变化,其中末次间冰期表现为相对温暖的气候特征。总之,利用钙结核的碳氧同位素可作为定量恢复古温度、古植被的一种有效方式,但需要注意与其他指标进行综合对比分析。     

Isotope composition of helium in eclogite from the Dabie Mountains, central China and its geological significance

Isotopic geochemical characteristics of helium in garnet and omphacite of eclogite in the Dabie Mountains are discussed. Concentrations of 3He and 4He in garnet and omphacite are 3.9×10 -14- 24.0×10 -14 and 0.48×10 -7-9.42×10 -7cm+3·g -1, respectively. Values of 3He/ 4He have a range of (1.19-4.63)×10 -7. Helium in the eclogite is derived from both mantle and crust. Isotopic geochemical data of helium indicate that eclogite in the Dabie Mountains might be formed in depleted mantle and the age of the cologite would be Indo-China epoch.     

Oxygen and carbon isotope records of calcareous nannofossils for the past 1 Ma in the southern South China Sea

Oxygen and carbon isotopic analyses have been carried out on calcareous nannofossils from ODP Site 1143 in the southern South China Sea. The results demonstrate that the δ18O values of calcareous nannofossils for the past 1 Ma vary systematically with those of planktonic and benthic foraminifera from the same site. But the average δ18O value of nannofossils is 1.869‰ higher than that of planktonic foraminifera, and is 3.855‰ lower than that of benthic foarminifera. There are systematic differences between the δ13C values of nannofossils, planktonic foarminifera and benthic foraminifera. The average δ13C of nannofossils is 0.756‰ higher than that of benthic foraminifera, and is 0.460‰ lower than that of planktonic foraminifera. The δ13C values of nannofossils exhibit a significant positive shift beginning near isotope stage 14 and lasting until stage 8. This study also finds that there is a close ralationship between the δ13C of nannofossils and the percentage of Florisphaera profunda. This implies that the δ13C of nannofossils can be used as an indicator of sea water surface paleoproductivity.     

Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong,Dabieshan: Implications for crust-mantle interaction

Oxygen isotopic compositions of zircons from pyroxenite (～145 Ma) of Daoshichong, Dabieshan have been measured by an ion microprobe. Both within the single grain and among different grains, oxygen isotopic ratios are homogeneous, δ 18O = (7.66‰±0.46)‰ (1 SD, 1 σ = 0.10, n = 22). High δ 18O values indicate that the mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combing with other geochemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involving in crust-mantle interaction is mafic lower crust, and the parent magma of pyroxenite possibly contain large proportion (>37%) of such lower crust.     

Coesite in the eclogite and schist of the Atantayi Valley,southwestern Tianshan,China

Coesite is an indicator mineral of ultra-high-pressure metamorphism. Since coesite was reported in the Habutengsu Valley, we have also found it in eclogite and schist from the Atantayi Valley in the southwestern Tianshan, China. Petrographic and micro-Raman analyses were carried out for the Atantayi metamorphic rocks and coesite was recognized in the predominant rock types, i.e. schist and eclogite, from three sections. The coesite-bearing schist consists mainly of garnet, Na-Ca amphibole, quartz, white mica and albite; the coesite-bearing eclogite is mainly composed of omphacite, garnet, glaucophane and zoisite. The coesite occurs as various mineral inclusions within porphyroblastic garnet. Findings of coesite in eclogite and associated schist indicate not only the regional in situ formation of the Atantayi ultra-high-pressure eclogite, but also the large areal extent of ultra-high-pressure metamorphism in southwestern Tianshan, extending up to 10 km north-south and 60-80 km east-west.