Coexistence of garnet blueschist and eclogite in South Tianshan,NW China: dependence of P–T evolution and bulk‐rock composition     

Z. L. Tian  C. J. Wei 《Journal of Metamorphic Geology》2014,32(7):743-764

Coexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeO_total + MnO + Na₂O)] (0.33–0.48) and lower A/CNK [=Al₂O₃/(CaO + Na₂O + K₂O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the P_max to the T_max stage and a post‐T_max decompression with slightly cooling stage, and similar P–T conditions at the T_max stage. The post‐T_max decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO₂) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO₂) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO₂). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.  相似文献   

江苏孟中榴辉岩中单斜辉石向石榴石相转变的初步研究     

陈宣华  王小凤  陈正乐  舒桂明  李小凤 《地质力学学报》1995,1(2):85-89

苏鲁超高压榴辉岩的成因一直是一个令人困惑的问题。本文通过对榴辉岩的进变质作用——榴辉岩相向石榴石岩相转变的初步研究,为苏鲁榴辉岩带的地幔转换带(高压可达10GPa)成因提供了最初的证据。   相似文献   

梵净山地区板溪群红子溪组钙质岩系中大理岩透镜体成因探讨     

彭成龙  陈建书  杨凯迪 《贵州地质》2011,(4):286-289

梵净山地区板溪群红子溪组钙质岩系中广泛发育大理岩透镜体。形成此岩系的控制因素可能为振荡频繁的构造运动或气候条件。通过仔细观察,发现大理岩透镜体有条带状、似层状断续产出2种,初步认为透镜体是成岩过程中差异性压实作用的产物,不是某种环境沉积的透镜体。  相似文献   

大别山北部蛇绿岩的地球化学制约   总被引：8，自引：1，他引：7  

刘贻灿  徐树桐 《矿物岩石》1999,19(1):68-73

常量元素,微量元素,稀土元素,Ｎｄ同位素及氧同位素地球化学特征表明,太别山北部变质镁铁－超镁铁质岩带中存在变质的蛇绿岩,它主要由变质橄榄岩,辉长岩（－辉绿岩？）和基性熔岩三部分组成,其中,变质的基性熔岩的亏损地幔模式年龄ｔＤＭ／Ｍａ＝１０３６．８～１２９３．８,εＮｄ（ｔ）＝７．２～７．７,表明它可能代表１０００Ｍａ左右形成于中等扩张速率（２ｃｍ／ａｄ左右）洋盆条件下的洋壳残片。  相似文献   

PTtD Path and Fluid Evolution of HighPressure Eclogites from the Dabie Mountains,Central East China     

ZHANG Zeming Institute of Geology  Chinese Academy of Geological Sciences  Beijing  China WEI Bize HAN Yujing YOU Zhendong China University of Geosciences  Wuhan  China 《Continental Dynamics》1999,(1)

１．Ｉｎｔｒｏｄｕｃｔｉｏｎ　ＴｈｅＱｉｎｌｉｎｇＤａｂｉｅｏｒｏｇｅｎｉｃｂｅｌｔｗａｓｆｏｒｍｅｄｂｙｃｏｌｌｉｓｉｏｎｂｅｔｗｅｅｎｔｈｅＮｏｒｔｈＣｈｉｎａａｎｄＹａｎｇｔｚｅｂｌｏｃｋｓ．Ｔｈｅｃｏｒｅｐａｒｔｏｆｔｈｅｏｒｏｇｅｎｉｃｂｅｌｔｃｏｎｓｉｓｔｓｏｆｓｅｖｅｒａｌｍｅｔａｍｏｒｐｈｉｃｒｏｃｋｇｒｏｕｐｓ,ｉｎｃｌｕｄｉｎｇｔｈｅＤａｂｉｅ（Ｔｏｎｇｂａｉ）ｃｏｍｐｌｅｘ,Ｈｏｎｇ’ａｎ（Ｓｕｓｏｎｇ）ｇｒｏｕｐ,ＳｕｊｉａｈｅｇｒｏｕｐａｎｄＳｕｉｘｉａｎ（Ｙａｏ…  相似文献   

Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (Western Alps, Italy)   总被引：3，自引：0，他引：3  

MESSIGA  KIENAST  REBAY  RICCARDI  & TRIBUZIO 《Journal of Metamorphic Geology》1999,17(3):287-299

Cr-rich magnesiochloritoid in the eclogitized ophiolites of the Monviso massif occurs in the least differentiated rocks of the gabbroic sequence (troctolites to melatroctolites). Chloritoid ( X _Mg=0.63–0.85; Cr≤0.55, atoms) co-exists with omphacite, talc and garnet. Minor, syn-eclogitic minerals are chromite, rutile and sometimes magnesite and Cr–Ti oxides.
Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs.
Syn-eclogitic volatile components are indicated by the development of OH-bearing minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), and supported by the presence of coarse-grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization, most likely during ocean-floor metamorphism. It is suggested that syn-eclogitic fluids formed by breakdown reactions of pre-eclogitic volatile-bearing minerals.
Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4  GPa and temperatures of 620±50  °C. The attainment of high-pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58  mol%). P–T  estimates point to a very low thermal gradient (about 9  °C km⁻¹), comparable to that deduced in the adjacent Dora-Maira ultra-high pressure unit.  相似文献   

The orientation of sorted stripes in the maritime subantarctic,Marion Island     

Stephen D. Holness 《地球表面变化过程与地形》2001,26(1):77-89

The orientation of sorted stripes developed in glacial deposits on Marion Island is examined. This study confirms earlier reports of preferential occurrence of sorted stripes on windward slopes. However, sorted stripes were also found to occur on low angle leeward slopes. Mechanisms for the orientation of sorted stripes by wind action are discussed. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

Water in garnet of garnetite (metarodingite) and eclogite from the Erzgebirge and the Lepontine Alps     

Esther Schmädicke  Jürgen Gose 《Journal of Metamorphic Geology》2020,38(9):905-933

Little is known about water in nominally anhydrous minerals of orogenic garnet peridotite and enclosed metabasic rocks. This study is focused on peridotite-hosted eclogite and garnetite (metarodingite) from the Erzgebirge (EG), Germany, and the Lepontine Alps (LA), Switzerland. Newly discovered, peridotite-hosted eclogite in the Erzgebirge occurs in the same ultra-high pressure (UHP) unit as gneiss-hosted coesite eclogite, from which it is petrologically indistinguishable. Garnet is present in all mafic and ultramafic high pressure (HP) rocks providing for an ideal proxy to compare the H₂O content of the different rock types. Garnet composition is very similar in EG and LA samples and depends on the rock type. Garnet from garnetite, compared to eclogite, contains more CaO (garnetite: 10.5–16.5 wt%; eclogite: 5–11 wt%) and is also characterized by an anomalous REE distribution. In contrast, the infrared (IR) spectra of garnet from both rock types reveal the same OH absorption bands that are also identical to those of previously studied peridotitic garnet from the same locations. Two groups of IR bands, SW I (3,650 ± 10 cm⁻¹) and SW II (3,570–3,630 cm⁻¹) are ascribed to structural hydroxyl (colloquially ‘water’). A third, broad band is present in about half of the analysed garnet domains and related to molecular water (MW) in submicroscopic fluid inclusions. The primary content of structural H₂O, preserved in garnet domains without fluid inclusions (and MW bands), varies systematically—depending on both the location and the rock type. Garnet from EG rocks contains more water compared to LA samples, and garnet from garnetite (EG: 121–241 wt.ppm H₂O; LA: 23–46 wt.ppm) hosts more water than eclogitic garnet (EG: 84 wt.ppm; LA: 4–11 wt.ppm). Higher contents of structural water (SW) are observed in domains with molecular water, in which the SW II band (being not restricted to HP conditions) is simultaneously enhanced. This implies that fluid influx during decompression not only led to fluid inclusions but also favoured the uptake of secondary SW. The results signify that garnet from all EG and LA samples was originally H₂O-undersaturated. Combining the data from eclogite, garnetite and previously studied peridotite, H₂O and CaO are positively correlated, pointing to the same degree of H₂O-undersaturation at peak metamorphism in all rock types. This ubiquitous water-deficiency cannot be reconciled with the derivation of any of these rocks from the lowermost part of the mantle wedge that was in contact with the subducting plate. This agrees with the previously inferred abyssal origin for part of the rocks from the LA (Cima di Gagnone). A similar origin has to be invoked for the Erzgebirge UHP unit. We suggest that all mafic and ultramafic rocks of this unit not only shared the same metamorphic evolution but also a common protolith origin, most probably on the ocean floor. This inference is supported by the presence of peridotite-hosted garnetite, representing metamorphosed rodingite.  相似文献   

东昆仑夏日哈木退变质榴辉岩地球化学、年代学特征及其地质意义          下载免费PDF全文

郭峰  王盘喜  王振宁  冯乃琦 《沉积与特提斯地质》2020,40(4):45-55

本文对东昆仑夏日哈木退变质榴辉岩开展岩石地球化学和年代学研究,探讨其原岩性质和构造背景。岩石SiO₂含量为46.77%~49.31%,Al₂O₃含量为13.6%~14.6%,TiO₂含量为0.93%~2.29%,全碱（K₂O+Na₂O）含量为1.61%~2.66%,原岩属亚碱性玄武岩系列中的拉斑玄武岩。（La/Yb）_N=0.98~1.63,（La/Sm）_N=0.98~1.41,δEu=1.10~1.28,稀土元素球粒陨石标准化图呈弱Eu正异常的右倾型,岩石相对富集大离子亲石元素Rb、Ba和不相容元素U,相对亏损高场强元素Zr、Hf,Hf/Th=0.59~0.97,Th/Yb=0.13~0.27,显示其原岩具E-MORB特征。退变质榴辉岩的锆石Th/U比值为0.002~0.091,为变质成因锆石,其锆石SIMS U-Pb年龄为414.2 ±7.3 Ma,代表了退变质榴辉岩的峰变质年龄,结合区域地质背景,认为该岩石形成于早泥盆世早期由碰撞作用向碰撞后伸展作用转换的构造环境。  相似文献   

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance     

Tomoki Taguchi  Shunsuke Endo  Yohei Igami  Akira Miyake 《Island Arc》2019,28(5)

The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm^?1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω₁ > 8.5 cm^?1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.  相似文献