Reaction Textures in a Suite of Spinel Granulites from the Eastern Ghats Belt, India: Evidence for Polymetamorphism, a Partial Petrogenetic Grid in the System KFMASH and the Roles of ZnO and Fe2O3   总被引：1，自引：0，他引：1  

DASGUPTA  SOMNATH; SENGUPTA  PULAK; EHL  JUERGEN; RAITH  M.; BARDHAN  S. 《Journal of Petrology》1995,36(2):435-461

Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. X_Mg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al₂O₃–SiO₂–K₂O–H₂Oat high fo₂, has been constructed and the effects of ZnO andFe₂O₃ on this grid have been explored Combining available experimentaland natural occurrence data, the high fo₂ invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author  相似文献   

富锂巨星的观测研究     

符晓婷  邓李才  Kirby Evan N 《天文学进展》2012,30(4)

锂在温度达到T＞2.5×106 K时即参与7Li(p,α)4He反应燃烧,经典恒星理论推测,当恒星演化到红巨星阶段时,由于向内非常延伸的对流包层的存在,锂的丰度A(Li)=12+ lg[n(Li)/n(H)]应为A(Li)＜0.5.但是,陆续有处于不同光谱型、不同演化阶段的富锂巨星被发现,其中一些巨星的锂丰度甚至远超出宇宙大爆炸的原初锂丰度,给经典恒星演化模型带来了难题.回顾了从恒星主序到巨星支的锂丰度观测研究历史,并详细介绍了我们对银河系矮星系中富锂巨星的最新发现,总结了所有富锂巨星的观测证据,以及为解决此难题所提出的理论解释.  相似文献   

Petrology of Granulites from Anakapalle--Evidence for Proterozoic Decompression in the Eastern Ghats, India   总被引：4，自引：0，他引：4  

DASGUPTA  SOMNATH; SANYAL  SANJOY; SENGUPTA  PULAK; FUKUOKA  M. 《Journal of Petrology》1994,35(2):433-459

The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py₃₇)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py₃₇)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py₃₀) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py₄₂) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-T_max of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.  相似文献   

Detecting changes in rainfall pattern and seasonality index vis-à-vis increasing water scarcity in Maharashtra     

PULAK GUHATHAKURTA  ELIZABETH SAJI 《Journal of Earth System Science》2013,122(3):639-649

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Impact of climate change on extreme rainfall events and flood risk in India   总被引：8，自引：0，他引：8  

P GUHATHAKURTA  O P SREEJITH  P A MENON 《Journal of Earth System Science》2011,120(3):359-373

The occurrence of exceptionally heavy rainfall events and associated flash floods in many areas during recent years motivate us to study long-term changes in extreme rainfall over India. The analysis of the frequency of rainy days, rain days and heavy rainfall days as well as one-day extreme rainfall and return period has been carried out in this study to observe the impact of climate change on extreme rainfall events and flood risk in India. The frequency of heavy rainfall events are decreasing in major parts of central and north India while they are increasing in peninsular, east and north east India. The study tries to bring out some of the interesting findings which are very useful for hydrological planning and disaster managements. Extreme rainfall and flood risk are increasing significantly in the country except some parts of central India.  相似文献   

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India   总被引：5，自引：0，他引：5  

SENGUPTA  PULAK; DASGUPTA  SOMNATH; BHATTACHARYA  P. K.; FUKUOKA  M.; CHAKRABORTI  SUBRATA; BHOWMICK  SANTANU 《Journal of Petrology》1990,31(5):971-996

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.  相似文献