Poisson’s ratio of eclogite: the role of retrogression
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Poisson's ratio
Newly found garnet-bearing mafic granulite in Yanai district, Ryoke belt occurs as blocks in gneissose tonalite. This granulite consists mainly of brown hornblende, orthopyroxene and plagioclase, with trace amounts of garnet, biotite and quartz. Garnet porphyroblast up to 3 cm in diameter is locally contained, and is surrounded by orthopyroxene and plagioclase symplectites that coexist with quartz. On the basis of microstructure and mineral assemblage in the garnet-bearing mafic granulite, it reveals that the reaction of garnet + quartz→orthopyroxene + plagioclase occurred at a decompressional process under the granulite facies condition.
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The Larsemann Hills are located in the central part of the Prydz tectonic belt in East Antarctica. The study of granulite metamorphism of this area is essential to understanding the tectonic nature of the Prydz belt. The petrographic study of the garnetbearing mafic granulite boulder collected from the region demonstrates that pre-peak prograde mineral assemblage( M1) consists of hb+ opx ± cpx + pl + bi + ilm ± q ± mt,and peak metamorphic assemblage( M2) comprises g + opx + cpx + hb + ilm ± mt ± q,whereas the symplectite opx + pl + bi + ilm ± mt represents a superimposed assemblage( M3) associated with a decompression event. Mineral chemistry shows that garnets and orthopyroxenes in the mafic granulite have weak compositional zonings. Through the use of THERMOCALC program,the thermodynamic modeling in the NCFMASHTO system has been undertaken for the mafic granulite.Combined with conventional thermobarometers and the average P-T estimates,P-T conditions of the different metamorphic stages are estimated as 650 ~ 750℃ /5. 5 ~ 6. 5kb( M1),850 ~ 950℃ /8 ~ 8. 5kb( M2),and 800 ~ 900℃ /5. 5 ~ 6. 5kb( M3),respectively.Its metamorphism evolution of the mafic granulite is a typical post-peak isothermal decompression( ITD) clockwise P-T path. Compared with the mafic granulites in this region,we argue that this sample may originate from basement in Larsemann Hills. Combined with available chronological data in the region,it is suggested that the peak metamorphism likely corresponds to the Greenville high-grade tectonism,whereas post-peak retrograde metamorphism may be related to the Pan-African high-grade tectonism,implying that the PanAfrican Prydz belt could be an intracontinental mobile belt.
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The types of the high-pressure(HP) mafic granulites from high-grade metamorphic basement of the Jiaobei terrane are mainly composed of Grt mafic granulites,Grt-Hy granulites and Grt amphibolites.The HP mafic granulites are mainly occurred in TTG gneisses or granitic gneisses as enclaves or deformed dikes in the Jiaobei terrane.Mobile elements such as the large ion lithophile elements(K,Na,Sr,Rb) of the HP mafic granulites changed in significant levels,high field strength elements(Th,Nb,Zr,Ti) and rare earth elements basically unchanged and keep stable during the metamorphic process.The HP mafic granulites from the Jiaobei terrane belong to tholeiitic-basaltic rock series,with the concentrations of SiO2 at 44.04%~53.54%,and Mg# at 35~60.The composition of the rare earth elements of the HP mafic granulites is characterized by the flat(ΣREE=21.13×10-6~78.49×10-6,(La/Yb)CN=1.03~2.86) and right-inclined(92.74×10-6~133.5×10-6,(La/Yb)CN=2.93~4.56) on the chondrite-normalized REE patterns with no Eu anomalies(Eu/Eu*=0.93~1.04).The variation of Cr and Ni concentrations is large,however Cr and Ni have strong correlation with MgO.All most samples of the HP mafic granulites share obvious negative anomaly in Nb,Zr and Ti of the Phanerozoic island arc basalts,with the positive eNd(t) of(+2.70~+4.77).The composition of the major and trace elements of the HP mafic granulites in the Jiaobei terrane is characterized by the island arc tholeiitic basaltic rocks,and the protoliths may be gabbro instrusions or diabase dykes,and the corresponding basic volcanic rocks in back-carc extension tectonic setting.
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Research Article| February 01, 1980 Adirondack mafic granulites and a model lower crust: Summary RICHARD O. SACK RICHARD O. SACK 1Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD O. SACK 1Department of Geological Sciences, Harvard University, Cambridge, Massachusetts 02138 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1980) 91 (2): 89–93. https://doi.org/10.1130/0016-7606(1980)91<89:AMGAAM>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation RICHARD O. SACK; Adirondack mafic granulites and a model lower crust: Summary. GSA Bulletin 1980;; 91 (2): 89–93. doi: https://doi.org/10.1130/0016-7606(1980)91<89:AMGAAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract No Abstract Available. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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The recrystallization of a tholeiitic basalt to garnet granulite and to eclogite has been experimentally examined at temperatures ranging from 800°C up to the solidus of the rock. In addition, the solidus of tholeiitic basalt has been redetermined. The slope of the boundary separating the field of garnet granulite frpm that of basalt is P kb = 0.014 T°C - 5.4 kb. The boundary separating the field of garnet granulite from the field of eclogite has the equation P kb = 0.020 T°C + 4.0 kb. The equation of the newly determined solidus of tholeiitic basalt is T = 6.0 P kb + 1105°C. Curves relating the densities of the mineral assemblages to temperatures and pressures of formation are shown. One relatively abrupt change in density occurs as basalt recrystallizes to garnet granulite, and a second relatively sharp density increase occurs as garnet granulite is recrystallized to eclogite.
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