Measured and calculated seismic velocities and densities for granulites from xenolith occurrences and adjacent exposed lower crustal sections: A comparative study from the North China craton
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Granulites from the Neogene xenolith‐bearing Hannuoba alkaline basalt and from the Manjinggou‐Wayaokou exposed lower crustal section in the Archean Huai 'an terrain, which occurs within and surrounds the Hannuoba basalt, provide a unique opportunity for a comparative study on petrophysical properties and composition of the lower crust represented by these two types of samples. P and S wave velocities and densities of 12 Hannuoba lower crustal xenoliths and one associated spinel Iherzolite xenolith as well as nine granulites and granulite‐facies metasedimentary rocks from the Archean Huai 'an terrain were measured in laboratory at pressures up to 600 MPa and temperatures up to 600°C. Calculations of P and S wave velocities were also made for the same suite of samples based on modal mineralogy and single‐crystal velocities whose variations with composition are considered by using microprobe analyses and velocities of end members. The measured and calculated V p at room temperature and 600 MPa, where the microcrack effect is considered to be almost eliminated, agree within 4% for rocks from the Manjinggou‐Wayaokou section and the adjacent Wutai‐Jining upper crustal to upper lower crustal section. In contrast, the xenoliths show systematically lower measured V p by up to 15% relative to calculated velocities, even if decompression‐induced products of kelyphite and glass are taken into account. The lower measured velocities for xenoliths are attributed to grain boundary alteration and residual porosity. This implies that although granulite xenoliths provide direct information about lower crustal constitution and chemical composition, they are not faithful samples for studying in situ seismic properties of the lower crust in terms of measured velocities due to alterations during their entrainment to the surface, which changes their physical properties significantly. In this respect, granulites from high‐grade terrains are better samples because they are not subjected to significant changes during their slow transport to the surface and because physical properties depend primarily on mineralogy in addition to pressure and temperature. On the other hand, calculated velocities for granulite xenoliths are consistent with velocities for granulites from terrains, suggesting that they can be also used to infer lower crust composition by correlating with results from seismic refraction studies.Keywords:
Xenolith
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.
Hornblende
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The original connections of Archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes. The Uauá block of the northern São Francisco craton may represent a fragment of a major Archean craton. Here, we report new paleomagnetic data from the 2.62 Ga Uauá tholeiitic mafic dyke swarm of the Uauá block in the northern São Francisco craton, Eastern Brazil. Our paleomagnetic results confirm the earlier results for these units, but our interpretation differs. We suggest that the obtained characteristic remanent magnetization for the 2.62 Ga swarm is of primary origin, supported by a provisionally-positive baked contact test. The corresponding paleomagnetic pole (25.2°N, 330.5°E, A95 = 8.1°, N = 20) takes the present northern part of the São Francisco craton to moderate latitudes. Based on the comparison of the paleolatitudes of cratons with high-quality paleomagnetic data and magmatic barcodes, we suggest that the northern part of the São Francisco craton could have been part of the proposed Supervaalbara supercraton during the Archean. Supervaalbara is proposed as including (but not limited to) the part of the São Francisco craton as well as the Superior, Wyoming, Kola + Karelia, Zimbabwe, Kaapvaal, Tanzania, Yilgarn, and Pilbara cratons.
Yilgarn Craton
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Radiogenic nuclide
Indian Shield
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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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Supercontinent
Dharwar Craton
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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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