Search results for "Migmatite"
showing 10 items of 32 documents
SHRIMP zircon U–Pb ages and REE partition for high-grade metamorphic rocks in the North Dabie complex: Insight into crustal evolution with respect to…
2012
Abstract We present SHRIMP U–Pb ages and REE data for zircon of two granulites, a garnet pyroxenite and a clinopyroxene amphibolite from the North Dabie complex. The North Dabie complex is a major fault-bounded petrotectonic unit in the Dabie continental collisional orogen between the South and North China Blocks. Zircon REE data are applied to link ages to specific petrological processes. In addition to SHRIMP zircon U–Pb dating, measurements were made on monazites from a HP–UHT metapsammitic granulite. Metamorphic zircon grains and coeval overgrowths on detrital cores from one granulite yielded a weighted mean 207Pb/206Pb age of 2042 ± 7 Ma, corresponding to peak HP/UHT metamorphism. The …
Metamorphic petrology and zircon geochronology of high-grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Pal…
2003
New data on the metamorphic petrology and zircon geochronology of high-grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan-African event. The metamorphic rocks are characterized by a clockwise P–T path, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite-bearing metapelites. The meta-granitoid rocks are of calc-alkaline composition, range in age fr…
Archaean Intracrustal Differentiation from Partial Melting of Metagabbro--Field and Geochemical Evidence from the Central Region of the Lewisian Comp…
2012
The central region of the mainland Lewisian gneiss complex of NW Scotland is a granulite-facies migmatite terrane. With the exception of ultramafic and rare calc-silicate rocks, all other lithologies partially melted during Neoarchaean, ultrahigh-temperature (Badcallian) metamorphism. The clearest evidence is preserved within large layered mafic^ultramafic bodies that exhibit macroscopic features diagnostic of anatexis. In situ partial melting of metagabbroic rocks produced patches and sheets of coarse-grained plagioclase-rich leucosome containing euhedral peritectic clinopyroxene.These leucosomes connect with larger, laterally continuous tonalite or trondhjemite sheets that record segregat…
Zur Genese der Migmatite des Beaume-Tales (Mittlere C�vennen, D�p. Ard�che)
1971
Samples of micaschists, gneisses, and migmatites from a sequence of metamorphic subfacies of the Gevennes Medianes (Dep. Ardeche, France) are characterized chemically by a) a high and only slightly varying Al2O3 content (m.v.=17.57 wt.-%, stand, dev.=1.804, var. coeff.=0.103). b) a negligible variation of the molar MgO/MgO+FeO ratio (m.v.=0.504, stand. dev.= 0.066, var. coeff.=0.130), and c) an insignificant variation of the molar CaO/CaO + Na2O ratio (m.v.=0.360, stand. dev.=0.014, var. coeff.=0.039).
Metamorphic evolution and zircon geochronology of early Proterozoic granulites in the Aravalli Mountains of northwestern India
2005
Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss–amphibolite–metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the ‘country rocks’ led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further…
SHRIMP U–Pb zircon and Sm–Nd garnet ages from the granulite-facies basement of SE Kenya: evidence for Neoproterozoic polycyclic assembly of the Mozam…
2007
The Taita Hills–Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20–30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630–645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850–960 Ma was derived from zircon cores. Sm–Nd garnet–whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630–645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana …
Melt production, redistribution and accumulation in mid-crustal source rocks, with implications for crustal-scale melt transfer
2014
Abstract Ascent of granitic melt initiates under suprasolidus conditions in the mid- to lower crust before continuing through subsolidus rocks to higher crustal levels. Whereas migration of melt in suprasolidus rocks can occur in pervasive net-like structures and involve relatively small melt volumes, ascent through the subsolidus crust requires more focused, dyke-like structures and larger volumes to prevent freezing. Migmatites in the Aus granulite terrain, southern Namibia, preserve evidence that large-scale melt redistribution and accumulation occurred in the near-source region under suprasolidus conditions. Melt that was mainly produced in metapelitic rocks utilised pervasive small-sca…
Single zircon ages of migmatitic gneisses and granulites in the Obudu Plateau: Timing of granulite-facies metamorphism in southeastern Nigeria
2006
Abstract A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded 207 Pb/ 206 Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from colli…
On the importance of minding one’s Ps and Ts: metamorphic processes and quantitative petrology
2010
This Special Issue comprises a selection of the papers given at a two-day discussion meeting held at the University of Melbourne, Australia in June 2009 to celebrate Roger Powells 60th birthday. At this milestone, it is fitting to review Rogers career to date. He has published 200 scientific papers on topics that range from low- to high-grade metamorphism, from low- to ultrahigh-pressure (UHP) metamor- phism, and from thermodynamics to kinetics. Most of Rogers papers are multi-authored and address important questions in the petrogenesis of metamorphic rocks. Roger is widely known for his work with Tim Holland to develop the most complete internally consistent dataset of thermodynamic proper…
False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data
2008
Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and ⁄ or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P- T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P-T stability fields of assemblages. Mineral equilibria…