Ordovician and Cretaceous tectonothermal history of the Southern Gemericum Unit from microprobe monazite geochronology (Western Carpathians, Slovakia)
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Biotite from paragneiss of Gneiss Point, McMurdo Sound, Antarctica is dated at 520 m y by the A 40 /K 40 method. Metamorphism of Precambrian sediments at the close of Precambrian or in Cambrian time is indicated, but additional work is needed.
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Abstract The content of monazite in the Egyptian black beach sand and coastal sand dunes is normally equal to or below 0.01 wt.%. The obtained high grade monazite concentrate includes three minor monazite groups in addition to major canary and lemon yellow coloured monazite: (i) the colourless to pinkish white coloured monazite; (ii) the opaque light to dark resinous, reddish brown and dark brown coloured monazite; and (iii) opaque yellowish red to brownish red coloured monazite grains group. These groups represent 3%, 4% and 2%, respectively, in the high grade monazite concentrate. A negligible amount of euhedral to subhedral black to brownish black chevkinite/perrierite mineral crystals was detected in the obtained monazite concentrate. The presence of these minor mineral groups affects the chemical composition of the obtained high grade monazite concentrate. The Ce 2 O 3 is the main REE in the studied monazite. In the colourless‐pinkish monazite grains, the analyzed REE are the following, in order of abundance; Ce > La > Nd > Pr > Sm > Gd > Dy. UO 2 ranges between 0.11 and 1.74 wt.%. The contents of Eu 2 O 3 is under the limit of detection while ThO 2 ranges between 3.99 and 8.58 wt.% with an average value of 5.57 wt.%. These grains are most probably igneous monazite from a highly differentiated granite. The resinous, brown monazite grains have lower Ce 2 O 3 content (24.63 wt.%) and much lower La 2 O 3 content (6.00 wt.%) but greater content of Eu 2 O 3 (0.41 wt.%) than those of the colourless‐pinkish monazite. These monazites have the lowest contents of Th, U and Ca among the three groups. The resinous, brown monazites are most probably formed by metamorphism or alteration leading to leaching or replacement of pre‐existing minerals. The red monazite group has a lower average Ce 2 O 3 content (25.28 wt.%) than the colourless‐pinkish variety (28.02 wt.%) but slightly greater than that of the resinous, brown ones. The red monazite group has the highest ThO 2 and UO 2 contents; 5.84 wt.% and 1.24 wt.%, respectively. It has the lowest monazite component mole fraction (0.75). The red monazite seems to have been formed by hydrothermal alteration of pre‐existing monazite and other mineral species bearing for Y, REE, Ca, Th and U. The two coupled substitution mechanisms: (Th, U) 4+ + Ca 2+ 2REE 3+ , and (Th, U) 4+ + Si 4+ REE 3+ + P 5+ , are obvious in the studied colourless‐pink monazite.
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(1973). Some problems of Precambrian geochronology in relation to metallogenic evolution. International Geology Review: Vol. 15, No. 12, pp. 1440-1444.
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U-Pb ages were determined on metamorphic garnets, monazites, sphenes, and rutiles in metapelites and marbles from the Adirondacks to determine the duration of metamorphism, the timing of mineral growth, and the cooling history. Garnet ages for the Lowlands range from 1168-1127 Ma, those from the central and southern Highlands from 1154-1013 Ma. The ages indicate that metamorphism may have begun synchronously throughout the Adirondacks, but high-grade conditions existed over a period of about 150 Ma in the Highlands and only about 40 Ma in the Lowlands. Metamorphism in the Highlands may not have occurred as a single event but rather in several discrete thermal pulses. An age of $$1153 \pm 3 Ma$$ was determined for garnets in the syn-regional metamorphic contact aureole of the Diana syenite, consistent with that of the syenite intrusion, $$1155 \pm 4 Ma$$ (U-Pb zircon, Grant et al. 1986). Garnets just outside the contact aureole give an age of $$1168 \pm 6 Ma$$. In the Lowlands, monazite yielded an age of $$1161 \pm 1 Ma$$, rutiles yielded ages of $$1005 \pm 2 Ma$$ and $$953 \pm 4 Ma$$, and sphene ages range from 1156 to 1103 Ma. In the Highlands, monazite yielded an age of $$1033 \pm 1 Ma$$, rutiles yielded ages of $$911 \pm 2 Ma$$ and $$885 \pm 2$$ and sphenes from 1033 Ma to 991 Ma. The rutile and monazite ages indicate that both terranes cooled at time-integrated rates of ca. 1.5°C/Ma for at least 150 Ma following the last phase of high-grade metamorphism. The Lowlands cooled to ca. 400°C by ca. 1000 Ma and the Highlands by ca. 900 Ma. The slow, nearly isobaric cooling (as deduced from garnet zoning and fluid inclusions indicates that during the final stages of metamorphism, the terranes were close to isostatic equilibrium, and vertical tectonic movement was limited. The mineral ages indicate that metamorphic pressures and temperatures recorded by thermo-barometry correspond to conditions attained polychronically over 150 Ma or more. Mineral ages combined with temperature estimates for peak metamorphism indicate that the closure temperature for the U-Pb system is >800°C in garnet, 640-730°C in monazite, and 500-670°C in sphene.
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