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Buffering of metamorphic reaction temperature by metamorphic fluids in mayuan group, northern fujian province
Abstract
Metamorphic rocks in Mayuan Group in northern Fujian Province are outcropped in the form of Barrow belts from chlorite, biotite, garnet, staurolite, kyanite to sillimanite zones. The calculations - from geothermometer indicate that the metamorphic temperature of kyanite zone (470°C- 500°C) is obviously lower than that of garnet zone and staurolite zone. It means that kyanite zone was formed before garnet zone and staurolite zone. This is evidently inconsistent with the traditional theory of metamorphic belts. Chemical equilibrium calculations of fluid composition and thermodynamical analysis of metamorphic reactions show that the lower metamorphic temperature of kyanite zone is resulted not from the effect of metamorphic pressure and retrogressive metamorphism but from the buffering of temperature of dehydration metamorphic reaction forming kyanite by fluids with lower XH2o. The buffered temperature range is up to 100°C-200°C. The paper discusses the buffering mechanism and buffering intensity from thermodynamics and emphasizes the important role of metamorphic fluids in the studies of metamorphic facies and metamorphic belts.
Mesozoic molybdenum deposits are distributed widely in Fujian, and associated with a certain amount of rhenium. This study explores Mesozoic molybdenum deposits in Fujian. It is found by analysis of characteristics of ore petrology and mineralogy of molybdenite of typical molybnum deposits (Pingdi, Shangxikeng, Luobuling, Makeng, etc.) in Fujian Province, five types of the molybdenum deposits have been determined, including the porphyry type, the magmatic hydrothermal type, the volcanic hydrothermal type, the tectonic breccia type and a few skarn type. Our systematic field investigation and petrography study finds that the ores mainly have coarse-medium fine grain, platy texture, small amount of thick tabular texture; the ore mineral(only molybdenite) is of leaden color, obvious polychroism, 2H polymorph (a small amount of 2H+3R, 3R), low contents of impurities including Re((N-N×100)×10-6, most(N-N×10)×10-6), which indicates the ores formed in moderate-high temperature condition and the ore originates from crust-mantle mixed source (most crust source); Mesozoic molybdenum mineralization is of universality in Fujian province; and as a kind of accompanying elements, Re is of some comprehensive utilization value.
Two empirical garnet-muscovite geothermometers, assuming no ferric iron (Model A) and 50% ferric iron (Model B) in muscovite, respectively, were calibrated under the physical conditions of P = 3.0-14.0 kbar and T = 530-700degreesC. The input temperatures and pressures were determined by simultaneously applying the garnet-biotite thermometer [Am. Mineral. 85 (2000) 881.] and the GASP geobarometer [Am. Mineral. 86 (2001) 1117] to natural metapelites. To confirm internal thermodynamic consistency, Holdaway's [Am. Mineral. 85 (2000) 881.] garnet mixing properties were adopted. Muscovite was treated as a symmetric Fe-Mg-Al-VI ternary solid solution, and its Margules parameters were derived in this work. The resulting two formulae reproduced the input garnet-biotite temperatures well within +/-50degreesC, and gave identical results for a great body of natural samples. Moreover, they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence, inverted metamorphic zone, and thermal contact aureole. Pressure estimation has almost no effect on the two formalisms of the garnet-muscovite geothermometer. Assuming analytical error of +/-5% for the relevant components of both garnet and muscovite, the total random uncertainty of the two formulations will generally be within +/-5degreesC. The two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700degreesC, low- to high-pressure, in the composition ranges Xalm = 0.51-0.82, Xpyr = 0.04-0.22, and Xgros = 0.03-0.24 in garnet, and Fe-tot = 0.03 -0.17, and Mg = 0.04-0.14 atoms p.f.u. in muscovite.
The Mayuan assemblage in the Cathaysia Block, Southeast China, consists of felsic paragneiss, pelitic schist, greenschist, amphibolite, marble, calcsilicate, and quartzite that underwent three episodes of deformation (D1-D3) and four episodes of metamorphism (M1-M4) in the early Neoproterozoic. The M1 assemblage consists of mineral inclusions defining an early foliation (S1) within porphyroblasts, represented by chlorite + muscovite + biotite + plagioclase + quartz in pelitic schist and actinolite + chlorite + epidote + albite ± quartz enclosed in amphibolite. M2 coincides with the development of the regional schistosity (S2) and represents the formation of the porphyroblasts and growth of matrix minerals, resulting in development of prograde metamorphic zones (chlorite-biotite, garnet, staurolite, and kyanite zones). M3 is simultaneous with the third phase of deformation (D3) and produced sillimanite-bearing mineral assemblages in pelitic schist and hornblende-bearing assemblages in amphibolite. The last metamorphic episode M4 gave rise to the retrogressive assemblage chlorite + muscovite in pelitic rock and actinolite + chlorite + epidote in amphibolite. The sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near-isothermal decompression clockwise P-T path for the Mayuan pelitic schists. Using the TWEEQU software program, the garnet-biotite thermometer and garnet-muscovite-biotite-plagioclase barometer yield P-T conditions for M1 of 5.5 to 6.0 kb and 450° to 500°C and conditions for the garnet, staurolite, and kyanite zones of M2 of 6.0 to 7.0 kb and 550° to 600°C, 6.0 to 7.5 kb and 600°C and 11.0 to 11.5 kb and 600°C. The P-T conditions of M3 were estimated at 570° to 625°C and 4.0 to 4.5 kb using the muscovite-biotite thermometer and hornblende-plagioclase geothermo-barometer. The garnet-chlorite thermometer yields temperatures of 300° to 400°C for M4, but the pressures of M4 cannot be quantitatively estimated because of the lack of a suitable geobarometer. These P-T estimates also define an near-isothermal decompression clockwise P-T path, which is involved in initial crustal thickening followed by rapid exhumation and final cooling, and is related to amalgamation of the Cathaysia and Yangtze Blocks to form the South China craton.
The garnet-muscovite geothermometer was refined through empirical calibration by using natural rocks metamorphosed under the
physical conditions of 238–1306 MPa and 490– 700°C. Input temperatures and pressures were determined through simultaneously
applying the garnet-biotite geothermometer and the garnet-biotite-plagioclase-quartz barometer, assuming that all FeO in muscovite
and garnet be ferrous. Garnet was treated as the asymmetric quaternary solid solution, and muscovite as the symmetric binary
solid solution. Input muscovite compositions include Fe atoms between 0.03–0.19 and Mg atoms between 0.04–0.16 on the basis
of 11 oxygen atoms, and input garnet compositions include spessartine fractions between 0.01–0.289, grossular fractions between
0.028–0.273, and the Fe/Mg ratio between 3.387–18.986. The resulting garnet-muscovite geothermometer reproduces temperatures
within ±50°C compared with the garnet-biotite thermometer. Total random error of ±37°C of the new thermometer may stem from
the pressure uncertainty of ±200 MPa, and uncertainties of ±5% of Fe and Mg components in muscovite, and ±5% of Fe, Mg, Mn
and Ca components in garnet, altogether. When there exist 10%, 20%, 30%, 40% and 50% Fe3+ in muscovite, respectively, the computed garnet-muscovite temperatures will be 1–6°C, 2–12°C, 3–16°C, 5–24°C and 7–29°C,
respectively, lower than those obtained when assuming that all FeO be ferrous. The new garnet-muscovite geothermometer can
efficiently reflect temperature change of typical prograde sequences and contact aureole rocks, and may be applied to low-
to high-grade and low- to high-pressure metamorphic rocks.
Time studies were performed in the quinary system Qz-Or-Ab-An-H2O at
\operatornameP \operatornameH 2 O = 5\operatorname{P} \operatorname{H} _2 O = 5
kbars and T=665 and 660 C. Starting material was a mixture of quartz, alkali feldspar Or80 and plagioclase An31. The compositions of plagioclases of run products were determined and compared with the plagioclase of stable solidus conditions.The solidus of the granite system was fixed at P
HO=5 kbars using various plagioclase — and appropriate alkali feldspar — compositions besides quartz in the starting mixture (Fig. 1).The results of time studies (Table 3 and Fig. 3) reveal metastable melting in the granite system Qz-Or-Ab-An-H2O. Plagioclase melts almost stoichiometrically. The new plagioclase compositions formed during melting of cotectic compositions approach the theoretically expected stable plagioclase compositions only extremely slowly. An extrapolation of the data achieved in run times of 5–1,500 h indicates attainment of equilibrium after 1014 years. Metastable melting of granitic compositions is not only considered as an experimental problem but also as a rock forming process in nature.