Regional Thermal Metamorphism Research Articles

Occurrence and genesis of minerals and their influences on pores and fractures in the high-rank coals

Pores and fractures constitute connected networks for fluid migration and production in or from coal reservoirs. The existence of minerals in coal not only increases the heterogeneity of the reservoir but also influences the development and connectivity of pores and fractures in the coal. By applying X-ray diffraction environmental scanning electron microscopy and field emission scanning electron microscopy with an energy dispersive X-ray spectrometer type, the occurrence modes and geneses of minerals were investigated in representative high-rank coals from the Qinshui basin, North China. Additionally, their impact on the development and connectivity of pores and fractures was explored in the high-rank coal. The results show that illite and kaolinite are the dominant minerals, comprising 57.74% of the mineral volume on average. The minerals fall into three genetic types, including detrital, syngenetic, and epigenetic, of which syngenetic minerals are the most common. The occurrence modes of the minerals depend on their geneses. The detrital minerals are mostly granular; the syngenetic minerals are agglomerates, stripped aggregates, and disseminated fine particles, and the epigenetic minerals are fracture fillings. At the micro-scale, the influences of the minerals on pores and fractures in the high-rank coals are mainly denoted by micro-fracture fillings, development of mineral pores, and generation of differential deformation pores and fractures. Differential deformation pores and fractures appear at the boundaries between clay and brittle minerals or clay and organic matter due to their physical and chemical property differences related to the regional thermal metamorphism of the coal. The coal samples are always connected by thermogenic pores and mineral pores, favoring connectivity of the pores and fractures in the coal. Moreover, the dissolved pores and intercrystalline pores are largely interconnected. This study focused on the effects of minerals on the pore and fracture structure of the high-rank coal reservoir. The results provide a better understanding of the migration networks and fluid mechanisms related to complex pores and fractures, particularly in high-rank coal reservoirs.

Metamorphic Evolution of the Ogcheon Belt, Korea: A Review and New Age Constraints

The Ogcheon metamorphic belt (OMB), which is often correlated with the Dabie-Sulu ultrahighpressure (UHP) belt in China, comprises Neoproterozoic to Paleozoic metasedimentary and -volcanic sequences representing a stack of synmetamorphic nappes. Regional metamorphic grade increases northwestward to produce the assemblage, biotite + garnet ± staurolite ± kyanite + plagioclase + quartz. Garnet porphyroblasts show chemical zoning typical for prograde metamorphism, with decreasing Mn and increasing Fe and Mg from core to rim. P-T conditions were estimated to be 4.2-9.4 kbar and 490-630°C, corresponding to the medium-pressure type. In addition, the GIBBS calculation suggests a clockwise P-T-t path, corroborating a crust-thickening event associated with regional peak metamorphism. Using various isotopic techniques, the timings for intracontinental rift volcanism and syntectonic metamorphism were determined. New SHRIMP U-Pb zircon ages from a felsic tuff reflect the Neoproterozoic rifting at ˜;750 Ma. Regional metamorphism has been recently dated at ˜;285 Ma, based upon several independent sets of radiometric ages: U-Pb ages of monazite inclusions in garnet, Pb-Pb whole-rock ages of black slates, chemical ages of uraninite, and SHRIMP U-Pb zircon ages from a granitic gneiss pebble in metadiamictite. Thus, the Ogcheon orogeny is newly defined as an earliest Permian event, preceding a high-pressure metamorphic event in the Imjingang belt, central Korea, by ˜;30 Ma. During the subsequent Songrim orogeny, at ˜;250-220 Ma, the OMB experienced a second regional-thermal metamorphism under greenschist-to amphibolite-facies conditions. This Triassic event is interpreted to correspond to a major collisional orogeny in the Dabie-Sulu belt. These multiple metamorphic events in the Ogcheon belt are readily correlated with those reported from not only the UHP belt in east-central China, but also the Hida and Renge belts in Japan. Thus, eastward extension of the Dabie-Sulu belt through the Korean Peninsula to the Hida belt (Ernst and Liou, 1995) remains a valid hypothesis.

Tectono‐metamorphic evolution of the Okcheon Metamorphic Belt, South Korea: Tectonic implications in East Asia

Abstract The tectonic history of the Okcheon Metamorphic Belt (OMB) is a key to understanding the tectonic relationship between South Korea, China and Japan. The petrochemistry of 150 psammitic rocks in the OMB indicates that the depositional environment progressively deepened towards the northwest. These data, combined with the distribution pattern of oxide minerals and the abundance of carbonaceous material, support a half‐graben basin model for the OMB. Biotite and muscovite K–Ar dates from metasediments in the central OMB range from 102 to 277 Ma. K–Ar ages of 142–194 Ma are widespread throughout the area, whereas the older ages of 216–277 Ma are restricted to the metasediments of the middle part of the central OMB. The younger (Cretaceous) ages are only found in metasediments that are situated near the Cretaceous granite intrusions. The 216–277 Ma dates from weakly deformed areas represent cooling ages of M1 intermediate pressure/temperature (P/T) metamorphism. The relationship between age distribution and deformation pattern indicates that the Jurassic muscovite and biotite dates can be interpreted as complete resetting ages, caused by thermal and deformational activities associated with Jurassic granite plutonism. Well‐defined 40Ar/39Ar plateau ages of 155–169 Ma for micas from both metasediments and granitic rocks can be correlated with the main Jurassic K–Ar mica ages (149–194 Ma). U–Pb zircon dates for biotite granite from the southwest OMB are 167–169 Ma. On the basis of the predominantly Jurassic igneous and metamorphic ages and the uniformity of d002 values for carbonaceous materials in the study area, it is suggested that the OMB has undergone amphibolite facies M2 metamorphism after M1 metamorphism. This low P/T M2 regional thermal metamorphism may have been caused by the regional intrusion of Jurassic granites. The OMB may have undergone tectono‐metamorphic evolution as follows: (i) the OMB was initiated as an intraplate rift in the Neoproterozoic during break‐up of Rodinia, and may represent the extension of Huanan aulacogen within the South China block; (ii) sedimentation continued from the Neoproterozoic to the Ordovician, perhaps with several unconformities; (iii) M1 intermediate P/T metamorphism occurred during the Late Paleozoic due to compression caused by collision between the North and South China blocks in an area peripheral to the collision zone; and (iv) during the Early to Middle Jurassic, north‐westward subduction of the Farallon‐Izanagi Plate under the Asian Plate resulted in widespread intrusion of granites, which triggered M2 low P/T regional thermal metamorphism in the OMB. This event also formed the dextral Honam shear zone at the boundary between the OMB and Precambrian Yeongnam massif.

Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden

The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The volcanic pile is intruded by a synvolcanic Jorn-type granitoid (Viterliden intrusive complex) and is overlain by a thick sequence of metasedimentary rocks (Vargfors Group). Mineralization occurs at two main stratigraphic levels, at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the Vargfors Group. The Kristineberg Cu–Zn mine is the largest deposit (approximately 21 Mt) and occurs at the base of the volcanic pile, close to the contact with the Viterliden intrusive complex. Four smaller deposits (Ravliden, Ravlidmyran, Horntrask and Nyliden) occur along the upper ore horizon. These deposits are thought to be related to a late intrusive phase of the Viterliden complex which cuts the altered volcanic rocks at the Kristineberg deposit. Within an area of about 50 km2 surrounding the Kristineberg deposit, felsic volcanic rocks between the two ore horizons are affected by extensive albite-destructive alteration (sodium depletion) and development of chlorite and muscovite (strong co-enrichment in magnesium and potassium). The Kristineberg deposit is enveloped by a large and partly transposed quartz–chlorite alteration zone, approximately 2 km in diameter, and a distal but coherent pyrite–quartz–muscovite alteration zone extending as far as 4 km from the deposit. Chlorite(±talc) in the mine area is notably magnesium-rich and contains anomalous F, Ba, Zn and Mn. High fluorine is also present in coexisting muscovite and phlogopite. The magnesium-rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic, volcanic-hosted Precambrian massive sulfide deposits. This may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones. Kyanite is developed locally in the chlorite-rich pipe at Kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot-wall rocks. Spectacular, andalusite-bearing quartz–muscovite schists and quartz–biotite–cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late Revsund granite. However, similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered. Deposits near the top of the felsic volcanic succession are characterized by magnesium-rich chlorite alteration in the foot wall and proximal calc-silicate assemblages (dolomite, calcite, tremolite, ±garnet, ±margarite) where the host sedimentary rocks are carbonate-rich. In general, the calc-silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target. The two main ore horizons in the Kristineberg area are not linked by any obvious discordant structures or alteration zones. However, mineral-chemical studies highlight several possible fluid flow pathways leading from the Kristineberg deposit to the Ravliden ore horizon, more than 2 km upsection. Overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore-related alteration. Electronic Supplementary Material is available at http://dx.doi.org/10.1007/s00126-002-0299-y. On that page (frame on the left side), a link takes you directly to the supplementary material.

Tectonic significance of the meta-evaporitic magnesite and scapolite deposits in the Seve Nappes, Sarek Mts., Swedish Caledonides

Magnesite and scapolite, hosted by diabase and amphibolite derived from diabase, occur in the Sarektjåkkå Nappe, Seve Nappe Complex in the Caledonides of northern Sweden. The Sarektjåkkå Nappe consists of a sedimentary sequence—the Favoritkammen Group—intruded by vast amounts of diabase, that constitute 70–80% of the nappe. The magnesite and scapolite layers are parts of the Spika Formation in the Favoritkammen Group. The Spika Formation formed in an evaporative setting of alternating influx of continental (magnesian carbonates) and marine water (halite and anhydrite/gypsum altered to scapolite during thermal metamorphism), possibly in rift-related basins in the Late Precambrian. The stratigraphy of the Favoritkammen Group indicates a progressively deepening basin, consistent with deposition in developing rift basins. The diabase dykes that cut the Spika Formation caused regional thermal metamorphism, but are not responsible for the magnesite formation. The lithology of the Sarektjåkkå Nappe thus records the evolution of a rift to the formation of a passive margin. That passive margin was detached and thrust over the Baltic Shield during the Caledonian Orogeny.

Metamorphic Facies Map of Alaska: ABSTRACT

A metamorphic-facies map of Alaska has been compiled, following the facies-determination scheme of the Working Group for the Cartography of the Metamorphic Belts of the World. Regionally metamorphosed rocks are divided into facies series where P/T gradients are known and into facies groups where only T is known. Metamorphic rock units also are defined by known or bracketed age(s) of metamorphism. Five regional maps have been prepared at a scale of 1:1,000,000; these maps will provide the basis for a final colored version of the map at a scale of 1:2,500,000. The maps are being prepared by the U.S. Geological Survey in cooperation with the Alaska Division of Geological and Geophysical Surveys. Precambrian metamorphism has been documented on the Seward Peninsula, in the Baird Mountains and the northeastern Kuskokwim Mountains, and in southwestern Alaska. Pre-Ordovician metamorphism affected the rocks in central Alaska and on southern Prince of Wales Island. Mid-Paleozoic metamorphism probably affected the rocks in east-central Alaska. Most of the metamorphic belts in Alaska developed during Mesozoic or early Tertiary time in conjunction with accretion of many terranes. Examples are Jurassic metamorphism in east-central Alaska, Early Cretaceous metamorphism in the southern Brooks Range and along the rim of the Yukon-Koyukuk basin, and Late Cretaceous to early Tertiary metamorphism in the central Alaska Range. Regional thermal metamorphism was associated with multiple episodes of Cretaceous plutonism in southeastern Alaska and with early Tertiary plutonism in the Chugach Mountains. Where possible, metamorphism is related to tectonism. Meeting participants are encouraged to comment on the present version of the metamorphic facies map. End_of_Article - Last_Page 663------------

Palaeomagnetism of Ketilidian metamorphic rocks of SW Greenland, and 1850-1600 m.y. apparent polar movements

Proterozoic supracrustal rocks of southwest Greenland and amphibolite dykes intruding the basement possess a thermal remanent magnetisation acquired during slow regional uplift and cooling between 1800 and 1600 m.y. following the Ketilidian mobile episode. Most samples from amphibolite dykes (mean palaeomagnetic pole 214°E, 31°N) possess a stable remanence associated with development of hematite during regional thermal metamorphism. Metavolcanics from the eastern part (eight sites, palaeomagnetic pole 230°E, 60°N, A95 = 15°) and western part (twelve sites, 279°E, 59°N, A95 = 17°) of Arsůk Island have magnetisations postdating folding and are related to KAr ages dating regional cooling (1700-1600 m.y.); magnetic properties are highly variable and partially stable remanence resides predominantly in pyrrhotite. These results agree in part with other palaeomagnetic results from the northern margin of the same craton, and currently available palaeomagnetic results assigned to the interval 1850-1600 m.y. are evaluated to define apparent polar wander movements. Two large polar movements are recognised during this interval with the possibility of a third at ca. 1800 m.y. It is concluded that apparent polar wander movements in Proterozoic times are most accurately described in terms of closed loops.

URANIUM-LEAD METHOD OF AGE DETERMINATION

A suite of samples of pitchblende, clausthalite, and galena from various mines in the Lake Athabasca uranium province was subjected to isotopic and chemical analyses as well as radon-leakage measurements in order to obtain apparent ages from the isotopic ratios Pb/sup 2-6//U/sup 238/, /pb/sup 206, Pb/ sup 210/pb/sup 206/ measurements were made to check equilibrium in the decay series. The apparent isotopic ages range from 220 to 1860 m.y. Aiteration effects which might explain these anomalies are considered. Uranium leaching and radon leakage are inadequate to explain these anomalies. The diverse ages can be calculated on the hypothesis of a single period of pitchblende deposition at 1.90 plus or minus 0.04 b.y. ago followed by two periods of exsolution of lead at about 1.2 and 0.15 b.y. This is consistent with the 207/206 ratio of the radiogenic lead in the clausthalite and galena which apparently formed at the times of recrystallization. However, the existing data do not preclude more than two periods of lead exsolution. An alternative hypothesis involving successive epochs of pitchblende deposition is more complex geologically and, without contemporaneous lead exsolution, cannot account for the observed isotopic relations. The recrystallization presumably requires only the elevation of the regionalmore » temperature to 150 to 300 deg C in the presence of water. The variable fractions of lead removed during the periods of recrystallization can be ascribed to variations in temperature and water content throughout the district. it is quite certain that (1) the initial uranium deposition occurred about 1.9 b.y. ago, (2) there have been at least two periods of lead exsolution, (3) one period of lead exsolution must have occurred subsequent to 0.22 b.y. ago; (4) another period of lead exsolution probably occurred about 1 ago. (5) alteration of subsurface pitchblende has been negligible during the last million years. These conclusions resulting from an intensive study of an important uranium province suggest some general principles which may be applied to other areas. If samples are chosen to avoid recent weathering effects and if the radon leakage is measured, the remaining anomalies in the isotopic ages are probably due to partial lead removal at some time or times during the geologic history of the mineral. In all cases the 207/206 age will be closest to the true age but will be minimal. The true age can be approximated if a large number of samples from a given locality are studied with their associated common leads. Uranium minerals that give accordant isotopic ages have a known age and indicate the absence of significant regional thermal metamorphism following formation.« less

Metamorphism and migmatisation in the Ythan Valley, Aberdeenshire

Old and new information is collated concerning the progressive metamorphic and migmatitic series from slate through knotted phyllite, andalusite-cordierite-schist, cordierite-sillimanite-gneiss to permeation-gneiss and finally migmatitic gneiss developed in the Ythan Valley. This series, the result of regional thermal metamorphism culminating in migmatisation, constitutes the Buchan Type of plutonic series. It differs from the classic Barrovian metamorphic zones and the Cromar Type of migmatite developed in regions farther south in that temperature control is dominant and so-called antistress minerals have a regional distribution.