摘要:

AbstractThe chronological characteristics of Alpine metamorphic rocks are described and Alpine metamorphic events are reinterpreted on the basis of chronological data for the western and central Alps from 1960 to 1992. Metamorphic rocks of the Lepontine, Gran San Bernardo, Piemonte, Internal Crystalline Massifs and Sesia‐Lanzo mostly date Alpine metamorphic events, but some (along with granitoids and gneisses from the Helvetic and Southern Alps) result from the Variscan, Caledonian or older events and thus predate the Alpine events. Radiometric age data from the Lepontine area show systematic age relations: U‐Pb monazite (23‐29 Ma), Rb‐Sr muscovite (15–40 Ma) and biotite (15–30 Ma), K‐Ar biotite (10‐30 Ma), muscovite (15–25 Ma) and hornblende (25‐35 Ma), and FT zircon (10‐20 Ma) and apatite (5‐15 Ma), which can be explained by the different closure temperatures of the isotopic systems. A 121 Ma U‐Pb zircon age for a coesite‐bearing whiteschist (metaquartzite) from the Dora‐Maira represents the peak of ultra‐high pressure metamorphism. Coesite‐free eclogites and blueschists related to ultra‐high pressure rocks in the Penninic crystalline massifs yield an40Ar‐39Ar plateau age of about 100 Ma for phengites, interpreted as the cooling age. From about 50 Ma, eclogites and glaucophane schists have also been reported from the Piemonte ophiolites and calcschists, suggesting the existence of a second high P/T metamorphic event. Alpine rocks therefore record three major metamorphic events: (i) ultra‐high and related high P/T metamorphism in the early Cretaceous, which is well preserved in continental material such as the Sesia‐Lanzo and the Penninic Internal Crystalline Massifs; (ii) a second high P/T metamorphic event in the Eocene, which is recognized in the ophiolites and calcschists of the Mesozoic Tethys; and (iii) medium P/T metamorphism, in which both types of high P/T metamorphic rocks were variably reset by Oligocene thermal events. Due to the mixture of minerals formed in the three metamorphic events, there is a possibility that almost all geochronological data reported from the Alpine metamorphic belt show mixed ages. Early Cretaceous subduction of a Tethyan mid‐ocean ridge and Eocene continental collision triggered of

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00087.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractApatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74‐90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K‐Ar ages and the Turonian‐Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100‐80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5‐3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene‐Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific‐Eurasian plate and the demise of the Kula‐Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00088.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMelting experiments have been carried out on an olivine andesite of Mt Yakushi‐Yama from the Miocene Setouchi volcanic belt in northeastern Shikoku, Japan. This andesite has been characterized by a low ratio of FeO*/Mg° (= 0.78). Phase relations have been determined within the pressure range of 2.8 to 19.3 kbar at 1000‐1300°C under anhydrous and water‐saturated conditions. At pressures less than 8.8 kbar, olivine is a liquidus phase. Orthopyroxene appears on the liquidus at 9.3 kbar under the anhydrous conditions. The multiple saturation point rises up to 17.5 kbar for water‐saturated experiments. The andesite melt coexists with olivine and orthopyroxene just below the liquidus at 8.8–9.3 kbar and 1230°C for dry conditions, and at 17.5 kbar and 1060°C under water‐saturated conditions. These experimental results indicate that the Yakushi‐Yama olivine andesite magma could coexist with a harzburgitic mantle at depths between about 30 and 60 km, and at temperatures between 1060 and 1230°C. Experimental data also suggest a possibility that a high magnesian andesite magma would be generated by a direct partial melting of the uppermost harzburgitic mantle under an

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00089.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00090.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractAlteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin‐fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite‐pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5‐15.5 Ma, determined by planktonic foraminifera assemblages of inter‐bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K‐Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The maximum Th of the fluid inclusions is 222°C and Tm indicates trapped fluids of up to 3.3 wt% equivalent NaCl (i.e. almost the same as seawater). A Th versus Tm plot indicates mixing occurred between hydrothermal fluids and formation water that has low salinity. Corrensite and chlorite form veins, and the temperatures of their formation, estimated by the extent of aluminium substitution into the tetrahedral site of chlorite, ranges between 165 and 245°C in the centre of the hydrothermal alteration zone (zone IIIa). This is consistent with the result of Th analyses. The deposition temperature of chlorite associated with prehnite in veins ranges between 190 and 215°C

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00091.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA Triassic formation in the Turpan‐Hami and Ordos basin of China gives two typical examples of occurrence of laumontite belonging to diagenetic facies. Sandstones of the Upper Triassic Yanchang Formation in the eastern sector of the Ordos basin are formed at the mature stage of mesodiagenesis at temperatures in the range of 71‐120°C, while the temperatures of the oil‐bearing beds Chang‐6 and Chang‐7 where laumontite is present are about 81‐88°C. The laumontite‐bearing beds of the Middle Triassic in the Hami depression are presently at the supermature stage of organic matter, the corresponding diagenetic temperature being about 140 °C. The term zeolite facies should be rejected, and the features of metamorphic stage should be clearly denned, such as closely packed grains, absence of pores and cements, and characteristic mineral assemblages including sericite, epidote, muscovite, illite (2M1), chlorite (IIb, β= 97°), dickite, pyrophyllite, graphite, chlorozeolite formed at temperatures of 200°C and Ro values of 2.5%; the stage is also characterized by schistosity and illite crystallinity of IC = 0.42°Δ 2θ. The anchimetamorphic zone is characterized by some characteristic minerals such as paragonite, rectorite, albite, laumontite, illite (1M), chlorite (Ib, P = 90°+ IIb, β= 97°) formed at temperatures of 130‐200°C and Ro values of 1.3‐2.5% as well as other quantitativ

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00092.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractVarious siliceous rocks are found in the Ohdoji, Akaishi and Maido Formations from the western Aomori basin, and the Yotsuzawa and Wadagawa Formations from the eastern Aomori basin of northern Honshu, Japan. These rocks are classified into diatomite, siliceous shale and chert.Diatomite is composed of abundant amorphous silica and has porosity between 50 and 65%. Siliceous shale is composed of a large amount of quartz, and has porosity ranging from 25 to 35%. Chert is chiefly composed of cristobalite or quartz, and has porosity between 20 to 30%.Average contents of total organic carbon, S1and S2generally increase from argillaceous rocks → diatomites → siliceous shales → cherts. Maturation of organic matter in these rocks is generally lower than that in average source rocks. Diatoms, which appeared in the late Cretaceous and became increasingly important in the Miocene, are the principal primary producers of organic matter in the marine environment during the Cenozoic. Excellent organic components and higher biological productivity show that diatoms might be the most important source of petroleum during the Neogene in Japan.Proteins, carbohydrates and lipids in diatoms have been transformed into fulvic acids, humic acids and humin by polycondensation and polymerization. Later, these humin materials could be changed into insoluble kerogen under the effect of mild temperature and pressure. A part of the lipids would transform to geochemical fossils (biomarkers). Amorphous silica in cells of diatoms would change to low‐cristobalite and low‐quartz by the increase of geothermal te

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00093.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractExtensional basins are formed in a variety of geotectonic settings and in some cases the infills of such basins show the effects of very low grade, burial‐type metamorphism. It is proposed that extensional basins with a low grade metamorphic infill can be divided into pre‐ and post‐orogenic types with a different causal mechanism for the metamorphism. In the pre‐orogenic setting the extensional process driven by mantle upwelling provides both the mechanism for basin formation and the development of the metamorphism in response to the enhanced thermal flux in such settings. The typical mineral parageneses and facies series found in these low grade metamorphic terranes are more compatible with the counter‐clockwise pressure‐temperature‐time loops postulated for such settings, rather than the clockwise loops associated typically with convergent settings. In the post‐orogenic settings, extensional basins formed in over‐thickened continental crust during orogenesis have infills in which very low grade metamorphism is also developed. It is proposed that such metamorphism is not linked to the extensional process itself. Instead, it is linked to crustal heating during the process of thickening and orogenesis and is expected to be found only in orogenic belts where widths are in

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00094.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1993.tb00095.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY