Thermal evolution of the Ryoke metamorphic belt, southwestern Japan: Tectonic and numerical modeling

Abstract The Ryoke metamorphic belt of southwestern Japan is composed of Cretaceous Ryoke granitoids and associated metamorphic rocks of low-pressure facies series. The Ryoke granitoids are divided into sheet-like bodies (e.g. Gamano granodiorite) and stock-like bodies. The Gamano granodiorite intruded concordantly into the high-grade metamorphic rocks without development of a contact metamorphic aureole, and the intrusion ages of the granodiorite are similar to the ages of thermal peak of the low pressure (low-P) metamorphism. It is suggested that the low-P Ryoke metamorphism resulted from the intrusion of the Gamano granodiorite. In this study, a simple 1-D numerical model of conductive heat transfer was used to evaluate the thermal effects of emplacement of the Gamano granodiorite. Calculated temperature-time ( T-t ) paths are characterized by a rapid increase of metamorphic temperature and a relatively short-lived period of high temperature. For example, the T-t path at the 15-km depth is characterized by a rapid average increase in temperature of 1.4 × 10-³°C/year and high temperatures for < ca 0.5 Ma. The calculated peak temperature for each depth is nearly equal to the petrologically estimated value for each correlated metamorphic zone. The results suggest that the magma-intrusion model is one possible thermal model for low-pressure facies series metamorphism.     

Magnetic parameters and variations in the composition of igneous rocks of the Franz Josef Land archipelago

A combined study of magnetic parameters of basalt and andesite samples is performed in the framework of geological investigations of the Franz Josef Land at the paleomagnetic laboratory of Munich University. The study included the determination of the coercivity, saturation magnetization, Curie points, natural remanent magnetization (NRM), and magnetic susceptibility and the examination of ferromagnetic minerals with a microscope. Data on the chemical composition of rocks are obtained for all samples, and radiological ages are determined for the majority of rocks.Thermomagnetic curves of samples are subdivided into four types depending on the composition of ferromagnetic NRM carriers.The data obtained point to multiple changes in the predominant composition of igneous rocks. Each stage of magmatism is characterized by a specific type of the ferromagnetic component in the rocks and, therefore, magnetomineralogical investigations can be used for differentiation and correlation of the igneous rocks.     

Origin of magnetic minerals and natural remanent magnetization of basaltic rocks from Lower Silesia,Poland

Summary Two groups of basaltic rocks with normal and reversed NRM were examined. It was found that in both groups the primary NRM, synchronous with the period of rock formation, is preserved. Relations of the primary NRM to the origin and the degree of oxidation of magnetic minerals were considered in detail.     

Remagnetization of the Early Paleozoic rocks from southern part of the Huabei basin (II)

The opinions vary on the secondary remagnetization mechanisms of carbonate rocks. The prevalent interpretations of magnetic remanences are either chemical in origin that invoked the large-scale migration of orogenic fluids. or thermoviscous magnetization involved burial diagenetic processes. The paleomagnetic study carried out in northwestern part of Henan Province (north of the Qinling orogenic belt) reveals that 90% of carbonate rocks have suffered from the secondary remagnetization. On the basis of rock magnetic experiments, the assemblage of the magnetic minerals, remanent carriers and its formation process, and the possible mechanisms of remagnetization in these carbonate rocks are discussed.     

Author index 1984 (volumes 67–71)

Magnetic properties of samples from Bell Island sedimentary rocks have been studied. X-ray analysis indicates that the main magnetic mineral is hematite in all samples. The other iron-bearing minerals identified are siderite and chamosite. Microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite. The intensity of natural remanent magnetization (NRM) varies in the range of (0.03–0.4 A m^?1), depending on the percentage of hematite. The thermal demagnetization curves of NRM show in some cases a sharp increase in magnetization at temperatures in the range 500–600°C. The peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating. X-ray analysis confirmed that the newly formed material is magnetite. Since the original NRM has been masked by the new intergrown material, this would result in a serious error in the determination of paleomagnetic pole positions. The samples showing this behaviour were not considered for paleomagnetic study. The samples containing oolitic hematite in a calcite matrix exhibit very high stability of NRM, including directional stability until almost 670°C. For these samples, a virtual pole position based on N = 6 samples (32 specimens) demagnetized to 665°C is 34°N, 114°E, not far from published Ordovician poles for the North American craton.     

Origin of the magnetization of Permo-Carboniferous sediments of Spitsbergen, Svalbard Archipelago

Rock magnetic investigations of Permo-Carboniferous carbonate sediments from two areas on Spitsbergen are described, conducted to identify the carriers of the NRM in these rocks. Since microscopic and magnetic separation techniques could not profitably be applied, the nature of magnetic minerals was investigated by thermal demagnetization of the NRM and decay of saturation isothermal remanence (I_rs) during heating to 600°C, as well as by the distribution of the median destructive fields of the NRM and observation of magnetic susceptibility after subsequent heatings. The results show that the NRM of these limestones resides mainly in magnetite, but creation of magnetic pyrrhotite and of fresh magnetite is observed during heating to 600°C. Presence of sulphides indicates that magnetite is an oxidation product of pyrite or of non-magnetic pyrrhotite. Examination of rock magnetic properties of limestones leads to the conclusion that most of the magnetite in the rocks of the Bellsund area is of detrital origin, whereas the rocks at Festningen contain magnetite derived from pyrite probably during an early stage of the diagenetic process.     

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province,China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.     

Inversion of titanomaghemite in oceanic basalt during heating

We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16-35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process.     

Remagnetization history of Middle Triassic Leikoupo Formation on Wangcang section in Sichuan Province,China

An intensive paleomagnetic investigation has been conducted on the Middle Triassic Leikoupo Formation on the Wangcang section (32.14°N, 103. 17°E). The results indicate the magnetic minerals are dominant by multidomain magnetite or maghemite, and the characteristic remnant magnetization revealed by stepwise thermal/alternating field demagnetization is close to the present-day geomagnetic direction of the sampling site. This suggests that dolomitization/thermal viscous magnetization is responsible for the remagnetization of this kind of rocks.

     

Remanent magnetization of a Pliensbachian limestone sequence at Bakonycsernye (Hungary)

Remanent coercivity spectra derived from IRM acquisition curves and thermal demagnetization of the IRM indicate that magnetite, haematite and minor amounts of goethite determine the magnetic properties of the Pliensbachian limestones at Bakonycsernye. These limestones have been sampled at approximately 7-cm intervals along a 10-m stratigraphic section which covers the whole Pliensbachian stage (Lower Jurassic) without any recognizable break in sedimentation. The primary natural remanent magnetization (NRM) is carried by detrital particles of magnetite and haematite, but it is seriously overprinted by a normal magnetization which originates from secondary haematite with a wide range of blocking temperatures. This haematite is believed to have formed diagenetically during one of the Mesozoic periods of normal polarity. However, the reversal pattern obtained after NRM thermal demagnetization at temperatures ≥450°C is thought to be characteristic of the Pliensbachian stage.     

Determination of paleointensity from sedimentary and igneous rocks by the method of alternating-field remagnetization

The remanent magnetization of igneous and sedimentary rocks, if not changed by heating or by alteration of magnetic minerals, keep the information about the intensity of that magnetic field in which initial magnetization took place.It has been determined that the dependence of anhysteretic remanent magnetization of such a rock on d.c. magnetic field permits us to find the paleointensity. A method of investigation of rock specimens by means of such remagnetization is suggested and applied to determining the paleointensity for a series of Permo-Triassic rocks.     

A continental crustal model and its geothermal implications

The following crustal model based on realistic estimates of metamorphic rock volumes and H₂O content is proposed as a basis for geothermal calculations: (1) a surface zone of intermediate metamorphic rocks containing granitic intrusions and grading downward into (2) a more felsic migmatite zone, (3) a lower crustal zone of approximately andesitic composition crystallized in granulite or possibly amphibolite facies. Heat production values and thickness for the three zones are 3 HGU, 5 HGU, 0.5–1.5 HGU and 8, 8, and 18 km respectively. If the surface heat flow is 1.2 HFU, the model predicts a temperature of only 407°C at the Moho and an upper mantle heat flow of 0.3–0.5 HFU. The low temperatures resulting from this model rule out a seismic low-velocity zone in the crust produced by thermal effects.     

Magnetic components contributing to the NRM of Middle Siwalik red beds

The components of the NRM in Middle Siwalik red beds are carried by two phases of hematite, a red pigment phase and a specular hematite phase. We present evidence in the form of a conglomerate test that the specularite phase carries a remanence acquired during or shortly after deposition. The red pigment, however, post-dates deposition, in many cases by at least one reversal boundary. This secondary component has a higher coercivity but a lower blocking temperature in these rocks and can therefore be selectively removed by thermal demagnetization to reveal a primary magnetization useful for magnetostratigraphic studies.     

Palaeomagnetic investigations on the Deccan Traps from Chincholi,Mysore State,India

Summary 38 oriented samples of Deccan Traps have been collected from the neighbourhood of Chincholi, Mysore State, India. The Natural Remanent Magnetisation of these rocks has been studied using an astatic magnetometer. It has been found that these rocks are magnetically reversed, the mean magnetic direction being N154°E in declination and 61° down in inclination. Thermoremanance studies conducted on four specimens showed that two specimens with weak NRM and a high secondary magnetisation have Curie temperatures around 560°C for the NRM and exhibited partial reversal of TRM at room temperature, while two specimens with high NRM and with little secondary magnetisation have Curie temperatures much lower than 560°C for the NRM.     

A multicomponent,dual-polarity palaeomagnetic regional overprint from the Moine of northwest Scotland

Palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems. The heavy mineral seams and associated psammite in the deformed and metamorphosed Lower Morar Division of the Moine Assemblage of northwest Scotland have a palaeomagnetic signature that is characteristic over a wide area. The various components of the magnetization are resolved by detailed thermal demagnetization. These components are post-tectonic, as they are not affected by any structural fabric. They may be classified according to blocking temperature and, when statistically distinguishable, show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander. Comparison to the European palaeomagnetic record suggests a late Silurian to early Devonian age of magnetization for the localities south of the Strath Conon Fault. The distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium-argon ages in this part of the British Caledonides. If the magnetization is purely of thermal origin, the palaeomagnetic record indicates higher temperatures at later time than does the potassium-argon record. This remagnetization event is related to the closing stages of the Caledonian orogeny.     

Evidences for multiple remagnetization of Proterozoic dykes from Iguerda inlier (Anti-Atlas Belt,Southern Morocco)

No paleomagnetic data exist for Paleo-Mesoproterozoic times of the West African Craton (WAC). Therefore, paleogeographic reconstructions for such old geological times are difficult to constrain. Gaps on the sedimentary record and intense remagnetizations are the major problems that paleomagnetic studies come across. Recent geochronological results for dyke swarms that intrude several Proterozoic inliers of WAC in the Anti-Atlas Belt (southern Morocco) revealed ages between Paleoproterozoic and early Neoproterozoic, opening for the first time a window of opportunity to conduct paleomagnetic studies and tentatively infer about the paleoposition of WAC during Proterozoic. On this scope we conducted a paleomagnetic study on seven Proterozoic dykes of the Iguerda inlier. The meaning of the obtained paleomagnetic directions was evaluated by rock magnetic and mineral analyses, complemented by petrographic observations. Our samples record the presence of a complex history of remagnetization, mostly assigned to several Phanerozoic thermal/chemical events, in particular to the late stages of Pan African orogeny (s.l.), to the Late Carboniferous Variscan orogeny, and even to more recent events. The recognized remagnetization processes are related to widespread metamorphic events under greenschist facies followed by low-temperature oxidation, both responsible for the formation of new magnetic phases, like magnetite and hematite. These events obliterated the primary (magmatic) thermo-remanent magnetization and promoted multiple remagnetizations of the dykes, thermally and chemically. For only one dyke the presence of primary magnetization is possible to infer, though not to confirm, and would place WAC at an equatorial position around 1750 Ma.     

Quality of pTRM acquisition in pyrrhotite bearing contact-metamorphic limestones: possibility of a continuous record of Earth magnetic field variations

The rockmagnetic and palaeomagnetic signal in pyrrhotite bearing limestones of different contact-metamorphic settings were investigated related to intrusions ranging from small sills to large magmatic complexes. Magnetic susceptibility, the pyrrhotite/magnetite ratio and thermal modelling serve as an investigative tool to define three metamorphic zonations: a contact zone of a mixed magnetic assemblage and low susceptibilities, a pyrrhotite bearing transitional zone, where full thermoremanent magnetisations (TRMs) are acquired due to temperatures above the Curie temperature of pyrrhotite (T_c,po), and a marginal zone containing pyrrhotite and magnetite generated at temperatures below T_c,po. The fact that TRMs can consist of independent pTRMs is successfully tested by modified Thellier experiments. It is shown that a metamorphic environment with low fluid circulation provides a scenario for the recording of independent pTRMs. Multicomponent behaviour of the NRM residing in samples from the transitional zone can therefore be interpreted as a continuous record of Earth magnetic field variations.     

Physical properties of Vilppula drill cores and petrographic analysis of associated breccias in Keurusselk? impact structure, central Finland

The Mesoproterozoic deeply eroded Keurusselk? impact structure in central Finland is situated within the ??1860?C1890 Ma Central Finland Granitoid Complex. An estimate for the original size of the structure is 30 km, yielding a 5 km wide central uplift with insitu shatter cones and shock metamorphic features in quartz. Petrophysical and rock magnetic properties of the three shallow drill cores (V-001, V-002 and V-003) in the vicinity of the central uplift are determined in order to assess the dimensions of the central uplifts magnetic anomalies. The drill core lithologies consist of schists (metagraywackes), metavolcanic rocks, gneisses and breccia. Petrophysical properties of the drill core rocks show average densities (D) of 2644?C2752 kg/m³, susceptibilities (??) of 160?C761 × 10^?6 SI and natural remanent magnetization (NRM) of 3?C306 mA/m and Koenigsberger Q ratios of 0.1?C10. Rock magnetic measurements with temperature dependence of susceptibility (??-T) curves and hysteresis indicated mostly paramagnetic behaviour. However, a fraction of fine-grained ferromagnetic minerals (pyrrhotite and magnetite) was detected from all lithologies. Breccia veins cutting the parautochthonous subcrater floor show lower values of petrophysical properties (D, ??, NRM, Q) and this could be related to the impact event. Amphiboles and micas in the breccia are strongly altered and replaced by secondary chlorite. Chloritization may indicate widespread impact-induced hydrothermal alteration of the target rocks or it may be related to regional tectonic shearing. However, planar deformation features in quartz, found from shatter cones in the central uplift area, are decorated with fluid inclusions indicating that alteration by post-impact processes was present.     

Giant Barkhausen jumps in Kamchatka basaltic lavas

Igneous rocks are shown to possess a magnetic fabric that admits the occurrence of giant Barkhausen jumps. It is also demonstrated that magnetic minerals of rocks can preserve information on a few magnetic fields (differing even in origin) and a few thermal magnetization temperatures.