Characteristic Magnetization Components Research Articles

Anomalous features of the geomagnetic field behavior at the end of the Cretaceous normal superchron based on the results of the study of the Turonian–Santonian in the Southwestern Crimea

In 394 samples characterizing 266 stratigraphic levels in four Turonian–Santonian sections in the Southwestern Crimea, characteristic remanent magnetization components (ChRM) formed at the stage of diagenesis is identified. The data obtained represent the record of the Paleocene geomagnetic variations of high amplitude (rms deviation S = 25.9° with a fixed cut-off angle of 45°, which is about twice as high as the model S for this latitude) in the sediments formed in ~5–6 Ma and are interpreted as anomalous behavior of the geomagnetic field in the Turonian, Coniacian, and Santonian.

NEW 254 ma PALEOMAGNETIC POLE OF THE EAST EUROPEAN PLATFORM: MOSCOW SYNECLISE, SUKHOBORKA AND SOSNOVKA SECTIONS

The paper presents the results of paleomagnetic and rock magnetic studied of upper Permian continental red beds of the Sukhoborka and Sosnovka sections of the south-eastern part of the Moscow syneclise on the right bank of the Vetluga river in Nizhniy Novgorod region, Russia. Arguments in favor of the primary nature of the characteristic component of magnetization are given. A first securely dated upper Permian (254 Ma) paleomagnetic pole of the east part of the East European Platform is calculated: slat = 56.731°N, slong = 45.748°E, N = 44, plat = 55.3°, plong = 158.1°, dp/dm = 3.5°/5.1°, paleolat = 34.6°.

Monte Alban periphery (Oaxaca, middle Mesoamerica): New evidences on the early occupation trough archaeomagnetic study of pottery kilns

Aimed to obtain absolute chronology data for the early stage of Monte Alban, magnetic mineralogy and archaeomagnetic investigation were carried out of recently discovered pottery kilns. Oriented samples belong to four rectangular kilns, both single and double chambers, and one fire pit at the piedmont peripherical area. Susceptibility vs. temperature curves show market thermal instability while large pseudo-single domain grains seem to carry the major part of remanence for the kiln sample. Fire pit samples, in contrast, seem to be dominated by antiferromagnetic hematite. Alternating field demagnetization allowed to reach primary, characteristic magnetization components with reasonably well-defined mean archaeodirections. Possible archaeomagnetic age intervals were obtained considering currently available global models and local curves. The dates are consistent with the most important periods of occupation at the site, although the results may indicate that there was also a significant cultural presence in the Early Classic Period. Recently discovered kilns indicate constant manufacture of pottery. Although we cannot currently define the intensity of production, we believe that pottery could be a specialized activity in peripherical areas of Monte Alban to exchange ceramics or for its local consumption. Consequently, we propose that the piedmont area was an economically autonomous community whose inhabitants dedicated themselves to various tasks throughout their occupation.

Palaeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European Platform, P. 2: magnetostratigraphy

We present new results of palaeomagnetic studies of the Vendian (Ediacaran) rocks of the Volyn Basalt Province revealed by six boreholes on the NW Ukraine. This is a continuation of previous studies of upper part of the Volyn series described by Bakhmutov et al. [2021]. In the recent works [Shcherbakova et al., 2020; Thallner et al., 2022], the results of palaeointensity determinations of the Volyn series basalts showed the ultra-low dipole moment of Earth’s magnetic field which coincident with other palaeomagnetic data for Ediacaran indicating an extremely weak geomagnetic field. Clear stratification and correlation of the basalt and tuff layers by magnetic parameters allow us to determine the magnetic field reversals throughout the stratigraphic succession of the Volyn series of about 400 m thick. In basalts, tuffs and baked contact rocks, a high-temperature characteristic component of remanent magnetization (ChRM) with all signs of primary magnetization have been isolated. For the directions of inclination of ChRM-components at least six magnetic polarity reversals were revealed. The geochronological ages of rocks is in the range of 580—545 Ma, but the errors in the age estimation are too large for separation of individual formations within the pulses of activity or differentiation of individual eruptions. Therefore the formation time of the entire stratum remains uncertain, and the key issue for interpretation of the magnetic polarity reversal frequency is the duration of accumulation of traps. Two possible interpretations of palaeomagnetic results are considered. In the first, which takes into account different stages of magmatic activity, the time interval of accumulation can be about 10 million years. Then the average frequency of geomagnetic inversions is close to mean for the Phanerozoic. We prefer the other interpretation when the formations of the Large Igneous Provinces have been occurred over a short time interval (e.g. 0.5 Myr). Taking into account the results of palaeointensity estimations for the same samples, which shown the extremely weak geomagnetic field, the hypothesis of the «hyperactivity» of the field with a frequency of at least 12 reversals per one million years on the end of the Ediacaran gain the additional confirmation.

New Early Neoproterozoic Paleomagnetic Constraints of the Northwestern China Blocks on the Periphery of Rodinia

AbstractRecent studies highlight the debates about the role of the northwestern China blocks, for example, north Tarim block, south Tarim block (STB), Altyn Tagh block (ATB) and Qaidam‐Qilian block (QQB) during the assembly of Rodinia. We herein present a combined paleomagnetic, geochronological, and geochemical study on 16 mafic dikes in the northern ATB. After removing a recent viscous remanent magnetization, three characteristic remanent magnetization components (CHs) were isolated. CH1 was identified in eight NW‐trending diabase dikes, one of which was newly dated at 895.3 ± 7.7 Ma through use of SHRIMP baddeleyite U‐Pb methodology. It yields a mean direction (D/I) of 123.3°/−42.2° (α95 = 12.1°). The paleopole (40.9°S/184.6°E, A95 = 11.7°) was calculated by averaging eight virtual geomagnetic poles (VGPs) from each dike. A partial baked‐contact test supports the primary origin of CH1. CH2 was isolated from three E‐W trending diabase dikes, pointing northwest with considerably shallower downward inclinations. CH3 was obtained from five lamprophyre dikes, directing southeast with moderate to shallow upward inclinations. However, the VGPs of either CH2 or CH3 are insufficient to average a precise mean pole. Geochemical analyses also reveal petrogenetic discrepancy among the dikes with distinct CHs. Combining the new ∼895 Ma pole of CH1 with the global paleomagnetic database and being aided by geological evidence, we propose that the STB‐ATB‐QQB and the north China craton were located along the northwestern side of Laurentia in Rodinia at ∼895 Ma, and suggest the connection might have been intact until ∼780 Ma.

Magnetostratigraphy and magnetic properties of the Jurassic to Lower Cretaceous Girón Group (northern Andes, Colombia)

AbstractWe report paleomagnetic results from the Jurassic to Lower Cretaceous continental sedimentary succession exposed in the eastern limb of the Los Yariguíes anticlinorium, Eastern Cordillera, Colombia. About 820 m of a stratigraphic section of the upper part of the Girón Group (Angostura del Río Lebrija and Los Santos Formations) was sampled to construct a magnetic polarity stratigraphy. A total of 199 independent samples that yield interpretable and acceptable data have a characteristic remanent magnetization component (ChRM) isolated between 400 °C and 680 °C in progressive thermal demagnetization. Demagnetization behavior and rock magnetic properties are interpreted to indicate that hematite is the principal magnetization carrier with a possible contribution by magnetite in some parts of the section. After tilt correction, 123 samples are of normal polarity (declination [D] = 44.9°, inclination [I] = +9.7°, R = 110.64, k = 9.87, and α95 = 4.3°), and the other 76 accepted samples are of reverse polarity (D = 216.4°, I = −6.1°, R = 68.29, k = 9.72, and α95 = 5.5°). The statistical reversal test conducted on virtual geomagnetic poles is positive (class B). Based on paleontologic age estimates for the Cumbre and Rosablanca Formations, we assume a Berriasian age for the Los Santos Formation. The magnetostratigraphic data from the Girón Group strata are interpreted to suggest an age for the sampled part of the section between early Kimmeridgian and early Valanginian (ca. 157–139 Ma). The age of the Angostura del Río Lebrija Formation is estimated as between early Kimmeridgian and early Tithonian (ca. 157–146.5 Ma). The age of the Los Santos Formation is estimated between early Tithonian and early Valanginian (146.5–139.3 Ma). With our proposed, but nonunique, correlation with the Geomagnetic Polarity Time Scale, the Jurassic-Cretaceous boundary is interpreted to be located within the Los Santos Formation. The Girón Group is characterized by two periods of high (>8 cm/k.y.) and two periods of low (< 2 cm/k.y.) sedimentation rates. An inferred clockwise rotation of ∼44°, based on paleomagnetic declination data from the Girón Group, is similar to rotation estimates reported in some previous studies in the general area, and this facet of deformation could be related to local and regional response to displacement along regional-scale strike-slip faults.

Paleomagnetism of the Guanyang Devonian sedimentary successions in Guangxi province, South China

The Devonian position of the South China Block (SCB) remains debated because apparently contradictory Devonian paleopoles have been documented. Here we report two characteristic remanent magnetization components from the mildly deformed Guanyang Devonian successions (Lochkovian-Famennian) within the cratonic interior of the SCB. Both components pass reversal tests and occur in different samples across the successions. Component A and associated paleopole DA (228.9°E, 37.7°N, A95 = 3.7°) are comparable with published Devonian data and were likely acquired before Permian-Triassic folding and likely reflect syn-depositional magnetization. Component B and its associated paleopole DB (204.6°E, 4.5°N, A95 = 5.5°) are distinct from published results and were probably acquired during minor tilting associated with the Late Devonian Liujiang orogeny. If paleopole DA was acquired prior to the Late Devonian Liujiang orogeny, the paleolatitude of the Guanyang successions varied little throughout the Devonian, but the azimuthal orientation of recorded declinations exhibits a 40° difference. The distinct positions of the two paleopoles are broadly consistent with existing enigmatic Devonian field records. Alternatively, given the uncertainty in the acquisition ages of both components in the Guanyang successions, a case can be made for component A being a secondary magnetization acquired during the Permian-Triassic Indosinian orogeny. Nonetheless, the similar distribution of Devonian paleopoles from Gondwana to those of cratonic interior of the SCB suggests a Devonian connection between the two continents, and adds further support for a non-uniformitarian geomagnetic field during this enigmatic time interval.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

ПАЛЕОМАГНЕТИЗМ СРЕДНЕЮРСКИХ ОСАДОЧНЫХ ПОРОД ЭЛЬГОНСКОЙ СВИТЫ УЛЬБАНСКОГО ТЕРРЕЙНА

The paper presents new findings of paleomagnetic studies on Middle Jurassic sedimentary rocks of the Elgon Formation of the Ulban Terrane from coastal outcrops along the Ulban Bay (53.5°N, 137.7°E). Demagnetization yielded a pre-folding characteristic component of magnetization. No inclination shallowing effect for the characteristic magnetization vector was revealed in sandstone specimens of the Elgon Formation. The coordinates of the paleomagnetic pole and the paleolatitude at which the studied rocks deposited were calculated: Plat = -34.3 (34.3)°; Plong = 161.2 (341.2)°; dm = 2.7 dp = 5.3, paleolatitude = 0.1° (+2.7°/-2.6°) S. The acquired paleomagnetic data show evidence for the deposition of rocks of the Ulban Terrane in the Middle Jurassic at the boundary between the Asian paleocontinent and the Paleo-Pacific which later formed part of the Sikhote-Alin Orogenic Belt.

Loess deposits in southern Tajikistan (Central Asia): Magnetic properties and paleoclimate

Abstract The continental accumulation of dust during the Quaternary led to the formation of widespread loess deposits in southern Tajikistan. In this area, the accumulation of loess is commonly associated with the occurrence of dust storms and the widespread distribution of loess provides evidence of dust storms becoming more frequent in arid Central Asia at least since the Early Pleistocene. Southern Tajikistan represents one of the largest loess deposits in Central Asia. We conducted a thorough study on the magnetostratigraphy, grain size, and magnetic susceptibility of the Chashmanigar section to reconstruct the stratigraphy of loess deposits and paleoclimate of Tajikistan. Based on our new data, the lower boundary of the basal ages of the Olduvai and Reunion subchrons were established for the studied Chashmanigar section. Rock magnetic analyses showed that the predominant ferrimagnetic minerals are large pseudo-single domain grains of magnetite, including limited maghemite. Standard demagnetization techniques yielded a characteristic component of natural remanent magnetization, which was used to obtain a well-defined magnetostratigraphy. In southern Tajikistan, paleosols consistently exhibit finer grain size distribution and higher magnetic susceptibility than loess horizons, suggesting that the environment of the Chashmanigar section was colder, drier, and dustier during glacial periods than during interglacial periods. Through correlation with astronomically tuned oxygen isotope records, sophisticated dating of the loess-paleosol sequence at Chashmanigar could be achieved, and the global significance of the recorded paleoclimatic variations could be revealed. The resulting grain size, magnetic susceptibility, and correlation with astronomically tuned oxygen isotope clearly provide information about the climatic pattern during the Early Pleistocene.

Верхнедевонские отложения (франский ярус) северо-запада Русской платформы: палеомагнитные данные

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

Paleomagnetism of the Mn wad deposit at Niimi hot springs, Hokkaido, Japan

The Niimi hot springs is located at the foot of Mt. Shyakunage in the active Niseko volcanic area, Hokkaido, Japan. The manganese (Mn) wad deposit was precipitated from the hot spring water by biomineralization processes. However, no precise direct age dates for the diagenesis of such Mn oxides have been reported. Here is reported the first paleomagnetic application to date Mn oxides on 144 disseminated Mn wad and 42 pyroclastic clay specimens. The Mn wad specimens were collected at 14 levels between ∼5.5 and ∼62 cm in thickness on a three-dimensional grid pattern, and the clay specimens were collected at seven levels between 85 and 118 cm in thickness. Alternating field step demagnetization isolates either one or two stable characteristic remanent magnetization (ChRM) components in the Mn wad specimens. The lower coercivity component resides in pseudosingle domain titanomagnetite and shows clustered ChRM directions at each sampling level. Conversely, the higher coercivity component resides in single domain titanomagnetite and shows more random directions. The higher coercivity component is likely carried by pyroclastic fragments that landed on the site. The remanent directions of the clay specimens reside in multidomain magnetite, are less stable, and show a scattered distribution. When the observed ChRM directions of the Mn wad specimens are compared with the paleosecular variation record from Lake Biwa in Japan, the Mn wad deposition records the secular variation and appears to have lasted at least ∼1600 years from ∼3.5 to ∼1.9 kyr BP.

A Paleomagnetic Study of Sariolian Conglomerates of the Onega Structure of the Karelian Protocraton: The Problem of Global Paleoproterozoic Remagnetization

A paleomagnetic study of Sariolian (2.4–2.3 Ga) conglomerates of the Onega structure distinguished two characteristic magnetization components. The average direction of the mid-temperature magnetization component has a concentrated distribution and coincides with the direction of the Svecofennian remagnetization within the Karelian protocraton. The directions of high-temperature magnetization components distinguished in conglomerates show a wide scatter of values that is evidence of the primary origin of this magnetization component. Two clusters of high-temperature components associated not only with the protolith composition, but also with the different conditions of rock transformation, in particular their fluid saturation, are distinguished.

Paleomagnetic studies sariolyiski conglomerates of the Onega structure of the Karelian craton: Paleoproterozoic global remagnetization

As a result of paleomagnetic studies Sariolian 2,4–2,3 Ga conglomerates of the Onega basin of the Karelian protoctaton, two characteristic components of magnetization have been separated. Mean direction of the medium-temperature component has a heap distribution and coincides with mean direction of the Svecofennian remagnetization within the Karelian protocraton. The directions of high-temperature magnetization components isolated in conglomerates have a significant spread, which indicates the primary nature of this magnetization component. Two clusters of high-temperature components associated not only with the composition of protolites, but also with different conditions of rock transformations, including their fluid saturation, are distinguished.

Paleoproterozoic Evolution of Mafic–Ultramafic Magmatism of the Kun-Manie Ore Field (Aldan-Stanovoy Shield): Evidence from Paleomagnetic Data

The results of petromagnetic and paleomagnetic studies of the Late Paleoproterozoic nickeliferous mafic–ultramafic rocks of the Kun-Manie ore field located in the southeast of the Aldan–Stanovoy Shield are reported. Our research is focused on intrusions of two morphogenetic types: the Black Giant and Meridional dykes and Kubuk and Iken sills. The petromagnetic data suggest a correlation between the attitude of the studied intrusions and the orientation of the axes of the anisotropy ellipsoid of the initial magnetic susceptibility, which can be used during geological survey, prospecting, and evaluation. Based on the data of our paleomagnetic studies, two stages in the Paleoproterozoic igneous activity were distinguished: the first is related to intrusion of sills, the second, to intrusion of dykes. The orientations of the characteristic component of magnetization isolated in all intrusive bodies of the complex were used to calculate the coordinates of the paleomagnetic pole. The position of the pole corrected for the opening of the Vilyui Rift in the Paleozoic is close to the range between 1700 and 1720 Ma of the apparent polar wander path for the Siberian Craton, which is in a good agreement with the geochronological data available.

Signatures of Reductive Magnetic Mineral Diagenesis From Unmixing of First‐Order Reversal Curves

AbstractDiagenetic alteration of magnetic minerals occurs in all sedimentary environments and tends to be severe in reducing environments. Magnetic minerals provide useful information about sedimentary diagenetic processes, which makes it valuable to use magnetic properties to identify the diagenetic environment in which the magnetic minerals occur and to inform interpretations of paleomagnetic recording or environmental processes. We use a newly developed first‐order reversal curve unmixing method on well‐studied samples to illustrate how magnetic properties can be used to assess diagenetic processes in reducing sedimentary environments. From our analysis of multiple data sets, consistent magnetic components are identified for each stage of reductive diagenesis. Relatively unaltered detrital and biogenic magnetic mineral assemblages in surficial oxic to manganous diagenetic environments undergo progressive dissolution with burial into ferruginous and sulfidic environments and largely disappear at the sulfate‐methane transition. Below the sulfate‐methane transition, a weak superparamagnetic to largely noninteracting stable single domain (SD) greigite component is observed in all studied data sets. Moderately interacting stable SD authigenic pyrrhotite and strongly interacting stable SD greigite are observed commonly in methanic environments. Recognition of these characteristic magnetic components enables identification of diagenetic processes and should help to constrain interpretation of magnetic mineral assemblages in future studies. A key question for future studies concerns whether stable SD greigite forms in the sulfidic or methanic zones, where formation in deeper methanic sediments will cause greater delays in paleomagnetic signal recording. Authigenic pyrrhotite forms in methanic environments, so it will usually record a delayed paleomagnetic signal.

On the validity of archeomagnetic dating method in Korea: a case study

An archeomagnetic study has been performed for six archeological relics of the Proto-Three Kingdoms period excavated in Jungdo Island, South Korea. Among a total of 90 samples from hearths remains of six dwelling sites, the characteristic remanent magnetization components were isolated from 70 samples using the progressive alternating field demagnetizations and considered to record faithfully a thermoremanent magnetization at the timing of the last cooling of the hearths. Two different approaches were made to determine the archeomagnetic ages: One is the conventional method using the relocated paleosecular variation (PSV) curve obtained from southwestern Japan. The other method adopts a Matlab archaeo_dating tool using a Bayesian statistics with global geomagnetic field prediction model (ARCH3K.1). Both archeomagnetic ages using two individual dating methods generally overlapped with the radiocarbon age ranges for each sites. However, no systematic relationship between archeomagnetic ages and radiocarbon ages was observed, implying that neither the relocated Japanese PSV data nor the global model accurately represent the Korean secular variation during the archeological period of this study. At present, archeomagnetic dating in Korea should be carefully applied in conjunction with other dating methods on the consideration of archeological context.

Paleomagnetic evidence for local oroclinal bending in the eastern Korean Peninsula

A paleomagnetic investigation has been carried out for the Lower Triassic Donggo Formation exposed in the Danyang area on the Yeongnam Massif to constrain local and regional tectonic history of the Taebaeksan Zone, Korean Peninsula. Among a total of 201 samples from 18 sites of the Donggo Formation, the characteristic remanent magnetization (ChRM) components were obtained from 166 samples. Field tests such as tilt test and reversal test reveal that the ChRM of the Donggo Formation is of primary in origin. The Early Triassic paleomagnetic pole position calculated from the site-mean directions of the primary magnetization is at 40.7°N, 17.8°E (A95 = 5.3°). Comparison of the reliable paleomagnetic poles from the Danyang (this study), Taebaek, and Yemi areas in the Taebaeksan Zone reveals a regional division of the pole position, indicating the clockwise vertical-axis rotation of the Taebaek area by 47.2 ± 11.1° with respect to the Danyang and Yemi areas after the formation of the Pyeongan Supergroup. In addition, there is a significant relation between strike deviations and declination deviations with some flattening of the regression line in the southern Taebaeksan Zone. Thus, it is interpreted that the Pyeongan Supergroup in this region had a primary curvature at the timing of sedimentation and subsequently experienced post-Early Triassic oroclinal bending of approximately 47°.

Paleointensity determination on Neoarchaean dikes within the Vodlozerskii terrane of the Karelian craton

The results of paleomagnetic studies and paleointensity determinations from two Neoarchaean Shala dikes with an age of ~2504 Ma, located within the Vodlozerskii terrane of the Karelian craton, are presented. The characteristic components of primary magnetization with shallow inclinations I = −5.7 and 1.9 are revealed; the reliability of the determinations is supported by two contact tests. High paleointensity values are obtained by the Thellier–Coe and Wilson techniques. The calculated values of the virtual dipole moment (11.5 and 13.8) × 1022 A m2 are noticeably higher than the present value of 7.8 × 1022 A m2. Our results, in combination with the previous data presented in the world database, support the hypothesized existence of a period of high paleointensity in the Late Archaean–Early Proterozoic.