Anisotropy Of Magnetic Susceptibility Measurements Research Articles

Reconstruction of the Magma Transport Patterns in the Permian-Triassic Siberian Traps from the Northwestern Siberian Platform on the Basis of Anisotropy of Magnetic Susceptibility Data

Patterns of magma transport during the emplacement of Large Igneous Provinces (LIPs) are extremely important for the understanding of their formation. The Permian-Triassic Siberian Traps LIP is considered to be one of the largest in the Phanerozoic; however, mechanisms of magma transfer within and under the crust are still poorly studied. This problem is vital for the reconstruction of the dynamics of magmatic activity and eruption styles, ascertaining the position of magmatic centers and feeding zones, and conception of ore deposits genesis. Here, we present the detailed results of anisotropy of magnetic susceptibility measurements for lava flows and intrusions from the Noril’sk and Kulumbe regions (the northwestern Siberian platform). We reconstructed patterns of magma flow based on the magnetic fabric analysis of more than 100 sites. Distribution of the magnetic lineation in the studied intrusions and flows points out that the lateral magma flow of NW-SE directions was predominant. Our results support the idea of a magma-controlling role of Noril’sk-Kharaelakh and Imangda-Letninskiy regional fault zones. Furthermore, the reconstructed geometry of magma transport in intrusions is contrasting with that in the Angara-Taseeva depression (the southern part of the LIP) due to the presence of the long-lived mobile zones in the northwestern Siberian platform.

Paleowind Directions over the Tarim Block during the Mesoproterozoic, Northwestern China

The Tarim Block is an ancient plate with a basement of ancient continental crust, which has been separated from the Rodinia supercontinent since the Neoproterozoic. During the Neoproterozoic, which lasted nearly 500 Myr, this block experienced significant evolutionary processes, such as proliferation, radioactive decay of elements, and gradual cooling and solidification. The investigation of Neoproterozoic paleogeography may shed light on the evolution of these geological events. In order to realize this potential, this study aimed to infer paleowind directions over the Tarim Block during each epoch of the Cryogenian–Ediacaran and to constrain the paleogeographic location of the Tarim Block. To this end, outcrop magnetic fabric data were employed to analyze the anisotropy of magnetic susceptibility within the Tarim Block. The anisotropy of magnetic susceptibility measurements yielded mean paleowind directions of 308° ± 69°, 277° ± 78°, and 256° ± 76° from the present north for the Early, Middle, and Late Cryogenian, respectively; the corresponding values for the Early and Late Ediacaran were 237° ± 77° and 254° ± 73° from the present north, respectively. Considering the rotation relationship of the Tarim Block from the Neoproterozoic to the present, the paleowind directions during the Early, Middle, and Late Cryogenian were ~55°, ~35°, and ~35° from the paleo-north, respectively. The paleowind directions during the Early and Late Ediacaran were ~35° and ~60° from paleo-north, respectively. By referring to the correspondence between the paleowind directions over the Tarim Block and trade winds in the Northern Hemisphere, this study provides evidence for the location of the Tarim Block during the Cryogenian–Ediacaran. The main contributions of this study can be summarized as follows: (1) paleowind patterns are established through the analysis of the anisotropy of magnetic susceptibility; (2) the paleogeographic location of the Tarim Block during the Cryogenian–Ediacaran is constrained; and (3) a reference for further study of the paleogeography of the Tarim Block during the Cryogenian–Ediacaran is provided.

New insights on the Neogene tectonic evolution of the Aksu Basin (SE Turkey) from the Anisotropy of Magnetic Susceptibility (AMS) and paleostress data

The junction between the Aegean and Cyprus arc in the southern Turkey forms a triangular-shaped morphological structure so-called Isparta Angle (IA). In Neogene time, the inner part of the Isparta Angle became overlain by sedimentary basins including Manavgat, Köprüçay and Aksu, characterized by marine clastics and carbonates.Aksu Basin, one of these marine basins, provides geological records about the Neogene crustal deformation and stress field in the Eastern Mediterranean region. Therefore, we kinematically assess if and when tectonic stress fields affected the Aksu basin in early Miocene to recent times. For this purpose, Anisotropy of Magnetic Susceptibility (AMS) data (AMS) of cored samples and fault-slip data from the brittle mesoscopic faults were collected. The ~490 oriented samples for Anisotropy of Magnetic Susceptibility measurements were analyzed and two different maximum magnetic lineation's directions are recognized; 1) N–S magnetic lineation in Pliocene and 2) from N–S to NW-SE magnetic lineation in the Miocene. The results inferred from more than 1000 fault-slip data collected from 83 different sites combined with AMS results demonstrate that the Aksu Basin developed under four different tectonic phases; 1) ~E-W extensional phase (related to basin formation), 2) ~N–S compressional (Lycian) phase, 3) ~(N)E-(S)W compressional (Aksu) phase and 4) N–S extensional (Recent) phase.

Tectonic and paleogeographic controls on development of the Early–Middle Ordovician Shanganning carbonate platform, Ordos Basin, North China

Lower–Middle Ordovician platform carbonates are important oil and gas reservoir rocks in the Ordos Basin of the North China craton (NCC). In recent years, numerous cores have been recovered from these units, but the depositional environments and facies architecture of this large carbonate platform system have not yet been reported in detail. This study integrates sedimentological and magnetic fabric data collected from outcrops and cores to document variation in microfacies and facies associations within the Shanganning carbonate platform (SCP), infer paleowind directions and the paleogeographic location of the SCP, and evaluate tectonic and paleogeographic controls on platform development. This analysis documented 25 microfacies belonging to 7 microfacies fabric and sediment types: carbonate mudstone, wackestone, packstone–grainstone, evaporite, bindstone–framestone, siliciclastics, and dolostone. Microfacies can be grouped into facies associations that represent tidal-flat, open-platform, restricted-platform, evaporite-platform, platform-margin, slope, and deep-basinal depositional environments. These environments are mainly distributed zonally around the L-shaped Qingyang paleo-uplift on the western and southern margins of the SCP. Anisotropy of magnetic susceptibility measurements at five platform-margin locales yielded paleocurrent directions of (all approximately) 318°–348°, 51°–81°, 42°–72°, 80°–110°, and 54°–84° from paleo-north after correction for a 120°–150° clockwise rotation of the NCC since the Ordovician. The depositional architecture of the SCP depended on a combination of tectonic and paleogeographic (wind) influences. Local tectonic movements controlled the spatial distribution of facies associations around the Qingyang paleo-uplift, which was characterized by open, restricted, and evaporite-platform facies in the platform interior and mainly slope and basinal facies on the outer-platform margin (to the west and south of the Qingyang paleo-uplift). Vertical tectonic movements resulted in cyclic submergence and emergence of the platform over its approximately 20-m.y.-long formation history. Paleo–trade winds influenced aspects of facies zonation across the platform, including growth of metazoan reefs on the windward platform margin and formation of oolitic grainstone and microbial reefs on the leeward platform margin. The facies model for the SCP developed in this study should provide a useful reference framework for future carbonate oil and gas exploration in Lower–Middle Ordovician units of the Ordos Basin. Furthermore, this study makes innovative use of magnetic anisotropy data, in combination with standard petrographic, core, and field data, to discern facies patterns and controls on a paleo-carbonate platform, which is an integrated methodology that may serve as a model for future studies of other carbonate platforms.

Measurements Combination of Elastic Anisotropy and Anisotropy of Magnetic Susceptibility on Case Study in Igneous Rock of Lava Flow Type from Ijen Volcanic Complex, East Java

Research about the anisotropy parameters of rock is important for geophysical investigations. This research was a preliminary study which aims to find the relationship between elastic anisotropy and anisotropy of magnetic susceptibility in igneous rock of lava flow type with MR1 code and to interpret preferred orientation in MR1 sample based on the integration of anisotropy measurement results with the supported results from micro-computed tomography (μCT) analysis, petrography analysis and geological data analysis. This research has never been done before. This study was divided into several main stages consist of elastic anisotropy measurements on a cube-shaped specimen to determine the direction of maximum velocity and elastic anisotropy parameter values, anisotropy of magnetic susceptibility measurements on six cylindrical specimens to determine the direction of maximum susceptibility (magnetic mineral orientation) and AMS parameter values, petrography analysis on the all sides of the cube in order to observe preferred orientation in mineral and μCT analysis in a cylindrical specimen to determine the pore trend of rock. This study showed the relationship between AMS and elastic anisotropy in MR1 sample which indicated by positive correlation between the direction of maximum susceptibility and maximum velocity. Based on the results, MR1 sample has elastic anisotropy degree ranges from 2,6 % - 6,6 % for P waves and 2,2 % - 4 % for S waves and anisotropy of magnetic susceptibility degree around 2,6 % - 5,4 %. The integration of all measurement results including the direction of maximum velocity, direction of maximum susceptibility, preferred orientation in mineral, pore trend, and dip direction of sheeting joint indicate preferred orientation in MR1 sample towards NE-SW. One possible cause of such preferred orientation is the direction of the lava flow. Further studies in this topic are needed to support this hypothesis.

Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith‐Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data

AbstractModes of magma emplacement may provide key information on geodynamic‐tectonic settings of magmatism, when the threshold of magma volume and duration of emplacement process is crossed, brittle/ductile crustal deformation behavior, and volumes and rates of magma transfer in the crust. The majority of upper‐crustal granitic intrusions are incrementally emplaced as laccoliths or sills, while midcrustal or deeper‐level granitic intrusions appear to be predominantly emplaced as sills or lopoliths, but relatively few examples have been studied. We present a case study on the Jurassic Qitianling pluton from South China, which was constructed in the upper part of the middle crust (at a depth of ~10–15 km). To characterize its emplacement mechanism and evolution, we have combined field and microscopic observations, Anisotropy of Magnetic Susceptibility measurements and Bouguer gravity anomaly modeling. The main findings are (1) the generally undeformed granite‐country rock contacts with a narrow thermal aureole, (2) irregularly wavy and gradational contacts between the main granite facies, (3) dominant subhorizontal magnetic fabrics in the granites with a scattered magnetic foliation, (4) rare occurrence of dykes, and (5) a subcircular surface and lopolith‐like pluton geometry. Accordingly, we suggest that the Qitianling pluton forms an ~25‐km wide and ~5‐km thick lopolith that was incrementally emplaced during a period of tectonic quiescence, most likely by under‐accretion of multiple magma sheets deflected from magma pulses that ascended through pipe‐like channels. The mineralogical and compositional variation of the pluton may be the result of in situ fractionation, fractionation at deeper crustal level, or variation in source‐derived melt compositions.

Internal fabrics of the Idaho batholith, USA

The Idaho batholith of the North American Cordillera is a large and long-lived silicic intrusive center. We studied fabrics at the regional scale within the different intrusive suites of the Idaho batholith, using microstructural characterization, anisotropy of magnetic susceptibility measurements, and shape preferred orientation analyses. Each studied outcrop was collocated with existing U-Pb zircon geochronology and ongoing (U-Th)/He zircon thermochronology results. Three new 40 Ar/ 39 Ar biotite ages, collocated with the existing U-Pb zircon ages, constrain the cooling rates within the batholith. The presence of dominantly magmatic microstructures allows us to interpret the results relative to the U-Pb zircon ages and observe spatial and temporal patterns of fabric development. The early (pre–80 Ma) intrusive suites exhibit solid-state microstructures that show a consistent orientation only in the western part of the batholith. Fabrics in the 83–67 Ma Atlanta peraluminous suite are weak and inconsistent in orientation, despite emplacement during regional contraction. We hypothesize that the lack of consistently oriented fabrics in the Atlanta lobe results from either: (1) topographic effects that caused local extensional/neutral strain environments in the upper parts of a crustal plateau; or (2) emplacement in thin, horizontal magmatic sheets. In contrast, fabrics within the 66–53 Ma Bitterroot peraluminous suite are well developed and consistently oriented (NW-striking; NE-dipping), recording localized contraction during magmatism.

Pore fabric geometry inferred from magnetic and acoustic anisotropies in rocks with various mineralogy, permeability and porosity

The ferrofluid impregnation technique combined with anisotropy of magnetic susceptibility measurements (AMSff) is one of the ways to analyze the 3-D geometry of the pore space in a rock and indirectly to infer the anisotropy of permeability. We applied this method on different types of rocks (sandstones and carbonates) with a range of different porosity values (10–30%) and permeability (1mD to 1D). To get additional information on both the pore aspect ratio and the directional anisotropy we used another technique, measuring the anisotropy of P-waves velocity (APV) in dry and water saturated conditions. Comparing between both methods shows that despite the good agreement in directional data, inferring the true shape of the porosity is not straightforward. Modeling the presence of an elastic anisotropy in the solid matrix for sandstones allows one to get more consistent values for the pore aspect ratio obtained from both APV and AMSff. However for the carbonate rocks, due to an intricate distribution of microstructures, the aspect ratios obtained show significant discrepancies between the two methods. The ferrofluid method is very sensitive to the quality of the impregnation and suffers from a major drawback which is the threshold size of investigation, limited by the size of the magnetite nanoparticles (10nm) and probably this method doesn't see all the porosity. On the other hand with acoustic methods, the range of porosity investigated is probably larger but several microstructural attributes can contribute to the elastic anisotropy which makes the pore shape effect more difficult to decipher. Therefore, we promote the combined use of both methods in order to get more reliable information on the pore shape in porous media.

The Pan-African quartz-syenite of Guider (north-Cameroon): Magnetic fabric and U–Pb dating of a late-orogenic emplacement

The small Guider pluton (70km2) in-between the cities of Maroua and Garoua (north Cameroon) is one of the quartz-syenite bodies that intrude Pan-African orthogneisses, and that are aligned along the western side of the NNE-directed Poli-Léré volcano-sedimentary corridor. The Guider syenite is here studied for its internal structures, using field and anisotropy of magnetic susceptibility measurements, and for its emplacement age of 593±4 Ma (U–Pb zircon), ∼20Ma younger than the beginning of gneissification of the country-rocks, mainly made of deformed granodiorites and diorites. In these orthogneisses, constantly NNE-trending vertical foliations, variously plunging stretching lineations and fold axes, as well as kinematic markers attest to a transpressive and possibly dextral regime. The Guider syenite, rich in K-feldspar and magnetite, displays typical magmatic microstructures with incipent solid-state features such as chess-board subgrains in quartz. Its magnetic fabric displays a well-defined feeding-zone in its centre and western side, attesting to its unrooted nature. However, N- to NE-trending lineations at its northern and southern ends, parallel to the overall linear structure of the country-rocks, point to its late-orogenic emplacement. Our results reveal that the NE-trending and dextral transpressive shear zone that acted in the Guider area ended after the emplacement of the syenite body; they specify the age of collage of the NW-Cameroon domain with the Mayo-Kébbi domain. They also show that the tectonic evolution of Guider area is closer to the E-Nigerian domain, to the west, than to its eastern neighbouring Mayo-Kébbi and Adamawa-Yadé domains. The connection of our sector with E-Brazil is briefly discussed.

Magnetic properties of micrometeorites

Most micrometeorites are strongly magnetic: the signal of a single micrometeorite may exceed the signal of a weakly magnetized standard sediment sample. Micrometeorites contain abundant magnetite, mostly produced by high‐temperature oxidation during atmospheric entry. In this study, we carried out measurements on 520 micrometeorites (505 melted cosmic spherules, 6 partially melted scoriaceous micrometeorites, and 9 unmelted micrometeorites). The natural remanent magnetization and the saturation isothermal remanent magnetization have been measured, followed by alternating field or thermal stepwise demagnetization. The natural remanent magnetization is in the range of 0.4–300 A/m for cosmic spherules; it is a stable thermal remanent magnetization acquired by quenching in the Earth's magnetic field. The range is 3.8–16 A/m for scoriaceous micrometeorites and 78–525 A/m for unmelted micrometeorites, which may have preserved a preatmospheric magnetization. The magnetic susceptibility is in the range of 0.005–2.9 SI for cosmic spherules and is in the range of 0.06–0.12 SI for scoriaceous and unmelted micrometeorites. Temperature‐dependent susceptibility analyses and thermal demagnetization indicate that magnetite is cation substituted in cosmic spherules. Different populations of magnetite grains may have different degrees of cation substitution within a single micrometeorite. Anisotropy of magnetic susceptibility measurements indicates that micrometeorites are strongly anisotropic (anisotropy degree >15%) and that most have oblate fabrics consistent with the parallel habit of magnetite in barred olivine cosmic spherules.

How are saucer‐shaped sills emplaced? Constraints from the Golden Valley Sill, South Africa

Magmatic sill intrusions develop saucer‐like geometries in layered sedimentary basins intruded by large volumes of magma. Saucer‐shaped sill emplacement models are based on the analysis of the intrusion geometry and their spatial relationships with potential feeders, not on magma flow pattern. The Karoo Basin of South Africa hosts hundreds of saucer‐shaped sills. Among these, the Golden Valley Sill is well exposed and displays connections with adjacent and nested saucers. A combination of detailed field observations and anisotropy of magnetic susceptibility measurements (AMS) were used to identify strain markers that can be interpreted in terms of magma flow directions. A total of 113 localities (six specimens per site), mostly including pairs of opposite sill margins, were sampled for anisotropy of magnetic susceptibility analyses. The magnetic properties were determined by measuring hysteresis loops and K‐T curves. The majority of the localities display well‐defined magnetic foliations that consistently dip outward from the center of the Golden Valley Sill. This orientation of the magnetic foliation most likely represents inflation/deflation cycles of the intruding sill that interacts with nonstatic enclosing walls. In addition, magma channels and ropy flow structures were identified and display an imbrication of the magnetic foliation indicating an outward magma flow direction. The observed magma flow geometries derived from macroscopic flow indicators and the AMS data correlate well and constrain an emplacement model for the Golden Valley Sill Complex. Our emplacement model integrates our observations with dynamic emplacement processes characterized by inflation and deflation cycles.

The form, distribution and anisotropy of magnetic susceptibility of Jurassic dykes in H.U. Sverdrupfjella, Dronning Maud Land, Antarctica. Implications for dyke swarm emplacement

The Mesozoic dyke swarms of Western Dronning Maud Land, Antarctica, form a minor intrusive component of the Karoo large igneous province. Five-hundred and sixty one dykes were recorded intruding Neoproterozoic gneisses and Middle Jurassic syenite plutons. 40Ar/ 39Ar geochronology data reveal two temporally distinct components: the 178–175 Ma, alkaline, Straumsvola dyke swarm that predominantly intrudes a nepheline syenite pluton; and the 206–204 Ma, tholeiitic, Jutulrøra dyke swarm found throughout the study area. The Straumsvola swarm exhibits highly variably dyke trends that display a restricted opening direction, interpreted to be the result of high magma pressure equal to the maximum principal stress. The Jutulrøra swarm displays a fan of dyke trends, with dyke thickness and spacing increasing away from the inferred point of fan convergence. Anisotropy of magnetic susceptibility measurements reveal vertical magma transport within both dyke swarms in the Straumsvola area, with the southern/outer exposures of the Jutulrøra swarm exhibiting lateral magma transport. Although associated with a long-lived, local igneous centre comparison of palaeostress estimates for the Straumsvola dyke swarm and contemporaneous dykes in Ahlmannryggen and Vestfjella, indicates the presence of a regional scale radial stress system in western Dronning Maud Land between 178–175 Ma, supporting a mantle plume origin for the Karoo large igneous province.

Palaeowind directions recorded in the youngest loess in Poland and western Ukraine as derived from anisotropy of magnetic susceptibility measurements

Nawrocki, J., Polechonska, O., Boguckij, A. & L anczont, M. 2006 (May): Palaeowind directions recorded in the youngest loess of Poland and western Ukraine as derived from anisotropy of magnetic susceptibility measurements. Boreas, Vol. 35, pp. 266/271. Oslo. ISSN 0300-9483. Studies of the anisotropy of magnetic susceptibility (AMS) were carried out in order to define the directions and strength of palaeowind during the sedimentation of the youngest loesses in Poland and western Ukraine. These sediments, like the Chinese and Alaskan loesses, appear suitable for application of the AMS method. The inclined mean minimum axes were considered as reflecting the prevailing palaeowind direction at the time of loess sedimentation. The majority of the sections studied indicate a palaeowind direction from W/SW to E/NE, with a mean azimuth of 2588. This direction corresponds to the strike of the Weichselian ice-sheet margin and is parallel to the axis of the lowland between the ice-sheet margin to the north and the Carpathian Mountains and the Podole Upland to the south. Further AMS studies of the oldest parts of loess sequences in Poland and western Ukraine may help in reconstruction of the atmospheric circulation in this part of Europe since c. 900 kyr BP.

Influences of urban fabric on pyroclastic density currents at Pompeii (Italy): 1. Flow direction and deposition

To assess ways in which the products of explosive eruptions interact with human settlements, we performed volcanological and rock magnetic analyses on the deposits of the A.D. 79 eruption at the Pompeii excavations (Italy). During this eruption the Roman town of Pompeii was covered by 2.5 m of fallout pumice and then partially destroyed by pyroclastic density currents (PDCs). Anisotropy of magnetic susceptibility measurements performed on the fine matrix of the deposits allowed the quantification of the variations in flow direction and emplacement mechanisms of the parental PDCs that entered the town. These results, integrated with volcanological field investigations, revealed that the presence of buildings, still protruding through the fallout deposits, strongly affected the distribution and accumulation of the erupted products. All of the PDCs that entered the town, even the most dilute ones, were density stratified currents in which interaction with the urban fabric occurred in the lower part of the current. The degree of interaction varied mainly as a function of obstacle height and density stratification within the current. For examples, the lower part of the EU4pf current left deposits up to 3 m thick and was able to interact with 2‐ to 4‐m‐high obstacles. However, a decrease in thickness and grain size of the deposits across the town indicates that even though the upper portion of the current was able to decouple from the lower portion, enabling it to flow over the town, it was not able to fully restore the sediment supply to the lower portion in order to maintain the deposition observed upon entry into the town.

Late Neoproterozoic paleomagnetic results from the Sugetbrak Formation of the Aksu area, Tarim basin (NW China) and their implications to paleogeographic reconstructions and the snowball Earth hypothesis

In order to better constrain the Neoproterozoic paleogeographic reconstruction of continents and to improve the understanding of the snowball Earth hypothesis, paleomagnetic investigations were carried out in the Aksu area of the northwestern Tarim basin. Forty-eight sites of samples were collected from the Sugetbrak and Chigebrak Formations. Twenty-four sites of sandstone and volcanic rock from the Sugetbrak Formation revealed stable characteristic remanent components (ChRm) isolated between 500 and 680 °C. The computed magnetic directions from these components are consistent and significantly distinguished from those of younger ages. Both normal and reverse polarities have been observed, and a positive fold test is revealed after bedding corrections at 95% confidence level. Anisotropy of magnetic susceptibility measurements shows primary sedimentary fabrics with weak anisotropy degrees. A paleomagnetic pole is calculated at 19.1°N, 149.7°E, k = 11.2, A 95 = 9.3° with n = 24, yielding a paleolatitude of ∼27°N for the sampling area. The chemo-stratigraphic correlation of this section with reference ones reveals an average age of ∼595 Ma. A new paleogeographic reconstruction has been attempted showing general low paleolatitude positions for the Australia, South China and Tarim blocks at about 600 Ma. This observation provides new evidences for the snowball Earth hypothesis.

Neogene and Quaternary ignimbrites in the area of Arequipa, Southern Peru: Stratigraphical and petrological correlations

This study focuses on the correlation of four newly identified rhyolitic ignimbrites located around Arequipa, which were previously described as a single ignimbrite sheet: (1) The Río Chili Ignimbrite (ca. 13.33 Ma) crops out in the Río Chili canyon and consists of a cooling unit of non-welded to partially welded, massive, crystal-rich lapilli tuff. This ignimbrite represents the base of the Neogene ignimbrite succession in the Arequipa area and may be stratigraphically equivalent to the 14–13 Ma Huaylillas Ignimbrites in southern Peru. (2) The second voluminous (∼ 20 km 3) and widespread La Joya Ignimbrite sheet (ca. 4.87 Ma) filled the Arequipa depression. Its probable source is now buried underneath the Chachani volcanic complex. The La Joya Ignimbrite covers both the Arequipa Batholith toward the Río Vitor piedmont and also part of the Altiplano to the East and North of the Chachani complex. (3) The Arequipa Airport Ignimbrite (AAI) sheet (ca. 1.6 Ma, ∼18 km 3), the third and latest infill of the Arequipa basin, consists of a lower white, columnar jointed, massive lapilli tuff, indurated by vapor-phase recrystallization, and is overlain by an upper, pink, lithic-rich, non-welded, massive lapilli-tuff. The source of this ignimbrite is also buried underneath the Chachani complex, as indicated by anisotropy of magnetic susceptibility measurements and component lithology. (4) The Yura non-welded pumice-flow deposits (ca. 1.02 Ma, 1.5 km 3) are restricted to the northern and western flanks of the Chachani complex and may correlate with tuffs of the Capillune Formation on the Altiplano. All ignimbrites are high-K calc-alkaline to alkaline rhyolites, which contain variable amounts of plagioclase, biotite, quartz, sanidine, and Fe–Ti oxides. The modal composition of mineral phases and chemistry of amphiboles and biotites are useful for distinguishing the ignimbrites. Specific geochemical fingerprints, such as Sr, Y, Rb, Ba, Cs, REE, and Sr isotopes were effective discriminator elements. Magmatic differentiation, crustal residence time and contamination, and source melting processes are discussed in order to explain compositional variations. The older La Joya Ignimbrite and Río Chili Ignimbrite reflect initial wetter crustal melts closer to granite composition, whereas with time the younger Arequipa Airport Ignimbrite and Yura Tuffs initial melts are slightly less evolved and dryer due to more extensive crustal melting. The voluminous (> 40 km 3) Neogene ignimbrites of the Arequipa area were produced by three major explosive volcanic pulses in the Central Volcanic Zone of southern Peru, whose genetic relation with tectonic uplift and crustal thickening of the Andean Cordillera are still a matter of debate.

Palaeowind directions recorded in the youngest loess in Poland and western Ukraine as derived from anisotropy of magnetic susceptibility measurements

Palaeowind directions recorded in the youngest loess in Poland and western Ukraine as derived from anisotropy of magnetic susceptibility measurements

Paleomagnetic study of Siluro‐Devonian volcanic rocks from the central Lachlan Orogen: Implications for the apparent pole wander path of Gondwana

The apparent pole wander (APW) path for Gondwana is still not clearly established, in particular, for Silurian‐Devonian times. A controversial debate places authors who argue for an “X path,” running directly through Africa on a reconstruction of Gondwana against those who advocate a large loop passing by southern South America, the “Y path.” Most of the paleomagnetic data used to draw this loop come from the Lachlan Orogen (Australia). A paleomagnetic study was carried out in the well‐dated Ambone and Ural volcanics in the central subprovince of Lachlan Orogen, New South Wales. Anisotropy of magnetic susceptibility measurements confirms detailed mapping of the region and shows that these massive dacitic sills and/or lava flows are flat lying. Among the different localities studied, only one yields interpretable paleomagnetic results. Two components of magnetization can be identified: a midtemperature direction yielding a corresponding pole in Australian coordinates λ = 67.9°S/ϕ = 084.4°E (B = 5; n = 21; dp = 17.5°/dm = 23.1°) and a high‐temperature direction with a corresponding VGP λ = 24.4°S/ϕ = 060.6°E (B = 5; n = 25; dp = 1.4°/dm = 2.5°). The first is interpreted as corresponding to an Early Carboniferous pole position and can be regarded as an overprint probably related to the Early Carboniferous Kanimblan orogenic event. The second does not correspond to any expected Silurian‐Devonian or younger pole position. This magnetization is thought to be primary in origin; however, secular variation has apparently not been averaged out in the single lava flow sampled. Therefore the earliest Devonian paleopole position probably lies in a 30° cone around the obtained VGP, and this position can only match the X‐type APW path for Gondwana. It is in particular very different from coeval poles obtained in the eastern subprovince of the Lachlan Orogen, and it is mostly used as key poles supporting the Silurian‐Devonian loop for the APW path of Gondwana. Therefore some poles from the Lachlan Orogen must be affected by rotation to explain these results. The Lachlan Orogen was hence not stable up to the mid‐Paleozoic, and data from this region should not be used as representative for Gondwana.

Palaeomagnetism of iron-formation from the late Palaeoproterozoic Frere Formation, Earaheedy Basin, Western Australia: palaeogeographic and tectonic implications

Abstract Oriented cores of unweathered Frere Formation (1.9–1.8 Ga), including shaly and peloidal iron-formation, were obtained from two holes drilled to 116 m depth on opposite limbs of an anticline in the southern Earaheedy Basin at the northeastern margin of the Yilgarn Craton. Anisotropy of magnetic susceptibility measurements indicated that the Frere Formation in this area was unaffected by regional metamorphism. The predominant magnetic mineral in the iron-formation is hematite but a small amount of magnetite also is present in some samples. Stepwise thermal demagnetisation was conducted for 263 specimens. Many specimens acquired spurious components of magnetisation after treatment to 300–500 °C, ascribable to the oxidation of siderite to magnetite on heating. However, other specimens displayed a stable, high-temperature (680 °C), dual-polarity component A carried by hematite that gave a bedding dip-corrected direction of D=314.3°, I=−24.8° (K=38.5, α95=2.6°, n=78). A fold test was positive at >99% level of confidence, indicating acquisition of remanence prior to Stanley folding in the Earaheedy Basin that occurred at ∼1.78 Ga near the end of Capricorn orogenesis. The presence of polarity reversals in the stratigraphic section from one drill hole and within some specimens argues for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the Frere Formation, located at 45.2°S, 220.0°E (dp=1.3°, dm=2.4°). Component A is ascribed to a chemical remanent magnetisation (CRM) acquired soon after deposition, and indicates a palaeolatitude of 13.0±1.5° for the Frere Formation. The Frere pole plots adjacent to, but is still significantly different from, late to post-Capricorn Orogeny overprint poles from the Hamersley Province of the southern Pilbara. However, vertical-axis relative anticlockwise rotation of 10–15° for the Frere pole brings it close to these Pilbara overprint poles. The data imply that the Earaheedy Basin and possibly the Yilgarn Craton underwent clockwise rotation of 10–15° relative to the Pilbara Craton during Stanley folding. This finding accords with evidence for tectonic settings marked by sinistral strike-slip deformation adjacent to the Yilgarn–Pilbara cratonic boundary during deposition and subsequent folding in the Earaheedy Basin. The data also imply that the West Australian and North Australian cratons were in close proximity by ∼1.8 Ga.

Paleomagnetism of the 1.88-Ga Sokoman Formation in the Schefferville–Knob Lake area, Québec, Canada, and implications for the genesis of iron oxide deposits in the central New Québec Orogen

Abstract Thirty-nine oriented block samples of iron-formation were collected at 13 sites, including opposite limbs of major folds, from the 1.88-Ga Sokoman Formation (Knob Lake Group) in the Schefferville–Knob Lake area of the central New Quebec Orogen, northern Quebec. The samples assayed up to 80.24% Fe2O3T (54.08% Fe), implying Fe-enrichment of the iron-formation up to ore grade. Anisotropy of magnetic susceptibility measurements on 245 standard specimens indicate a well preserved bedding-parallel fabric in the iron-formation, suggesting minimal alteration of the magnetic mineralogy since deposition and/or a mimetic secondary magnetic mineralogy. The iron-formation has not been internally deformed since the magnetic mineralogy was established. Analyses by variable-field translation balance and X-ray diffraction showed that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in most samples. Following low-temperature pre-treatment as appropriate, stepwise thermal and alternating-field demagnetization of 218 specimens revealed a low-temperature, post-folding component (maximum Tub≈400 °C, D=27.1°, I=20.1°, α95=10.9°, from seven sites; pole position of 40.6°S, 257.0°E), and components carried by magnetite (maximum Tub≈580 °C, D=35.8°, I=3.9°, α95=9.1°, from 10 sites; pole position of 29.6°S, 250.9°E) and hematite (maximum Tub≈680 °C, D=40.0°, I=1.6°, α95=18.6°, from seven sites; pole position of 26.8°S, 247.0°E). The components carried by magnetite and hematite are pre-, syn- and post-folding depending on the sampling site, indicating that the magnetization was acquired continuously with deformation in the New Quebec Orogen at 1.84–1.83 Ga. No evidence was found for acquisition of magnetization during the Mesozoic, when many of the iron oxide orebodies in the Schefferville–Knob Lake area are thought to have formed. Our findings imply that an episode of Fe-enrichment of iron-formation in the Sokoman Formation involved the circulation of hydrothermal fluids related to late Paleoproterozoic orogenesis. Such orogenic circulation of fluids may have contributed to the development of hematitic orebodies in the central New Quebec Orogen.