Does AMS constrain granite emplacement fabrics? Evidence of decoupling between AMS and K-feldspar fabrics during post-emplacement crystallization of interstitial melt.
A multistage model for the origin of magmatic fabrics in porphyritic granites is proposed. It is based on a comparative study of the anisotropy of magnetic susceptibility (AMS), preferred orientation of large K-feldspar phenocrysts and quantitative microstructure analysis in the late Variscan Land's End Granite (Cornwall, UK). There is a decoupling between the AMS method and the feldspar phenocryst fabric indicating that each fabric reflects different parts of the magma cooling history and related stress field. The AMS method has a high sensitivity to post-emplacement regional tectonic overprinting while the feldspar phenocryst fabric records magma emplacement and has a low sensitivity to post-emplacement deformation. The post-emplacement crystallization of interstitial melt and re-orientation of groundmass micas significantly modifies the AMS fabric and reduces its 'memory' of emplacement-related processes. It may explain the commonly observed low intensity of granite AMS fabrics. Fabric features, intensity and symmetries are evaluated with respect to different scales of observation and sample location. The general agreement between AMS and K-feldspar fabrics at the pluton scale contrasts with important discrepancies in fabric intensity and mutual orientations at outcrop scale. The AMS fabric is predominantly homogeneous and stable at pluton scale while the K-feldspar shows a complex fabric pattern, characterized by meter-scale variations in orientation, symmetry and intensity. We suggest that the AMS fabric is controlled by late regional deformation, overprinting the partially crystallized and weakly anisotropic matrix. In contrast, the K-feldspar fabric reflects heterogeneous flow of the phenocryst-rich magma, mainly related to magma chamber construction and pluton emplacement. The model of fabric development associated with a pure-shear overprint on a variable intensive vertical fabric was confirmed by numerical modeling.
Lateral Emplacement of the Western Mourne Granite, N.Ireland, From AMS Fabric Data
Field observations and anisotropy of magnetic susceptibility (AMS) measurements of oriented blocks from the Palaeogene Western Mourne granite pluton indicate the presence of a weak fabric. Visible fabrics, determined from the preferred alignment of feldspar phenocryst long axes in outcrop, trend between NNE and north but it is unclear whether these are gently plunging lineations or the trace of dipping foliations. Texturally the granite shows little evidence for plastic strain suggesting that the observed fabric delineates magma flow. AMS fabrics are dominantly oblate, defining sub-horizontal foliations parallel to gently dipping margins and gently plunging lineations that trend SSW-NNE and diverge northward. These data so far point to an emplacement model for the Western Mourne granite that describes a laccolith, fed laterally from the SSW. This mirrors the NNE directed lateral emplacement of the adjacent Eastern Mourne granite (Stevenson et al. 2007, J Geol Soc Lond, 164, 99-110) suggesting that these two centres share a common feeder zone outside the Mourne area and located to the SSE, coincident with a 50mGal peak gravity anomaly close to the coast (GSNI data). Contemporaneous mafic dykes that outcrop along this stretch of coast exhibit xenoliths of mafic cumulate that, together with the gravity anomaly, suggest there may be an unexposed mafic pluton in this area. Given the genetic links between mafic and felsic magmas in this region, the coincidence of the projected Mourne granite feeder zone and possible buried mafic centre, leads to a model in which the Mourne granites were emplaced NNE and NW as a gently dipping sheet, up dip, from this unexposed mafic centre. This model raises the possibility that the other Palaeogene igneous centres in NE Ireland (Slieve Gullion and Carlingford) may be laterally linked. The main implication of significant lateral movement of magma in the upper crust is that the location of igneous centres in the upper crust or volcanic edifice at the surface may not reflect the point at the base of the crust where magma was generated.
Strong Tectonic Rotation Evidenced by the Study of a Widespread Remagnetization in the Corbières Thrust Sheet (France)
This study was carried out in the eastern part of the French Pyrénées where the tectonic structures, E-W oriented, bend northward at the Corbières virgation. This change affects the North-Pyrenean zone that overlains the foreland at the North Pyrenean thrust and its eastern extension, the Corbières thrust sheet. In this area, the Corbières thrust sheet is formed by the Mesozoic cover overlying down flat the Cenozoic formations of the Aude basin. The studied sites belongs to the North Pyrenean zone and to the Corbières thrust sheet and correspond to ages from Lias to Lower Cretaceous. Rock-magnetism analyses indicate that magnetite is the main magnetization carrier. Natural Remanent Magnetization mostly corresponds to a viscous component A and a component B determined during treatments from 300 to 400-500°C. Some samples also present a higher temperature component C, which cannot be isolated because of strong mineralogical alteration during thermal demagnetization at these temperatures or of too weak magnetization intensity. Dip variation in neighboring sites allows fold tests indicating synfolding remagnetization (i.e. acquired between or during different tectonic phases) for the component B. Direction of the latter has been then determined using small circles method for groups of neighboring sites. Previous study presented last year and made in the large Saint-Paul de Fenouillet syncline showed that western E-W part of the syncline is only locally affected by weak rotations while a large counterclockwise rotation affected all the eastern part of the syncline at the Corbières virgation during the Tertiary tectonic phase. New data have been acquired in numerous sites from the Corbières thrust sheet south from the Aude River. They point out that the different units of the Corbières thrust sheet also show large counterclockwise rotation. The emplacement of the the Corbières thrust sheet then appears as the result of a large movement of rotation of the North Pyrenean cover rather than a simple northeastward translation of a cover. Use of small circles method also yields the reconstruction of folds at the intermediate stage of folding. Main folding periods were the Upper Cretaceous and the Eocene. In the western part of the Saint-Paul de Fenouillet syncline, the main folding is of Tertiary age, but the relative importance of the two phases of folding is much more variable in the eastern part of the syncline and in the Corbières thrust sheet of more southern origin.
Magnetic signature and fabric of serpentinized mantle rocks in the Betic-Rif Arc and tectonic implications
Serpentinized peridotites from the Betic-Rif Arc constitute the largest exposure of subcontinental mantle. Exhumation of these large peridotitic bodies is still controversial and therefore extensive structural studies as well as paleomagnetic studies have been carried out. However, the origin of the magnetic susceptibility and magnetic remanence as well as the timing of NRM acquisition have not been established yet. A comprehensive rock magnetic study combined with a low-field AMS study on a collection of samples along the arc have been carried out. These include thermomagnetic curves, hysteresis parameters, IRM acquisition and subsequent back field curves and mathematical modelling of the derived coercivity spectra and FORC diagrams. The results show the presence of two distinct magnetic populations of very different coercivity, one most likely associated with small magnetite inclusions in olivine, pyroxene etc, crystals and the other being a soft component associated with the serpentinization process, taken place at two stages of the exumation/emplacement. AMS has been determined at all sites and separation of magnetic subfabrics by high-field methods was carried out in selected samples. The results show a magnetic lineation that outlines the trace of the arc although it is systematically different from the magnetic lineation in the field. The magnetic foliation is also shallower than the structural foliation being the first dominated by the serpentinization.
Tracking the Deformation Along a Shear Zone Using Anisotropy of Magnetic Susceptibility
Magnetic sub-fabrics (i.e., diamagnetic, paramagnetic) are investigated for a set of calcite mylonites, collected along a large-scale shear zone (Morcles nappe) in southwest Switzerland, adopting the method described by Schmidt et al. (2007: GJI, 168, 40 - 47). The mylonites consist mainly of matrix calcite, where second phases (i.e. minerals other than calcite) make up less than 5 percent of the total volume. The most important second phases from an AMS perspective are the paramagnetic minerals white mica, chlorite and pyrite. Substitution of paramagnetic iron and manganese in the calcite lattice also occurs. Rock magnetic tests infer that the ferromagnetic content is negligible. The crystallographic preferred orientation (CPO) for the samples was measured with x-ray diffraction goniometry (XRD) and electron backscatter diffraction (EBSD). Generally CPOs show c-axes maxima perpendicular to the shear plane and a-axes concentration along a great circle sub- parallel to the shear plane. With decreasing metamorphic grade the CPO strength decreases. The AMS measured in low- and high-fields, at room temperature, display both normal and inverse magnetic fabrics, with susceptibility shape ellipsoids in the oblate and prolate fields. An inverse magnetic fabric is predominant when the AMS for the mylonites is measured at 77 K, with shape ellipsoids in the prolate field. A relationship between CPO and AMS is not obvious from these results. However, from the separated magnetic sub-fabrics it is possible to establish a quantitative relationship between AMS and CPO. The paramagnetic sub-fabric is related to the specimen bulk susceptibility (i.e., due to sample chemistry), whereas the diamagnetic sub-fabric agrees with the calcite CPO. The strongest CPO, observed in the root zone of the nappe where metamorphic grade is highest (greenschist facies, ~395C) and displacement is large, give a separated diamagnetic anisotropy with site mean k1 - k3 = 4.38E-7 +/- 1.00E-7 (2sigma) SI. The weakest CPO, near the front of the shear zone where metamorphic conditions were sub-greenschist facies (~280C), similarly correspond to a lower separated diamagnetic anisotropy, with site mean k1 - k3 = 2.35E-7 +/- 6.62E-8 (2sigma) SI. Intermediate magnetic fabrics, resulting from the combination of paramagnetic and diamagnetic sub-fabrics, are difficult to interpret but can be resolved through successful separation. In our case the separated diamagnetic sub-fabric is a quantitative representation of the calcite CPO. Separation of magnetic sub-fabrics can in general be performed when the details of matrix mineral and second phases are known.
High-Temperature Magnetic Fabric Development from Deformation of Magnetite in Shear Experiments
We present results from deformation experiments on synthetic shear zones deformed at high temperature (T=1000-1200°C) designed to study magnetic fabric development under plastic conditions. Samples composed of magnetite-plagioclase aggregates were synthesized by hot-pressing at 1200°C resulting in aluminum-substituted magnetite of composition Fe3-xAlxO4 where x=0.2 (AM20). These samples were subsequently deformed in co-axial simple shear to shear strains of up to γ=3. Extremely large increases in magnetic anisotropy measured by both susceptibility and anhysteretic remanence (AMS and AARM) can be explained by increases in magnetic grain shape anisotropy, indicating internal strain in the magnetic grains. Weak crystalline preferred orientation of the magnetic grains in some samples indicates that dislocation creep may be active for some fraction of grains but does not predominate. Significant changes in acquisition of isothermal remanence and increases in coercivity after deformation are likely to be stress-induced and reflect increases in the internal stress state of the magnetic grains.
Source of Magnetic Anisotropy in Andesitic Rocks: A case study from Java, Indonesia
We have studied scores of intrusive and extrusive andesitic rocks from 21 sites in Java, Indonesia to gain insight into how magnetic anisotropy was developed in these rocks. The study includes measurement of anisotropy of magnetic susceptibility (AMS), anisotropy of anhysteretic susceptibility (AAS), Curie temperature, magnetic hysteresis parameters, and XRF geochemical analyses. The percent anisotropy of AMS in these rocks varies from 1.4 to 8.4%, while the percent anisotropy of AAS varies from 15% to 69%. Magnetic lineation is predominant in ten sites, while magnetic foliation is predominant in the remaining eleven sites. No direct relationship was noted between magnetic lineation or foliation and structure. Measurement of Curie temperature shows that samples from some sites have a single magnetic phase while others show the presence of two to three magnetic phases; some of them have a Curie temperature as low as 208 C suggesting high Ti content. Assuming that the predominant magnetic mineral is magnetite, estimation of grain size made by plotting anhysteretic susceptibility as a function of low-field susceptibility suggests that only five sites are dominated by magnetic grains smaller than 1 micrometer. This was supported by measurement of hysteresis parameters. Cross-parameter analyses show that the number of magnetic phases in the rocks seems to affect the AMS but not the AAS parameters. Meanwhile variations in grain size affect both AMS and AAS. For samples with smaller magnetic grains, AMS degree of anisotropy increases as grain size increases while the AAS degree of anisotropy decreases as grain size increases. For samples with larger magnetic the degree of anisotropy the degree of anisotropy the degree of anisotropy grains, both AMS and AAS degree of anisotropy decrease as grain size increases. Geochemical analyses show that the degree of anisotropy tends to be larger for andesites with FeO content of less than 9%.
Modes of Basaltic Magma Flow and Propagation Through the Crust as Inferred From the Foum Zguid Dyke and its Host Sedimentary Rocks (Southern Morocco)
Dykes are the main feeders of mantle-derived magmas that cross the lithosphere and eventually reach the surface. Therefore, the study of flow in dykes is crucial in our understanding of how magmas migrate from the mantle source to the Earth's surface, through the crust, which we can access and study. This work focuses on magma flow and propagation of the Foum Zguid dyke (NE-SW, sub-vertical) through the upper crust, and how the sedimentary host rocks reacted to the intrusion mechanically and thermally. The dyke intruded an extensive fracture that shows no significant displacement parallel to the fracture surface. The petrofabric of igneous and host sedimentary rocks was determined by measurement of anisotropy of magnetic susceptibility (AMS). These results were complemented by detailed rock magnetic, structural and microscopic (petrography and SEM/EDS) studies. Important variations of the bulk magnetic properties and of the magnetic fabric for the igneous and sedimentary rocks are strictly related to the distance of the samples to the dyke wall. From AMS data of the igneous body it is possible to infer a dominant sub-vertical flow near the margins that changes to one steeper oblique towards NE for domains in the dyke middle. AMS fabric closest to the dyke wall likely records: i) mechanical processes associated with the first stages of opening and propagation of the dyke tip, or ii) later mechanical processes related to thickening of the dyke, with the chilled margins working as host- rocks. The forceful intrusion of a thick dyke induces deformation in the host sedimentary rock in two typical ways: (i) by homogeneous flattening, or (ii) by folding of sedimentary strata. AMS records such mechanical effects on the host rock, with a gradual transition from a bedding-parallel magnetic foliation away from the dyke to a dyke-parallel magnetic foliation close to the contacts (newly formed hematite crystallized under forceful injection). AMS evolution in the host sediment is accompanied by a significant change in the bulk magnetic properties, indicating important chemical and thermal effects related to the intrusive process.