GP21D-0794
Anisotropy of Magnetic Susceptibility and Petrography of the Ross of Mull Granite, NW Scotland: Implications for Ascent and Emplacement of a Reversely-Zoned Intrusion
The Ross of Mull granite is a ca. 400 Ma reversely-zoned granite-granodiorite-diorite pluton. Field and petrographic observations suggest that late-stage intrusion of hot mafic magmas, evidenced by centrally located swarms of diorite enclaves with superb mixing and mingling textures, had profound effects on the cooling granite mush. These features, together with exceptional coastal exposure, provide a unique opportunity to study magma flow within a petrologically dynamic and potentially long-lived magmatic system. One hundred and thirty two sites were sampled across the Ross of Mull granite for anisotropy of magnetic susceptibility (AMS) study in order to constrain the internal architecture and post emplacement deformation of the intrusion. The principal objectives of the study are to develop an intrusive model for the pluton, attempt to determine the emplacement direction of the magma, and the observed zonation pattern. Curie temperature measurements, estimated from experiments on the temperature dependence of susceptibility, yield values of 594C, consistent with average bulk magnetic susceptibilities of 10.78 × 10-3 SI, that suggest a low-Ti Fe-oxide or mixture of oxides as the principal magnetic phases. Hysteresis data at ambient and high temperatures reveal a narrow range of response with low Mrs/Ms and high Hcr/Hc values consistent with a multidomain Fe-Ti oxide phase. The AMS fabrics are internally consistent at the site level. The corrected degree of anisotropy ranges from 5 to 15 percent and anisotropies are variable in shape between oblate and prolate. In oblate sites, the well-constrained foliation dips steeply west, whereas in prolate sites, the lineation plunges moderately southeast and southwest. The fabric data have begun to describe that magma possibly originated in the southwest but with flow modified or controlled by the movements on the Sound of Iona Fault (west-dipping oblate fabrics) before flowing eastward or upward to the east (southwest and southeast steep plunging lineations). Additional AMS data will be used to further constrain magma flow and may also yield insights into post-emplacement deformation or tilting.
GP21D-0795
Dome-shaped Form for the Granite Pluton of the Lavras do Sul Intrusive Complex (South Brazil) Revealed by Magnetic Fabrics
Magnetic fabric and rock magnetism studies were performed on "isotropic" granites from the main intrusion of the Lavras do Sul intrusive Complex pluton (LSIC, Rio Grande do Sul). This intrusion is a roughly N-S elongated ellipsoidal pluton (12x18km) composed of alkali-calcic and alkaline granitoids, with the alkaline granites at the margin of the pluton. Magnetic fabrics were determined by applying both anisotropy of low- field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The AARM fabrics are coaxial with AMS fabrics. The parallelism between the AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the granites. Several rock-magnetism experiments performed in one specimen from each sampled site show that for all sites magnetic susceptibility is dominantly carried by ferromagnetic minerals, and mainly coarse-grained magnetite carries the magnetic fabrics. Fabric patterns (lineation and foliation) in the granites were successful determined by applying the magnetic methods. Magnetic lineations (Kmax) are gently plunging and roughly parallel to pluton elongation whereas the magnetic foliations (normal to Kmin) tend to follow the contacts between the granites. They are outerwardly gently dipping inside the pluton and become either steeply southwesterly dipping or vertical towards its margin. The lack of solid-state deformation at outcrops and at thin sections scales precludes deformation after full crystallization of the granites from the LSIC pluton. This evidence allows us to interpret magnetic fabric in the analyzed granites as primarily the result of internal magma chamber dynamics reflecting magma flow. The foliation pattern displays a dome-shaped form for the granites. However, the alkaline granites (syenogranite and perthite granite), which outcrop in the south of the studied area have an inward-dipping foliation whereas for the other granites it dips outwards. It suggests that either the alkaline granites were affected by faults after the pluton emplacement or they have been emplaced later along the border of the pluton. A model that can explain the geometry and the fabrics found for the LSIC pluton could be doming (inflection) caused by resurgence of volcano-plutonic subsidence system.
GP21D-0796
Application Of Continuous Wavelet Transform On Aeromagnetic Data To Identify Volcanic Rocks
This paper focuses on the application of continuous wavelet transform on aeromagnetic data, to locate and characterize volcanic rocks. The studied structure is sited in the north centre of the Huanghua depression in the Bohaiwan basin of east China. As channels of magmatism activities, the faults have caused multi-stage magma outpouring and intrusion, forming igneous rocks of different series of strata. As a traditional frequency decomposition method, the discrete wavelet transform is unable to localize frequency variations over time. To handle this problem, the short time Fourier transform method is widely used for the decomposition of non-stationary signals. One problem with this approach is that the fixed width ¡®window function' results in limited resolution. Therefore, the continuous wavelet transform decomposition was used as an alternative approach to overcome this resolution problem. In the continuous wavelet transform, the signal is multiplied with a function similar to a ¡®window function' but the width of the window is not fixed. The time window width is allowed to vary depending upon the frequency that is being considered. As for the magnetic anomalies of igneous rocks, they have different frequencies due to their depths; by analyzing the complex wavelet-based time-frequency characteristics of certain frequencies, we can identify the residual anomalies caused by volcanic rocks in different depths. The theoretical results show that local high frequency spectrum anomalies are the reflection of magnetic sources, and different scales (or different center frequencies) reflect different source depths, with larger scales for deeper sources. Therefore, by analyzing the complex wavelet-based frequency spectrum under different centre frequencies, we can analyze the distribution of magnetic field sources. Then the continuous wavelet transform was applied on the RTP aeromagnetic data of our study area. The data processing results present a detailed description of the distribution and genesis of volcanic rocks. Finally, based on our magnetic data interpretation results and the tectonic structures of our study area, we are supportive of one geologic model for the genesis of the Cenozoic volcanic rocks.
GP21D-0797
Magma Flow Sense in Mafic Dikes: is Grain-Size Dependence an Alternative to the "Imbrication Fabric" model?
Magma flow directions have been inferred for hundreds of mafic dikes in studies using the Anisotropy of Magnetic Susceptibility (AMS). This technique is particularly valuable when macroscopic (elongated vesicules) or microscopic (microfabrics) evidence of flow is lacking. The magma flow sense, however, is generally more difficult to establish without time intensive petrofabric studies. The observation of symmetrically oblique AMS foliations along the margins of specific dikes led to the model; coined "imbrication fabric" by Knight and Walker [1988] after their study of the Koolau volcano in Hawai'i. Complications in the interpretation of AMS fabrics may arise due to (1) magnetically inverse fabrics caused by single domain (SD) magnetite [Cadman et al., 1992], (2) the interplay between two incongruent subfabrics (e.g., mafic silicates and Fe-Ti oxides), (3) subsolidus (e.g., deuteric alteration) formation of Fe-Ti oxides and (4) Fe-Ti oxides growth under thermal migration. AMS fabrics where the magnetic foliation (K1-K2 plane) or the magnetic lineation (K1 axis) is at high angle to the dike wall are generally considered anomalous. We use a theoretical model to investigate the implications of cooling-related grain size variations in a tholeiitic dike crystallizing primary magnetite. The dike walls are parallel and vertical. The central part of the dike hosts multi-domain magnetite (MD) grains elongated parallel to a horizontal magma flow direction. In the case of purely conductive cooling against a cold host rock, the grain size of magnetite decreases significantly towards the dike margins. The volume percentage of SD magnetite (inverse AMS) increases towards the margin while that of MD magnetite (normal AMS) increases toward the interior. The model dike displays a symmetric AMS fabrics pattern characterized by normal AMS in the central zone, intermediate AMS fabrics on both lateral zones and inverse AMS fabrics along both marginal zones. The host-rock temperature, magma temperature and dike width exert fundamental control on the width of the respective zones. The predicted profile of P, the degree of magnetic anisotropy, across the dike shows two symmetrical minima corresponding to the intermediate AMS fabrics zones. This specific profile of P provides a test for the origin of AMS in mafic dikes without the need for numerous, time-intensive AARM measurements.
GP21D-0798
Origin of the variations in magnetic susceptibility with depth in the Barcroft granodiorite pluton, White Mountains, California
The 3D variations of magnetic susceptibility (Km) within plutons result from various factors. In ferromagnetic rocks, Km reflects the abundance of Fe-Ti oxides (magnetite, ilmenite), whereas in paramagnetic rocks, Km reflects the abundance of mafic silicates. These variations are a proxy for magmatic differentiation and/or oxygen fugacity. Magmatic processes, including multiple injections, hybridization, host-rock assimilation, also affect Km variations. The Barcroft granodiorite pluton offers exposures over 2000 m in elevation and is an good target to investigate the variations of Km across the pluton and with elevation. Previous missions yielded 622 samples from 76 stations for laboratory measurement of Km. In 2008, 1200 new measurements were collected directly in the field from 120 stations between 1650 and 4000 m elevation. Detailed surveys were performed at outcrop scale to evaluate the impact of the younger McCaffee intrusion on the background Km. The combined datasets display a broad positive correlation between magnetic susceptibility and elevation. The rock types also appear to vary with elevation but are more difficult to quantify. Specific stations where late-stage magmatic (or hydrothermal) fluid alteration display a significant drop in Km. At the outcrop scale, small dikes and dikelets of the McCaffee Creek granites cut through the Barcroft granodiorite and are associated with a decrease in Km towards the dike. The new contoured map of Km shows a high degree of correlation between local topographic features (such as deep canyons) and magnetic susceptibility. The magnetic susceptibility of the Barcroft granodiorite varies at the outcrop scale as a result of post- solidification processes. However these variations are embedded in a broader magnetic susceptibility trend due primarily to elevation and these variations reflect the magnetic stratification of the pluton. Systematic determination of the Curie temperature in selected samples (currently underway) will provide an estimate of the average ulvöspinel content in titanomagnetite, which is a proxy for oxygen fugacity. In turns this will help determining if the vertical variation of Km is primarily caused by magmatic differentiation or an increase in oxygen fugacity towards the top of the intrusion.
GP21D-0799
Comparison of Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Anhysteretic Remanent Magnetization (AARM) in Intrusive Igneous Rocks
Anisotropy of magnetic susceptibility (AMS) data have been used extensively to infer magma flow directions in intrusive igneous rocks, including granitic plutons and mafic dikes. Anisotropy of anhysteretic remanent magnetization (AARM) has been used to measure magnetic mineral fabrics in metamorphic rocks as a strain indicator and in volcanic rocks as a flow fabric measurement. AARM has not been utilized to assess magma flow in intrusive igneous rocks. AMS and AARM data from the same samples in a selected group of intrusive rocks are compared to determine the utility of both magnetic fabric techniques for interpreting magma flow direction. Samples from the Philipsburg Batholith (granodiorite) in SW Montana, Shonkin Sag Laccolith (shonkinite) in north-central Montana, Spanish Peaks complex (mafic dikes), Colorado, and the Oman Ophiolite (gabbro dikes and peridotite) all yield interpretable magnetic fabric data. In all but one sample, the percent anisotropy is greater in AARM data than in AMS data and in some samples over 10X greater. These results suggest that AARM may give more information about rock fabric than the more commonly used AMS. The relationship between AMS and AARM fabrics varies with different rock types and within rock types. AMS fabrics for well defined sites are generally oblate in granodiorite and shonkinite. AARM fabrics are almost always prolate and show three different relationships to the AMS foliation; 1)the maximum AARM direction is in the AMS foliation plane, 2)the intermediate AARM direction is in the AMS foliation plane, or 3)both maximum and intermediate directions of the AARM fabric lie in the AMS foliation. In granodiorite and some shonkinite samples, the maximum AARM direction (i.e. lineation) is similar in direction to the intermediate direction of the AMS ellipse. Although this suggests that the magnetite in the granodiorite is dominantly single-domain, rock magnetic tests and petrography show considerable multi-domain magnetite. The differences between AMS and AARM fabrics must be due to another cause, such as distribution anisotropy or contributions to the AMS fabric from ferromagnesian silicates. Alternatively the AMS fabric could be an intermediate fabric due to the combined affects of MD magnetite (normal fabric) and SD magnetite (inverse fabric). Understanding the relationships between the two magnetic fabric techniques could lead to a better utilization of the techniques, either independently or in combination, and a better understanding of magma flow in intrusive igneous rocks.
GP21D-0800
Magnetic stratification and the internal structure of layered intrusions
Magnetic methods can be used to investigate the internal structure and to constrain the magmatic history of mafic layered intrusions. Borehole cores provide a nearly continuous record of the vertical zonation of magnetic properties (i.e., magnetic stratification) across the compositional layering. Two intrusions with contrasting petrogenetic histories were investigated: the Insizwa sill (South Africa), an open system characterized by multiple magma pulses, and the Sonju Lake intrusion (Minnesota), a nearly perfect closed system. Both intrusions display vertical variations of their magnetic properties (magnetic susceptibility, AMS, degree of anisotropy, and shape factor). Magnetic units were initially defined based on magnetic susceptibility and its variability and were further refined using additional magnetic parameters. Magnetic stratification occurs at scales from a few meters to several 10's of meters and the boundaries between magnetic units range from gradual to abrupt. Petrologic and rock magnetic studies indicate that modal variations in primary magnetite/titanomagnetite and/or paramagnetic minerals (olivine, pyroxene) control the magnetic susceptibility. In the open system case (Insizwa), magnetic stratification correlates well with the prominent petrologic and geochemical layering. Within units appearing compositionally homogeneous (i.e., no visible macroscopic layering), magnetic stratification is also observed. The abrupt changes in susceptibility are interpreted to result from magma replenishment in a growing magma chamber. A magma recharge event could shift petrologic equilibrium and postpone magnetite crystallization, leading to a sudden decrease in susceptibility. In the closed system case (Sonju Lake), many layers are characterized by cyclical trends in susceptibility; an upward increase followed by an upward decrease. Cyclic trends may result from fractional crystallization, during which an initial upward increase in magnetic susceptibility is predicted due to the volumetric increase in Fe-silicates. At the onset of magnetite crystallization, a dramatic increase in magnetic susceptibility occurs followed by a gradual decrease due to a decreasing abundance of magnetite.
GP21D-0801
Rapid subsidence and formation of thick volcanic sections at magma-rich spreading centers: Paleomagnetic and AMS evidence from north-central Iceland
Surface exposures at modern spreading centers do not clearly elucidate how relatively thick (up to 0.8 km) sections of volcanics accumulate within the relatively narrow (<1 km) neovolcanic zones of fast-superfast spreading ridges. High rates of areally focused crustal subsidence are required to explain the fact that each flow within these thick lava piles originated at the surface of the neovolcanic zone. Excellent three- dimensional exposures within the central Vatnsdalsfjall range in north-central Iceland may provide insights into the style and rates of crustal subsidence at relatively magma-rich spreading centers. Regionally, the Vatnsdalsfjall region exposes gently (5° -10°) west-dipping lavas comprising part of the Blönduós flexure zone, one of several Tertiary age regions in Iceland believed to represent abandoned spreading centers. Locally, however, the volcanic section quickly and dramatically transitions from the slight regional dip to westward dips of 45° -60° degrees forming a structure previously mapped as the Hvammer monocline by Katie Ackerly. Ackerly also noted that the upper part of the volcanic section within the monocline appeared to thicken westward suggesting that they may have erupted during formation of the structure. Our subsequent mapping shows that the monocline represents the eastern edge of an ~6km- long, north/south-elongate structural basin that dies out along strike to the north and south. We measured and sampled a ~0.5km thick volcanic section across the monocline to constrain its structural formation and subsidence rate. Paleomagnetic remanence shows that dips in the lower part of the monoclinal section can be fully explained through structural tilting. Within the uppermost, westward-thickening part of the monocline ramp section, however, structural tilting accounts for only a part (~35° -40°) of the volcanic dip. Likewise, anisotropy of magnetic susceptibility suggests a distinct change in flow behavior between the lower and upper volcanic sections of the monocline. The lower section contains scattered k{max} flow directions while the upper section has dominantly west-plunging k{max} directions. We interpret these data collectively to suggest that at least the upper 50% of lavas within the measured section erupted during formation of the monocline and flowed down along its then, ~20° west-dipping ramp. Two previously reported 40Ar/39Ar dates on lavas that bracket the syn-monocline section suggest that approximately 0.25km of lava filled the basin during an interval between 1.14-0.14m.y.. The resulting subsidence rates range between ~0.5 to nearly 2km/my. More significantly, subsidence within the basin kept approximate pace with effusion rate which appears to be a feature of faster spreading rate ridges in the modern oceans. The monclinally bounded Vatnsdalsfjall basin may also provide a model for presence of moderate, ridge-facing dips in upper crustal units observed within tectonic windows through fast-spread crust such as at Hess Deep.