Palaeomagnetism of the Torridonian of Rhum, Scotland: evidence for limited uplift of the central intrusive complex

doi:10.1016/0012-821X(87)90142-7

Earth and Planetary Science Letters

Volume 85, Issue 4, October 1987, Pages 473-487

https://doi.org/10.1016/0012-821X(87)90142-7 Get rights and content

Abstract

The Tertiary Central Intrusive of Rhum was emplaced into the Torridonian Sandstone, causing a heat influx into the sediment. Detailed progressive demagnetisation of sandstones from the thermal aureole of the intrusion show that they reached maximum temperatures which decrease systematically with distance in a manner indicative of conduction-dominated cooling. Temperatures reached near to the intrusion are higher than would be predicted theoretically, probably as a result of the presence of smaller plugs of magma at depth which are only rarely exposed at the surface. The shape and magnitude of the thermal aureole suggest that upward movement of the Central Intrusive along the bounding fault was limited.

Primary remanence in Torridonian sandstones unaffected by the intrusion is carried by both magnetite and haematite, often with both occurring in the same specimen. It is suspected that only magnetite carries a truly primary (detrital?) remanence; the haematite probably formed diagenetically (largely by oxidation of detrital magnetite) at some time shortly after deposition.

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Cited by (12)

The peridotite plugs of Rum: Crystal settling and fabric development in magma conduits
2012, Lithos
Citation Excerpt :
No aureole rocks are exposed around the Mam Tuath plug, while the local exposures of the West Sgaorishal aureole are incomplete, preventing acquisition of detailed observations such as those presented by Holness (1999) for the Kinloch Glen peridotite plug. The edge of the contact aureole within the Torridonian arkose is defined by the bleaching of the originally pink arkose (Holness, 1999), due to the reduction of hematite to magnetite (Robinson and McClelland, 1987). 12 m from the contact at West Sgaorishal (GR NM 34448 02178) detrital muscovite grains are partially replaced by turbid aggregates while muscovite is still present 19 m away.
The Rum Igneous Complex contains numerous circular to elongate exposures of peridotite cumulates within the layered series and the country rock. The extent of thermal effects on quartzo-feldspathic country rock surrounding the Mam Tuath and West Sgaorishal plugs demonstrates that these are pipe-like bodies, inclined at various angles to the vertical, that remained at magmatic temperatures for periods of up to several months. Tabular olivine grains within the plugs preserve a weak sub-horizontal fabric that is stronger in the plug centres than at their margins. The olivine mode increases from ~ 50 vol.% at the plug margins to ~ 85 vol.% at the centre. The spatial arrangement of the olivine grains is consistent with their being clustered, with the increase in mode towards the plug centres achieved by a combination of overgrowth and mechanical re-arrangement. Comparison of fabrics with those of the Ard Mheall peridotites of the Rum Western Layered Intrusion is consistent with porosity reduction in the Ard Mheall cumulates also having been primarily achieved by overgrowth and mechanical re-arrangement: compaction via intra-crystalline deformation did not occur. We suggest that the Rum ultramafic plugs are deep-level cross-sections presenting the ultimate ‘snap-shots’ of volcanic conduits through which olivine-rich magmas flowed for substantial periods, probably giving rise to fire-fountains of the kind associated with Hawaiian eruptions. The exposed parts of the plugs are now miniature cumulate bodies that developed in situ from the settling of olivine phenocrysts once flow had ceased within the conduit.
Self-reversed magnetization held by martite in basalt flows from the 1.1-billion-year-old Keweenawan rift, Canada
2011, Earth and Planetary Science Letters
Citation Excerpt :
In order for the component to be recognized, it needs to be comparable in magnitude to remanence held by primary magnetite and/or parallel hematite, and thermal demagnetization must proceed at high resolution (< 20 °C). Antiparallel directions held by hematite have been observed in many studies of hematite-rich oxidized sediments (“red beds”; e.g. Robinson and McClelland, 1987; Rösler et al., 1997; Roy and Park, 1972; Schmidt and Williams, 1995; Turner, 1981; Turner et al., 1984). The explanation given for these antiparallel remanences is that the samples contain hematite that formed significantly after deposition following a geomagnetic reversal.
In some basalt flows of the 1.1-billion-year-old Keweenawan Rift exposed at Mamainse Point, Ontario, there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession. The paleomagnetic and geological context of this component demonstrates that it is not a chemical overprint whose remanence is attributable to a subsequently reversed geomagnetic field, but that the component is a self-reversal of the primary magnetization. Here we use rock magnetic experiments and Mössbauer spectroscopy to show that this phase occurs in the most oxidized flows and is held by a fine-grained population of hematite that acquired its self-reversed remanence through interactions with a phase of lower blocking temperature. We propose that this self-reversed hematite is a manifestation of the self-reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and that has been attributed to negative exchange coupling across the crystal lattices. We suggest that hematite formed in association with iron-silicates in the basalt flows carries a remanence reflective of the field in which it formed, but that martite (hematite pseudomorphed after magnetite) which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self-reversed remanence. This study marks the second time that a naturally occurring self-reversed magnetization has been attributed to this mechanism and marks the oldest reported instance of any type of self-reversed remanence. The martite that forms through this process has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years.
Palaeomagnetic study of the (late Mesoproterozoic) Torridon Group, NW Scotland: Age, magnetostratigraphy, tectonic setting and partial magnetic overprinting by Caledonian orogeny
2005, Precambrian Research
Citation Excerpt :
The B remanence is the magnetisation in the Torridon Group held predominantly by magnetite (Table 2) and clearly defined in the lower blocking temperature segments of many vector plots. It is apparent in the results of Robinson and McCLelland (1987) although an inclination contrast with A− went unrecognised. The petrologic significance of magnetite in the Torridon Group remains unclear.
The (late Mesoproterozoic) Torridonian Supergroup is a predominantly fluviatile ∼7 km thick succession of red sediments deposited onto (Palaeoproterozoic) Lewisian metamorphic terrane of the Hebridean Craton in NW Scotland. Although a subject of investigation since the earliest days of palaeomagnetism, the age and constitution of the remanent magnetisation in the Upper Torridonian (Torridon Group) have remained unclear. In this study we undertake detailed statigraphically controlled sampling through the succession and conduct progressive thermal demagnetisation to resolve component structures. Rock magnetic analysis shows that magnetite, although volumetrically small, is a significant component of the ferromagnetism in these sediments in addition to specularite and pigmentary hematite. Unlike the Lower Torridonian (Stoer Group), which was fully lithified prior to late Grenville deformation (∼1100 Ma), the Torridon Group was not influenced by tectonism until Early Palaeozoic times and records a complex history of partial, and sometimes total, later overprinting. The A remanence is extensively overprinted by a B remanence with E to S directed intermediate to steep positive directions and subordinate reversed representatives and less commonly by a C remanence with NE+ direction. B is resolved mainly in the magnetite spectrum and incorporates a swathe of directions coincident with mid Ordovician palaeofields predicted from Baltica and adjoining Caledonian terranes. It is therefore interpreted as a remanence residing in small amounts of authigenic magnetite acquired during burial of this margin of the Hebridean Craton by nappe stacking during the Grampian Orogeny (∼470–430 Ma) following final closure of the Iapetus Ocean. Due to the merging of the B population with the A+ direction, A has not been defined as an antiparallel axis by former studies and can be inseparable by cleaning. However, defining A to include components with I < 50° at 140 sites yields a mean of D/I = 115/+31° (α₉₅ = 2.9°) for A+ and D/I = 293/−25° (α₉₅ = 3.5°) for A−. The poles at 238°E, 1°S (dp/dm = 2.0/3.8°) and 235°E, 1°N (dp/dm = 1.8/3.3°) are close to the Laurentian APWP at 1039–1025 Ma on the pre-drift reconstruction and constrain sedimentation within the 1060 to 994–977 Ma range of previous evidence. Only minor polar movement is detected through the entire thickness of the Torridon Group and suggests a sedimentation rate of the order of 0.5 m/1000 years for this clastic succession. A magnetic stratigraphy in A suggested by early work is confirmed in at least 13 polarity zones and identifies a rapid reversal rate preserved in specular hematite fixed by post-depositional mechanisms postdating slumping and sand volcano formation in wet sediment. The palaeomagnetic age of the A remanence links Torridonian sedimentation to rifting and subsidence accompanying break up of the Laurentian and Fennoscandian Shields at ∼1050–950 Ma following the Grenville Orogeny. Rifting between the two shields was followed by rotation and rewelding into a Neoproterozoic configuration. These events correlate with transition in Torridon Group palaeoenvironments from enclosed basin, to open basin with river systems feeding onto a passive margin, to marine transgression, and finally return to enclosed basin conditions. Diagenesis continued in post-Palaeozoic times and a D remanence with N–S shallow axis is sporadically preserved in authigenic hematite and interpreted as mainly a Variscan acquisition.
The Rum Layered Suite
1996, Developments in Petrology
Palaeocene igneous activity in the Rum Central Complex culminated in the formation of the ultrabasic and gabbroic rocks of the Layered Suite. Its three components, the Eastern Layered Series, the Western Layered Series and the Central Series, represent a continuum in time during which replenishments of picritic (MgO 15-20 wt.%) and basaltic magmas ponded in thin sill-like bodies at the Lewisian gneiss - Torridonian sandstone unconformity, each contributing incrementally to a layered cumulate sequence. The magmas were probably guided during ascent by the long-lived Long Loch Fault. Peridotite (olivine - chrome-spinel) cumulates formed from picritic magma. The residual (basaltic) magma mixed with resident residual magma from earlier batches, and with small amounts of siliceous rheomorphic melts from country rocks, forming (isotopically contaminated) allivalitic (= troctolitic), plagioclase - olivine cumulates or, less commonly, gabbroic (plagioclase - olivine - clinopyroxene) cumulates. Residual basaltic magma was probably also intruded as gabbroic sheets and plugs, and extruded as lavas. Widespread slump and shear structures indicate mechanical instability of unconsolidated cumulate mushes, especially in the allivalites. Ultrabasic breccias are common in the Central Series, and are attributed to (i) disruption of earlier cumulates as new batches of magma rose along an elongate, north-south feeder zone and (ii) collapse of cumulates into this zone during episodic magma withdrawal. Equilibrated textures, lack of compositional zoning in olivine and pyroxene, offsets between compositional and modal variation at unit and other lithological boundaries, the occurrence of finger structures and other replacement features, and the compositional modification of ultrabasic rocks adjoining late-stage gabbroic veins, all attest to the pervasive influence of migrating intercumulus liquids during crystallization and consolidation of the cumulates.
Multiple fluid migration events along the Moine Thrust Zone, Scotland
2005, Journal of the Geological Society
Palaeomagnetic dating of clastic dykes in Proterozoic basement, NW Scotland: Evidence for syndepositional faulting during deposition of the Torridonian
2005, Scottish Journal of Geology

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^*: Now at Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, U.K.

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Palaeomagnetism of the Torridonian of Rhum, Scotland: evidence for limited uplift of the central intrusive complex

Abstract

Phys. Earth Planet. Inter.

The Torridonian succession of the Isle of Rhum

Geol. Mag.

Rhum: Solid Geology

Evidence for the age of deposition of the Torridonian sediments of Northwest Scotland

Scott. J. Geol.

Palaeogeography of the Torridonian Applecross Group

Nature

An oxygen and hydrogen isotope study of the Skaergaard Intrusion and its country rocks: a description of a 55 m.y. old fossil hydrothermal system

J. Petrol.

18O/16O, D/H and14C/12C studies of the Tertiary Complex of Skye, Scotland

Am. J. Sci.

18O-depleted rocks from the Tertiary Complex of the Isle of Mull, Scotland

Earth Planet. Sci. Lett.

Stable isotope evidence for deep meteoric/hydrothermal circulation: Island of Rhum, Inner Hebrides, Scotland

Analysis of the palaeomagnetism of the Torridonian Sandstone Series of Northwest Scotland

Philos. Trans. R. Soc. London, Ser. A

The origin of the palaeomagnetism of the Torridonian Sandstone Series of Northwest Scotland

Philos. Trans. R. Soc. London, Ser. A

Palaeomagnetism of Precambrian sedimentary rocks from NW Scotland and the Apparent Polar Wandering Path of Laurentia

Geophys. J. R. Astron. Soc.

Palaeomagnetic studies of the Torridonian sediments, NW Scotland

Scott. J. Geol.

Palaeomagnetism of the British Tertiary Igneous Province; Rhun and Canna

Geophys. J. R. Astron. Soc.

¹⁸O/¹⁶O, D/H and¹⁴C/¹²C studies of the Tertiary Complex of Skye, Scotland

¹⁸O-depleted rocks from the Tertiary Complex of the Isle of Mull, Scotland