C. H. Emeleus

Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting

Abstract A lithospherically weak zone embracing the southern margin of the Greenland Archaean craton and the Ketilidian Julianehab batholith underwent repeated rifting during the interval 1350–1140 Ma, accompanying breakup of Palaeopangaea. The Gardar Province comprises the area affected by the faulting and asssociated alkaline magmatism. While an estimated 2–5 km of Proterozoic cover has been eroded, rift-fill successions have been preserved in early fault-bounded basins. The orientation of dyke swarms changed from WNW–ESE to nearly NE–SW during Gardar times. The principal swarms are inferred to occupy zones of lithospheric thinning and graben development. Central-type intrusive complexes, largely of syenites and nepheline syenites, reached shallow levels. The principal magmatic evolution was from transitional olivine basalt through to phonolites. Accompanying silica-oversaturated magma generation involved greater degrees of crustal assimilation. Anorthositic xenoliths in the Gardar intrusions imply the presence of an extensive anorthositic complex at depth, regarded as an integral part of the North American Proterozoic massif anorthosite association. The most primitive Gardar basalts are themselves relatively evolved, probably as a result of olivine±pyroxene fractionation during crustal underplating. The Gardar basic rocks are troctolitic with elevated Al 2 O 3 /CaO ratios: their incompatible element patterns suggest a significant input from lithospheric sources. The Ca-deficient nature of the Gardar basalts is attributed to an origin involving lithospheric mantle depleted by previous melting events. Trace element and isotopic signatures suggest considerable heterogeneity in the mantle sources which are ascribed to differential metasomatism of clinopyroxene-poor peridotite sources by small-fraction asthenospheric melts. The ultramafic lamprophyre/carbonatite association that recurred throughout the period is inferred to have originated from melting of metasomites deep within the lithospheric mantle. Affinities between the alkaline intrusions over an interval of >100 Ma characterize the Gardar as a coherent magmatic province and support the contention that the magmas are largely of lithospheric origin. The energy required to generate the very large requisite volumes of primitive magmas may have been supplied by successive mantle plumes. The Gardar magmatism pre- and post-dates the ∼1.27 Ga Mackenzie Igneous Events of North America but wholly pre-dates the ∼1.1 Ga Keweenawan magmatism associated with the Midcontinent Rift.

Rapid eruption of Skye lavas inferred from precise U-Pb and Ar-Ar dating of the Rum and Cuillin plutonic complexes

The interpretation of rocks of the British Tertiary Volcanic Province has played an important role in the historical development of many concepts in igneous petrology. Exposures of lavas, sub-volcanic rocks and plutonic complexes have allowed a detailed understanding of the field relationships between such units in the context of flood-basalt magmatism. Nevertheless, age control has been a source of much controversy and a limiting factor in comparing these relationships to recent developments in the theoretical modelling of magmatism within continents. Here we report precise 206Pb/238U zircon ages of 60.53 ± 0.08 Myr (2σ) for the Rum basic/ultrabasic pluton and 58.91 ± 0.07 Myr for the Cuillin gabbros, Skye, which tightly constrain eruption of the greater than 1.5-km-thick Skye lavas to a maximum duration of 1.6 ± 0.2 Myr. These dates yield magma production rates for the Skye lavas of about 2.2 × 10−3 km3 yr−1 comparable with rates inferred for individual magmatic centres produced by melting related to mantle plumes below ocean basins. In addition, the approximately 30 km of lithospheric thinning suggested by magma chemistry is required to have occurred in less than 2 Myr.

The Rum Layered Suite

Abstract 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.

Precise 40Ar/39Ar age for the initiation of Palaeogene volcanism in the Inner Hebrides and its regional significance

Sanidine crystals from two tuff layers at the base of the Palaeocene Eigg Lava Formation on the Isle of Muck, Inner Hebrides, yield step-wise Ar release plateau ages of 62.8 ± 0.6 (2 σ) and 62.4 ±0.6 Ma using the laser 40Ar/39Ar dating technique. These determinations, which are in accord with, but significantly more precise than earlier results, provide a definitive age for the inception of igneous activity in the Small Isles. From stratigraphical inferences, the Muck ages provide precise, maximum constraints on the possible age of Skye magmatism and possibly most of the British Tertiary Volcanic Province. The new ages significantly predate the oldest ages determined so far for Tertiary magmatism in NE Greenland but are similar to ages obtained from W Greenland and offshore SE Greenland and support the concept that the initiation of regional volcanism was primarily controlled by lithospheric thin spots. The Muck sanidine age appears to precisely date the first manifestation of the impingement of the Iceland mantle plume on the lithosphere beneath NW Scotland.

Picritic magmas and the Rum ultramafic complex, Scotland

Three small picritic dykes, intruded at a late stage in the evolution of the Rum basic–ultra-basic complex, Inner Hebrides, shed new light on the nature of the magmas responsible for the main complex. The magmas are of transitional (mildly alkalic) type, generated by relatively small-fraction (6–7 %) melting of a depleted mantle source. Melting is tentatively deduced to have commenced at ±100 km, straddling the garnet–spinel transition. Of the three samples, one (M9) is remarkable for the preservation of very primitive characteristics, with olivines of Fo93 containing highly aluminous spinels; these appear unique within the British Tertiary Volcanic Province. Sr, Nd and Pb isotopes indicate only minor (≤ 4 %) contamination with Precambrian crustal lithologies, reflecting the rapidity of ascent of the magma batches. The Rum picrites have 187Os/188Os ratios and trace element characteristics comparable to those of recent picrites from Iceland, suggesting minimal temporal change of the depleted parts of the Iceland plume over 60 Ma. Movements of the Long Loch Fault may have been instrumental in causing decompression melting of the spreading Iceland plume-head and facilitating ascent of the melts to near-surface levels.

The Tertiary lavas and sediments of Northwest Rhum, Inner Hebrides

Several small outliers of Tertiary lavas and sediments rest with strong unconformity on a buried landscape eroded from Torridonian sediments and Tertiary granophyre. Erosion continued during the period of sediment and lava accumulation. Four formations are recognized; these are, in order of increasing age, the Orval Formation (hawaiite and basaltic hawaiite lavas), the Guirdil Formation (icelandite lavas, interbedded conglomerates), the Upper Fionchra Formation (tholeiitic basaltic andesite lavas, hyaloclastite deposits, basal conglomerate) and the Lower Fionchra Formation (alkali and transitional basalt, basaltic hawaiite and hawaiite lava flows, basal conglomerate); each is separated by an erosional interval. Clasts in the conglomerates reveal a history of erosion of a terrain exposing gneisses, Torridonian sediments, igneous rocks derived from the Rhum Tertiary Central Complex (including allivalites), and Tertiary lavas of local origin but also including, in the oldest conglomerates, tholeiitic basalts not now preserved on or near Rhum. Prior to and during lava and sediment accumulation, erosion on Rhum had cut down to a level similar to that of the present day, although not to the extent that high-grade thermally altered rocks, which are a marked feature of the Central Complex, were being eroded in any quantity. A sequence of east–west trending valleys, possibly initiated on the line of the earlier Main Ring Fault, drained the area of the Central Complex which then, as now, must have been high ground. Small lakes occasionally formed in the valleys allowing the accumulation of fine-grained sediment with plant remains, and promoting the formation of hyaloclastite deposits when buried by later flows. No source for any of the lava formations is preserved on Rhum; they are thought to have come from feeders north of Rhum, possibly near Canna, and to have ponded against the hills and valleys near and in the Central Complex. The oldest tholeiitic lavas, not now found in situ , were followed by alkali and transitional flows compositionally similar to the Skye Main Lava Series but characteristically feldsparphyric; the most mafic also contain phenocrysts of magnesian olivine (with included Cr-Al-rich spinels) and aluminous spinel. Both the early alkalic/transitional basalts and the youngest hawaiites and basaltic hawaiites equilibrated at pressures Apart from a few N–S to NW–SE-trending basalt dykes, the lava formations represent the youngest Tertiary igneous event on Rhum.

The contact zone of the Rhum ultrabasic intrusion: evidence of peridotite formation from magnesian magmas

The Eastern Layered Series of the Rhum ultrabasic complex formed in situ, and was not tectonically emplaced as previously suggested. Field relations along the contact show no evidence for the existence of a late ‘Marginal Gabbro’, instead peridotite and allivalite layers can be traced to within 2 m of the surrounding partially melted country rocks. Local preservation of a chilled margin to one peridotite layer indicates that the parental magma consisted of an olivine tholeiite liquid carrying approximately 19% (vol.) olivine crystals in suspension.

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

The Western Granite is the largest of several granitic bodies around the margin of the Rum Central Igneous Complex. We report palaeomagnetic and anisotropy of magnetic susceptibility (AMS) data that bear on the emplacement and deformation of the Western Granite. The collection includes samples from 27 sites throughout the Western Granite, five sites in adjacent feldspathic peridotite, and two sites in intermediate to mafic hybrid contact aureole rocks. Palaeomagnetic data from 22 of the 27 sites in the granite provide an in situ group mean D = 213.2°, I = −69.5°, α95 = 5.5° that is discordant to an early Paleocene reverse polarity expected field (about 184°, −66°, α95 = 4.3°). The discrepancy is eliminated by removing an inferred 15° of northwest-side-down tilting about a best fit horizontal tilt axis trending 040°. Data from the younger peridotite and hybrid rocks of the Rum Layered Suite provide an in situ group mean of D = 182.6°, I = −64.8°, α95 = 4.0°, which is statistically indistinguishable from an early Paleocene expected field, and imply no post-emplacement tilting of these rocks since remanence acquisition. The inferred tilt recorded in the Western Granite, which did not affect the younger Layered Suite, suggests that emplacement of the ultrabasic rocks resulted in roof uplift and associated tilt of the Western Granite to make space for mafic magma emplacement. Magnetic fabric magnitude and susceptibility parameters yield two subtle groupings in the Western Granite AMS data set. Group 1 data, defined by rocks from exposures to the east and south, have comparatively high bulk susceptibilities (Kmean, 29.51 × 10−3 in SI system), stronger anisotropies (Pj, 1.031) and oblate susceptibility ellipsoids. Group 2 data, from rocks in the west part of the pluton, have lower values of Kmean (15.89 × 10−3 SI) and Pj (1.014), and triaxial susceptibility ellipsoids. Magnetic lineations argue for emplacement of the granite as a tabular sheet from the south–southeast toward the north and west. Moderate to steeply outward-dipping magnetic foliations, together with deflection of the country rock bedding in the north, are consistent with doming accompanying magma emplacement.

The Southern Mountains Zone, Isle of Rum, Scotland: volcanic and sedimentary processes upon an uplifted and subsided magma chamber roof

The Southern Mountains Zone of the Rum Central Complex lies inside a major ring fault and comprises an intricate association of country-rock outcrops, breccias and rhyodacite. The breccias and rhyodacite were long thought to be products of subterranean explosion and intrusion, respectively. Here, we report new observations that support re-interpretation of these units as mass movement deposits and ignimbrites. The most abundant breccias (Coire Dubh-type) consist mainly of country-rock clasts

Layered Alkaline Igneous Rocks of the Gardar Province, South Greenland

Abstract The Gardar Province comprises a Proterozoic suite of intrusive and extrusive igneous rocks, produced in an extensional continental environment between 1350-1130 Ma. The principal magmatic lineages can be related to the evolution of aluminous transitional olivine basalt, via trachyte through to comenditic and phonolitic residues. Some of the residual products are peralkaline and occasionally agpaitic. Some twelve major intrusive complexes characterize the province; whereas these are dominantly salic (syenites, nepheline syenites, and granites), intermediate to basic intrusions also occur. Layered cumulates are prominent within virtually all of the larger intrusions. Layering takes the form of modal layering, phase layering, cryptic layering, and igneous lamination, typically involving parallel orientation of tabular feldspar crystals. Xenolithic or autolithic slabs of roof rocks, or concentrated zones of roof-derived xenoliths, lying concordant with the modal layering and/or igneous lamination, can constitute a fifth element contributing to the layering. The layering typically dips inwards towards linear or point foci (according to the dyke- or stock-like nature of the intrusion). Marginal border groups of varying relative widths, commonly surround the layered cumulates composing the central portion. Layering can be low-angled ‘saucer-shaped’ or relatively steep (30–45°), giving V-shaped cross-sections. Modal layering is generally manifested as subordinate mafic layers within dominantly feld-spathic sequences. Some of this layering is inferred to have occurred through in situ growth as a consequence of intermittent cessation of feldspar nucleation. Additionally, gravitational sorting of ferromagnesian crystals from less dense felsic minerals produced modal layering. Other processes also appear to have operated to generate modal layering in specific instances. Cumulus minerals (or mineral aggregates) are inferred to have been capable of settling though their host melts. However, in the Ilimaussaq agpaitic magmas, sodalite floated. Additionally, sodic labradorite anorthosite xenoliths and megacrysts also appear to have floated in basaltic and hawaiitic melts. Structures commonly present which resemble those of clastic sedimentary rocks include normal (modal) grading, cross-bedding, angular unconformities, channels, and load structures. Stacks of channels (“trough stacks”) occur in several intrusions. Slump structures, involving both incoherent mafic cumulus mush and consolidated cumulate breccias, are also common. Such features are recognizable, not only in gabbroic cumulates, but in syenogabbroic, syenitic, nepheline syenitic (including agpaitic), and also granitic cumulates. Low viscosity of the magmas is thought to have been of paramount importance in permitting efficient density sorting between melts, crystals, and rocks. Figure 1. Sketch map showing principal intrusions exhibiting cumulate layering in the Gardar Province, south Greenland.