The East Greenland rifted volcanic margin

The Palaeogene North Atlantic Igneous Province is among the largest igneous provinces in the world and this review of the East Greenland sector includes large amounts of information amassed since previous reviews around 1990.The main area of igneous rocks extends from Kangerlussuaq (c. 67°N) to Scoresby Sund (c. 70°N), where basalts extend over c. 65 000 km2, with a second area from Hold with Hope (c. 73°N) to Shannon (c. 75°N). In addition, the Ocean Drilling Project penetrated basalt at five sites off South-East Greenland. Up to 7 km thickness of basaltic lavas have been stratigraphically and chemically described and their ages determined. A wide spectrum of intrusions are clustered around Kangerlussuaq, Kialeeq (c. 66°N) and Mesters Vig (c. 72°N). Layered gabbros are numerous (e.g. the Skaergaard and Kap Edvard Holm intrusions), as are under- and oversaturated syenites, besides small amounts of nephelinite-derived products, such as the Gardiner complex (c. 69°N) with carbonatites and silicate rocks rich in melilite, perovskite etc. Felsic extrusive rocks are sparse. A single, sanidine-bearing tuff found over an extensive area of the North Atlantic is thought to be sourced from the Gardiner complex.The province is famous for its coast-parallel dyke swarm, analogous to the sheeted dyke swarm of ophiolites, its associated coastal flexure, and many other dyke swarms, commonly related to central intrusive complexes as in Iceland. The dyke swarms provide time markers, tracers of magmatic evolution and evidence of extensional events. A set of dykes with harzburgite nodules gives unique insight into the Archaean subcontinental lithosphere.Radiometric dating indicates extrusion of huge volumes of basalt over a short time interval, but the overall life of the province was prolonged, beginning with basaltic magmas at c. 60 Ma and continuing to the quartz porphyry stock at Malmbjerg (c. 72°N) at c. 26 Ma. Indeed, activity was renewed in the Miocene with the emplacement of small volumes of basalts of the Vindtoppen Formation to the south of Scoresby Sund.Although the basalts were extruded close to sea level, this part of East Greenland is a plateau raised to c. 2 km, but the timing of uplift is controversial. Superimposed on the plateau is a major dome at Kangerlussuaq.East Greenland presents a rich interplay between magmatic and tectonic events reflecting the birth of the North Atlantic Ocean. It was active over a much longer period (36 Ma) than other parts of the province (5 Ma in the Hebrides, Northern Ireland and the Faroe Islands) and contains a wider range of products, including carbonatites, and felsic rocks tend to be granitic rather than syenitic. As expected, there are many similarities with Iceland, the present-day expression of activity in the province. Differences are readily explained by higher production rates and the thicker lithospheric lid during the early stages of development in East Greenland. The igneous and related activity clearly results from plate-tectonic factors, but the relationship is not understood in detail. In particular, the nature of the underlying mantle processes, primarily the presence or absence of a plume, is still not resolved

Geological Investigations in East Greenland. Part X.

The Kap Edvard Holm complex, situated on the west side of the Kangerdlugssuaq, is one of the major units of the east Greenland Thulean igneous province. The largest part of the complex consists of layered gabbros that, like the Skaergaard intrusion, were emplaced after the formation of the early Tertiary basalts but prior to the large monoclinal coastal flexure. Subsequently the layered gabbros were intruded by the syenites of Kap Deichmann and Kap Boswell. Three separate and major injections of basic magma were involved in the formation of the layered complex, and gave rise to an earlier, Lower Layered Series to the north, a Middle Layered Series occupying the central, and a later Upper Layered Series in the southern part of the complex. The present contribution is concerned solely with the earliest basic rocks of the complex, the Lower Layered Series, which form a group of rhythmically banded cumulates some 3900 m in thickness. There is no chilled margin exposed and at the only locality in which the Lower Layered Series are exposed close to the Pre-Cambrian gneisses, an earlier fine-grained marginal gabbro, 100 m in thickness, lies between the rocks of the layered series and the gneisses. The mineralogy of the series is described and includes details of the plagioclase zoning at various heights in the layered succession, the compositional trends of the clinopyroxenes and olivines, and also some details of the primary and secondary amphiboles, and of the iron oxides. Orthopyroxene does not occur, either as a cumulus or interculus phase in the rocks of the layered series. The Lower Layered Series is divided into a lower and an upper unit, the Lower Zone A and Lower Zone B cumulates respectively. The lower unit has a maximum thickness of 700 m and consists of plagioclase-augite-olivine orthocumulates and includes both extreme feldspar- and ferromagnesian-rich bands. The chemical composition of Lower Zone A is illustrated by three analyses of the cumulates and includes that of a feldspar adcumulate layer. Lower Zone B is 3200 m in thickness and displays progressive cryptic layering from the base of the zone upwards in the succession for 1300 m. Above this level, at a height of 2050 m in the layered succession, the trend of the cryptic layering is reversed, and these higher cumulates, the composition of which are illustrated by eight analyses, show progressively decreasing mafic and felsicindices. The deduced composition of the original magma from which the cumulates crystallized is compared with the initial chilled margin of the Skaergaard intrusion. The Lower Layered Series magma is less tholeiitic in character than that of the Skaergaard, and has affinities with transitional alkali olivine basalt magma.The Kap Edvard Holm complex, situated on the west side of the Kangerdlugssuaq, is one of the major units of the east Greenland Thulean igneous province. The largest part of the complex consists of layered gabbros that, like the Skaergaard intrusion, were emplaced after the formation of the early Tertiary basalts but prior to the large monoclinal coastal flexure. Subsequently the layered gabbros were intruded by the syenites of Kap Deichmann and Kap Boswell. Three separate and major injections of basic magma were involved in the formation of the layered complex, and gave rise to an earlier, Lower Layered Series to the north, a Middle Layered Series occupying the central, and a later Upper Layered Series in the southern part of the complex. The present contribution is concerned solely with the earliest basic rocks of the complex, the Lower Layered Series, which form a group of rhythmically banded cumulates some 3900 m in thickness. There is no chilled margin exposed and at the only locality in which the Lower Layered Series are exposed close to the Pre-Cambrian gneisses, an earlier fine-grained marginal gabbro, 100 m in thickness, lies between the rocks of the layered series and the gneisses. The mineralogy of the series is described and includes details of the plagioclase zoning at various heights in the layered succession, the compositional trends of the clinopyroxenes and olivines, and also some details of the primary and secondary amphiboles, and of the iron oxides. Orthopyroxene does not occur, either as a cumulus or interculus phase in the rocks of the layered series. The Lower Layered Series is divided into a lower and an upper unit, the Lower Zone A and Lower Zone B cumulates respectively. The lower unit has a maximum thickness of 700 m and consists of plagioclase-augite-olivine orthocumulates and includes both extreme feldspar- and ferromagnesian-rich bands. The chemical composition of Lower Zone A is illustrated by three analyses of the cumulates and includes that of a feldspar adcumulate layer. Lower Zone B is 3200 m in thickness and displays progressive cryptic layering from the base of the zone upwards in the succession for 1300 m. Above this level, at a height of 2050 m in the layered succession, the trend of the cryptic layering is reversed, and these higher cumulates, the composition of which are illustrated by eight analyses, show progressively decreasing mafic and felsicindices. The deduced composition of the original magma from which the cumulates crystallized is compared with the initial chilled margin of the Skaergaard intrusion. The Lower Layered Series magma is less tholeiitic in character than that of the Skaergaard, and has affinities with transitional alkali olivine basalt magma.The Kap Edvard Holm complex, situated on the west side of the Kangerdlugssuaq, is one of the major units of the east Greenland Thulean igneous province. The largest part of the complex consists of layered gabbros that, like the Skaergaard intrusion, were emplaced after the formation of the early Tertiary basalts but prior to the large monoclinal coastal flexure. Subsequently the layered gabbros were intruded by the syenites of Kap Deichmann and Kap Boswell. Three separate and major injections of basic magma were involved in the formation of the layered complex, and gave rise to an earlier, Lower Layered Series to the north, a Middle Layered Series occupying the central, and a later Upper Layered Series in the southern part of the complex. The present contribution is concerned solely with the earliest basic rocks of the complex, the Lower Layered Series, which form a group of rhythmically banded cumulates some 3900 m in thickness. There is no chilled margin exposed and at the only locality in which the Lower Layered Series are exposed close to the Pre-Cambrian gneisses, an earlier fine-grained marginal gabbro, 100 m in thickness, lies between the rocks of the layered series and the gneisses. The mineralogy of the series is described and includes details of the plagioclase zoning at various heights in the layered succession, the compositional trends of the clinopyroxenes and olivines, and also some details of the primary and secondary amphiboles, and of the iron oxides. Orthopyroxene does not occur, either as a cumulus or interculus phase in the rocks of the layered series. The Lower Layered Series is divided into a lower and an upper unit, the Lower Zone A and Lower Zone B cumulates respectively. The lower unit has a maximum thickness of 700 m and consists of plagioclase-augite-olivine orthocumulates and includes both extreme feldspar- and ferromagnesian-rich bands. The chemical composition of Lower Zone A is illustrated by three analyses of the cumulates and includes that of a feldspar adcumulate layer. Lower Zone B is 3200 m in thickness and displays progressive cryptic layering from the base of the zone upwards in the succession for 1300 m. Above this level, at a height of 2050 m in the layered succession, the trend of the cryptic layering is reversed, and these higher cumulates, the composition of which are illustrated by eight analyses, show progressively decreasing mafic and felsicindices. The deduced composition of the original magma from which the cumulates crystallized is compared with the initial chilled margin of the Skaergaard intrusion. The Lower Layered Series magma is less tholeiitic in character than that of the Skaergaard, and has affinities with transitional alkali olivine basalt magma

Low-pressure, water-assisted anatexis of basic dykes in a contact metamorphic aureole, Fuerteventura (Canary Islands): oxygen isotope evidence for a meteoric fluid origin

Migmatites produced by low-pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite-gabbro intrusion (PX2), on Fuerteventura. Dykes outside and inside the aureole record interaction with meteoric water, with low or negative δ18O whole-rock values (+0.2 to −3.4‰), decreasing towards the contact. Recrystallised plagioclase, diopside, biotite and oxides, from within the aureole, show a similar evolution with lowest δ18O values (−2.8, −4.2, −4.4 and −7.6‰, respectively) in the migmatite zone, close to the intrusion. Relict clinopyroxene phenocrysts preserved in all dykes, retain typically magmatic δ18O values up to the anatectic zone, where the values are lower and more heterogeneous. Low δ18O values, decreasing towards the intrusion, can be ascribed to the advection of meteoric water during magma emplacement, with increasing fluid/rock ratios (higher dyke intensities towards the intrusion acting as fluid-pathways) and higher temperatures promoting increasing exchange during recrystallisatio

Petrology and evolution of the Northern part of the Rhum Ultrabasic Complex

Volume One onl

Ubiquitous dendritic olivine constructs initial crystal framework of mafic magma chamber

peer reviewedLayered intrusions are fossilized mafic magma chambers in the Earth's crust. The pathways that led to crystallization and solidification of layered intrusions have been hotly debated as the growth model of primocrysts (the earliest-formed crystals) in mafic magma chambers remains enigmatic. In this study, we carried out high-resolution elemental mapping of mm-scale olivine primocrysts from the Sept Iles layered intrusion (Canada), the third largest one in the world, with a focus on phosphorus (P) zoning of olivine. Our results reveal that complex P zoning of olivine with intense dissolution textures is ubiquitous in the ∼4.7 km-thick Layered Series of the intrusion. The P-rich zones of olivine are featured with dendritic, hopper and sector-zoned patterns, which are attributed to significant magma undercooling. Thermal modeling based on a 1-D conductive cooling model suggests that initially hot parental magma intruding into cold country rocks would result in high degrees of undercooling (-ΔT >60 °C) in the margins (i.e., floor, roof and sidewalls) of magma chamber, facilitating rapid growth of dendritic olivine, which may be then spread within the magma chamber by dynamic convection and crucial to construct initial crystal framework of a solidifying magma chamber. Additionally, diffusion modeling based on the P gradients in olivine suggests a minimum cooling rate of 2.7 to 3.3×10−3 °C/year in the center of the intrusion, similar to the averaged cooling rate of other layered intrusions (e.g., Bushveld, Stillwater and Skaergaard) reported in previous studies. This indicates that rapid cooling (ca. 10−2 to 10−3 °C/year) at high temperature (>800 °C) may be predominant regardless of the size of magma chambers. Our study demonstrates that P zoning of olivine is powerful in decoding crystallization and thermal histories of mafic-ultramafic intrusions

The geology and geochemistry of the north-western portion of the Usushwana Complex, South-Eastern Transvaal

The 2.9 Ga old Usushwana Complex in the Piet Retief-Amsterdam area (south-eastern Transvaal) represents an exposed segment of a layered intrusion. It has the form of a dyke-like body elongated in a northwesterly direction, and extends to an estimated depth of 3000 -5500 m. Lithologically, the Complex consists of a cumulate succession of mafic rocks capped by granitoids and has intruded along the contact between the basement and the supracrustal sequences of the Kaapvaal Craton. Differentiation of an already contaminated gabbroic magma resulted in an ordered stratigraphic sequence comprising progressively more evolved lithotypes, with at least two imperfect cyclic units developed over a stratigraphic thickness of about 700 metres (Hlelo River Section). Meso- to orthocumulate textured gabbros and quartz gabbros grade upwards into magnetite- and apatite-bearing quartz gabbros, interlayered with discontinuous magnetitite horizons. The gabbros in turn grade into hornblende-rich, granophyric granodiorites. The differentiation process is regarded as having been considerably enhanced by the assimilation of acidic material, derived by partial melting of the felsic country rocks at the roof of the magma chamber. Recrystallisation of these rocks gave rise to the microgranites that locally overlie the granodiorites. Mineralogical, textural and geochemical features indicate a relatively advanced fractionation stage, suggesting that the exposed sequence of the Usushwana Complex in the study area represents the upper portion of the intrusion. No significant mineralised occurrences were identified. However, on the basis of similarities between the Usushwana Complex and other mafic layered intrusions which host significant ore deposits, it is suggested that economic concentrations of base metal(Cu-Ni) sulphides, PGE and chromitites are likely to be developed at lower stratigraphic levels

Temporal, lithospheric and magmatic process Controls on ni, cu and platinum-group element (PGE) mineralisation: A case study from Scotland

A temporal and spatial relationship between plume magmatism, cratonic lithosphere and the occurrences of orthomagmatic Ni-Cu and platinum-group element (PGE) sulphide mineralisation has been documented in the literature. However the underlying causes for this correlation have yet to be resolved – is there an inherent feature of the cratonic lithosphere and its mantle ‘keel’ that controls mineralisation? Or is this correlation purely a preservational bias in the geological record? Scotland has experienced multiple tectono-magmatic events and provides an ideal testing ground, or ‘framework’, in which to assess the role of lithospheric mantle on chalcophile element (Ni and Cu) and precious metal (PGE and Au) abundances through time. Given the well-documented geological history of the region (including several suites of mantle xenoliths), coupled with exploration campaigns in Greenland (with which Scotland has comparable geology), this thesis aims to assess the contributions and influences of lithospheric mantle vs. asthenospheric mantle during melting and mineralisation. It also evaluates the Ni-Cu-PGE mineralisation potential for Scotland, particularly in a Noril’sk-type conduit-hosted setting within the British Palaeogene Igneous Province (BPIP). The earliest major tectono-magmatic event following cratonisation of the North Atlantic Craton (NAC) occurred c. 2.4 Ga during Palaeoproterozoic extension, forming the maficultramafic Scourie Dyke Swarm. Despite evidence for lithospheric mantle melting at this time, the subcontinental lithospheric mantle (SCLM) below the Scottish portion of the NAC did not become severely depleted in sulphides or PGE. Instead, spinel lherzolite mantle xenoliths from this region (e.g., Loch Roag) record an influx of carbonatite-associated sulphides at this time, enriched in PGE, and providing a deeper indication of continental extension that may be correlated to carbonatitic intrusions in Greenland. Subsequent collision and orogenesis of the NAC in the late Palaeoproterozoic (c. 1.9 to 1.7 Ga) represents a second significant tectonomagmatic event, recorded in the Scottish SCLM as sulphide (re-)melting and formation of discrete Pt-sulphide minerals (cooperite). Hence the lithospheric mantle here became appreciably enriched in precious metals during the Palaeoproterozoic, but crucially this preserved multiple co-existing populations of sulphides, distinct in their petrographic setting and geochemistry. Cratonic basement and associated mantle lithosphere are absent in the southern terranes of Scotland. This provides a direct comparison between lithospheric mantle geochemistry for Archaean-Palaeoproterozoic terranes north of the Great Glen Fault vs. Palaeozoic terranes south of the Great Glen Fault. Rifting of Rodinia and opening of the Iapetus Ocean in the late Neoproterozoic thus marks a significant change in geodynamic setting. This is especially apparent in the concentration of cobalt in lithospheric mantle sulphides, which appears to be inherently linked with the formation and/or later destruction (subduction) of oceanic crust during the Grampian event of the Caledonian orogeny. The impingement of the proto-Icelandic mantle plume initiated in the Palaeogene at the base of the NAC lithospheric mantle keel of Scotland and Greenland. The earliest Palaeogene magmas are enriched in Pt (i.e., have a high Pt/Pd ratio), whilst subsequent magmas associated with the opening of the Atlantic Ocean have successively lower Pt/Pd ratios. High Pt/Pd ratios are therefore coincident with magmas that have intruded through cratonic lithosphere. The SCLM at the margin of this region is known to be Pt-enriched (with cooperite) and therefore the changing Pt/Pd ratio of North Atlantic Igneous Province magmas suggests a fundamental interaction between the mantle plume and pre-enriched SCLM. Thus, whilst the concentration of metals, particularly Ni and Cu, is largely based on the high degree of asthenospheric mantle melting associated with the plume itself, the ratio of precious metals, such as Pt/Pd, can be strongly influenced by SCLM geochemistry. Overall, the intricate subtleties of metasomatic signatures recorded by mantle xenolith sulphides (or populations of sulphides) could allow for metallogenic ‘mapping’ of the upper mantle. This may identify areas of geochemical and mineralogical ‘preconditioning’, and together with geophysical constraints such as major lithospheric lineaments, it may be possible to establish the craton-specific fertility of a region. Finally, in order for orthomagmatic sulphide mineralisation to occur, magmas must achieve sulphur saturation in the upper crust, forming an immiscible sulphide liquid and thereby collecting PGE and chalcophile elements, possibly to economic grades. Thus a crucial part of assessing the mineralisation potential of a region must entail an investigation into the causes and locations of S-saturation. Given that crustal sulphur contamination is a common trigger for magmatic S-saturation, this thesis establishes the first S-isotopic (δ34S) framework for western Scotland in order to identify areas of sulphur contamination in the BPIP. In Scotland, the most readily available and S-rich rocks occur in the Mesozoic Hebrides Basin. Sulphur contamination of BPIP rocks is widespread and both S-saturation and S-undersaturation can be observed, suggesting that the region may be extremely fertile for orthomagmatic Ni-Cu-PGE mineralisation. By reconstructing the Hebrides Basin stratigraphy we can assess locations of contamination, even if these are above the current level of exposure (and since removed by erosion), and in some situations sulphide liquid sinking may be demonstrated, suggesting further possibilities for mineralisation present ‘up-stream’ in magmatic conduits. In conclusion, the Scottish BPIP represents a new exploration frontier not yet identified by industry for orthomagmatic Ni-Cu-PGE mineralisation. The conclusions are based on approximately 500 rock samples from across Scotland, which have been analysed for major elements and over thirty trace elements (including PGE) and S-isotopes. All data are available on an accompanying CD