Rocks In Greenland Research Articles

Restoration of the Caledonian Baltoscandian margin from balanced cross-sections: the problem of excess continental crust

ABSTRACTConsideration of six balanced cross-sections through parts of the Finnmark Caledonides, N Norway indicates that shortening varies between 25% and 75%. A restored long cross-section across the width of the orogen, constructed with the aid of a branch line map, demonstrates a foreland propagating thrust system, with earlier formed more internal metamorphic nappes thrust SE 330 km under ductile conditions and then carried piggyback ESE a further 296 km on later brittle thrust sheets. Total shortening is 78·7% with a translation of the most internal thrust sheet of 626 km.The restored section suggests that: (1) the rate of propagation of deformation from hinterland to foreland is c. 2·27 cm y−1; (2) incorporation of basement into the nappes resulted from inversion of extensional faults formed during Iapetus rifting; (3) during rifting a Finnmark basement ridge separated a 220 km wide southeasterly Gaissa basin from the passive Iapetus continental margin which was at least 423 km wide; (4) the Finnmark Caledonides resulted from a continent-microcontinent collision which obducted continental crust at least 600 km across the Baltic margin; and (5) the Caledonian Baltoscandian margin prior to Iapetus suturing extended at least 400 km W of the Norwegian coast. On a Bullard reconstruction this overlaps with Laurentian rocks in Greenland. The excess continental crust is accounted for by shortening of the Baltoscandian margin during collision.

Origin and Subglacial Erosion of Part of the Coastal Highland of East Antarctica

The origin and erosional history of the eastern end of the coastal highlands of East Antarctica within Mac. Robertson, Kemp and Enderby Lands (50°-64°E) has been investigated using 13,000 km of ice-thickness-radar traversing. The highlands are interpreted as the residual of widespread 1-km-altitude crust within Gondwanaland, the low altitude of the adjacent coastal lowlands as due to crustal thinning caused by continental breakup, and the low altitude of the areas inland as due to depression caused by the weight of the present ice sheet. The coastal highlands are therefore a normal continental-edge feature of a large ice sheet. The highlands have an important influence on preserving the ice sheet, and on ice-sheet dynamics. The topography and present-day ice-base temperatures of the highland and coastal lowland are consistent with a model containing three zones of glacial erosion proposed by Sugden from exposed rocks in Greenland. In this model there is glacial erosion only on those rock surfaces where the basal ice is at the pressure melting point and so allows basal sliding of the ice sheet. In the region studied, the average denudation and uplift resulting from this glacial erosion is between 0.3 and 0.6 km. The high nunataks are the result of local uplift of up to 1.5 km, in an area with dissection of up to 3 km.

Palaeomagnetism of nine dated Phanerozoic dykes in south-east Greenland

Summary Nine basic dykes were sampled near Angmagssalik, east Greenland. Specimens have been treated by alternating field demagnetization in 11 steps up to 3000 (peak) oersted (300 ml). The ‘cleaned’ direction at all sites is recognized after treatment at 150 oersted. All specimens are reversely magnetized. The mean of the site mean directions has declination = 182°.0, inclination =−66°.9, it = 45, α95= 7°.7. This direction yields a palaeomagnetic pole (reversed) at 73°.4N, 139°.5E (dp= 10°.7, dm= 12°.9) which is near, but significantly different from, that derived from lower Tertiary rocks in Greenland, namely 63°.2N, 184°.6E (A95= 4°.5). K-Ar ages of the nine dykes, based upon whole-rock and mineral separates, range from mid-Tertiary to Cambrian. It is impossible to reconcile these ages with the palaeomagnetic results. The palaeomagnetic evidence, supported by geological inference, suggests that all nine dykes are members of the east Greenland lower Tertiary dyke swarm, designated THOL1, of probable age c. 52 Ma. The difference between the poles given above can be explained by supposing that the sampling area has tipped about a horizontal axis directed along 013°/193°, the angle of rotation being 13° (± 11°) anti-clockwise, when the axis is viewed along 013°. This local effect could have been due to block faulting when the north-east Atlantic started to open, or may be attributed to upwarping of the coast due to the weight of the ice-cap inland.

Early Precambrian tonalite-trondhjemite sialic nuclei

Early Precambrian batholiths evolved by diapiric intrusion of near-liquidus to superheated tonalitic and trondhjemitic magmas into an early greenstones crust. Distribution patterns of enclaves and xenolith screens derived from the latter provide reference markers which define the internal geometry and detailed structure of the “gregarious batholiths” (Macgregor) as polydomal multi-lobal bodies. Near-liquidus temperatures are suggested by the digestion of vast volumes of ultramafic—mafic crust by the acid magmas. Tracing of xenolith trains between low and high grade metamorphic terrains provide key evidence for coeval relations between granite—greenstone type terrains and amphibolite to granulite facies infracrustal root zones of the latter. The formation of the plutonic tonalite—trondhjemite suite was accompanied by dacitic to rhyolitic extrusions, the acid volcanic lenses being located above early greenstone units intruded by the batholiths and below upper greenstone sequences which postdate these intrusions. The geochemical characteristics of high-level and deep-level tonalites and trondhjemites are compared. Both suites display very wide compositional spectra, but data from high-grade terrains tend to define a more basic field than data from granite—greenstone terrains. Effects of source compositions on the geochemistry of the acid plutonic rocks are pointed out. Tonalites dominate in South African terrains whereas trondhjemites dominate in Western Australian terrains — a difference conceivably related to the more ultramafic composition of source rocks represented by early greenstone units in southern Africa. Granodiorites and potassic granites form a comparatively minor component of Archaean batholiths, and may occur in the following forms: (1) bands of augen gneiss in high-grade terrains; (2) components of trondhjemitic to granitic gneisses in high-level plutons; and (3) discrete post-tectonic intrusions typically emplaced at high levels of the batholiths and along older tonalite—greenstone contacts. Migmatites characteristically form in close spatial association with xenolith-rich zones, probably due to depression of the solidus consequent on water addition related to dehydration of the xenoliths. A derivation of the acid sodic magmas by anatexis of sialic materials is inconsistent with geochemical evidence and petrological theory. In contrast, the commonly low to very low LIL element levels and REE evidence indicate derivation by about 30–50% melting of basic rocks. Marked trace element anomalies are characteristic of some Archaean plutonic suites, e.g. very high Sr in some Western Australian rocks, low Rb in some Lewisian (Scotland) and South African rocks, U depletion in South African and southwestern Greenland suites, high Li in some Pilbara rocks and high Zr in some southwestern Greenland rocks. However, the only consistent anomaly observed to date is a well-pronounced depletion in Y and heavy REE, suggesting extensive equilibration of the acid melts with eclogite and/or amphibolite. Uniformitarian interpretations of the Archaean are questioned in the light of the evidence for high temperature and pressure, the unique tectonic style of diapirism and the low initial 87Sr/ 86Sr as compared to Proterozoic plutonic suites. The diachronous nucleation of tonalite—trondhjemite plutons during the Archaean is seen as the major process effecting a transformation of an early Archaean sima into sial.

The Early Precambrian of Scotland and Related Rocks of Greenland

The Early Precambrian of Scotland and Related Rocks of Greenland

Molybdenite in the Fiskenæsset anorthosite complex, southern West Greenland

During the past few seasons mapping of early Precambrian rocks in the Fiskenæsset region (Kalsbeek, this report), the author has noted the widespread occurrence of molybdenite in the Majorqap qava outcrop of the Fiskenæsset anorthosite complex. This outcrop lies 35 km north-east of Fiskenæsset town, north of Fiskenæsfjorden (Myers, 1973). EIsewhere in the Archaean rocks of Greenland, substantial amounts of molybdenite have only been found in supracrustal amphibolites in the Godthåb region by Kryolitselskabet Øresund A/S at Ivisartoq (Nielsen, 1973) and V.R. McGregor at Narssaq.

A geochronological study of the Lewisian from Loch Laxford to Durness, Sutherland, N. W. Scotland

Potassium-argon biotite and hornblende ages from near Laxford Bridge, Sutherland, range up to 1770 m.y.: higher apparent ages of 2060 and 1840 m.y. occur in the biotite pods along the southern margin of the Laxford assemblage. A rubidium-strontium isochron from whole-rock specimens from the gneisses of the Laxford assemblage gives 1850 ± 50 m.y., initial 87 Sr/ 86 Sr = 0.7061 ± 0.0005 (T 1 2 = 4.85. 10 10 y). It is suggested that 1850 ± 50 m.y. is a minimum age for the climax of the Laxfordian metamorphic episode, and that this metamorphism had effectively terminated by 1750 m.y. the highest K-Ar age of a mineral from the type area. Younger apparent ages of minerals from and close to the type area spread down to 1575 ± 50 m.y., associated with late effects of the metamorphism, continued pegmatite-formation, and a general slow-cooling. Closure of radiogenic systems occurred later at Durness than at Laxford Bridge. Comparison with the sparse data from nearby shield areas suggests a closer correlation of the Lewisian with comparable rocks in Greenland than with Fennoscandia.

Devonian Rocks of Greenland and Svalbard: ABSTRACT

In Greenland and Arctic Scandinavia, Devonian sedimentary rocks crop out in only two areas: (1) east Greenland between lat. 71½°N and 74½°N; and (2) Svalbard, between lat. 76½°N and 80°N in Vestspitsbergen. These strata are nonmarine clastics. Aggregate thicknesses are large, up to 7,000 m in Greenland and 5,000-6,500 m in Vestspitsbergen. In both areas, the successions are unconformable on deformed pre-Devonian, Caledonian rocks. Both areas are well known for their fossil vertebrates, on which the following age determinations depend. In Greenland the successions can be assigned to 5 major units, each with a maximum thickness of 1,000-3,000 m: (1) A unit that includes the Vilddal Group, of red and gray siltstone, conglomerate, sandstone, volcanic rock, and granite intrusions; (2) a unit of conglomerates, and red and gray-green sandstone; (3) the Kap Kolthoff Formation of gray-green sandstone; (4) Kap Graah Group of red sandstone, siltstone, and volcanic rock; and (5) the Mt. Celsius Supergroup of red and gray-green siltstone with some sandstone. Units (1) and (2) are dated as Givetian (late Middle Devonian), and units (4) and (5) are dated as Famennian (late Upper Devonian). Unconformities between and within these units occur, particularly in the northeast part of the area, and the rocks were folded again before de osition of the nonmarine Carboniferous. In Svalbard, the succession consists of 4 major units, each with a maximum thickness of 1,000-3,000 m: (1) the Siktefjellet Group of conglomerate and sandstone beds; (2) the Red Bay Group of conglomerates and sandstone beds; (3) the Wood Bay Formation of sandstone and red siltstone; and (4) a supergroup (Grey Hoek and Wijde Bay Formations, Mimer Valley Group) of gray conglomerate, sandstone, and siltstone. There is no fossil evidence for the date of (1) which was strongly folded before the deposition of (2). Unit (2) is dated as Gedinnian (early Lower Devonian), and the succession is continuous through (3) to (4), which ranges up to Givetian (late Middle Devonian) in age. These units were folded strongly before the deposition of Carboniferous sediments. End_of_Article - Last_Page 2481------------

Discussion

Mr D. B ridgwater said that the regional pattern of events described by the authors was of considerable interest and their approach had widespread application outside the Lewisian of north-west Scotland. Studies on a similar regional scale were in progress on the Pre-Cambrian rocks of Greenland (for the most recent summary of research, see the 1966 report of activities of the Geological Survey of Greenland, Rapp. Grønlands geol. Unders. 11 ). The major units recognized in West Greenland were: a large central segment, the last major thermal event of which occurred about 2500 m.y. ago, flanked by younger fold belts (the Nagssugtoquidian, 1650–1800m.y. to the north; and the Ketilidian 1500–1800 m.y. to the south). These major dividions corresponded closely in time to the early Scourian and Laxfordian of north-west Scotland. The main features of relevance to the present discussion were the large amounts of early rocks reworked in the younger fold belts and the prevalence of concordant anorthosites of igneous origin (Windley 1966, Rapp. Grønlands. geol. Unders. 11 ), which the speaker thought had probably been formed earlier than 2700 m.y. ago. Mapping of the Pre-Cambrian of south-east Greenland, which might eventually be expected to provide the best area for comparison with north-west Scotland, had only just started on a regional basis. Preliminary age-determinations (Wager & Hamilton Nature, Lond., 1964) suggested that the last major plutonic event in the Angmagssalik area took place about 1600 m.y. ago. It is already clear, however, that the area is structurally complex and probably contains