The Mesoproterozoic Midsommersø dolerites and associated high-silica intrusions, North Greenland: crustal melting, contamination and hydrothermal alteration

Abstract

The Midsommersø dolerites and the flood basalts of the Zig-Zag Dal Basalt Formation, eastern North Greenland, represent a major Mesoproterozoic (∼1,380 Ma) igneous event. The intrusive rocks form large sheets within a succession of feldspathic sandstones which underlie the basalts. The geochemistry of the basalts has recently been re-investigated and reported elsewhere in this Journal; here we present new trace element and Nd-, Sr- and Pb-isotopic data for the intrusive rocks. Unlike the basalts, the intrusions yield evidence of considerable interaction and contamination with upper crustal rocks, especially the sandstones. High-silica rocks (80–90 wt% SiO₂) occur in sheets, up to 60 m thick. They were formed by mobilisation of sandstones, and indicate a very high rate of emplacement of hot basic magma into the sandstones at depth. These mobilised sandstones (‘rheopsammites’) are among the most SiO₂-rich intrusive rocks on earth. Sheets of remobilised granitoid rocks from the crystalline basement (∼70% SiO₂) are also present. Hydrothermal activity, associated with the igneous event, significantly changed the compositions of the silicic rocks as well as that of many dolerites. Sheets of hydrothermally altered (‘red’) dolerites and silicic rocks invariably have borders of dark, fresh dolerite; this is interpreted to be the result of intrusion from zoned magma chambers. Nd isotope data confirm the crustal origin of the silicic rocks as well as the contamination of some dolerites by components derived from crustal sources, while Sr- and Pb-isotopic systems are strongly affected by the hydrothermal alteration, and give little information on the petrogenesis of the rocks. Recent loss of Sr from hydrothermally altered rocks further affected the Sr isotope systems, and earlier age determinations by the Rb–Sr whole-rock isochron method (1,230 Ma) have proved to be in error. The dolerites and the basalts are geochemically very similar, but most dolerites have moderately negative Eu anomalies that are not observed in the basalts. Eu anomalies in the dolerites could be related to contamination by sandstone at depth, but it is not clear why the basalts escaped a similar contamination.

Appendix: Sample descriptions; for further details see Kalsbeek and Jepsen (1983)

Kap Ejnar Mikkelsen 1. Near Kap Ejnar Mikkelsen (81°55.3′N–31°51.6′W) a 190-m thick horizontal sheet of very fresh dolerite crops out in the steep wall of Independence Fjord. For a detailed description and a chemical profile of this intrusion see Kalsbeek and Jepsen (1983). Sample nos. 273241 and 273247 were collected from the lower part of the sheet exposed in a large landslide, at respectively 40 and 85 m above the lower contact.

Independence Fjord. This intrusion at 81°54.9′N–32°05.3′W is a subhorizontal sheet of variable doleritic rocks, ∼180 m thick. Apart from the bottom chill zone, the lower part of the intrusion is weathered to gravel; fresh rocks are present from 100 m upwards. Normal dolerites occur from 100 to 140 m, followed by red-and-greenish weathering granophyric dolerites. The upper 5 m consist of very fine-grained dark dolerite. Sample #273228 (+140 m) is a normal doleritic rock, #273230 (+160 m) a red granophyric dolerite, and #273236 represents the upper chill ∼1 m below the contact with the sandstones. A chemical profile through the intrusion is shown in Kalsbeek and Jepsen (1983).

Vildtland 1. An isolated outcrop at 81°43.1′N–32°49.8′W exposes brick-red doleritic rocks. Contacts with the country rocks are not exposed, but there is a gradation of the red dolerite into normal dark-grey dolerite. The latter probably represents the border of the intrusion. Samples from this locality showed a wide range in Rb–Sr ratios, and yielded a Rb–Sr whole-rock isochron age of 1,230 ± 20 Ma. Three samples were selected for further study: #273390 represents the dark border (?) dolerite; #273392 and #273398 are ‘red dolerites’.

Zig-Zag Dal 1. At 80°56.6′N–26°19.9′W a thick (> 100 m; top not exposed) subhorizontal sheet of variable doleritic rocks crops out in a steep mountain side. At 80 m above the base there is, over an interval of some 2 m, a gradation from normal dark-grey dolerite to red doleritic rocks. Sample #273482 was collected from near the lower contact (+1.5 m), #273487 (+60 m) is a grey dolerite, and #273495 is a brick-red granophyric dolerite from higher up (> +100 m; precise height not known).

Kap Ejnar Mikkelsen 2. At 81°55.9′N–31°53.6′W a steeply inclined sheet of red granophyric rocks, with numerous inclusions of quartzite, occurs between a major dolerite and sandstones; contacts are not exposed. Samples #273214 and #273219 were chosen at random from a larger collection of very similar samples taken at this locality.

Astrup Fjord. A ∼95 m thick, subhorizontal intrusive sheet is exposed within sandstone at 81°46.8′N–29°29.4′W, 10 km south of Astrup Fjord. It consists of 20 m dolerite at the bottom, followed by a thin zone (5 m) of granophyric dolerite, ∼65 m of rheopsammite and, at the top, 5 m of dolerite. Contacts between the dolerites and the sandstones are sharp and chilled; all contacts within the intrusion are gradational over short distances. Sample #197402 represents the lower dolerite (5 m above the contact), #197405 is a red granophyric dolerite (+24 m), #197406 (+25 m) represents the transition between the red granophyric rock and the rheopsammites, and #197408 (+80 m) is a rheopsammite.

Vildtland 2. At 81°43.5′N–32°40.6′W a 25-m thick sheet of rheopsammitic rocks with doleritic borders is well exposed. The lower dolerite (5 m) is chilled against the sandstones and has a gradual transition over ∼50 cm into rheopsammite. The upper dolerite is 1.5 m thick; it has a sharp chilled contact towards the sandstones and a narrow transitional contact with the central rheopsammites. Sample #273364 (+3 m) is a dolerite; #273367 (+10 m) and #273370 (+18 m) are typical rheopsammites. A chemical profile through the intrusion is shown in Kalsbeek and Jepsen (1983).

Zig-Zag Dal 2. At 80°57.1′N–26°17.5′W several flat-lying sheets of rheopsammite are rather poorly exposed in subhorizontal outcrops, and their geometry is therefore not well known. At least three separate intrusions, a few tens of metres across, and each with dark doleritic borders, are present. The two samples selected for this study, #273525 and #273532, come from two different intrusions.

Crystalline basement and sandstones. Three samples from basement rocks were taken from glacially transported boulders, which are scattered over the whole study area, and originate from underneath the Inland Ice. Representative sandstone samples were collected from various localities within the region.