Independent ages of magmatic and hydrothermal activity in alkaline igneous rocks: The Motzfeldt Centre, Gardar Province, South Greenland

Abstract

The Motzfeldt Centre is one of four major alkaline centres belonging to the Igaliko complex, part of the Mid-Proterozoic Gardar province of South Greenland. Motzfeldt comprises syenites and nepheline syenites displaying complex magmatic and subsolidus histories. Ta, Nb and REE-rich zones are associated with a pervasive hydrothermal alteration process in the North and East of the complex, part of a subdivision of the Centre called the Motzfeldt Sø Formation (MSF). The zircons from the MSF show textures that are both magmatic and altered, whereas the textures of pyrochlore have a predominantly subsolidus chemistry. We compare and contrast the radiogenic isotope geochemistry of the zircon and pyrochlore to obtain independent age estimates of the magmatic and hydrothermal episodes. Selected analyses of magmatic zircons are concordant with an age of 1,273 ± 6 Ma. Altered zircons are normally discordant with one intercept at 1,261 ± 28 Ma and the other at 349 ± 49 Ma. This younger age would appear on face value to date the hydrothermal event at Motzfeldt to *900 Ma after the magmatism. However, the Pb-Pb isotope systematics from altered pyrochlore define an isochron age of 1,267 ± 6 Ma for isotopic closure of the pyrochlore following alteration during the subsolidus. There is no known magmatic event in Southern Greenland at *350 Ma and discordant points have high common lead. We therefore infer that the younger intercept age is spurious. The overlap between the magmatic U-Pb zircon and the Pb-Pb pyrochlore age suggests that alteration of the Motzfeldt centre occurred shortly after magmatism.

From Mantle to Motzfeldt: A genetic model for syenite-hosted Ta,Nb-mineralisation

“…5). There is no significant difference between data in 'fresh' and 'altered' regions, in contrast to U-Pb data (McCreath et al 2012) where altered regions are predominantly discordant. A two-tailed student's 't' test assuming equal variance on the means of altered and unaltered initial 176 Hf/ 177 Hf ratios shows no significant difference (P=0.36 and is >> 0.05).…”

Europe's rare earth element resource potential: An overview of REE metallogenetic provinces and their geodynamic setting

“…Formation of REE deposits in alkaline to peralkaline igneous rocks and carbonatites is typically due to magmatic and/or hydrothermal processes (Wall and Mariano, 1996;Kogarko et al, 2002;Salvi and Williams-Jones, 2006;Schilling et al, 2011;Sheard et al, 2012;McCreath et al, 2012). Alkaline silicate and carbonatite magmatism are associated with small degrees of partial melting of enriched mantle, Table 1 Significant localities or groups of localities described in this paper, classified as resource (those with a formal REE resource estimate compliant with the JORC or NI-43-101 reporting codes); deposit (those for which an economic resource is likely to be present and may be identified by future exploration); occurrence (those in which the REE are enriched but which are unlikely to be economic); and by-product (those in which the REE could be economic as a by-product of another commodity).…”

“…However, the mineralisation is associated with the latest intrusive phase, comprising agpaitic peralkaline sheets that cut hydrothermally altered nepheline syenites, largely around the margins of the centre. Metasomatic alteration in older syenites was related to these latest, highly-fractionated agpaitic magmas (McCreath et al, 2012). The potential resources of REE and HFSE are thus hosted both in the altered syenites and in the peralkaline sheets themselves.…”

Metallogeny of Greenland

“…The syenite intrusions are cross cut by sub-horizontal microsyenite sheets, and syeno-gabbro and larvikite dikes ( Fig. 25; Tukiainen et al, 1984;McCreath et al, 2012). The magmatic evolution started with crystallization of syenite containing amphibole, clinopyroxene, feldspar, nepheline, Fe-Ti oxides, zircon, apatite, fluorite and locally olivine at 850-600°C and 1-2 kbar from a melt sourced in the mantle with only little crustal contamination (Schönenberger and Markl, 2008).…”

“…Hydrothermal alteration and ZrNb-Ta mineralization are considered to be related to the intrusion of microsyenite sheets and devolatilization of the volatile-and incompatible element-rich magmas (Thomassen, 1988). This hydrothermal alteration was estimated by Pb-Pb dating of pyrochlore at 1267 ± 6 Ma, which is within error with the age of syenite intrusion at 1273 ± 6 Ma (U-Pb zircon age) (McCreath et al, 2012). The hydrothermal fluid evolved from an originally reducing magmatic fluid into oxidized conditions probably due to intense mixing with meteoric water, stabilizing hematite in the alteration assemblage (Schönenberger and Markl, 2008).…”