Geological mapping and diamond exploration in northern Quebec and Labrador has revealed an undeformed ultramafic dyke swarm in the northern Torngat Mountains. The dyke rocks are dominated by an olivine-phlogopite mineralogy and contain varying amounts of primary carbonate. Their mineralogy, mineral compositional trends and the presence of typomorphic minerals (e.g. kimzeyitic garnet), indicate that these dykes comprise an ultramafic lamprophyre suite grading into carbonatite. Recognized rock varieties are aillikite, mela-aillikite and subordinate carbonatite. Carbonatite and aillikite have in common high carbonate content and a lack of clinopyroxene. In contrast, mela-aillikites are richer in mafic silicate minerals, in particular clinopyroxene and amphibole, and contain only small amounts of primary carbonate. The modal mineralogy and textures of the dyke varieties are gradational, indicating that they represent end-members in a compositional continuum.
The Torngat ultramafic lamprophyres are characterized by high but variable MgO (10–25 wt.%), CaO (5–20 wt.%), TiO2 (3–10 wt.%) and K2O (1–4 wt.%), but low SiO2 (22–37 wt.%) and Al2O3 (2–6 wt.%). Higher SiO2, Al2O3, Na2O and lower CO2 content distinguish the mela-aillikites from the aillikites. Whereas the bulk rock major and trace element concentrations of the aillikites and mela-aillikites overlap, there is no fractional crystallization relation between them. The major and trace element characteristics imply related parental magmas, with minor olivine and Cr-spinel fractionation accounting for intra-group variation.
The Torngat ultramafic lamprophyres have a Neoproterozoic age and are spatially and compositionally closely related with the Neoproterozoic ultramafic lamprophyres from central West Greenland. Ultramafic potassic-to-carbonatitic magmatism occurred in both eastern Laurentia and western Baltica during the Late Neoproterozoic. It can be inferred from the emplacement ages of the alkaline complexes and timing of Late Proterozoic processes in the North Atlantic region that this volatile-rich, deep-seated igneous activity was a distal effect of the breakup of Rodinia. This occurred during and/or after the rift-to-drift transition that led to the opening of the Iapetus Ocean. 相似文献
Sixteen kimberlite boulders were collected from three sites on the Munro and Misema River Eskers in the Kirkland Lake kimberlite field and one site on the Sharp Lake esker in the Lake Timiskaming kimberlite field. The boulders were processed for heavy-mineral concentrates from which grains of Mg-ilmenite, chromite, garnet, clinopyroxene and olivine were picked, counted and analyzed by electron microprobe. Based on relative abundances and composition of these mineral phases, the boulders could be assigned to six mineralogically different groups, five for the Kirkland Lake area and one for the Lake Timiskaming area. Their indicator mineral composition and abundances are compared to existing data for known kimberlites in both the Kirkland Lake and Lake Timiskaming areas. Six boulders from the Munro Esker form a compositionally homogeneous group (I) in which the Mg-ilmenite population is very similar to that of the A1 kimberlite, located 7–12 km N (up-ice), directly adjacent to the Munro esker in the Kirkland Lake kimberlite field. U–Pb perovskite ages of three of the group I boulders overlap with that of the A1 kimberlite. Three other boulders recovered from the same localities in the Munro Esker also show some broad similarities in Mg-ilmenite composition and age to the A1 kimberlite. However, they are sufficiently different in mineral abundances and composition from each other and from the A1 kimberlite to assign them to different groups (II–IV). Their sources could be different phases of the same kimberlite or—more likely—three different, hitherto unknown kimberlites up-ice of the sample localities along the Munro Esker in the Kirkland Lake kimberlite field. A single boulder from the Misema River esker, Kirkland Lake, has mineral compositions that do not match any of the known kimberlites from the Kirkland Lake field. This suggests another unknown kimberlite exists in the area up-ice of the Larder Lake pit along the Misema River esker. Six boulders from the Sharp Lake esker, within the Lake Timiskaming field, form a homogeneous group with distinct mineral compositions unmatched by any of the known kimberlites in the Lake Timiskaming field. U–Pb perovskite age determinations on two of these boulders support this notion. These boulders are likely derived from an unknown kimberlite source up-ice from the Seed kimberlite, 4 km NW of the Sharp Lake pit, since indicator minerals with identical compositions to those of the Sharp Lake boulders have been found in till samples collected down-ice from Seed. Based on abundance and composition of indicator minerals, most importantly Mg-ilmenite, and supported by U–Pb age dating of perovskite, we conclude that the sources of 10 of the 16 boulders must be several hitherto unknown kimberlite bodies in the Kirkland Lake and Lake Timiskaming kimberlite fields. 相似文献
Abstract Calcretes can be observed on the surface of old moraines around Batura Glacier in the upper Hunza Valley, Karakoram Mountains, Pakistan. They develop as a calcareous crust cementing small gravels under boulders. In order to understand the genesis of the calcrete crust, a variety of methods were employed: (i) study of mineralogy and geochemistry of a calcrete crust precipitated on the lateral moraine using X-ray diffractometer and electron probe microanalysis; (ii) analysis of solute chemistry of surface water and ice bodies around the Batura Glacier; and (iii) accelerator mass spectrometry 14C dating of the crust itself. The results indicate that the calcrete crust has definite laminated layers composed of a fine-grain and compact calcite layer, and a mineral fragment layer. The chemical composition of the calcite layer is approximately 60% CaO and 1% MgO. The mineral fragment layer consists of rounded grain materials up to 0.2 mm in diameter. It shows a graded bedding structure with fine grains of quartz, albite and muscovite. Meanwhile, as the Paleozoic Pasu limestone is distributed around the terminal of Batura Glacier, Ca cations dissolve in the melt water of the glacier. Accordingly, the calcrete crust is precipitated by decreases in CO2 partial pressure from glacier ice and evaporation of the melt water, including high concentration of Ca2+ at ephemeral streams and small ponds stagnating between the moraine and glacial ice. On the basis of the AMS 14C age, the calcrete is considered to have formed approximately 8200 calibrated years bp under the Batura glacial stage. 相似文献
Palaeomagnetic data are presented from the southern Volodarsk-Volynsky Massif (VVM) of the Korosten Pluton, the Ukrainian Shield. Laboratory experiments (AF and thermal demagnetization, IRM acquisition, thermal separation), field tests (consistency and secular variation methods) and optical observations indicate that single domain and nearly single domain magnetite is the dominant carrier of a primary TRM in the anorthosites. Palaeomagnetic poles from the three sampling sites (Golovino and Turchinka quarries) are indistinguishable at the 95% confidence level and have been combined to yield a mean pole at Plat = 30 °N, Plon = 178 °E, a95 = 3.4 °.In the large slow cooling Korosten Pluton the U-Pb zircon/baddeleyite (Uzb) technique gives an age for the anorthosites, which are not equivalent to the time of magnetic blocking. Based on integrated analysis of geochronologic information and blocking-temperature data for magnetic minerals proposed by Briden et al. (1993), a first attempt has been undertaken to estimate the palaeomagnetic pole age from the Mesoproterozoic anorthosites. The Korosten Pluton has cooled from 850 °C (the closure temperature of U-Pb systematics in zircon/baddeleyite) to 350 °C (the closure temperature of K-Ar systematics in biotite) during 150 Ma after the emplacement of the anorthosites. Assuming a uniform cooling of the intrusion yields a rate of 3.3 °C/Ma. The cooling rate for the granites is 3.1 °C/Ma. The mafic and acid rocks have an average rate of 3.2 °C/Ma. Using the cooling gradient for the VVM (3.2 °C/Ma) and the mean natural blocking temperature of magnetite (520 °C) can be determined a remanence age. The estimate for TRM acquisition is 1656 ± 10.0 Ma.The magnetic pole for the VVM is in good agreement with the mean pole from the Baltic quartz porphyry dykes with an age of 1630 – 1648 Ma. The VVM pole is best dated and requires a revision of the latest paleogeographic reconstructions for the Fennoscandian and Ukrainian Shields at 1770 and 1650 Ma. (Pesonen et al., 2003). 相似文献