Three metamorphic events in the precambrian P-T-t history of the Transangarian Yenisey ridge recorded in garnet grains in metapelites
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Riphean
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The Paleoproterozoic Foxe Fold Belt (FFB) is composed
of the Penrhyn Group, a Paleoproterozoic passive margin sequence,
and supracrustal Archean basement. These units were interfolded and
metamorphosed at amphibolite to lower granulite facies conditions
during the 1883-1865 Ma Foxe orogeny, a part of the Trans-Hudson
orogeny. The purpose of this study was to constrain the timing of
metamorphism and deformation within the Penrhyn Group, in order to
determine the role of the Foxe orogeny within the Trans-Hudson
orogeny. Petrographic analysis, P-T-X pseudosections, monazite
composition, and in-situ electron microprobe U-Th-Pb geochronology
from sampled metapelites were used to determine the age and
significance of metamorphic and deformation events related to
monazite age populations. Population 1 is composed of 1926 ± 8 Ma
monazite interpreted as detrital. Population 2 consists of 1892 ± 9
Ma monazite, the youngest detrital ages seen in the Penrhyn Group.
Population 3 is composed of 1853 ± 5 Ma high-Y + HREE monazite
predating an episode of pervasive garnet growth. Population 4
contains 1839 ± 8 Ma lower-Y + HREE monazite related to pervasive
garnet growth. Population 5 is 1819 ± 16 Ma lowest-Y + HREE
monazite with high LREE and Th/U, linked to the interpreted peak
reaction: Bt + Sil + Pl = Grt + Crd + Kfs + melt. Monazite
constraints on deformation fabrics indicate that deformation was
ongoing locally as early as 1853 ± 9 Ma and continued until at
least 1814 ± 14 Ma, pre- to syn-peak metamorphism. Rare 1794-1776
Ma monazite is interpreted to constrain the age of retrograde
metamorphism as the Trans-Hudson orogeny waned. These data support
interpreted clockwise P-T-t-D paths consistent with metamorphism
initiated by crustal thickening in an orogenic belt.
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Abstract The remnants of three major orogenic cycles have been recognised within the Lewisian complex of north-western Scotland, the Scourian, Inverian and Laxfordian orogenies having taken place, respectively, about 2600 + to 2460 m. y., 2200 to ?2000 m. y. and 1600 to 1300 m. y. ago. After the deposition of the Torridonian (and Moine) series, the Knoydartian orogeny took place about ?950 to 740 m. y. ago. Evidence from the Precambrian of Canada and the northern United States of America indicates the existence, of five orogenic episodes—2730 to 2450 m. y. (Kenoran), 2200 to 2100 m. y. (?Penokean), 1900 to 1700 m. y. (Hudsonian), 1520? to 1220? m. y. (Elsonian) and 1090 to 770 m. y. (Grenville). The marked correspondence of dates for the Precambrian orogenic episodes of Scotland and Canada and their similarity with those of peaks of world wide mineral ages (Gastil 1960) and with dates for orogenic episodes in Sweden (Welin 1966) and the Baltic Shield (Polkanov & Gerling 1961) suggests an overall correlation: Scourian—Kenoran—Saamian, Inverian—?Penokean—Belomorian, Hudsonian—Svecofennian (Karelian), Laxfordian—Elsonian—Gothian, Knoydartian—Grenville—Sveconorwegian—Riphean. Such a correlation may form the basis of a subdivision of the Precambrian.
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The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP–HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750–800 °C and 5–6 kbar, followed by near-isothermal decompression down to 3–4 kbar. The analysed monazites yielded 207Pb/206Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the ≥1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89–1.86 Ga fits well with the high-grade metamorphic event at 1.89–1.88 Ga observed elsewhere in southern and central Finland. The 1.87–1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83–1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80–1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones.
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Basal Gneiss Region, west Norway gives an Rb-Sr whole-rock isochron age of 1775 ±57 m.y. A foliated granodiorite in more homogeneous gneisses (Fetvatn gneiss) gives an age of 960 ±lO m.y. Similar ages from other areas indicate that most of the Basal Gneiss Region original ed during the Svecofennian and Sveconorwegian orogenies. The role of the Caledonian orogeny in the evolution of the Basal Gneiss Region remains unsettled. The 960 m.y. intrusion possesses a pronounced schistosity indicating recrys talliza tion in a stress field during the Caledonian orogeny or perhaps a late stage of the Sveconorwegian orogeny. The diminished role of the Caledonian orogeny in the formation of portions of the North Atlantic Caledonian System should modify theories for the evolution of this orogen.
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