Mesoarchaean Peridotite-Norite Cumulates of Sw Greenland – the Miaggoq Ultramafic Complex
Aliz ZemenyCarson KinneyChris YakymchukChristopher L. KirklandNicholas J. GardinerKristoffer SzilasHugo K.H. Olierook
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Abstract:
Several studies focused on the ultramafic bodies of the Archaean continental crust in southern Greenland in order to gain information on early Earth petrogenetic, metamorphic and metasomatic processes. This research provides the first petrological dataset of the Miaggoq Ultramafic Complex (~1 km2) in the Akia terrane, with a minimum age of 2997 ± 15 Ma. It comprises ultramafic (dunite, peridotite) and mafic (orthopyroxenite, norite) rocks along with chromitites and provides a window into Mesoarchaean mantle compositions. Field observations, such as chromitite bands, mineral layering, and orthopyroxenite oikocrysts in peridotites coupled with chemical analysis displaying high abundance of chromites in the dunitic rocks and high forsterite contents (Mg# 91 to 92.5) of the olivines, all point to a layered cumulate origin for the Miaggoq body. Pseudosection calculations along with geothermometry estimations reveal peak metamorphic conditions of 850–1100 °C at pressures of 0.7–1.25 GPa under anhydrous conditions followed by a possible metamorphic overprint at 650–800°C and 0.7 GPa with relatively dry melting (0.025–0.125 wt.% H2O). MELTS fractional crystallization coupled with cumulate modelling approximated the compositional trends with conditions on ~3 kbar with 1 wt.% H2O. This research concludes that the Miaggoq body represents a layered cumulate complex that was derived by large degrees of partial melting of the mantle with possible assimilation (synonymous with contamination) of basalts in the crust. Overall, this study provides complementary data for the topic of Mesoarchaean cumulate bodies of the Akia Terrane and their petrological processes.Keywords:
Peridotite
Ultramafic rock
Norite
Ultramafic rock
Norite
Layered intrusion
Peridotite
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Ultramafic rock
Felsic
Mantle plume
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Chromite
Chromitite
Pegmatite
Norite
Layered intrusion
Massif
Pentlandite
Dike
Ultramafic rock
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Peridotite
Metasomatism
Chromite
Massif
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Four ultramafic lamprophyre to kimberlite intrusive sites have been recognized in the Archean Nain Province of northern Labrador. Two sites are located within the Hopedale Block, at (1) capes Aillik–Makkovik and (2) Ford's Bight, and two are located within the Saglek Block, in the (3) Saglek and (4) northern Torngat Mountains areas. To date, no diamonds have been found in either intrusive site, but geochemical analysis has been, at best, rudimentary. Group 1 has been previously described as kimberlitic, at least in part, but, to date, no detailed geochemical evaluation of diamond indicator minerals has been completed. The intrusive rocks of Group 2 have only recently been identified as part of a possible kimberlite diatreme; there are no whole-rock or mineral geochemical data yet derived for this suite. The Group 3 suite has been defined as kimberlitic for some time, but spatial and geochemical data have been restricted to unpublished reports. Group 4 dykes were originally classified as ultramafic lamprophyres, but mineral assemblages of some dykes indicate that they are macrocrystal hypabyssal phlogopite–perovskite archetypal kimberlites, whereas the remainder are olivine–phlogopite–calcite ultramafic lamprophyres, possibly melilitites. Sample size, however, was very small for definitive determinations, and one kimberlite has a strike extension classified as lamprophyre. Within the kimberlites, the following minerals were identified: (1) lherzolitic, eclogitic and megacrystal garnets (megacrystal garnets were also found in some of the lamprophyres), (2) clinopyroxenes, which plot in Fipke's diamondiferous CPX domain (as does one lamprophyre), (3) orthopyroxenes with < 1% to 1.5 % Al2O3, and (4) olivines with Fo contents of ~ 88-92 (some of the lamprophyres have two populations of olivine with clusters of similar high Fo values). Orthopyroxene and clinopyroxene geothermobarometers suggest a possible harzburgite chemistry for some mineral separates, and also that the postulated geotherm for the intrusive history of the dykes is permissive of diamond stability.
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Phlogopite
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Abstract The Upper Critical Zone of the Rustenburg Layered Suite (RLS) in the Swartklip Sector, north-western Bushveld Complex, is considerably attenuated relative to other parts of the Complex. The interval between the UG2 chromitite and the Merensky Reef amounts to as little as 25 m in places. Within this interval, the aggregate thickness of orthopyroxenite-dominated ultramafic layers hosting the UG1 and UG2 chromitites and the Merensky and Bastard reefs does not differ significantly from the area around Rustenburg, to the south. The total thickness of ultramafic lithologies is, in fact, increased by the presence of the 3 to 5 m thick olivine-rich Pseudo Reef Unit, which is developed between the UG2 and Merensky Reef units in the Swartklip Sector, but does not occur in any significant form elsewhere in the Bushveld intrusion. The substantial thinning of the succession is due almost entirely to the fact that plagioclase-rich rocks (norite and anorthosite) between the ultramafic layers are radically thinned in the Swartklip Sector relative to virtually all other parts of the Bushveld Complex. The ultramafic layers, although dominated by orthopyroxenite, are characterized by higher proportions of olivine than in other parts of the Bushveld Complex. In our logging of the substantial number of exploration drill cores that form the basis of this study, we have found it expedient to define stratigraphic units that are either exclusively plagioclase-rich (norite and anorthosite) or plagioclase-poor (consisting of varying proportions of orthopyroxenite, harzburgite and chromitite). This effectively binary system of lithological classification has no overt genetic connotations. Our nomenclature has, in fact, enabled us to rigorously document the nature of contacts between ultramafic and plagioclase-rich units, and thus to identify unconformities between the ultramafic units (orthopyroxenite and harzburgite) and intervening noritic and anorthositic units, which have in the past been ascribed to localized thermo-chemical erosion of pre-existing plagioclase-rich cumulates. Apart from the well-documented evidence of erosional unconformities at the basal contacts of ultramafic units, we also provide evidence for unconformities at the tops of these units.
Chromitite
Ultramafic rock
Anorthosite
Norite
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Chromite
Pyroxene
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The article presents the results of petrostructural and mineralogical studies of olivine grains from ultrabasic and basic rocks of different genesis. In particular, they correspond to cumulative dunites of the Yoko-Dovyren layered massif (Northern Cisbaikalia), restite hyperbasites of the Velvet massif (Kuznetskiy Alatau), and xenoliths of peridotites from basalts of the Canary Islands (Spain). The relationship between the petrostructural and mineralogical features of ultrabasic rocks is shown, which makes it possible to identify their cumulative and restite varieties. An important element of the petrostructure of ultrabasites is the orientation of olivine, which reflects either the conditions for the formation of primary crystals in the magmatic melt, or the conditions for its recrystallization as a result of plastic deformations during exhumation to the Earth's surface. The mineral composition of rocks is an additional feature that reflects the real conditions of formation. In the presence of basic plagioclase, it is already quite difficult to speak about the restitic nature of these ultrabasic rocks. On the other hand, plastic deformations of olivine are characteristic of restitic hyperbasites, in which plagioclase is absent. The results of mineralogical studies in ultrabasic xenoliths of the Canary Islands showed the presence of basic plagioclase (labradorite), as well as chrysolite-type olivine (12-16 Fa), which corresponds to the attributes of ultrabasic layered intrusions such as the Yoko-Dovyren dunite-troctolite-gabbro pluton. For restite hyperbasites (by the example of the Kuznetsk Alatau), the iron content of olivine does not exceed 9-10% of the fayalite end, while plagioclase and clinopyroxene are absent. A characteristic feature of the xenoliths of the Canary Islands is the presence of endiopside, which is present in layered intrusions, but is absent in the restrained fragments of the upper and lower mantle. It is assumed that the xenoliths in the basalts of the Canary Islands are not of a mantle nature, but are fragments of a deep magma chamber. The studies of the optical orientation of olivine in xenoliths of the rocks of Lanzarote Island confirm these conclusions. The geochemical parameters of ultramafic xenoliths on Tenerife Island may well correspond to deeper formations.
Ultramafic rock
Massif
Anorthosite
Norite
Xenolith
Pyroxene
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Peridotite
Xenolith
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Diorite
Anorthosite
Norite
Massif
Charnockite
Porphyritic
Metasomatism
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