Stable (C, O) and radiogenic (Sr, Nd) isotopic evidence for REE-carbonatite formation processes in Petyayan-Vara (Vuoriyarvi massif, NW Russia)
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Keywords:
Carbonatite
Massif
Radiogenic nuclide
Metasomatism
Ultramafic rock
Petrogenesis
Carbonatites in association with Cretaceous ultramafic and alkaline rocks are exposed in Sung Valley, East Khasi Hills, Meghalaya. Carbonatites and ultramafic rocks have been petrographically and geochemically studied. Ultramafic rocks are mostly pyroxenites, while the carbonatites are apatite-carbonatites and silico-carbonatites. Pyroxenites are characterized by high SiO2 (avg. 52 wt.%), high MgO (avg. 16 wt.%), low Nb (avg. 4 ppm), low Zr (avg. 45 ppm) compared to carbonatites. The carbonatites are characterized by low to moderate SiO2 (39–44 wt.%), high CaO (13.3 to 24.3 wt.%), low MgO (2 to 10 wt.%), high Nb (19–39 ppm) and high Zr (342–822 ppm) contents. The concentration of total Rare Earth Element (©REE) in carbonatite ranges from (520 to 820 ppm). ©REE for pyroxenites averages 101 ppm. Primordial Mantle (PM) normalized multi-elemental patterns for the pyroxenites and carbonatites show flat patterns along with negative anomalies at Sr, Ti and Y. Both the rock types display an enriched pattern compared to PM though the carbonatites display a higher level of enrichment (10–90 times PM). Chondrite normalized REE patterns of carbonatite display a flat pattern [(La/Yb)N =1.6–9.8] along with negative anomalies at Nd. Similar fractionated REE patterns are also displayed by pyroxenites [(La/Yb)N = avg. 4.5] but with a prominent negative Eu anomaly. Carbonatites display a higher level of REE enrichment in both the rock types (100 to 800 times chondrite). Geochemical characteristics therefore suggest that magmas for the pyroxenites as derived from a garnet bearing deeper mantle; while that for the carbonatites was derived from a carbonated peridotite.
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Ultramafic rock
Petrogenesis
Rare-earth element
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Petrogenesis
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Petrogenesis
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Carbonatite
Metasomatism
Massif
Ultramafic rock
Kola peninsula
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The introductions to each of the preceding
chapters (Sections I) have reviewed the conclusions reached, and provided
some cross references to other chapters.
This final contribution reviews in general
and historical terms some of the problems of the petrogenesis of ultramafic and ultrabasic
rocks. The greater part of the review
deals with the petrogenesis of the alpine-type
ultramafic rocks, because these have
served as a focus of controversy for many
years. The variety of ultramafic rock associations
in different tectonic environments,
and of the processes involved in
their formation, have been emphasized
throughout the book. Both are important
in considering petrogenesis. Although interpretation
of processes may require extensive
study, rock associations are more
easily distinguished; but the distinctions
have not always been made. Hess (1955,
p. 394) drew attention to this with the following
statement: Gross errors have probably
resulted from applying conclusions
drawn from facts related to mica peridotites
to alpine peridotites and vice versa.
If mica peridotite had been called humpty-dumptyite,
these probably would not have
arisen.
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The Tapira alkaline complex is the southernmost of a series of carbonatite- bearing intrusions occurring in the Alto Paranaiba region, western Minas Gerais State, Brazil. Together with kamafugites, lamproites and kimberlites, these complexes form part of the Late-Cretaceous Alto Paranaiba Igneous Province (APIP). The Tapira igneous complex is emplaced into rocks of the Late-Proterozoic Brasilia mobile belt, adjacent to a major cratonic area (the Sāo Francisco craton).The complex is formed by the amalgamation of several intrusions, comprising mainly ultramafic rocks (wehrlites and bebedourites), with subordinate syenite, carbonatite and melilitolite. At least two separate units of ultramafic rocks (B1 and B2) and five episodes of carbonatite intrusion (CI to C5) are recognised. The plutonic rocks are crosscut by fine-grained ultramafic and carbonatite dykes. Two varieties of ultramafic dykes are recognised: phlogopite-picrites are the most primitive rocks in the complex; low-Cr dykes are more evolved, and typically lack olivine. The ultramafic dykes are carbonate-rich, and may contain carbonate ocelli, indicating that immiscibility of carbonatite liquid occurred early in the evolution of the complex. The ultramafic dykes are chemically similar to the APIP kamafugites. The primitive Tapira magmas underwent some differentiation in the crust, before their final emplacement. Crystal fractionation from the phlogopite-picrite magma may have produced olivine and chromite-rich cumulates, but these rocks are under- represented in the complex. Crystal fractionation from low-Cr dykes may have produced the bebedourites. The Tapira complex contains examples of carbonatites that originated by either liquid immiscibility or crystal fractionation. These contrasting petrogenetic mechanisms have produced distinct geochemical and mineralogical signatures, which have been used to pinpoint specific events in the evolution of the complex, and to test the consanguinity of carbonatites and associated silicate rocks.
Carbonatite
Ultramafic rock
Phlogopite
Chromite
Large igneous province
Silicic
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Ultramafic rock
Massif
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Abstract Numerous ultramafic rocks occur as lens-shaped bodies in the Archaean continental crust exposed in southern West Greenland. As some of the oldest exposed ultramafic bodies, determining their origin, as mantle segments or magmatic cumulates, is an important yet controversial issue. The origin of these Archaean ultramafic rocks remains unclear, in-part because these rocks have undergone metasomatic modification since their formation, yet the effects of this metasomatism have so far not been assessed in detail, despite being crucial for understanding their geochemical evolution. Here, we examined the petrology, mineral and whole-rock chemistry of the largest ultramafic body located within the Mesoarchaean Akia terrane, known as the Ulamertoq ultramafic body, to elucidate the poly-metamorphic and metasomatic events that overprinted the protolith. Pronounced lithologic zoning from hydrous mineral-rich layers to orthopyroxene-rich ultramafic rocks at the boundaries between ultramafic rocks and the granitoid country rocks was formed locally by metasomatic reactions related to the granitoids. The main body of ultramafic rocks, far from the contacts, can be classified into four types based on mineral assemblage and chemistry. The fine-grained orthopyroxene aggregates and large poikilitic orthopyroxenes have low Cr2O3 and CaO contents, suggesting a secondary origin. Trace element compositions of orthopyroxene and/or amphibole in the main ultramafic rocks indicate that at least three types of metasomatic agents were required to form these minerals and the associated whole-rock chemical variations within the ultramafic body. Variations represent differences in the proportions of metasomatic orthopyroxene and/or amphibole and phlogopite added to a dunitic protolith. The main body of Ulamertoq ultramafics experienced metasomatism under granulite-facies. Retrograde cooling occurred, to 650°C–850°C and <1.8 GPa prior to local metasomatism via country-rock reaction. The presence of titanian clinohumite and its associated mineral assemblage in the least-metasomatised dunites suggest the possibility that the main ultramafic rocks went through a hydration/dehydration process at ~800°C–900°C and <2 GPa prior to metasomatic modification. This study demonstrates that it is important to consider the effects of multi-stage metasomatism and metamorphism in order to elucidate the origin of the Archaean ultramafic rocks in Greenland and elsewhere.
Ultramafic rock
Metasomatism
Amphibole
Peridotite
Phlogopite
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Carbonatite
Ultramafic rock
Metasomatism
Phlogopite
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