Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

1999 ◽  
Vol 134 (4) ◽  
pp. 325-341 ◽  
Author(s):  
Friedrich Lucassen ◽  
Sven Lewerenz ◽  
Gerhard Franz ◽  
José Viramonte ◽  
Klaus Mezger
Keyword(s):  
Lower Crustal ◽  
Granulite Xenoliths

Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths: mechanisms and rates

2009 ◽  
Vol 159 (3) ◽  
pp. 293-314 ◽  
Author(s):  
Júlia Dégi ◽  
Rainer Abart ◽  
Kálmán Török ◽  
Enikő Bali ◽  
Richard Wirth ◽  
...  
Keyword(s):  
Mafic Granulite ◽  
Lower Crustal ◽  
Granulite Xenoliths

Lower crustal contamination of Deccan Traps magmas: evidence from tholeiitic dykes and granulite xenoliths from western India

2008 ◽  
Vol 93 (3-4) ◽  
pp. 243-272 ◽  
Author(s):  
A. G. Dessai ◽  
H. Downes ◽  
F.-J. López-Moro ◽  
M. López-Plaza
Keyword(s):  
Crustal Contamination ◽  
Deccan Traps ◽  
Western India ◽  
Lower Crustal ◽  
Granulite Xenoliths

scholarly journals Eclogite and Garnet Pyroxenite Xenoliths from Kimberlites Emplaced Along the Southern Margin of the Kaapvaal Craton, Southern Africa: Mantle or Lower Crustal Fragments?

2020 ◽  
Vol 61 (4) ◽  
Author(s):  
Anton Le Roex ◽  
Christel Tinguely ◽  
Michel Gregoire
Keyword(s):  
Southern Africa ◽  
Kaapvaal Craton ◽  
Crustal Shortening ◽  
Southern Margin ◽  
Accessory Phase ◽  
Garnet Granulite ◽  
Crustal Origin ◽  
Lower Crustal ◽  
Ree Patterns ◽  
Granulite Xenoliths

Abstract Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000 °C, at pressures of 1·7 ± 0·4 GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55 km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835 °C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2 Ga.

Lower crustal granulite xenoliths in carbonatite volcanoes of the Western Rift of East Africa

1987 ◽  
Vol 51 (363) ◽  
pp. 621-633 ◽  
Author(s):  
C. W. Thomas ◽  
P. H. Nixon
Keyword(s):  
East Africa ◽  
Alkali Feldspar ◽  
Fractional Crystallisation ◽  
Metamorphic Grade ◽  
Eu Anomaly ◽  
South West ◽  
Garnet Granulite ◽  
Lower Crustal ◽  
Basaltic Melts ◽  
Granulite Xenoliths

AbstractSporadic small garnet granulite and two-pyroxene granulite xenoliths found in the carbonatite tuffs and lavas near Fort Portal, South West Uganda, are chiefly silica-saturated and rich in Al2O3 (> 20 wt. %) and Na2O (c. 2 to 4 wt. %). Three REE patterns are distinguished: LREE enriched—HREE depleted with a positive Eu anomaly; LREE depleted—HREE relatively enriched and flat; and LREE slightly enriched with a very weak Eu anomaly and high overall REE. The xenoliths are considered to represent original basaltic melts and fractional crystallisation products, varying with the dominance of the clinopyroxene, plagioclase or olivine crystallising phase. It is thought that REE abundances were established before metamorphism.The clinopyroxenes are low-jadeitic augites, the orthopyroxenes, aluminous hypersthenes and the garnets, pyrope-almandine with constant grossularite. Plagioclase varies with increasing metamorphic grade from labradorite to andesine-oligoclase. Scapolite (meionite), alkali-feldspar, quartz, mica, amphibole, rutile and apatite are minor phases and some appear to be metasomatic.Calculated temperatures of metamorphic equilibration range from 580 to 800°C at pressures > 4 kbar for the two-pyroxene granulites and > 6 kbar for the garnet granulites. A known geophysical discontinuity marking a density change at 16 km in the Western Rift may be due to the presence of two-pyroxene granulite, calculated to become garnet-bearing at depths greater than 23 km. The absence of proven omphacite-bearing eclogite xenoliths (with no plagioclase) indicates that the greatest depth of crustal sampling by the carbonatite in the Fort Portal field is about 25 km which could be the depth of the Moho in this area.

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