PRESSURE-TEMPERATURE EVOLUTION OF LOWER CRUSTAL-UPPER MANTLE XENOLITHS FROM THE COLORADO PLATEAU TRANSITION ZONE

AbstractThe Transdanubian Volcanic Region (TVR) is composed mainly of Pliocene alkali basalts, basanites, olivine basalts and olivine tholeiites, as well as rare nephelinites. The partial melting and genesis of alkali basaltic liquids is a consequence of an upwelling of the upper mantle which also caused thinning of the lithosphere and recent sinking of the Pannonian Basin.Four different types of lower crustal and upper-mantle xenoliths are found within the TVR: garnet-free and garnet-bearing granulites, clinopyroxenites and spinel lherzolites. We present mineralogical and geochemical data on granulite facies and clinopyroxenite xenoliths from three localities in the Hungarian part of the TVR (Bondoróhegy, Szentbékálla and Szigliget). It is concluded that, whilst the protoliths of the granulite facies xenoliths were tholeiitic igneous rocks and could be part of an ancient crust, the clinopyroxenite xenoliths represent recent underplating and may have formed from an alkali basaltic liquid similar to the host lava. Planar contact relations between clinopyroxenites and spinel lherzolites as observed in composite xenoliths, as well as high Al-contents in clinopyroxenes, point to a high-pressure genesis in the upper mantle for these rocks. In contrast, geobarometrical estimates yielded only a moderate pressure range characteristic of lower crustal levels for the garnet-free granulite xenoliths (7–9 kbar). Nevertheless, two-pyroxene geothermometry yielded high temperatures of equilibration (>900°C) for these xenoliths, probably caused by advective heat transfer connected with underplating and in agreement with the high present-day geothermal gradient of this region. In the Central Range localities only garnet-free granulite xenoliths occur, whereas at the border of the TVR both garnet-free and garnet-bearing granulite facies nodules are found. It is possible that the incoming of garnet is retarded by higher temperatures in the lower crust below the Central Range.It is also suggested that the difference in seismically measured crustal thickness between the Central Range and adjacent basin areas may be connected with different thermal conditions below these regions and that the seismically defined Moho and the petrological Moho do not necessarily coincide.

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Evidence for an upper mantle low velocity zone beneath the southern Basin and Range-Colorado Plateau transition zone

Geophysical Research Letters ◽

10.1029/93gl01660 ◽

1994 ◽

Vol 21 (7) ◽

pp. 509-512 ◽

Cited By ~ 18

Author(s):

H. M. Benz ◽

J. McCarthy

Keyword(s):

Transition Zone ◽

Upper Mantle ◽

Colorado Plateau ◽

Basin And Range ◽

Low Velocity ◽

Low Velocity Zone ◽

Southern Basin ◽

Velocity Zone

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Compositional data on partial melt products in upper mantle and lower crustal xenoliths and their host basaltic rocks, world localities and Cima volcanic field, San Bernardino County, California

Open-File Report ◽

10.3133/ofr95585 ◽

1995 ◽

Author(s):

A.V. McGuire ◽

Howard Gordon Wilshire

Keyword(s):

Upper Mantle ◽

Compositional Data ◽

Volcanic Field ◽

San Bernardino County ◽

Basaltic Rocks ◽

Partial Melt ◽

San Bernardino ◽

Crustal Xenoliths ◽

Lower Crustal ◽

Cima Volcanic Field

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Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

Scientific Reports ◽

10.1038/s41598-021-90844-w ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Costanza Bonadiman ◽

Valentina Brombin ◽

Giovanni B. Andreozzi ◽

Piera Benna ◽

Massimo Coltorti ◽

...

Keyword(s):

Oxygen Fugacity ◽

Upper Mantle ◽

Mantle Xenoliths ◽

Spinel Peridotite ◽

Simultaneous Formation ◽

Empirical Potentials ◽

Elastic Data ◽

Mineralogical Study ◽

Semi Empirical ◽

T Locus

AbstractThe occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict that the P–T locus of the equilibrium reaction pargasite + forsterite = Na-phlogopite + 2 diopside + spinel, over the range 1.3–3.0 GPa/540–1500 K, yields a negative Clapeyron slope of -0.003 GPa K–1 (on average). The intersection of the P–T locus of supposed equilibrium with the new mantle geotherm calculated in this work allowed us to state that the Mount Leura xenoliths achieved equilibrium at 2.3 GPa /1190 K, that represents a plausible depth of ~ 70 km. Metasomatic K-Na-OH rich fluids stabilize hydrous phases. This has been modelled by the following equilibrium equation: 2 (K,Na)-phlogopite + forsterite = 7/2 enstatite + spinel + fluid (components: Na2O,K2O,H2O). Using quantum-mechanics, semi-empirical potentials, lattice dynamics and observed thermo-elastic data, we concluded that K-Na-OH rich fluids are not effective metasomatic agents to convey alkali species across the upper mantle, as the fluids are highly reactive with the ultramafic system and favour the rapid formation of phlogopite and amphibole. In addition, oxygen fugacity estimates of the Mount Leura mantle xenoliths [Δ(FMQ) = –1.97 ± 0.35; –1.83 ± 0.36] indicate a more reducing mantle environment than what is expected from the occurrence of phlogopite and amphibole in spinel-bearing peridotites. This is accounted for by our model of full molecular dissociation of the fluid and incorporation of the O-H-K-Na species into (OH)-K-Na-bearing mineral phases (phlogopite and amphibole), that leads to a peridotite metasomatized ambient characterized by reduced oxygen fugacity.

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