Geochemistry of granulite-facies lower crustal xenoliths: implications for the geological history of the lower continental crust below the Eifel, West Germany

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

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Evolution of the lower continental crust: granulite facies xenoliths from the Eifel, West Germany

Nature ◽

10.1038/311368a0 ◽

1984 ◽

Vol 311 (5984) ◽

pp. 368-370 ◽

Cited By ~ 57

Author(s):

H.-G. Stosch ◽

G. W. Lugmair

Keyword(s):

Continental Crust ◽

Granulite Facies ◽

West Germany ◽

Lower Continental Crust

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Lower crustal zircons reveal Neogene metamorphism beneath the Pannonian Basin (Hungary)

Open Geosciences ◽

10.1515/geo-2015-0028 ◽

2015 ◽

Vol 7 (1) ◽

Cited By ~ 3

Author(s):

Hilary Downes ◽

Andrew Carter ◽

Richard Armstrong ◽

Gabor Dobosi ◽

Antal Embey-Isztin

Keyword(s):

Pannonian Basin ◽

Granulite Facies ◽

Blocking Temperature ◽

Late Devonian ◽

Alkali Basalts ◽

Volcanic Deposits ◽

Eruption Age ◽

Crustal Xenoliths ◽

Lower Crustal ◽

Variscan Basement

AbstractNeogene alkaline intraplate volcanic deposits in the Pannonian Basin (Hungary) contain many lower crustal granulite-facies xenoliths. U-Pb ages have been determined for zircons separated from a metasedimentary xenolith, using LA-ICPMS and SHRIMP techniques. The zircons show typical metamorphic characteristics and are not related to the hostmagmatism. The oldest age recorded is late Devonian, probably related to Variscan basement lithologies. Several grains yield Mesozoic dates for their cores, which may correspond to periods of orogenic activity. Most of the zircons show young ages, with some being Palaeocene-Eocene, but the majority being younger than 30Ma. The youngest zircons are Pliocene (5.1-4.2 Ma) and coincide with the age of eruptions of the host alkali basalts. Such young zircons, so close to the eruption age, are unusual in lower crustal xenoliths, and imply that the heat flow in the base of the Pannonian Basin was sufficiently high to keep many of them close to their blocking temperature. This suggests that metamorphism is continuing in the lower crust of the region at the present day.

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Contaminating melt flow in magmatic peridotites from the lower continental crust (Rocca d'Argimonia sequence, Ivrea–Verbano Zone)

European Journal of Mineralogy ◽

10.5194/ejm-32-587-2020 ◽

2020 ◽

Vol 32 (6) ◽

pp. 587-612

Author(s):

Marta Antonicelli ◽

Riccardo Tribuzio ◽

Tong Liu ◽

Fu-Yuan Wu

Keyword(s):

Continental Crust ◽

Granulite Facies ◽

Variscan Orogeny ◽

Lower Permian ◽

Melt Flow ◽

Slow Cooling ◽

Lower Continental Crust ◽

Magmatic Origin ◽

Mafic Complex ◽

Geochemical Investigation

Abstract. The lower continental crust section of the Ivrea–Verbano Zone (Italian Alps) was intruded by a ∼ 8 km thick gabbroic–dioritic body (Ivrea Mafic Complex) in the Upper Carboniferous–Lower Permian, in conjunction with the post-collisional transtensional regime related to the Variscan orogeny. In the deepest levels of the Ivrea Mafic Complex, several peridotite–pyroxenite sequences considered of magmatic origin are exposed. We present here a petrological–geochemical investigation of the peridotites from the largest magmatic ultramafic sequence of the Ivrea Mafic Complex, locally called Rocca d'Argimonia. In spite of the widespread subsolidus re-equilibration under granulite facies conditions, most likely reflecting a slow cooling evolution in the lower continental crust, the Rocca d'Argimonia peridotites (dunites to harzburgites and minor clinopyroxene-poor lherzolites) typically retain structures and microstructures of magmatic origin. In particular, the harzburgites and the lherzolites typically show poikilitic orthopyroxenes enclosing partially dissolved olivine and minor spinel. Olivine has forsterite proportion diminishing from the dunites to the harzburgites and the lherzolites (90 mol % to 85 mol %) and negatively correlating with δ18O (+5.8 ‰ to +6.6 ‰). Gabbronorite dykes locally crosscut the peridotites and show millimetre-scale thick, orthopyroxenite to websterite reaction zones along the contact with host rocks. We propose that the Rocca d'Argimonia peridotites record a process of reactive melt flow through a melt-poor olivine-rich crystal mush or a pre-existing dunite. This process was most likely responsible for the olivine dissolution shown by the poikilitic orthopyroxenes in the harzburgites–lherzolites. We infer that the reactively migrating melts possessed a substantial crustal component and operated at least at the scale of ∼ 100 m.

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The Athabasca Granulite Terrane and Evidence for Dynamic Behavior of Lower Continental Crust

Annual Review of Earth and Planetary Sciences ◽

10.1146/annurev-earth-063016-020625 ◽

2018 ◽

Vol 46 (1) ◽

pp. 353-386 ◽

Cited By ~ 5

Author(s):

Gregory Dumond ◽

Michael L. Williams ◽

Sean P. Regan

Keyword(s):

Continental Crust ◽

Lower Crust ◽

Tectonic Evolution ◽

Paradigm Shift ◽

Lower Continental Crust ◽

Deep Crust ◽

And Behavior ◽

Lower Crustal ◽

New Evidence ◽

Lower Crustal Xenolith

Deeply exhumed granulite terranes have long been considered nonrepresentative of lower continental crust largely because their bulk compositions do not match the lower crustal xenolith record. A paradigm shift in our understanding of deep crust has since occurred with new evidence for a more felsic and compositionally heterogeneous lower crust than previously recognized. The >20,000-km2Athabasca granulite terrane locally provides a >700-Myr-old window into this type of lower crust, prior to being exhumed and uplifted to the surface between 1.9 and 1.7 Ga. We review over 20 years of research on this terrane with an emphasis on what these findings may tell us about the origin and behavior of lower continental crust, in general, in addition to placing constraints on the tectonic evolution of the western Canadian Shield between 2.6 and 1.7 Ga. The results reveal a dynamic lower continental crust that evolved compositionally and rheologically with time.

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