Universität zu Köln

Pfaender, Joerg A., Jung, Stefan, Kluegel, Andreas, Muenker, Carsten, Romer, Rolf L., Sperner, Blanka and Rohrmueller, Johann (2018). Recurrent Local Melting of Metasomatised Lithospheric Mantle in Response to Continental Rifting: Constraints from Basanites and Nephelinites/Melilitites from SE Germany. J. Petrol., 59 (4). S. 667 - 695. OXFORD: OXFORD UNIV PRESS. ISSN 1460-2415

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Abstract

Cenozoic primitive basanites, nephelinites and melilitites from the Heldburg region, SE Germany, are high-MgO magmas (8.5-14.1 wt % MgO), with low SiO2 (34.2-47.1 wt %) and low to moderately high Al2O3 (9.0-15.5 wt %) and CaO (8.7-12.7 wt %). The Ni and Cr contents of most samples are up to 470 ppm and 640 ppm, respectively, and match those inferred for primary melts. In multi-element diagrams, all samples are highly enriched in incompatible trace elements with chondrite-normalised La/Yb = 19-45, strongly depleted in Rb and K, with primitive mantle normalised K/La = 0.15-0.72, and moderately depleted in Pb. The initial Sr-Nd-Hf isotope compositions (Sr-87/Sr-86 = 0.7033-0.7051, Nd-143/Nd-144 = 0.51279-0.51288 and Hf-176/Hf-177 = 0.28284-0.28294) fall within the range observed for other Tertiary volcanic rocks of the Central European Volcanic Province, whereas Pb-208/Pb-204 and Pb-206/Pb-204 (38.42-38.88 and 18.49-18.98) are distinctly lower at comparable Pb-207/Pb-204 (15.60-15.65). Trace element modelling and pressure-temperature estimates based on major element compositions and experimental data suggest that the nephelinites/melilitites formed within the lowermost lithospheric mantle, close to the lithosphere-asthenosphere boundary, by similar to 3-5% partial melting of a highly enriched, metasomatised, carbonated phlogopite-bearing garnet-lherzolite at temperatures <1250 degrees C and pressures of similar to 2.8 GPa. This corresponds to a melting depth of less than similar to 85 km. Formation and eruption of these magmas, based on Ar-40/Ar-39 dating, started in the late Eocene (38.0 Ma) and lasted until the late Oligocene (25.4 Ma). Basanite eruptions occurred in the same area in the middle Miocene, about 7.7 Myr after nephelinite/melilitite generation has ceased, and lasted from 17.7 to 13.1 Ma. The basanites were generated at lower pressures (2.2-1.7 GPa) at similar temperatures (similar to 1220-1250 degrees C) within the spinel stability field in the lithospheric mantle by 2-6% partial melting. Isotope and trace element systematics indicate that the lithospheric mantle source of the Heldburg magmas was affected by metasomatism associated with long-lasting subduction of oceanic and continental crust during the Variscan orogeny. Aqueous or supercritical fluids that formed at temperatures <1000 degrees C and pressures of likely >4 GPa infiltrated the thermal boundary layer at the base of the lithospheric mantle and imprinted a crustal lead isotope, and to a minor extent crustal Sr, Nd and Hf isotope signatures. They also reduced Nb/U, Ce/Pb, Lu/Hf, Sm/Nd, U/Pb and Th/Pb, but increased Rb/Sr and Nb/Ta and amplified the enrichment of LILE and LREE relative to HREE. This lead to the highly-enriched trace element patterns observed in both sample suites, and to overall less radiogenic Pb-206/Pb-204 and 208Pb/204Pb compared to other continental basalts in Central Europe, and to less radiogenic Hf-176/Hf-177 and Nd-143/Nd-144 that plot distinctly below the terrestrial mantle array. Temporal evolution of magmatism in the Heldburg region coincides with the changing Tertiary intraplate stress field in Central Europe, which developed in response to the Alpine orogeny. Magmatism was most probably caused in response to lithosphere deformation and perturbation of the thermal boundary layer, and not by actively upwelling asthenosphere.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Pfaender, Joerg A. UNSPECIFIED UNSPECIFIED UNSPECIFIED Jung, Stefan UNSPECIFIED UNSPECIFIED UNSPECIFIED Kluegel, Andreas UNSPECIFIED UNSPECIFIED UNSPECIFIED Muenker, Carsten UNSPECIFIED UNSPECIFIED UNSPECIFIED Romer, Rolf L. UNSPECIFIED UNSPECIFIED UNSPECIFIED Sperner, Blanka UNSPECIFIED UNSPECIFIED UNSPECIFIED Rohrmueller, Johann UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-190072
DOI:	10.1093/petrology/egy041
Journal or Publication Title:	J. Petrol.
Volume:	59
Number:	4
Page Range:	S. 667 - 695
Date:	2018
Publisher:	OXFORD UNIV PRESS
Place of Publication:	OXFORD
ISSN:	1460-2415
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language EUROPEAN VOLCANIC PROVINCE; TRACE-ELEMENT SYSTEMATICS; OCEAN ISLAND BASALTS; AREA CENTRAL GERMANY; CARBONATE METASOMATISM; FRACTIONAL CRYSTALLIZATION; ISOTOPIC SYSTEMATICS; PERIDOTITE XENOLITHS; ALKALINE MAGMATISM; GARNET LHERZOLITE Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/19007