Universität zu Köln

Jung, Stefan, Brandt, Soenke, Bast, Rebecca, Scherer, Erik E. and Berndt, Jasper (2019). Metamorphic petrology of a high-T/low-P granulite terrane (Damara belt, Namibia) - Constraints from pseudosection modelling and high-precision Lu-Hf garnet-whole rock dating. J. Metamorph. Geol., 37 (1). S. 41 - 70. HOBOKEN: WILEY. ISSN 1525-1314

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Abstract

Migmatites comprise a minor volume of the high-grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K-feldspar, and leucosomes, which are sometimes garnet- and cordierite-bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak-metamorphic grade throughout the area is in the granulite facies (similar to 5 kbar at similar to 800 degrees C). Cordierite coronas around garnet suggest some decompression from peak-metamorphic conditions and rare andalusite records late, near-isobaric cooling to P-T path is consistent with minor crustal thickening through continent-continent collision followed by limited post-collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of similar to 35-40 degrees C/km at medium pressures. New high-precision Lu-Hf garnet-whole rock dates are 530 +/- 13 Ma, 522.0 +/- 0.8 Ma, 520.8 +/- 3.6 Ma, and 500.3 +/- 4.3 Ma for the migmatites that record temperatures of similar to 800 degrees C. This indicates that high-grade metamorphism lasted for c. 20-30 Ma, which is compatible with previous estimates using Sm-Nd garnet-whole rock systematics. In previous studies on Damara orogen migmatites where both Sm-Nd and Lu-Hf chronometers have been applied, the dates (c. 520-510 Ma) agree within their small uncertainties (0.6-0.8% for Sm-Nd and 0.1-0.2% for Lu-Hf). This implies rapid cooling after high-grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re-dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high-T metamorphism. In addition, high contents of heat-producing elements K, Th, and U may have raised peak temperatures by 150-200 degrees C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre-scale leucosomes to decametre-scale leucogranite sheets within the high-grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high-grade regional metamorphism and Pan-African magmatism. In view of the close association of the studied high-T migmatites with hundreds of synmetamorphic high-T granites that invaded the terrane as metre- to decametre-wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late-stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Jung, Stefan UNSPECIFIED UNSPECIFIED UNSPECIFIED Brandt, Soenke UNSPECIFIED UNSPECIFIED UNSPECIFIED Bast, Rebecca UNSPECIFIED UNSPECIFIED UNSPECIFIED Scherer, Erik E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Berndt, Jasper UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-140394
DOI:	10.1111/jmg.12448
Journal or Publication Title:	J. Metamorph. Geol.
Volume:	37
Number:	1
Page Range:	S. 41 - 70
Date:	2019
Publisher:	WILEY
Place of Publication:	HOBOKEN
ISSN:	1525-1314
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language BASEMENT-DOMINATED TERRANES; TEMPERATURE TIME PATHS; RB-SR GEOCHRONOLOGY; SM-ND SYSTEM; U-PB; REGIONAL METAMORPHISM; OROGENIC BELTS; MINERAL ASSEMBLAGES; CONTINENTAL-CRUST; THERMAL EVOLUTION Multiple languages Geology Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/14039