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Titolo:
MELT ENRICHMENT OF SHALLOW DEPLETED MANTLE - A DETAILED PETROLOGICAL,TRACE-ELEMENT AND ISOTOPIC STUDY OF MANTLE-DERIVED XENOLITHS AND MEGACRYSTS FROM THE CAMEROON LINE
Autore:
LEE DC; HALLIDAY AN; DAVIES GR; ESSENE EJ; FITTON JG; TEMDJIM R;
Indirizzi:
UNIV MICHIGAN,DEPT GEOL SCI,2534 G C LITTLE BLDG ANN ARBOR MI 48109 VRIJE UNIV AMSTERDAM,FAC AARDWETENSCHAPPEN 1081 HV AMSTERDAM NETHERLANDS UNIV EDINBURGH,DEPT GEOL & GEOPHYS EDINBURGH EH9 3JW MIDLOTHIAN SCOTLAND UNIV YAOUNDE,DEPT EARTH SCI YAOUNDE CAMEROON
Titolo Testata:
Journal of Petrology
fascicolo: 2, volume: 37, anno: 1996,
pagine: 415 - 441
SICI:
0022-3530(1996)37:2<415:MEOSDM>2.0.ZU;2-V
Fonte:
ISI
Lingua:
ENG
Soggetto:
SPINEL PERIDOTITE XENOLITHS; RARE-EARTH ELEMENTS; OCEAN ISLAND BASALTS; WEST-AFRICA; ULTRAMAFIC INCLUSIONS; LITHOSPHERE BENEATH; GARNET PYROXENITE; CONTINENTAL RIFT; GEOCHEMICAL DATA; ALKALI BASALTS;
Keywords:
CAMEROON LINE; MANTLE XENOLITHS; MEGACRYSTS; REE; ISOTOPIC COMPOSITION; TRACE ELEMENT;
Tipo documento:
Review
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
123
Recensione:
Indirizzi per estratti:
Citazione:
D.C. Lee et al., "MELT ENRICHMENT OF SHALLOW DEPLETED MANTLE - A DETAILED PETROLOGICAL,TRACE-ELEMENT AND ISOTOPIC STUDY OF MANTLE-DERIVED XENOLITHS AND MEGACRYSTS FROM THE CAMEROON LINE", Journal of Petrology, 37(2), 1996, pp. 415-441

Abstract

Major element, trace element and Sr-Nd-Pb isotopic compositions of ultramafic xenoliths and megacrysts from the continental Cameroon line provide evidence for metasomatism of the upper-most lithospheric mantleby enriched melts during the Mesozoic. The megacrysts probably crystallized within the lower continental crust from melts similar to the host magmas. All the xenoliths originated as depleted residues after theextraction of basaltic melts, but some indicate evidence of interaction with enriched partial melts before entrainment. The U-Pb isotopic data on garnet are consistent with cooling through > 900 degrees C at >300 Ma. The Sm-Nd isotope systematics in constituent phases appear tohave been in equilibrium on a xenolith scale at the time of entrainment, indicating derivation from mantle that remained at temperatures > 600 degrees C until eruption. Spinel lherzolites that show simple light rare earth element (LREE) depletions are characterized by isotopic compositions that are comparable with, but slightly more depleted than Atlantic NMORB, suggesting that the unmetasomatized sub-continental lithosphere of the Cameroon line may be isotopically similar to that of sub-oceanic lithosphere. The Nd-depleted mantle model ages of these xenoliths are consistent with late Proterozoic depletion, similar in ageto much of the overlying continental crust. In contrast, samples thathave LREE-enriched clinopyroxenes (La/Yb = 4.7-9.4) contain trace amounts of amphibole, are enriched in U and have more radiogenic Pb and Sr. These xenoliths yield U-Pb and Sm-Nd model ages consistent with Mesozoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Clinopyroxenes record three orders of magnitude enrichment in U and LREEaccompanied by progressive K depletion associated with the growth of trace amphibole, with K/U ratios that range from 12 000 to 1. The ratios of the trace elements thought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopic compositions. Mass balance calculations suggest that the melts responsible for the most recent enrichment of the lithosphere had higher La/Yb and U/Pb than Cameroon line host magmas, and were probably the Product of small degrees of partial melting associated with the earlieststages of the breakup of Pangea.

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Documento generato il 24/11/20 alle ore 08:27:03