Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia)

Geochronological data, major and trace element abundances, Nd and Sr isotop e ratios, delta O-18 whole rock values and Pb isotope ratios from leached f eldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the gr anites from outcrop 10 km N ' Omaruru, reversely discordant U-Pb monazite d ata give Pb-207/U-235 ages of 511 +/- 2 Ma and 517 +/- 2 Ma, similar to pre viously published estimates for the time of regional high grade metamorphis m in the Central Zone. Based on textural and compositional variations, garn ets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P-T estimates from these xenocrystic ga rnets are significantly higher (800 degreesC/9-10 kbar) than regional estim ates (700 degreesC/5 kbar), the monazite ages are interpreted to date the p eak of regional metamorphism in the source of the granites. Sm-Nd garnet-wh ole rock ages are between similar to 500 and similar to 490 Ma indicating t he age of extraction of the granites from their deep crustal sources. For t he granites from Oetmoed, both Sm-Nd and Pb-Pb ages obtained on igneous gar nets range from similar to 500 to similar to 490 Ma. These ages are interpr eted as emplacement ages and are significantly younger than the previously proposed age of similar to 520 Ma for these granites based on Rb/Sr whole r ock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial Sr-87/Sr-86 ratios (> 0.716), high delta O-18 values of > 13.8 part s per thousand, negative initial epsilon (Nd) values between -4 and -7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intru ded. The large range of Pb isotope ratios and the lack of correlation betwe en Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically h ighly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these grani tes might have been caused by crustal thickening and thrusting of crustal s heets enriched in heat-producing elements. Very limited fluxing of volatile s from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of t he lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that hav e been derived at least in part from a mantle source supports this model. ( C) 2001 Elsevier Science B.V. All rights reserved.