Isotopic equilibrium/disequilibrium in granites,metasedimentary rocks and migmatites(Damara orogen,Namibia)-a consequence of polymetamorphism and melting

摘要

The overwhelming part of the continental crust in the high-grade part of the Damara orogen of Namibia consists of S-type granites,metasedimentary rocks and migmatites.At Oetmoed(central Damara orogen)two different S-type granites occur.Their negative epsilon_(Nd)values(-3.3 to-5.9),moderately high initial ~(87)Sr/~(86)Sr ratios(0.714-0.731),moderately high ~(206)Pb/~(204)PbPb(18.21-18.70)and ~(208)Pb/~(204)Pb(37.74-37.89)isotope ratios suggest that they originated by melting of mainly mid-Proterozoic metasedimentary material.Metasedimentary country rocks have initial epsilon_(Nd)of -4.2 to - 5.6,initial ~(87)Sr/~(86)Sr of 0.718-0.725,~(206)pb/~(204)pb ratios of 18.32-18.69 and ~(208)Pb/~(204)Pb ratios of 37.91-38.45 compatible with their variation in Rb/Sr,U/Pb and Th/ Pb ratios.Some migmatites and residual metasedimentary xenoliths tend to have more variable epsilon_(Nd)values(initial epsilon_(Nd):-4.2 to-7.1),initial Sr isotope ratios(~(87)Sr/~(86)Sr:0.708-0.735)and less radiogenic ~(206)Pb/~(204)Pb(18.22-18.53)and ~(208)Pb/~(204)Pb(37.78-38.10)isotope compositions than the metasedimentary rocks.On a Rb-Sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c.550 Ma which is interpreted to indicate major fractionation of the Rb-Sr system at that time.However,initial ~(87)Sr/~(86)Sr ratios of the melanosomes of the stromatic migmatites(calculated for their U-Pb monazite and Sm-Nd garnet ages of c.510 Ma)are more radiogenic(~(87)Sr/~(86)Sr:0.725)than those obtained on their corresponding leucosomes(~(87)Sr/~(86)Sr:0.718)implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the Rb-Sr system.However,the leucosomes have similar Nd isotope characteristics than the inferred residues(melanosomes)indicating the robustness of the Sm-Nd isotope system during high-grade metamorphism and melting.On a Rb-Sr isochron plot residual metasedimentary xenoliths show residual slopes of c.66 Ma(calculated for an U-Pb monazite age of 470 Ma)again indicating major fractionation of Rb/Sr at c.540 Ma.However,at 540 Ma,these xenoliths have unradiogenic Sr isotope compositions of c.0.7052,indicating depleted metasedimentary sources at depth.Based on the distinct Pb isotope composition of the metasedimentary rocks and S-type granites,metasedimentary rocks similar to the country rocks are unlikely sources for the S-type granites.Moreover,a combination of Sr,Nd,Pb and O isotopes favours a three-component mixing model(metasedimentary rocks,altered volcanogenic material,meta-igneous crust)that may explain the isotopicvariabilty of the granites.The mid-crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre-existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central Damara orogen of Namibia.