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Geochronolgy NORSIM research proposal (1 March 2000)
Recent experimental petrology in jotunites associated with massif-type anorthosite complexes, including jotunites from Rogaland, suggests that the parent magmas of anorthosite-charnockite suites derived by partial melting of lower crustal (10–13 kbar) gabbro-noritic source rocks (Longhi et al., 1999). These results challenge classical models of one stage mantle origin. Available Sm-Nd, Rb-Sr and Pb-Pb isotopic data from anorthosites and associated rocks from Rogaland (Demaiffe et al., 1986; Duchesne et al., 1989; Nielsen et al., 1996) are unable to discriminate between a mantle or a lower crustal origin. Recent Re-Os isotope data (Schiellerup et al., 2000) show that Os has a radiogenic initial composition, and are consistent with a lower crustal origin. Interpretations of isotopic data so far have been complicated by uncertainty regarding the importance and nature of middle-crustal contamination at the level of intrusion (ca. 15 km). The variability of the isotopic and geochemical signatures of undifferentiated anorthosite and jotunite lithologies can be attributed to variation in the source rock composition, and no convincing evidence for middle-crustal contamination exists in these rocks. For differentiated quartz-mangerite to charnockite lithologies, the existence of middle-crustal contamination is well established (Demaiffe et al., 1986; Nielsen et al., 1996), but the age and isotopic composition of the contaminants are poorly constrained. Conventional zircon dating of anorthosite to jotunite lithologies in Rogaland constraints the intrusion of the whole magmatic complex at 0.93–0.92 Ga and also points to the occurrence of inherited zircons with a poorly defined age of 1.69 to 1.24 Ga (Schärer et al., 1996). In differentiated mangerite to charnockite lithologies, high-U zircon cores, probably corresponding to inherited material, are widespread (Duchesne et al., 1987). Old zircon cores could correspond to residual material inherited from the source or to wall rock assimilation during intrusion at middle-crustal level. We propose to search for inherited zircons in jotunites and quartz-mangerites and measure their age. The age distributions of inherited zircons, if present, will help characterize the age of the contaminants at different stages of differentiation. They will possibly constrain the age of the source rocks, although it is unclear today whether the source rocks of the mafic magmas could be zircon-bearing. These ages will be compared to the age of potential contaminant rocks in the exposed crust in S. Norway. Potential contaminants include sediment sequences for which a rather large database of detrital zircon ages is now available (Åhäll et al., 1998; Bingen et al., submitted; de Haas et al., 1999; Knudsen et al., 1997). The ages of inherited zircons are necessary to constrain available genetic models based on isotopic data, and to derive reasonable isotopic compositions for both the parent magmas of the anorthosite-charnockite suite and the crustal contaminants. These ages are thus key information to build up an integrated petrologic, isotopic, geologic and metallogenic model for the genesis of massive anorthosite in Rogaland and elsewhere. Three jotunite rocks and four differentiated quartz-mangerite to charnockite rocks are selected in well mapped and well characterized rock units. Geochemical data and zircon concentrates are available for the samples.
References Åhäll, K.-I., Cornell, D.H., and Armstrong, R. (1998) Ion probe zircon dating of metasedimentary units across the Skagerrak: new constraints for early Mesoproterozoic growth of the Baltic Shield. Precambrian Research, 87, 117–134. Bingen, B., Birkeland, A., Nordgulen, Ø., and Sigmond, E.M.O. (submitted) Correlation of Proterozoic supracrustal sequences of SW Scandinavia using detrital zircon geochronology: evidence for transpressive juxtaposition of terranes during the Sveconorwegian orogeny. GeoCanada 2000. de Haas, G.J.L.M., Andersen, T., and Vestin, J. (1999) Detrital zircon geochronology: new evidence for an old model for accretion of the SW Baltic Shield. Journal of Geology, 107, 569–586. Demaiffe, D., and Hertogen, J. (1981) Rare earth geochemistry and strontium isotopic composition of a massif-type anorthositic-charnockitic body: the Hidra massif (Rogaland, SW Norway). Geochimica et Cosmochimica Acta, 45, 1545–1561. Demaiffe, D., Weis, D., Michot, J., and Duchesne, J.-C. (1986) Isotopic constraints on the genesis of the Rogaland anorthositic suite (SW Norway). Chemical Geology, 57, 167–179. Duchesne, J.-C., Caruba, R., and Iacconi, P. (1987) Zircon in charnockitic rocks from Rogaland (southwest Norway): petrogenetic implications. Lithos, 20, 357–368. Duchesne, J.-C., Roelands, I., Demaiffe, D., and Weis, D. (1985) Petrogenesis of monzonoritic dykes in the Egersund-Ogna anorthosite (Rogaland, SW Norway): trace elements and isotopic (Sr, Pb) constraints. Contributions to Mineralogy and Petrology, 90, 214–225. Duchesne, J.-C., Roelandts, I., Demaiffe, D., Hertogen, J., Gijbels, R., and De Winter, J. (1974) Rare-earth data on monzonoritic rocks related to anorthosites and their bearing on the nature of the parental magma of the anorthositic series. Earth and Planetary Science Letters, 24, 325–335. Duchesne, J.-C., and Wilmart, E. (1997) Igneous charnockites and related rocks from the Bjerkreim-Sokndal layered intrusion (SW Norway): a jotunite (hypersthene monzodiorite)-derived A-type granitoid suite. Journal of Petrology, 38, 337–369. Duchesne, J.-C., Wilmart, E., Demaiffe, D., and Hertogen, J. (1989) Monzonorites from Rogaland (southwest Norway): a series of rocks coeval but not comagmatic with massif-type anorthosites. Precambrian Research, 45, 111–128. Knudsen, T.-L., Andersen, T., Whitehouse, M.J., and Vestin, J. (1997) Detrital zircon ages from southern Norway - implications for the Proterozoic evolution of the southwestern Baltic Shield. Contributions to Mineralogy and Petrology, 130, 47–58. Longhi, J., Vander Auwera, J., Fram, M.S., and Duchesne, J.-C. (1999) Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks. Journal of Petrology, 40, 339–362. Nielsen, F.M., Campbell, I.H., McCulloch, M., and Wilson, J.R. (1996) A strontium isotopic investigation of the Bjerkreim-Sokndal layered intrusion, Southwest Norway. Journal of Petrology, 37, 171–193. Schärer, U., Wilmart, E., and Duchesne, J.-C. (1996) The short duration and anorogenic character of anorthosite magmatism: U–Pb dating of the Rogaland complex, Norway. Earth and Planetary Science Letters, 139, 335–350. Schiellerup, H., Lambert, D., Robins, B., and Presvik, T. (2000) Re–Os, Sm–Nd and Rb–Sr isotopic evidence for the genesis of massif-type anorthosites and Fe-Ti-rich intrusions in Rogaland, Southwest Norway. 24 Nordiske Geologiske Vintermøte, 6.-9. 1. 2000, p. 150. Norwegian Geological Society, Geonytt, Trondheim.
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