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A High-K, Mantle Derived Plutonic Suite from 'Linga', near Arequipa (Peru) |
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| Authors: | LE BEL L; COCHERIE A; BAUBRON J -C; FOUILLAC A M; HAWKESWORTH C J |
| Institution: | 1G.I.S. B.R.G.M.-C.N.R.S., I A rue de la F?rollerie 45045 Orleans Cedex, France 2B.R.G.M. D?partment M.G.A. BP 6009, 45060 Orleans Cedex, France 3Department of Earth Sciences, The Open University Milton Keynes MK7 6AA, United Kingdom |
| Abstract: | The Linga Group consists of a suite of Cretaceous high-K calc-alkalinerocks intruded into 2?0 Ga old basement in S.E. Peru. The rocksrange in composition from gabbros, through diorites and grandioritesto granites. Microprobe, major and trace element and isotopedata suggest the suite evolved by fractional crystallization,with plagioclase as the dominant phase and with surprisinglylittle interaction with the pre-existing crust. The rocks yieldan Rb/Sr whole rock isochron of 68 ? 3 Ma with an initial Sr-isotoperatio 0.70516 ? 8, and from three Nd-isotope analyses initial Nd = – 1?4 to – 2?0.  18 O increases from 5?0 permil in a gabbro to 7?0 per mil in a granite, and models arepresented which suggest that the suite evolved from parentalmagmas which had 18O = 5?8 to 6?0 per mil, 1.5 per cent K2O,63 p.p.m. Rb, 582 p.p.m. Sr, 35 p.p.m. Ce, 0.38 p.p.m. Ta, 87Sr/86Sr= 0.7052, and 143Nd/144Nd = 0.51247. Trace element considerationsimply that these magmas contain contributions from incompatibleelement enriched upper mantle material and from a componentassociated with subduction. The latter is thought to reflectthe preferential mobilization of alkali and alkaline earth elementsby H2O-rich fluids from the subducted slab, and preliminarycalculations indicate that it was responsible for {small tilde}45 per cent of the Sr and {small tilde} 80 per cent of the Kin the more primitive Linga rocks. However, the isotope datasuggest that while the fluids may be from the subducted slab,the elements in this component may also have been mobilizedfrom the overlying mantle wedge. |
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