Nd, Sr-isotopic provenance and trace element geochemistry of Amazonian foreland basin fluvial sands, Bolivia and Peru: implications for ensialic Andean orogeny

Abstract

Nd and Sr isotopes and the trace element contents, including the rare earths, were determined for fluvial sands of lithic arenite composition from the Madre de Dios foreland basin of Bolivia and Peru. On standard petrologic ternary diagrams, the sands fall in the recycled orogen provenance field and thus are similar to typical ancient foreland basin composition. The average rare earth elemental pattern of the sands is identical to the upper continental crustal average, as estimated from post-Archean composite shales of different continents. Ratios ofTh/U, Co/Th, La/Sc and Th/Sc of the fluvial sands are intermediate between an average magmatic arc and an upper crustal average compositions. The dispersion of some trace elemental patterns in the sands can be attributed to fractionation of dense minerals, including zircon, during the sedimentation process. The variations of Nd isotopes in conjunction with the petrographic parameters of lithic metamorphic (Lm) and volcanic (Lv) fragments allow a two-fold classification of the sands. These two sand types can be interpreted in terms of mixing among three different provenances: one volcanic rock-suite with less negativeε Nd(O) parameter than the other volcanic suite, and a third metasedimentary source withε Nd(O) value of around −12, which is considered to be similar to the average western Brazilian shield composition. Thus the overall compositions of the sands has been modeled as mechanical mixtures of two components, an Andean magmatic arc and the Brazilian shield-derived metasediments. The model is strongly supported by a plot ofε Nd(O) versusε Sr(O) of the sands. In this plot, the Type 1 and 2 sands define two coherent hyperbolic trends contiguous with two different portions of the Andean magmatic trend. This relationship has been interpreted to indicate that the observed Andean magmatic trend in anε Sr(O)-ε Nd(O) diagram is the result of varying degrees of contamination of a “primitive arc-type” magma by the Precambrian continental crust of the western Brazilian Shield. The depleted mantle average Nd model age of 1.46 Ga for the fluvial sands reflects the average age of the Brazilian continental crustal source. The development of the Andean orogenic belt has been discussed schematically with the isotopic data of the sands. The model describes a trailing edge prism of sediments, derived from the Brazilian Shield during the late Paleozoic-early Mesozoic. The prism becomes part of the fold-thrust belt during the Andean orogeny in the Neogene, when the foreland basin develops with the basin fill partly derived from the fold-thrust belt. The sedimentary rocks in the fold-thrust belt are also a major source of contaminants for the Andean magmas. The contiguous nature of the Andean magmatic trend and the fluvial sand data in theε Sr(O)-ε Nd(O) diagram suggests that the ensialic Andean magmatic arc has remained connected to its parent continent, the western Brazilian Shield, throughout the development of the Andean orogeny.