Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes

Details Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes Isotope and trace element evidence for late-stage intra-crustal melting in the High Andes

Apr 1982

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982e%26psl..58..240h&link_type=abstract

Earth and Planetary Science Letters, Volume 58, Issue 2, p. 240-254.

Computer Science

34

Scientific paper

Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found. 87Sr/86Sr=0.7095-0.7081 at Purico, 0.7079-0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas: 143Nd/144Nd=0.51222-0.51236 overall. The Purico lavas are characterised by higher SiO2, Rb/Sr, 87Sr/86Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and 143Nd/144Nd than most Andean igneous suites. There is no indication of selective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low 87Sr/86Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO2 in rocks with 87Sr/86Sr=0.7081-0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (~60%SiO2) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.

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