Temporal evolution of mafic magmas in the southern Puna of the Central Andes.

KAY, S.M. AND COIRA, B.

Baltimore, USA

Congreso; Joint Assembly Suppl. AGU 2006; 2006

American Geophysical Union

       Latest Miocene to Quaternary backarc magmatism in the southern Puna is characterized by basaltic andesitic to andesitic flows and silicic ignimbrites that track the evolving history of the underlying mantle and crustal lithosphere. Mafic basaltic andesite to andesite flows typically erupt along faults and are notable for their glassy character, plagioclase and quartz xenocrysts, and relatively high Mg numbers and Cr and Ni contents. Similar lavas are generally absent further north on the plateau. The most mafic lavas (53 to 55% SiO2) in the southern Puna generally have La/Yb ratios from 10 to 25 and show a temporal change from more arc-like to more intraplate-like La/Ta ratios (36 to 55 at 6.7 to 4.3 Ma; 20 to 35 at < 3 Ma). All have 87Sr/86Sr ratios above 0.705 and 143Nd/144Nd ratios below 0.51262. Low Ba/La ratios (most < 15) show little influence of slab-derived fluid mobile elements. Small volume late Miocene to early Pliocene glassy andesites and dacites in the same region have La/Ta ratios of 45 to 78 and La/Yb ratios up to 75 to 100. The high La/Ta ratios of these lavas are particularly notable when compared with La/Ta ratios of 27 to 36 in mafic frontal arc lavas (~ 53% SiO2, La/Yb = 17 to 24) in the southern Central Volcanic Zone to the west. Temporal changes in the backarc mafic magmas require a change from more arc-like to more intraplate-like mantle sources. The overall eruption and temporal chemical pattern can be reconciled with an episode of lower crustal delamination following a subdued episode of shallow subduction. Contributions from crustal material removed by forearc crustal erosion as the frontal arc to the west migrated up to 45 km eastward must also be considered. Other characteristics of the mafic lavas can be explained by rapid mixing of mantle-derived basalt and pooled lower crustal melt fractionating at mid crustal depth. These features are consistent with seismic data that suggest the presence of silicic magma at mid-crustal levels.        Latest Miocene to Quaternary backarc magmatism in the southern Puna is characterized by basaltic andesitic to andesitic flows and silicic ignimbrites that track the evolving history of the underlying mantle and crustal lithosphere. Mafic basaltic andesite to andesite flows typically erupt along faults and are notable for their glassy character, plagioclase and quartz xenocrysts, and relatively high Mg numbers and Cr and Ni contents. Similar lavas are generally absent further north on the plateau. The most mafic lavas (53 to 55% SiO2) in the southern Puna generally have La/Yb ratios from 10 to 25 and show a temporal change from more arc-like to more intraplate-like La/Ta ratios (36 to 55 at 6.7 to 4.3 Ma; 20 to 35 at < 3 Ma). All have 87Sr/86Sr ratios above 0.705 and 143Nd/144Nd ratios below 0.51262. Low Ba/La ratios (most < 15) show little influence of slab-derived fluid mobile elements. Small volume late Miocene to early Pliocene glassy andesites and dacites in the same region have La/Ta ratios of 45 to 78 and La/Yb ratios up to 75 to 100. The high La/Ta ratios of these lavas are particularly notable when compared with La/Ta ratios of 27 to 36 in mafic frontal arc lavas (~ 53% SiO2, La/Yb = 17 to 24) in the southern Central Volcanic Zone to the west. Temporal changes in the backarc mafic magmas require a change from more arc-like to more intraplate-like mantle sources. The overall eruption and temporal chemical pattern can be reconciled with an episode of lower crustal delamination following a subdued episode of shallow subduction. Contributions from crustal material removed by forearc crustal erosion as the frontal arc to the west migrated up to 45 km eastward must also be considered. Other characteristics of the mafic lavas can be explained by rapid mixing of mantle-derived basalt and pooled lower crustal melt fractionating at mid crustal depth. These features are consistent with seismic data that suggest the presence of silicic magma at mid-crustal levels.        Latest Miocene to Quaternary backarc magmatism in the southern Puna is characterized by basaltic andesitic to andesitic flows and silicic ignimbrites that track the evolving history of the underlying mantle and crustal lithosphere. Mafic basaltic andesite to andesite flows typically erupt along faults and are notable for their glassy character, plagioclase and quartz xenocrysts, and relatively high Mg numbers and Cr and Ni contents. Similar lavas are generally absent further north on the plateau. The most mafic lavas (53 to 55% SiO2) in the southern Puna generally have La/Yb ratios from 10 to 25 and show a temporal change from more arc-like to more intraplate-like La/Ta ratios (36 to 55 at 6.7 to 4.3 Ma; 20 to 35 at < 3 Ma). All have 87Sr/86Sr ratios above 0.705 and 143Nd/144Nd ratios below 0.51262. Low Ba/La ratios (most < 15) show little influence of slab-derived fluid mobile elements. Small volume late Miocene to early Pliocene glassy andesites and dacites in the same region have La/Ta ratios of 45 to 78 and La/Yb ratios up to 75 to 100. The high La/Ta ratios of these lavas are particularly notable when compared with La/Ta ratios of 27 to 36 in mafic frontal arc lavas (~ 53% SiO2, La/Yb = 17 to 24) in the southern Central Volcanic Zone to the west. Temporal changes in the backarc mafic magmas require a change from more arc-like to more intraplate-like mantle sources. The overall eruption and temporal chemical pattern can be reconciled with an episode of lower crustal delamination following a subdued episode of shallow subduction. Contributions from crustal material removed by forearc crustal erosion as the frontal arc to the west migrated up to 45 km eastward must also be considered. Other characteristics of the mafic lavas can be explained by rapid mixing of mantle-derived basalt and pooled lower crustal melt fractionating at mid crustal depth. These features are consistent with seismic data that suggest the presence of silicic magma at mid-crustal levels.