Part II – Geochemistry of the central Andean rear arc
In part I, I discussed current ideas about the formation of the non-collisional central Andean plateau. Most of the ideas presented were based on evidence from seismic studies and geodynamical modeling. Here I discuss the role of geochemistry and what it can tell us about the relationship between plateau formation and volcanism. The starting idea is that the chemical composition of lavas from volcanic eruptions can reveal information about the mechanism that triggered melting in the mantle. From here, we can try to fit this information into the hypothesized models of lithospheric dynamics.
Arc vs. rear-arc geochemical similarities in the central Altiplano region
This research focuses on the central region of the central Andes known as the central Altiplano. I define this region as occurring south of the Arica bend between the latitudes of ~18 and 21.5ºS, bound by the arc front centers of Parinacota and Aucanquilcha, and extending eastward to the Eastern Cordillera.
The geochemical differences between the arc front and the rear arc are not strongly pronounced. This is demonstrated in the figure below by the similarities in elemental concentrations (normalized to normal mid-ocean ridge basalt) between the arc front (lavas of Parinacota (Hora et al., 2009)) and the Pleistocene rear-arc center of Tunupa (my data). Also shown for comparison are data from ~11 Ma Huayrana, Pleistocene Quillacas (Mcleod et al., 2012), and ~25 Ma Chiar Kkollu basalts (Hoke and Lamb, 2007).
Only the late Oligocene Chiar Kkollu basalt shows the relatively smooth pattern similar to ocean island basalts (OIB). All other Quaternary lavas lavas (some more than others) have ‘spiky’ distributions common to subduction zone lavas, raising an important question:
1) Does the similarity in chemistry between late Miocene and younger rear arc and arc front volcanoes indicate a similar melting mechanism?
Assuming that the arc front volcanoes are due to volatiles (mainly water) released from the subducting Nazca plate, it is difficult to imagine these same volatiles extending 100s of km into the rear arc (see Grove et al. 2009). The similarities, however, do not preclude a similar style of melting. Perhaps the rear-arc magmatism was initiated by volatiles released by the breakdown of hydrous minerals in the mantle lithosphere. Minerals such as amphibole and biotite were likely present in this region due to a period of flat-slab subduction in the Eocene-Oligocene (James and Sacks, 1999). Melting induced by the breakdown of hydrous minerals during mountain building events has been proposed elsewhere, including the Iranian plateau (Allen et al., 2013) and may also be involved in potassic lavas from the Tibetan plateau. The Iranian and Tibetan lavas are also characterized by spiky elemental distributions on normalized diagrams and may indicate involvement of hydrous mantle lithosphere. The breakdown of hydrous minerals may occur during lithospheric thickening, or during lithospheric removal, as shown below.
This diagram is modified from diagrams in Myers et al. (1998) and Beck and Zandt (2002), who interpreted seismic data collected across the plateau to reveal significant variations in the depth of the crust and mantle lithosphere, which may be related to lithospheric removal. The actual process is likely much more complex. Recent dynamic modeling by Krystopowicz and Currie (2013) suggests one plausible scenario of lithospheric removal over a time span of ~23 million years, roughly the same amount of time since the end of flat-slab subduction in the central Altiplano region.
My current research at Durham University involves looking closely at the differences in ages and chemistry between the arc front and rear-arc lavas to further examine the hypothesis of removal-induced magmatism.