CUMULATES FROM THE PANAMA CANAL REGION: SQUEEZING THE AMPHIBOLE “SPONGE”

The role of a mid- to lower crustal amphibole sponge and amphibole fractionation has been recently acknowledged as a key component in the evolution of magmatism and recycling in volcanic arcs (Davidson et al., 2007, Geology, p787-790). The geochemical fingerprint of amphibole (e.g. Dy/Yb) is pervasive in most arcs, however it is generally absent as a significant phenocryst phase in erupted magmatic products. Exhumed arc terranes (e.g. Baltimore Mafic Complex, Alaska) commonly contain amphibole bearing cumulates and these illustrate the important role of amphibole in the sub-arc crust. However, for young arcs, amphibole-bearing cumulate xenoliths are the only available window into these mid- to lower crustal processes. The Panama Canal Region lies at the eastern end of the Central American Arc – a key location in ongoing efforts to characterize lithospheric evolution of volcanic arcs. Significant Miocene volcanic activity and unprecedented exposure associated with the canal excavation and quarrying has made the region important for understanding Miocene volcanic activity within the arc. This study focuses on amphibole-bearing cumulates and antecrysts hosted within Miocene hypabyssal andesitic intrusions. These andesitic rocks have trace element patterns typical of Central American arc-front volcanism. Cumulate nodules are ~6 cm in diameter and are almost entirely composed of 5-10mm amphibole crystals (dominantly ferri-tschermakite). Individual megacrysts (8-10 mm) and the amphibole crystals within the cumulate nodules are frequently zoned with a high MgO core and a lower MgO rim. Antecrysts (mm-scale) exhibit variable and sometimes oscillatory zoning in resorbed layers that suggest multiple mixing episodes. On the basis of co-variation of TiO₂ and MgO from the amphiboles at least two distinct magma types with either compositional or thermal heterogeneity are required. Pressure estimates indicate a limited range of crustal depth for these mixing processes. I suggest these amphibole cumulates and the other antecrystic cargo are derived from a complex mid-lower crustal magmatic differentiation system with multiple magma injection events. These amphiboles therefore represent the previously hypothesized ‘amphibole sponge’ that plays a central role in the evolution of arc magmas.