A study of rare earth element (REE)-SiO2 variations in felsic liquids generated by basalt fractionation and amphibolite melting:: a potential test for discriminating between the two different processes

The origin of felsic magmas (> 63% SiO2) in intra-oceanic arc settings is still a matter of debate. Two very different processes are currently invoked to explain their origin. These include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite. Because both fractionation and melting can lead to similar major element, trace element and isotopic characteristics in felsic magmas, such lines of evidence have been generally unsuccessful in discriminating between the two processes. A commonly under-appreciated aspect of rare earth element (REE) solid-liquid partitioning behavior is that D (REE) for most common igneous minerals (especially hornblende) increase significantly with increasing liquid SiO2 contents. For some minerals (e.g., hornblende and augite), REE partitioning can change from incomptatible (D < 1) at low liquid SiO2 to compatible (D > 1) at high liquid SiO2. When this behavior is incorporated into carefully constrained mass-balance models for mafic (basaltic) amphibolite melting, intermediate (andesitic) amphibolite melting, lower or mid to upper crustal hornblende-present basalt fractionation, and mid to upper crustal hornblende-absent basalt fractionation the following general predictions emerge for felsic magmas (e.g., similar to 63 to 76% SiO2). Partial melting of either mafic or intermediate amphibolite should, regardless of the type of melting (equilibrium, fractional, accumulated fractional) yield REE abundances that remain essentially constant and then decrease, or steadily decrease with increasing liquid SiO2 content. At high liquid SiO2 contents LREE abundances should be slightly enriched to slightly depleted (i.e., C-l/C-o similar to 2 to 0.2) while HREE abundances should be slightly depleted (C-l/C-o similar to 1 to 0.2). Lower crustal hornblende-bearing basalt fractionation should yield roughly constant REE abundances with increasing liquid SiO2 and exhibit only slight enrichment (C-l/C-o similar to 1.2). Mid to upper crustal hornblende-bearing basalt fractionation should yield steadily increasing LREE abundances but constant and then decreasing HREE abundances. At high liquid SiO2 contents LREE abundances may range from non-enriched to highly enriched (C-l/C-o similar to 1 to 5) while HREE abundances are generally non-enriched to only slightly enriched (C-l/C-o similar to 1 to 2). Hornblende-absent basalt fractionation should yield steadily increasing REE abundances with increasing liquid SiO2 contents. At high SiO2 contents both LREE and HREE are highly enriched (C-l/C-o similar to 3 to 4). It is proposed that these model predictions constitute a viable test for determining a fractionation or amphibolite melting origin for felsic magmas in intra-oceanic arc environments where continental crust is absent.