The role of blueschist stored in shallow lithosphere in the generation of postcollisional orogenic magmas

In regions with small ocean plates and continental blocks, such as the Mediterranean in the Cenozoic and modern Indonesia, many blueschists may be stored at shallow depth in the mantle lithosphere that is newly formed during the amalgamation of small continental blocks and oceans. Melting of these blueschists at low pressures in the mixed lithosphere of this type during postcollisional relaxation may be involved in the generation of postcollisional orogenic magmas. Here, we have conducted a series of melting experiments on three blueschist samples with differing protoliths at 800–850°C, 2 GPa to investigate phase relations, mineral compositions, and trace element partitioning (using laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]/nanoscale secondary ion mass spectrometry [NanoSIMS]), including the first results for clinozoisite. Dacitic/rhyolitic melts (SiO₂ 60 to 66 wt%) containing ~13 wt% volatiles are produced in all runs. Mn-rich and Mn-poor almandine garnets exhibit distinct rare earth element (REE) distribution patterns, while Mn-rich garnets are effective preconcentrators of heavy REEs (HREEs) for later metamorphic processes. Melts produced from fluid-affected blueschists of terrigenous origin show the most large-ion lithophile element (LILE) enrichment, whereas those from terrigenous blueschists with no fluid history exhibit the lowest K/Th and Ba/Th and strongest Ce/Pb and Nb/U fractionations. Melts of blueschists derived from mid-ocean ridge basalt (MORB) have relatively consistent, MORB-like Ba/Th and Nb/U (~3) but the lowest Ce/Pb ratios (3.3). V/Sc and Zr/Hf ratios remain constant at ≥850°C regardless of the origin of the blueschist. Dehydration melting in new mantle lithosphere where blueschist is stored at shallow depth will significantly affect trace element behavior and should be considered carefully when examining chemical cycling and magmatism in postcollisional orogenic settings.