Deciphering multiple episodes of partial melting, metasomatic and remelting processes in the Eastern Pyrenean orogenic mantle massif

Abstract

Secular variations in the composition of the subcontinental lithospheric mantle (SCLM) are intimately controlled by multiple geological processes, including diverse melting mechanisms and complex metasomatic processes. We present comprehensive analyses of whole rock and mineral chemistry, along with Re-Os isotopic system of the Eastern Pyrenean orogenic mantle massifs, supported by quantitative modeling through alphaMELTS thermodynamic software. The Eastern Pyrenean lherzolites display consistent negative correlations of TiO2/Al2O3, TiO2 versus MgO and relatively high and unfractionated heavy rare earth elements. These chemical features shed light on the diverse melting mechanisms responsible for these rocks, encompassing passive continuous melting in regions of lithospheric extension and decompression melting linked to the upwelling asthenospheric mantle. Some refractory harzburgites exhibit elevated TiO2/Al2O3, TiO2 contents and moderate light rare earth element enrichment. This suggests chromatographic metasomatism due to the reactive porous flow of evolved melts/fluids in the upper part of the lithosphere during their ascent to the surface. An isochron analogy between 187Os/188Os and Al2O3 yields an age of ∼ 1.5 Ga, constraining the timing of partial melting responsible for the formation of SCLM beneath the Eastern Pyrenean region. We then expand the scope of our study to encompass on– and off-cratonic SCLM on a global scale. This expanded analysis explores the variations in melting mechanisms across different tectonic settings and geological epochs and scrutinizes the role of diverse metasomatic processes in shaping the characteristics of the lithospheric mantle and its longevity on a broad scale. Silicate metasomatism typically produces fertile peridotites at the asthenosphere-lithosphere boundary, making them vulnerable to thermomechanical erosion, whereas cryptic metasomatism, commonly observed in refractory mantle rocks induced by evolved melts at decreasing melt-rock ratios (e.g., hydrocarbon-bearing silicate melt, carbonatite melt etc.), generally forms enrichment of highly incompatible elements but less significant influence on the mineral assemblages and major element geochemistry of the on-cratonic refractory SCLM, making them remain stable for extended periods.