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Phase mixing in the Lanzo upper mantle shear zone (Italy)

How fine-grained, well-mixed (ultra)mylonitic layers in the upper mantle form is still unclear. Here, microfabrics displaying the transition from porphyroclasts to mixed assemblages were analyzed regarding their phase assemblage, grain size and shape, mixing patterns and crystallographic orientations for insights into the interplay of phase mixing and recrystallization. Olivine recrystallization starts in porphyroclastic textures and continues until ultramylonites. Ol microfabrics are divided in neoblasts in multiphase assemblages, recrystallized porphyroclasts and neoblast layers. Both latter microfabrics are ±monomineralic (ol~90 area%) with low mixing intensities (>90% ol-ol boundaries). However, olivine is the dominant mixing phase during opx/cpx porphyroclast recrystallization. Up to mylonitic textures orthopyroxenes occur as porphyroclasts and bands. In mylonites both recrystallize, forming an instant fine-grained mixture of ol (~56 area%, ~13µm) and opx (~37 area%, ~14µm). The phase mixing intensity is high (63% phase boundaries). Clinopyroxene porphyroclasts recrystallize from protomylonites onwards forming an instant mixture of cpx (~61 area%, ~21µm) and ol (~31 area%, ~18µm) in mylonites. Cpx generally displays bigger grain sizes and higher abundances than opx in ol+opx assemblages. Mixing intensities are high (66% phase boundaries). Beside few thoroughly mixed areas, cpx/opx+ol bands are still distinguishable in (ultra)mylonitic layers. They can largely be traced back to recrystallized opx/cpx porphyroclasts. At their boundaries, high abundances of ol neoblasts are present, mixing with host porphyroclast neoblasts. Opx and cpx porphyroclasts that recrystallized isolated from ol-bearing matrix have significantly lower mixing intensities. Phase mixing is therefore assumed to occur simultaneous during porphyroclast recrystallization, and to be strongly dependent on ol nucleation.


Sören Tholen & Jolien Linckens
Goethe-Universität Frankfurt, Germany
GeoUtrecht 2020