Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalites (Italian Southern Alps)

Abstract

Pseudotachylytes and cataclasites are present along a strike-slip fault zone in the tonalites of the Adamello intrusion (Italian Southern Alps). Ambient conditions during faulting were 0.25–0.3 GPa and 250–300 °C. Pseudotachylyte veins thicker than 6 mm are zoned and consist of two symmetric microlitic domains towards the vein walls and a central spherulitic domain. The thickness of the microlitic domains (x) increases linearly with the half total thickness (a) of the pseudotachylyte vein (commonly <2 cm) according to the relation: x=(0.29±0.12)a. The spherulitic and microlitic domains have similar chemical composition but the microlitic domain has a lower amount of plagioclase clasts. A numerical model explains zoning as the result of the different cooling rates and clast/melt interactions at the center and periphery of thick veins. Zoning is compatible with the injection of a single pulse of superheated friction-induced melt (Tmelt≅1450 °C). Melt temperatures estimated by the clast/matrix ratio (O'Hara, 2001) are considerably lower (316–577 °C). It is suggested that the difference in the temperature estimates reflects a more complex slip history than a single seismic slip event along the fault during production of cataclasites/pseudotachylytes.