Some Like It Cold: The “Conundrum of Samail” Revisited

Abstract

The "conundrum of Samail" (Hacker & Gnos, 1997) refers to the interaction of "hot" processes of subduction initiation with "cold" high-pressure (HP) metamorphism in the Oman Mountains. Hacker and Gnos (1997) discussed the example of the far-traveled, non-HP Samail Ophiolite and its underlying high-temperature (HT) metamorphic sole that formed above an infant, hot, intraoceanic subduction zone and overlies the cold, continental Saih Hatat HP complex that originated in a mature subduction setting (Figures 1 and 2).The onset of intraoceanic subduction is associated with the formation of a metamorphic sole (i.e., HT rocks subsequently welded to the base of the suprasubduction-zone ophiolite) reflecting elevated, transient thermal gradients (e.g., Dewey & Bird, 1971;Stern & Bloomer, 1992), which in the intraoceanic Samail subduction zone at the northeastern Arabian margin in Oman were up to ∼30°C km −1 (e.g., Garber et al., 2020;Searle & Cox, 2002). The available age data constraining high-grade metamorphism and partial melting of the metamorphic sole and subsequent formation of the Samail Ophiolite (Garber et al., 2020;Guilmette et al., 2018;Rioux et al., 2021;Soret et al., 2022) demonstrate hot subduction-zone processes between 105 and 93 Ma (Figure 3). Compiled data from 70 subduction zones by Lallemand and Arcay (2021) and numerical modeling by Peacock (1990) indicate that it takes 5-15 Ma between hot subduction-zone initiation and self-sustained, steady-state subduction characterized by a stable, low-thermal gradient of ∼10°C km −1 .The almost completely exposed Saih Hatat window (Figure 2) represents a formidable example of a HP complex and provides a one-of-a-kind natural laboratory important for understanding subduction-zone processes. A shortcoming of the Saih Hatat HP rocks is its controversial and incomplete geochronology. Almost all geochronologic