Dating the prograde metamorphism of high-pressure oceanic rocks with combined lawsonite and garnet 176Lu-176Hf geochronology: A feasibility study (Halilbagi Unit, Central Anatolia)
Final Report Abstract
Subduction, the process by which the Earth crust sinks into the mantle, can be explored through studies of the mineralogical and chemical transformation of rocks that were once buried and then exhumed back to the surface. Blueschist and eclogite are the characteristic rocks that form during subduction and are presently exposed along ancient plate boundaries, where oceans closed. Research on blueschist and eclogite provides important insights into chemical and mechanical processes taking place in the upper part of subducting plates and their role in global chemical cycles and earthquake generation. The timescale of processes taking place during subduction has long remained difficult to determine due to subsequent transformations commonly affecting rocks during their uplift towards the surface. Over the past decade, Lu–Hf isotopic dating (based on the decay of 176Lu into 176Hf) has been applied to the mineral garnet, which forms in blueschist and eclogite mainly during burial and is relatively resistant to such alteration. Recent studies have shown that lawsonite too, another mineral diagnostic of subduction, can be dated using Lu–Hf isotopes, offering a new tool to study subduction. Lawsonite, however, is rarely preserved, and its routine use for Lu–Hf dating still needs to be evaluated in the light of new analytical data. This project evaluated the possibility to date garnet and lawsonite from individual blueschist and eclogite samples. Garnet yielded robust Lu-Hf isotopic dates that have important geological implications, whereas lawsonite results were a mixed success. Geologically-consistent Lu–Hf lawsonite dates were obtained only from the sample where lawsonite grew along with garnet during subduction. Further investigation is required to understand the inconsistency of the other lawsonite results. In line with previous studies, we argue that, within the investigated complex, blueschist and eclogite lenses evolved along different P–T paths, which would record changes of the thermal regime of the former subduction zone. Our results show that, in the studied area, subduction was initiated at ca. 104 million years ago (Ma) below relatively young oceanic crust and then cooled down until ~87 Ma when a continental margin was subducted. Our Lu–Hf garnet dates (104, 92, 90, 87, and 86 Ma) track the progressive cooling over ~15 million years of this then-juvenile subduction zone. This result has important implications for our understanding of the evolution of young subduction systems, such as in the Caribbeans, the Malay Archipelago, and the western Mediterranean. Two surprises are worth to be mentioned: The thermal evolution of a juvenile subduction zone has been revealed by Lu–Hf garnet geochronology. Lawsonite yielded geologically consistent Lu–Hf dates only when it formed early in the metamorphic history, i.e. along with prograde garnet.
