Internal P–T–t Structure of Subduction Complexes — Insights from Lu–Hf Geochronology on Garnet and Lawsonite (Halilbağı, Central Anatolia)

Abstract:

The subduction complex near Halilbağı (Central Anatolia) is among the best sites to investigate deep-seated tectonic, petrologic, and geochemical processes taking place in subduction zones. The Halilbağı Unit comprises slices of lawsonite- and/or epidote-bearing blueschist and eclogite, as well as meta-chert and marble. The unit is overlain by an ophiolitic slab and underlain by a HP/LT metamorphosed carbonate platform. Previous studies of the Halilbağı Unit suggested tectonic blocks were metamorphosed under diverse peak conditions, but shared a common exhumation P–T path marked by syn-decompression cooling (Davis and Whitney, 2006; Çetinkaplan et al., 2008).

To better understand the internal structure and dynamics of this subduction complex, we carried out Lu–Hf geochronology on garnet (grt) and lawsonite (lws) from a variety of HP oceanic rocks, as well as the sub-ophiolitic metamorphic sole. Our results suggest that intra-oceanic subduction started at ~110 Ma (grt–amph isochron from a grt amphibolite). Less than 23 Myr later, the subduction interface was refrigerated enough to allow clockwise P–T loops (~87 Ma peak grt–matrix isochron for a lws+grt-bearing eclogitic blueschist) and syn-decompression cooling (~79 Ma retrograde lws–matrix isochron) of subducted oceanic rocks.

We will present further results for several HP metamorphic sub-facies (e.g., epidote (ep) eclogite, lws+ep blueschist, lws blueschist, lws eclogite). Such data may allow unraveling whether the co-occurrence of “warm” (i.e., ep-bearing), and “cold” (i.e., lws-bearing) HP rocks in the same locality results from (a) sampling of distinct levels of the subduction slab, (b) thermal maturation of the juvenile subduction zone, or (c) inaccurate P–T estimates. Novel natural constraints are thus expected on the dynamics of the Halilbağı Unit and of subduction complexes in general.

 

Çetinkaplan M., Candan O., Oberhänsli R. and Bousquet R., 2008. Pressure–Temperature Evolution of Lawsonite Eclogite in Sivrihisar, Tavşanlı Zone–Turkey. Lithos, 104(1–4): 12–32.

Davis P. B. and Whitney D. L., 2006. Petrogenesis of Lawsonite and Epidote Eclogite and Blueschist, Sivrihisar Massif, Turkey. Journal of Metamorphic Geology, 24(9): 823–849.