(37) Petrogenesis of Lawsonite + Chlorite Rocks, Sivrihisar Massif, Turkey

In paleosubduction zones, blackwall or “hybrid” rocks are commonly found along the contacts between ultramafic rocks and mafic or sedimentary rocks. As the compositions of these rocks do not correspond to any... [ view full abstract ]

In paleosubduction zones, blackwall or “hybrid” rocks are commonly found along the contacts between ultramafic rocks and mafic or sedimentary rocks. As the compositions of these rocks do not correspond to any pre-metamorphic equivalent (sediment, oceanic crust, peridotite), they have been interpreted to form from two (or more) different precursor rocks as a result of tectonic mixing and/or fluid-rock interaction. Many of these blackwall rocks are chlorite- and talc-rich, but few preserve lawsonite, a trace element and water-rich (~ 11 wt.% H₂O) mineral that is stable at pressures exceeding 2.5 GPa. In the Tavşanlı Zone, a Late Cretaceous subduction complex formed during the closure of the Neo-Tethys ocean, lawsonite-bearing blackwall rocks are found at the contact between serpentinite lenses and metamafic rocks. To understand the origins and pressure-temperature (P-T) evolution of these rocks, we conducted a petrologic and geochemical study of three representative lawsonite + chlorite-rich rocks. Two of the samples consist almost entirely of chlorite + lawsonite (with accessory titanite and epidote-group minerals), and the third sample consists of chlorite + lawsonite + garnet + epidote, with minor pyroxene. In the latter sample, the presence of epidote inclusions in garnet indicates that part of the prograde P-T path was in the stability field of epidote. Compared to metamafic rocks in the Tavşanlı Zone, the lawsonite + chlorite-rich samples have low SiO₂ (35 – 40 wt.%), Na₂O (0.08 – 0.37 wt.%), and K₂O (0 – 0.02 wt.%) contents, and high MgO (13 – 22 wt.%) and Al₂O₃ (18 – 25 wt.%) contents. With the exception of these elements, the lawsonite + chlorite-rich rocks are compositionally similar to metamafic rocks, indicating that they may be metasomatized mafic rocks. Thermodynamic modelling of these bulk rock compositions and examination of mineral compositions and zoning patterns will be used to elucidate the relative timing and P-T conditions at which these rocks formed.