242-1 Probing the Fluid Histories and Protolith(s) of Chlorite-Rich Hybrid Rocks from the Sivrihisar Massif (Tavşanlı Zone, Northwest Türkiye) using Bulk Rock Geochemistry and Oxygen Isotopes

Abstract:

Metasomatism is the fluid-driven alteration of a rock's bulk chemistry. Hybrid rocks, those that cease to match a common crustal protolith, are an excellent window into fluid movement in subduction zones. Many of these fluids are released by the high pressure, low temperature (HP/LT) breakdown of hydrous minerals such as serpentine, chlorite, and lawsonite, a calcium aluminum silicate rarely preserved because of its limited stability range.

The Sivrihisar Massif is in one of largest and best exposed lawsonite blueschist belts, the Tavşanlı Zone of NW Türkiye. Composed mainly of coherent marble, schist, quartzite and metabasite units, it contains hundreds of meter-scale blueschist and eclogite pods. Lawsonite- and chlorite-rich hybrid rocks occur at pod margins and other lithologic contacts, particularly associated with serpentinite.

Nine Sivrihisar samples, mostly hybrid rocks, were tested for bulk rock major and trace element concentrations. These are combined with pre-existing, mostly unpublished data from twenty other samples – six eclogites, six serpentinites, two metasediments, and a blueschist. Oxygen and hydrogen isotopes were also collected on the serpentinites. Textural and mineralogical analysis is being carried out with a standard petrographic microscope.

Based on lithologic similarity, two rough groups of hybrid rocks are identified. Group 1 rocks consist of chlorite + lawsonite ± epidote ± garnet in varying proportions, while those in Group 2 are dominantly chlorite and may be monomineralic. Some of the latter contain minor magnetite. Both groups may include accessory titanite, rutile (often rimmed by titanite), and apatite. Preliminary bulk-rock geochemical analysis suggests that Group 1 is most similar to the eclogites and Group 2 to the serpentinites. MgO and LOI for both groups are between eclogite (low) and serpentinite (high), with Group 2 > Group 1. CaO, Na₂O, K₂O, and P₂O₅ in Group 2 approximately equal the serpentinites and are generally higher in Group 1.  Serpentinites are the most consistent lithology in major element concentrations. Trace elements of Group 1 approximately parallel the eclogites, blueschist, and metasediments, while Group 2 is between these and serpentinite. Both hybrid groups have equal or higher Al₂O₃ and lower SiO₂ than every other rock type sampled.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-22406

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