عنوان

Exhumation of blueschist-facies assemblages from western Turkey :

TLets326182

Exhumation of blueschist-facies assemblages from western Turkey :

[Thesis]

Sherlock, Sarah Christine

the significance of ⁴⁰Ar-³⁹Ar ages and excess argon in a HPLT terrain

Open University

1999

Thesis (Ph.D.)

1999

The Tavsanli Zone of northwest Turkey is a tract of metacherts, metapelites and metabasite lithologies that have undergone a single metamorphic cycle in the lawsonite-blueschist, epidote-blueschist and low-temperature eclogite sub-facies. These deeply subducted rocks which formed during late Cretaceous closure of the Tethys Ocean reached minimum pressures of 24 ± 2 kbar and temperatures of approximately 500° C in the eastern regions, and 20 ± 2 kbar and 400° C in the central and western regions. The difference in metamorphic pressures between the east and west regions of the Tavsanli Zone are attributed to different locations of the rocks within the subducting slab. Syn-subduction exhumation brought the rocks up from depths of 75 km to the base of the continental crust. Metamorphism took place under fluid-absent conditions, in part due to the incorporation of large quantities of water into the lawsonite. Subduction ended by approximately 70 Ma and the ensuing exhumation during continent-continent collision took place by thrusting. Rb-Sr and ⁴⁰Ar-³⁹Ar geochronological techniques have been applied to phengites from the Tavsanli Zone and indicate that white mica crystallisation and the onset of exhumation occurred at 79 to 82 Ma. Ultra-violet laser, infra-red laser and furnace step-heating ⁴⁰Ar-³⁹Ar techniques confirm that the rocks of the Tavsanli Zone contain excess argon, which has probably built up in the grain-boundary network during the single metamorphic cycle. The variable concentrations of excess argon correlate with lithological type, reflecting low mobility of argon. This is consistent with vapour-absent metamorphism as inferred from metamorphic petrology.

Tavsanli zone; Metamorphism; Tethys Ocean

Sherlock, Sarah Christine

Open University

p

[Thesis]

276903

a

Y