Paleozoic Basement Structure of Northwest Anatolia (an Approach)

Summary

The principle Paleozoic basement structure of NW-Anatolia is that of large-scaled linear segments, which have undergone different structural and sedimentary evolutions. The segments are composed of either high-grade metamorphic rocks or ultramafitites, being mantled by low-grade metasedimentary strata. The ultramafitite includes harzburgite, lherzolite, dunite with various grades of serpentinization, subordinate hypabyssal intrusions of gabbro, norite, diorite, and rodingites. Textural and compositional criteria suggest them to have suffered metamorphic processes. The over-all distribution pattern and contact relationships to disconformably overlying metamorphic and non-metamorphic rock units involve the fact that the ultramafitite is a differentiated mantle portion to constitute a simatic crust. Its emplacement dates Pre-Ordovician (or Precambrian). High-grade green-schist facies metamorphic rocks crop out in isolated polygonal masses surrounded by ultramafitite into clear-cut faulted boundaries. The rock succession can be generalized into augen-gneiss, banded gneiss, schist and marble. The gradational contacts and interlayers of different rock types indicate the parent rocks to be mainly clastic sediments. It is suggested that the high-grade metamorphic rocks constitute a sialic crustal layer underlain by a simatic layer (= differentiated mantle portion). The low-grade metasedimentary rocks occur within five distinct stratigraphic units, in ascending order: lawsonite-albite schist facies rocks, low-grade greenschist facies metaclastics, metacarbonates, metavolcanics and glaucophanitic greenschist facies metaclastics-metavolcanics. The units, interrupted by disconformities, overlie the erosional surface of the ultramafitite, and superimpose the high-grade metamorphics. A Pre-Ordovician age is assigned the low-grade metamorphics.