Early Cambrian back-arc volcanism in the western Taurides, Turkey: implications for rifting along the northern Gondwanan margin


Gursu S., Goncuoglu M.

GEOLOGICAL MAGAZINE, cilt.142, ss.617-631, 2005 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 142 Konu: 5
  • Basım Tarihi: 2005
  • Doi Numarası: 10.1017/s0016756805000919
  • Dergi Adı: GEOLOGICAL MAGAZINE
  • Sayfa Sayıları: ss.617-631

Özet

The Lower Cambrian (Tommotian) Gogebakan Formation in western Central Anatolia is made up of slightly metamorphosed continental to shallow marine elastic rocks with pillowed and massive spilitic lavas and dolerite dykes. Spilitic lavas, commonly amygdaloidal, are albite-and pyroxene-phyric with the metamorphic mineral paragenesis albite + calcite + sericite +/- epidote +/- tremolite +/- chlorite. Dolerite dykes mainly include plagioclase and pyroxene as primary minerals and tremolite epidote chlorite as low-grade secondary minerals. Geochemical data show that the spilitic lavas and dolerite dykes are sub-alkaline, of oceanic tholeiitic basalt character and display a tholeiitic fractional trend, characterized by an increase in FeO/MgO and Zr and TiO2 in variation diagrams. They are characterized by relatively high Zr/Y (2-4.5), relatively high Th/Yb (0.15-1.0) and La/Nb (0.5-2.5). Both show marked negative Nb and Ti anomalies relative to Th and La (Ce), implying a subduction-related chemistry. Chondrite-normalized REE patterns display slight enrichment of light REE (spilitic lavas (La/Yb)(N) = 0.79-1.56; dolerite dykes (La/Yb)(N) = 0.89-3.50) fairly comparable with MORB. The geochemical similarity of the spilitic lavas and dolerite dykes suggests a co-genetic origin. La/Nb ratios of both types are slightly higher than average MORB values and were possibly formed in the early stages of back-arc basin development. Petrogenetic modelling suggests the mafic rocks of the formation were formed by 9% batch melting of spinel 1herzolite in shallower depths (c. 60 km). Taken together the data suggest that the Early Cambrian mafic rocks of the Taurus units were developed in a back-are basin along the northern edge of Gondwana above the southward-subducting oceanic lithosphere and may represent initial rifting that resulted in separation of the peri-Gondwanan terranes.