Hide/Show Apps

Petrology and geochronology of the igneous rocks from the Çangaldağ metamorphic complex and the Çangaldağ pluton (Central Pontides, Turkey)

Çimen, Okay
The Central Pontide Structural Complex (CPSC) in northern Turkey forms an imbricated unit that consists of a number of variable metamorphic oceanic assemblages between the Istanbul-Zonguldak and Sakarya Composite terranes. The Çangaldağ Complex comprising the Çangaldağ Metamorphic Complex and the Çangaldağ Pluton is one of the members of this body and occurs as an E-W trending 30 km long and 15 km wide belt to the north of the Kastamonu-Boyabat Tertiary basin. The Çangaldağ Metamorphic Complex consists predominantly of variably deformed and metamorphosed igneous rocks ranging from felsic to mafic in composition. The main volcanic lithologies are basalt, tuff, diabase, andesite and rhyodacite associated with sediments. The mafic and intermediate volcanic rocks were mainly subjected to greenschist facies metamorphism as evidenced by the assemblage of alb+act+chl+ep. The felsic phases exhibit blastoporphyritic textures with musc+ alb+ chl as the metamorphic parageneses. Geochemically, all metavolcanic rocks display the typical features of subduction-related magmas as evidenced by variable enrichment in LREE over HFSE. LA-ICP-MS U-Pb geochronological data from zircons of three different meta-rhyodacite samples yield Middle Jurassic ages between 156 ± 3 Ma and 176 ± 6 Ma. Additionally, in-situ 176Hf/177Hf initial ratios (between 0.28281 ± 0.00003 and 0.28276 ± 0.00003) on the same zircons correspond to TDM model ages between 538 and 678 Ma, which suggest partial melting of a depleted mantle, affected by the Cadomian arc magmatism. This Cadomian signature is common in Gondwana-derived terranes. The Çangaldağ Pluton to the northeast of the Çangaldağ Complex is bounded by a strike-slip fault towards the Çangaldağ Metamorphic Complex. The pluton comprises non-metamorphic diorite, dacite porphyry, and lesser amount of granitic rocks. The dioritic rocks are surrounded by dacite porphyries, which indicate a zoned nature (mafic core with felsic rim) of the complex. Geochemically, various rock types from this pluton exhibit calc-alkaline features and are akin to volcanic arc magmatics displaying LILE/HFSE enrichment coupled with Nb depletion. The available data from the pluton suggest derivation primarily from partial melting of an amphibolitic source. The diorites and dacite porphyries exhibit geochemical features of I-type granites, whereas the granitic rocks show typical features of S-type granites. LA-ICP-MS U-Pb dating of zircons from dacite porphyry and granite samples yielded ages of 161 ± 6 Ma and 170 ± 2 Ma, respectively. Corresponding in-situ 176Hf/177Hf initial ratios are 0.28276 ± 0.00003 and 0.28213 ± 0.00002 for the dacite porphyry and granite, respectively. These results support their generation in a marginal arc system with the involvement of a subduction-modified mantle source. TDM model ages vary between 674 and 1512 Ma and suggest that the Çangaldağ Pluton magmatism has formed by partial melting of Neoproterozoic/Mesoproterozoic crustal rocks, which are common in Gondwana-derived terranes. Overall the geochemical and geochronological data confirm the presence of an island-arc (Çangaldağ Metamorphic Complex) and a continental-arc (Çangaldağ Pluton) system during the Middle Jurassic period formed by the partial closing within the Intra-Pontide Ocean.