AN EXAMPLE FOR A PRE-EARLY ORDOVICIAN ARC MAGMATISM FROM NORTH TURKEY: GEOCHEMICAL STUDY OF THE ÇAŞURTEPE FORMATION (BOLU, W PONTIDES)

Three different units are exposed underneath the Palaeozoic sequence of W Pontides in the Bolu-Yedigöller area. These are, from the structural base to the top: i) high grade metamorphic rocks (the Sünnice group), ii) granitoids and, iii) a volcanic sequence (the Çaşurtepe formation) into which the granitoids intruded. The granitoidic intrusions are a part of an extensive group of intrusions, called the "Bolu Granitoid Complex" (BGC) and, together with the Çaşurtepe formation, they crop out structurally on top of the Sunnice group along an NE-SW trending, NW-dipping tectonic contact. The granitoids are cut by a number of lamprophyre dykes. The Çaşurtepe formation, the main subject of this paper, comprises of andesitic lavas at the base, overlain by an ignimbrite series in which rhyolitic volcaniclastics are dominant. Both the volcanic sequence and the granitoids were metamorphosed to greenschist facies and as a result, an albite+epidote+chlorite+actinolite mineral assemblage was developed, together with relict igneous minerals. An extensive pyrite mineralization is developed along sinuous shear zones in locally developed intense hydrothermal alteration areas. Massive lavas of the volcanic sequence are calc-alkaline andesitic and locally rhyo-dasitic in composition with high SiO2 content (> % 54). These have LIL-element enrichment relative to N-type MORB and show Nb depletion relative to LREE (La, Ce, Nd). The dikes within the granites have similar chemical characteristics. One sample analyzed from the Çaşurtepe formation gave 87Sr/86Sr 550 Ma model age value of 0.706482,143Nd/144Nd model value of 0.512450 and £Nd value of 10.2. Both major and trace elements of volcanic rocks indicate that the lavas are products of calcalkaline active margin arc volcanism, developed above a subduction zone. 143Nd/144Nd - 87Sr/86Sr isotope ratios depart from typical MORB values and are compatible with those of intra-oceanic arcs. The Sünnice group, the Çaşurtepe formation, and the BGC are unconformably overlain by Lower Ordovician continental elastics of the Palaeozoic of Istanbul. Therefore, the data presented here point out to the existence of subduction-related magmatism during the pre- Early Ordovician period in W Pontides

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ISSN: 0026-4563
Yayın Aralığı: Yılda 3 Sayı
Başlangıç: 1950
Yayıncı: Cahit DÖNMEZ

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