E. Yalçın Ersoy

40Ar/39Ar geochronology, geochemistry and petrology of volcanic rocks from the Simav Graben, western Turkey

AbstractMajor and trace element compositions with Sr–Nd isotopic ratios, as well as Ar–Ar radiometric ages of the Miocene volcanic rocks from the Neogene units around Simav region (western Anatolia), are used to discuss the genetic relationship between (1) high-voluminous Lower–Middle Miocene high-potassic, calc-alkaline (HKCA) and (2) Middle Miocene small-voluminous high-MgO shoshonitic–ultrapotassic (SHO–UK) magmatic units in the region. All the HKCA rocks, including basaltic to rhyolitic (and granitic) samples, share similar trace element characteristics (enrichments of LILE and LREE and depletions in HFSE as a common feature of orogenic magmatic rocks), with subtle differences in their 87Sr/86Sr(i) ratios (basalts and rhyolites ~0.708, dacites ~0.710). Most of the samples of the high-MgO SHO–UK group are classified as shoshonite and latite, with some lamproites, sharing similar geochemical features with the other ultrapotassic rocks of the Mediterranean. All the rock groups have similar and high abundances of incompatible trace elements, and radiogenic Sr. Geochemical modeling of the trace element and isotopic ratios of the samples reveals that both the SHO–UK and HKCA groups were derived from a common mantle source which had been highly metasomatized and enriched by continental materials during partial subduction of the crustal metamorphic slices in a continental collision setting. The geochemical variations of these rocks were mainly controlled by source characteristics (such as heterogeneity) and variable degrees of partial melting and subsequent effects of fractional crystallization, with low degrees of crustal contamination. The HKCA series were derived by higher degrees of partial melting of the lithospheric mantle source than the SHO–UK rocks. The HKCA rocks then underwent two-stage fractional crystallization (clinopyroxene-dominated followed by feldspar-dominated fractionating mineral assemblages) to form the high-K calc-alkaline basalt to rhyolite series, whereas the SHO–UK rocks experienced comparatively little fractional crystallization. A tectonic scenario involving the rapidly extending and thinning of orogenic crust is compatible with the time-dependent compositional variation of the magmatic rocks.

Soma Havzası ve Çevresindeki Miyosen Volkanizmasının Petrografik ve Jeokimyasal Özellikleri, Batı Anadolu, Türkiye

Bati Anadolu’daki onemli linyit yataklarini iceren Soma havzasi ve cevresinde yuzlek veren Miyosen yasli volkanik birimler baslica uc gruba ayrilir. Bunlar: (i) B, KB ve GB’da havza istifi ile kismen giriklilik sunan ve onlari uyumlu uzerleyen alt ve orta Miyosen yasli Kozak-Yuntdagi volkanitleri ile KD’da Bigadic volkanosedimanter havzasina ait Sindirgi-Gelembe volkanitleri, (ii) havza icinde Dededag volkanitleri ve (iii) havza istifini kesen gec Miyosen Adilkoy volkanitleridir. Bu calismada Kozak-Yuntdagi ve Dededag volkanitleri ile Adilkoy volkanitlerinin petrografik ve jeokimyasal ozellikleri incelenmis ve petrolojik evrimleri tartisilmistir. Kozak-Yuntdagi volkanitleri ortac-yuksek potasyumlu kalk-alkali andezit, latit, dasit, trakidasit ve riyolitlerden, Dededag volkanitleri ortac potasyumlu kalk-alkali andezitlerden ve Adilkoy volkanitleri ortac potasyumlu kalk-alkali bazaltik andezitlerden olusur. Tum volkanik birimler ilksel mantoya gore normallestirilmis iz element diyagramlarinda, Nb, Ta ve Ti gibi yuksek degerlikli katyonlar bakimindan Rb ve Ba gibi genis iyon capli elementlere gore tuketilme sunar. Silika iceriklerine gore incelenen ana ve iz element degisim diyagramlari ve iz element oranlari Kozak-Yuntdagi volkanitlerinin Dededag volkanitlerinden ayrimlasmali kristalizasyon ve muhtemelen kabuksal kirlenme surecleri esliginde turedigini gosterir. Gec Miyosen yasli Adilkoy volkanitlerinin jeokimyasal ozellikleri, bu volkanik birimlerin Kuvaterner Kula volkanitleri gibi baslica astenosferik kokene sahip olan volkanik birimlerden ve diger gec Miyosen yasli ve astenosferik katki iceren volkanitlerinden farkli oldugunu gosterir. Bu yonuyle, bolgede gec Miyosen’den itibaren tum bazaltik volkanitlerde etkisi oldugu dusunulen astenosferik katkinin Soma havzasi icin gecerli olmadigi ve havzadaki volkanizmanin Miyosen boyunca yalnizca litosferik mantodan turedigi sonucuna ulasilmistir