Geochemical features and K-Ar age data from metadetrital rocks and high-K metasomatized metarhyolites in the Afyon- Bolkardag Zone (llgin-Konya), SW Turkey

摘要

The Afyon-Bolkardag Zone (ABZ) is made up of two main metamorphic sequences - a Paleozoic sequence and a Meso-zoic sequence, both containing metadetrital rocks and metacarbonates. Although the metasediments have similar tectonic settings and sedimentary provenances, the Mesozoic sequence has transgressive characteristics and includes dolomitic metacarbonates and metavolcanics. In the Ilgin (Konya) area, the low-grade metavolcanics were originally rhyolitic lavas and tuffs. Metarhyolites are extensively K-feldspathised. K-metasomatism has produced K-feldspar porphyroblasts, sericitization in the matrix and has modified the K/Na, K/Rb and Rb/Sr ratios. The metarhyolites are extremely K-rich (6-11 wt.% K_2O), with one exception that is marginally alkaline. They are subalkaline and calcalkaline in character. The REE data define two groups of metarhyolites having relatively high and low heavy REE abundances. The results indicate an origin of the high-K rhyolites in an extensional tectonic regime during the early-Alpine rifting stage, which could be related to the opening of the northern branch of the Neo-Tethys on the northern margin of Gondwana as a consequence of a southward subduction of the Paleo-Tethys. The mineral assemblage of the Paleozoic metasedimentary sequence is quartz, albite, chlorite, muscovite, biotite, calcite, stilpnomelane, and chloritoid, whilst the absence of stilpnomelane and chloritoid in the Mesozoic metasedimentary sequence demonstrates low-grade greenschist metamorphism and also decreasing metamorphic grade upwards. P-T conditions vary around 380-410 °C and ≥ 5 kbar for Paleozoic and Mesozoic mica schists and associated metarhyolite. One sample from the Paleozoic sequence yielded aphengite K-Ar age value of 96.9+ 1.4 Ma, whilst three samples from the Mesozoic sequence yielded phengite K-Ar age values of 70.2 + 1.0 Ma, 65.6 ± 1.0 Ma and 64.4 + 1.0 Ma. These age values are likely a consequence of incomplete resetting of the K-Ar system in the phyllosilicates during Alpine (Late Cretaceous-present) low-grade metamorphism in this region. Three metarhyolites from the Mesozoic sequence yielded whole-rock K-Ar age values of 60.4 + 0.9 Ma, 62.6 + 0.9 Ma and 64 + 2 Ma which also could reflect Alpine metamorphism and metasomatism.