Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Niğde Massif, Turkey

Abstract

Protracted heating and melting of orogenic middle crust followed by emplacement of a syn-extensional pluton during decompression and cooling are demonstrated for a metamorphic core complex in central Turkey. New geochronologic data for the Niğde Massif of the Central Anatolian Crystalline Complex document an interval of melting, crystallization, and Pb loss at elevated ambient temperature (85–92 Ma, determined by U–Pb SHRIMP) within a single sample of peraluminous granite. Crustal melting was syn- to post-peak metamorphism (725 °C), dated at 91.0±2.0 Ma by SHRIMP analysis of zircon rims in sillimanite schist that is part of the country rock to the pluton. Metamorphic monazite in sillimanite schist yields a younger age of 84.7±0.7 Ma for matrix grains and inclusions in garnet. Hornblende 40Ar/ 39Ar ages from amphibolites interlayered with the sillimanite schists coincide with the timing of emplacement of magma into metamorphic country rock that had experienced cooling and decompression to <12 km and ∼550 °C. The similarity of 40Ar– 39Ar ages for biotite in the pluton (79 Ma) and hornblende (79–81 Ma), biotite (74–76 Ma), and muscovite (78–83 Ma) in the metamorphic country rock document rapid cooling (≥100 °C/my) following intrusion. The coincidence of the youngest U–Pb magmatic zircon ages and the oldest 40Ar– 39Ar ages, combined with structural evidence for normal-sense shearing and tectonic unroofing in the Late Cretaceous, suggests that intrusion was driven by extension. We propose that crustal melting occurred in the middle crust during prograde metamorphism and/or initial decompression, and that ascent and emplacement of magma as a pluton were only triggered once extension occurred and removed upper crustal rocks.