Late Cretaceous alkaline magmas of the Eastern Pontides Orogenic Belt (NE Turkey): A review with new geological, geochemical and geochronological data

Abstract

The Eastern Pontides Orogenic Belt (EPOB) in northeastern Turkey is a continental arc that is part of the Himalayan-Alpine orogeny. Late Cretaceous magmatism in this belt is dominated by high-K calc-alkaline, shoshonitic, and alkaline rocks that occur over an area of ~600 km (E-W) and ~200 km (N-S). We present new geochemical data (whole rock major and trace element analyses, mineral compositions, U-Pb zircon and titanite ages, zircon trace element data, and Sr-Nd-Pb isotopic analyses) for 153 samples, with compiled data for an additional 265 samples. Magmatism is diachronous across the arc. In the north, Cycle-I (92–84 Ma) commenced with tholeiitic and calc-alkaline basalts and andesites, and continued with felsic extrusive rocks. Cycle-II (84–70 Ma) includes mainly intermediate and felsic extrusive rocks. Over time,the low-K granitoids progressively give way to calc-alkaline granitoids in the north. In the south, Cycle-I (84–78 Ma) includes high-K trachyandesites, trachytes and felsic extrusive rocks. Cycle-II (78–65 Ma) comprises leucitites, trachybasalts, and other K-rich rocks. Shoshonitic granitoids in the south show ages between 80 and 65 Ma. Clinopyroxene compositions in trachyandesites and leucitites indicate initial crystallization at ~38 km depth and 1120 °C. Amphibole joins the phase assemblage at ~8 km depth and T < 950 °C. All samples have LILE and LREE enrichments and negative Nb, Ta, and Ti, and positive Pb anomalies, consistent with their subduction-related origin. Isotopic data indicate mixing between magmas from heterogeneous, enriched subcontinental mantle sources and those from a range of crustal sources. Mantle-derived and crustally-derived magmas are clearly separated by (Gd/Yb)N. Chemical zoning of the EPOB, from tholeiitic and calc-alkaline rocks to high-K calc-alkaline, shoshonitic, and alkaline rocks, from north to south, and the timing of magmatism younging southward, are consistent with a model of southward subduction in the Late Cretaceous.