Xenoliths in Late Cretaceous to Early Paleocene adakites of the Eastern Pontides Orogenic Belt, NE Turkey

Abstract

Late Cretaceous to Early Paleocene hypabyssal intrusives near Bayburt in northeastern Turkey, in the Eastern Pontides Orogenic Belt (EPOB), host a variety of xenoliths, including clinopyroxenite, hornblendite, hornblende- and plagioclase-rich gabbro, quartz diorite and amphibole-rich diorite, quartz syenite, and amphibolite. We report major and trace element data for both host rocks and xenoliths, along with zircon U-Pb and amphibole 40Ar/ 39Ar ages, zircon trace element data, mineral compositions, and Sr-Nd-Pb-Hf isotopic data. Host rock compositions range from trachyandesite to dacite and rhyodacite, but major element (e.g. Mg-number) and trace element systematics (e.g. La/YbN) indicate that they are not related by fractional crystallization. Host rocks belong to the high-K calc alkaline and calc-alkaline series, and have the signature of arc rocks (depleted Nb, Ta, Ti, and elevated Pb). Trace element data (Sr/Y and La/Yb) indicate that the dacitic rocks are adakitic. Host rock ages cluster around 70 ± 2 Ma; one intrusion contains zircons that suggest a ~62 Ma crystallization age. Hornblendite xenoliths show ages of 76, 78, and 81 Ma, whereas quartz syenites range from 66 to 70 Ma. The younger ages (62, 66, 70 Ma) fall in an apparent magmatic gap, and document the oldest adakitic magmatism in the EPOB. Mineral compositions indicate initiation of crystallization in the host rocks at ~9 kb and 1250 °C, with progressive crystallization continuing to ~2 kb and 800 °C. Host rocks and xenoliths have similar ranges of 87Sr/86Sri (0.7044–0.7062), 143Nd/144Ndi (−1.5 < εNd < 1.5), and Pbi (18.28 <206Pb/204Pb < 18.68) compositions. The clinopyroxenite, hornblendite, and gabbro xenoliths are broadly cognate with the host rocks, and are cumulate fragments from an arc-root magma system. The quartz diorite, quartz syenite, and amphibolite xenoliths have different histories. Some of the quartz syenites are near eutectic compositions in the H2O saturated q-or-ab system at ~5 kb. The host rocks and xenoliths derive from crustal magma chambers and dominantly crustal melting in the back-arc region of the EPOB.