Saraçköy Volcanic Suite: implications for the subductional phase of arc evolution in the Galatean Arc Complex, Ankara, Turkey

Abstract

A sequence of upper Campanian volcanic rocks, hitherto considered as part of the Miocene calc-alkaline Galatean volcanic sequence, crops out near Saraçköy, 40 km north of Ankara. These volcanic rocks, unconformably overlain by Eocene nummulitic limestones, comprise thin flows and volcaniclastic deposits. They can be divided compositionally into three distinct subunits: (i) basal massive trachyandesitic volcaniclastic rocks with a thin flow at the top, overlain by (ii) mixed volcaniclastic rocks and thin amygdaloidal flows of alkali basaltic composition, with (iii) trachyandesitic flows and subordinate volcaniclastic rocks at the top. This alkaline volcanic activity, associated with late Campanian forearc peripheral sedimentation, is indicative of late Campanian extension within a collisional regime, pre-dating the compression accompanying Galatean arc development.Indeed, the composition of the Saraçköy Volcanic Suite and field evidence are consistent with a slab roll-back process in an extensional supra-subduction setting which resulted from the northward-subducting, northern Neotethyan ocean floor during the late Campanian. They also imply that the present-day Galatean volcanic plateau is a magmatic arc development in three stages: (i) late Campanian-Lutetian subductional phase, (ii) post-Lutetian collisional phase, and (iii) Oligocene-Miocene postcollisional phase.

Zircon ages from the Beypazarı granitoid pluton (north central Turkey): tectonic implications

“…Three of the ages obtained from aplite dike zircons are concordant (61.7 ± 3.6 Ma, 5.6 ± .3 Ma and 5.0 ± .3 Ma). Although we speculate that the aplite data may document alteration and/or metamorphism rather than timing a specific geologic event, the two concordant Pliocene ages merit further investigation given the Late Miocene-Early Pliocene volcanism in the GAC (Keller et al, 1992;Koçyiğit et al, 2003).…”

“…Volcanism in the GAC is reported during this time (K-Ar hornblende, 76 ± 2.4-73.2 ± 1.2 Ma) (Keller et al, 1992;Koçyiğit et al, 2003). Based on geochemical similarities between the Beypazarı granitoid and the GAC, the coeval melts may originate from a similar source.…”

“…The CL images of the sample provide evidence of metamorphism or fluid-driven alteration rather than crystallization (Figure 12) (Hoskin & Schaltegger, 2003;Watson, Cherniak, Hanchar, Harrison & Wark, 1997). Late Miocene-Early Pliocene volcanism occurred in the nearby GAC (Keller et al, 1992;Koçyiğit et al, 2003), and present-day evidence for extension and fault-driven fluid-rock interaction in the southwest Beypazarı granitoid are represented by the Kapullu Bath Spring, a deeply circulating geothermal feature characterized by normal faults (Diker et al, 2006).…”

“…Low radiogenic Ar is reported for these ages (37-74%), and the sample locations are not published. K-Ar hornblende ages reported for volcanic rocks in the nearby Galatean Arc Complex (GAC) (Figure 1) are 76.4 ± 2.4 Ma (Koçyiğit, Winchester, Bozkurt & Holland, 2003) and 73.2 ± 1.2 Ma (Keller, Jung, Eckhardt & Kreuzer, 1992).…”

“…The subducting Tavşanlı slab was likely the source of mantle melts that induced the parental magmas of the Beypazarı granitoid and the GAC, which has been interpreted as the back-arc of a supra-subduction zone setting (Koçyiğit et al, 2003). A possible reason for the overlap in Late Cretaceous ages is concurrent magmatism from a similar source in response to rollback of the subducting Tavşanlı slab, which could also account for back-arc extension in the GAC (Koçyiğit et al, 2003). Alternatively, the Late Cretaceous zircons in the Sivrihisar granitoid may be related to exhumation of the HP/LT assemblages and were entrained in the rising melts during intrusion of the granitoid.…”