Paleo-exhumation histories of the Sakarya and the Istanbul Zones of the Western Pontides, the Almacık Block and its surroundings, NW Turkey

The Almacık Block is a tectonic sliver formed during the activity of the North Anatolian Fault Zone (NAFZ). It includes two important tectonic zones of the Western Pontides (NW Anatolia): the İstanbul and the Sakarya Zones meet along a suture zone called the Intra-Pontide Suture Zone (IPSZ). The units within the İstanbul Zone are exposed in the eastern part of the Almacık Block, whereas the rocks of the Sakarya Zone are exposed in the west. In the presented study, some of the magmatic rocks have been analysed using the zircon U-Pb method: these rocks have previously been either incorrectly or not dated. The U-Pb results have shown that some of the granitic rocks in the İstanbul Zone, which intruded into the amphibolites of the Intra-Pontide Suture Zone, have a crystallization age of 559–556 Ma. A granitoid block in the Upper Cretaceous wild flysch formed over the units of the İstanbul Zone had an age of ~566 Ma. Two distinct granitoid bodies in the Sakarya Zone were dated at 404.5 ± 3.9 Ma (Early Devonian) and 161.8 ± 0.82 Ma (Late Jurassic) ages. The palaeo-exhumation evolutions of the zones were obtained using the zircon (U-Th)/He technique (ZrHe) to understand the differences in the geological evolution of the individual zones. ZrHe ages of the samples obtained from the İstanbul Zone indicate a pronounced Early Cretaceous palaeo-exhumation period. The Middle-Upper Jurassic Mudurnu Formation in the Sakarya Zone gave ZrHe ages close to its sedimentation age, which indicates that the major part of the unit was not buried too deep to open the ZrHe closure system. Unlike those the İstanbul Zone, the main ZrHe age peaks of the Sakarya Zone are ~202, 160, 98, 74, and 52 Ma. Finally, a sample of the metamorphic rocks that were assigned to the IPSZ yielded a peak age of ~63 Ma, which indicates that the final closure of the Intra-Pontide Ocean (IPO) took place during the Early Tertiary.

阿尔马奇克地块（Almacık Block）是北安纳托利亚断裂带（North Anatolian Fault Zone, NAFZ）活动期间形成的构造岩片。该地块涵盖安纳托利亚西北部西庞蒂德造山带（Western Pontides）的两个核心构造带：伊斯坦布尔构造带与萨卡里亚构造带沿庞蒂德内部缝合带（Intra-Pontide Suture Zone, IPSZ）缝合相接。伊斯坦布尔构造带的地质体暴露于阿尔马奇克地块东部，萨卡里亚构造带的岩石则出露于地块西侧。在本研究中，研究团队采用锆石U-Pb定年法（zircon U-Pb method）对部分此前定年有误或未开展定年的岩浆岩展开分析。U-Pb定年结果表明，侵入于庞蒂德内部缝合带角闪岩中的伊斯坦布尔构造带部分花岗岩类岩石的结晶年龄介于559~556 Ma之间。产于伊斯坦布尔构造带地层之上的上白垩统混杂复理石中的花岗质岩块，其形成年龄约为566 Ma。萨卡里亚构造带内两处独立的花岗质岩体的定年结果分别为404.5±3.9 Ma（早泥盆世）与161.8±0.82 Ma（晚侏罗世）。为明晰各构造带地质演化的差异，研究借助锆石（U-Th）/He热年代学技术（zircon (U-Th)/He technique, ZrHe）获取了各带的古剥蚀演化历程。伊斯坦布尔构造带样品的ZrHe年龄数据显示其存在显著的早白垩世古剥蚀事件。萨卡里亚构造带的中-上侏罗统穆杜尔努组的ZrHe年龄与其沉积年龄近乎一致，表明该地层主体未被深埋至足以开启ZrHe封闭体系的深度。与伊斯坦布尔构造带不同，萨卡里亚构造带的主要ZrHe年龄峰值依次约为202 Ma、160 Ma、98 Ma、74 Ma及52 Ma。最后一件归属于庞蒂德内部缝合带的变质岩样品获得了约63 Ma的峰值年龄，这表明庞蒂德内部洋（Intra-Pontide Ocean, IPO）的最终闭合发生于早第三纪时期。