Geochemistry and petrogenesis of Raviz-Shanabad intrusions (SE UDMB): an evidence for Late Eocene magmatism

The SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB), known as Dehaj-Sarduieh Magmatic Belt, includes a huge pile of Palaeogene volcano-sedimentary rocks and Eocene to Miocene mafic to felsic intrusions. In the middle of this belt (in Raviz-Shanabad, W Rafsanjan) mafic-intermediate intrusions (as stocks and/or dikes-sills) are abundant and have intruded into the volcano-sedimentary rocks of Dehaj-Sarduieh. The thickness of dikes range from 0.5 to >5 metres and the minimum diameter of the intrusions is ~10 m. These rocks are highly porphyritic and have gabbroic to dioritic composition. Petrographic data indicate that the main rock-forming minerals include plagioclase (labradorite to anorthite based on the new electron microprobe data), pyroxene (augite), and opaque (magnetite and titanomagnetite) minerals, set in a granular groundmass of the same minerals. New zircon U-Pb data show that the age of these rocks is 34.39 ± 0.31 Ma (Late Eocene). These rocks show calc-alkaline characteristics and their geochemical signatures are compatible with arcs with a derivation from subduction zones. Estimated temperature and pressure from Clinopyroxene of these rocks suggest a temperature in the range of 1050° to 1150°C and a pressure of >2 kbar. These data show the formation depth of these intrusions should be ~18 km. Initial ⁸⁷Sr/⁸⁶Sr ratio for these rocks varies from 0.70403 to 0.70409, whereas ¹⁴³Nd/¹⁴⁴Nd isotope ratio changes between 0.5128 and 0.5129, corresponding to εNd(i) values of +3.5 to +6.6. These geochemical and isotopic evidences show that the melts originated by 5% partial melting of amphibole spinel peridotite mantle source. Petrographic observations as well as mineral chemistry and isotopic studies indicate that assimilation-fractional crystallization (AFC) has probably played a dominant role in the evolution of the Raviz-Shanabad intrusions. We suggest these intrusions formed as a part of the Late Eocene high-flux magmatism in SE segment of the UDMB in an active continental margin, which was related to the subduction of Neo-Tethys oceanic lithosphere beneath the Central Iranian microcontinent.

被称为德哈杰-萨尔杜耶岩浆带（Dehaj-Sarduieh Magmatic Belt）的乌鲁米耶-多赫塔尔岩浆带（Urumieh-Dokhtar Magmatic Belt, UDMB）东南段，发育巨厚的古近纪火山沉积岩系，以及始新世至中新世的基性-长英质侵入岩体。该带中部位于拉维兹-沙纳巴德（Raviz-Shanabad，拉夫桑詹西部）一带，发育大量基性-中性侵入岩（以岩株和/或岩墙-岩床形式产出），并侵入于德哈杰-萨尔杜耶组火山沉积岩系之中。岩墙厚度介于0.5米至大于5米之间，侵入岩体的最小直径约为10米。此类岩石具显著斑状结构，岩石类型以辉长岩至闪长岩为主。岩相学数据显示，其主要造岩矿物包括斜长石（根据最新电子探针（electron microprobe）数据，其成分从拉长石至钙长石）、辉石（普通辉石）以及不透明矿物（磁铁矿与钛磁铁矿），这些矿物赋存于由同种矿物组成的粒状基质中。最新锆石U-Pb（zircon U-Pb）定年结果显示，此类岩石的形成年龄为34.39±0.31 Ma，对应晚始新世（Late Eocene）。该类岩石具有典型的钙碱性（calc-alkaline）地球化学特征，其地球化学属性与俯冲带成因的岛弧岩浆岩一致。基于单斜辉石（Clinopyroxene）的温压估算结果显示，其形成温度介于1050℃至1150℃之间，压力大于2千巴（kbar）。据此计算，此类侵入岩体的形成深度约为18千米。此类岩石的初始⁸⁷Sr/⁸⁶Sr比值介于0.70403至0.70409之间，而初始¹⁴³Nd/¹⁴⁴Nd同位素比值则介于0.5128至0.5129之间，对应的εNd(i)值为+3.5至+6.6。上述地球化学与同位素证据表明，其岩浆源区为角闪石尖晶石二辉橄榄岩地幔，岩浆由该源区经历5%的部分熔融（partial melting）形成。岩相学观测、矿物化学与同位素研究结果均显示，同化混染-分离结晶（assimilation-fractional crystallization, AFC）作用可能在拉维兹-沙纳巴德侵入岩体的演化过程中占据主导地位。我们认为，此类侵入岩体是UDMB东南段晚始新世高通量岩浆作用的组成部分，其形成于活动大陆边缘环境，与新特提斯洋（Neo-Tethys）岩石圈俯冲到伊朗中部微大陆（Central Iranian microcontinent）之下的构造背景相关。