سنگ‌زایی و جایگاه زمین‌ساختی مجموعه آذرین درونیِ بَکتِر (جنوب سنقر، باختر ایران)

مجموعه‌ آذرین درونی بَکتِر در شهرستان سنقر، استان کرمانشاه و در پهنه زمین‌ساختی سنندج-سیرجان جای دارد. این مجموعه شامل گابرو، سینیت، آلکالی‌سینیت و کوارتز سینیت است. در بررسی‌های سنگ‌نگاریِ سینیت‌ها، بافت‌های هیپ‌ایدیومورف گرانولار، پویی‌کیلیتیک، پرتیت و گرانوفیری و همچنین، بافت غربالی دیده می‌شود. آلکالی‌فلدسپار، پلاژیوکلاز، کلینوپیروکسن، آمفیبول و بیوتیت از کانی‌های اصلی سینیت‌ها هستند. ویژگی‌های بافتی مانند بافت‌های ناتعادلی (مانند بافت غربالی در پلاژیوکلاز) را می‎توان پیامد تحولات ماگمایی در هنگام جایگزینی در آشیانه‌های ماگمایی متعدد و آمیختگیِ فازهای جدایش‌یافته در این آشیانه‌های فرعی با پوسته و ماگمای مافیک دانست. ذوب‌بخشی و آمیختگی مهم‌ترین عوامل پیدایش سنگ‌های آذرین درونی منطقه هستند. نسبت la/yb در برابر la رخداد این فرایند را آشکار می‌کند. داده‌های به‌دست‌آمده در این پژوهش شامل غنی‌شدگی عنصرهای lree در مقایسه با hree، آنومالی مثبت pb، تهی‌شدگی عنصرهای nb و ti و دیگر داده‌ها، نقش احتمالی آلایش پوسته‌ای در بخش‌های اسیدی مجموعه بَکتِر است روندهای دیده‌شده در تحول ماگمای سازنده این توده آذرین درونی وابستگی مذاب‌های مادر این توده آذرین درونی به فازهای کششی پس از برخورد و پشت‌کمان ماگمایی را نشان می‌دهند. الگوهای زمین‌شیمی نشان می‌دهند ماگمای مادر مجموعه سنگی بکتر شامل بخش اسیدی (سینیتی) و بخش بازیک (گابرویی) از یک گوشته غنی‌شده یا دگرنهادشده با ترکیب گارنت لرزولیت با درجه ذوب‌بخشی 5 تا 15 درصد پدید آمده است. با توجه به بررسی نمودارهای زمین‌شیمیایی، توده‌های آذرین درونی بکتر در پی سازوکار زمین‌ساختی کششی یا نازک‌شدگی پوسته‌ای در یک پهنه پسابرخوردی میان دو پوسته قاره‌ای رخ داده‌اند. این سنگ‌ها در پی فرورانش پوسته اقیانوسیِ نئوتتیس به زیر صفحه ایران مرکزی در پهنه دگرگونی-ماگمایی و پس از برخورد پوسته قاره‌ای ایران مرکزی و عربی در پهنه سنندج-سیرجان پدید آمده‌اند.

petrogenesis and tectonic implications of bakter intrusive complex (south sonqor, west of iran)

introductionthe sanandaj-sirjan zone formed a piece of the northeastern section of the gondwana continent during the paleozoic era and separated it from the asian plate by the paleo-tethys ocean (golonka., 2004). during the middle and late triassic, coinciding the closure of the paleo-tethys, a rift called the neo-tethys sea developed along the zagros region, separating the sanandaj-sirjan zone from the arabian plate (part of gondwana). crustal subduction is thought to have initiated in the late triassic to early jurassic (berberian, 1981). this subduction led to the deformation of rocks and emplacement of intrusive masses during the late triassic in the southern part of the sanandaj-sirjan zone, and the emplacement of various masses from gabbroic to granitic rocks during the earlier times in the northern part of the zone from the middle jurassic (shahbazi et al., 2010; mahmoudi et al., 2011) to the middle cretaceous (ghalamghash et al., 2009) and early eocene (ahadnejad et al., 2010). the closure of the neo-tethys sea and the subduction of the oceanic crust are often been associated with the emplacement of ophiolites along the zagros zone during the late cretaceous to paleocene (agard et al., 2005), in addition to some granitoid bodies of the upper eocene to oligocene. according to (berberian 1981 and azizi and moinevaziri, 2009), these bodies and their ages indicate the continuation of the oceanic crustal termination and the collision between the arabian plate and central iran during the neogene.several valuable studies have been conducted on the magmatism of intrusive masses within the sanandaj-sirjan zone, including the southern dehgalan (sarjoughian et al., 2015), northwest of azna (shabanian et al., 2009), cheshme sefid (davoudian et al., 2007), qorveh-mobarakabad (azizi and asahara., 2013), southeastern saqqez, and almoghlaq batholith.regional geologythe study area is located in the northwestern part of the sanandaj-sirjan zone, southeast of the city of sanandaj in kermanshah province. it lies within the geographical longitudes of ’35°47 to ’30°47 east and latitudes of ’35°34 to ’38°34 north, within the southwestern portion of the 1:100,000 sanandaj geological sheet. the upper cretaceous phyllitic units are intruded by syenitic-gabbroic intrusive masses and have undergone metamorphism and transformation into hornfels due to neighboring intrusive masses and associated metamorphism. the geological map at a scale of 1:25,000 (mokhtari and kohestani, 2018) has delineated the rock units of the area (figure 1).research methodologyfollowing the field studies and mapping with satellite images, field observations were conducted. microscopic studies were carried out on the collected samples. following the examination of thin sections of the rocks, 12 samples exhibiting the least evidence of secondary processes, including mineral alteration, cavity filling, and the presence of secondary veins and fractures, were selected. subsequently, the samples were dispatchedare sent to the zara-azma laboratory for chemical analysis. this entailed thethrough this analysis determination of the abundance of trace and rare earth elements using are determined by applying the inductively coupled plasma mass spectrometry (icp-ms) and major oxides using the fusion method.petrography:the study area consists of gabbro, syenite, alkali syenite, and quartz syenite. the dominant textures are hypidiomorphic granular, poikilitic, perthitic, and granophyric in the syenites, as well as sieve-textured fabric. all rock types trending northeast-southwest. gabbroic intrusions:gabbroic intrusions are present in the southern part of the baetar intrusive complex and at the margin of the syenitic intrusion. in the northern margin of the syenitic intrusion, two small outcrops of gabbroic intrusion are observed, where one is in contact with the syenitic intrusion, and the other is within the units of hornfelsed shale, siltstone, and metamorphos sandstone of upper cretaceous era. syenitic intrusion:in the central region of the bakter intrusive complex, the outcrops of syenitic intrusion are widespead. the syenitic intrusion is in close proximity to the gabbroic intrusion, particularly in the southern region.geochemistrybased on whole rock geochemical data, especially the la/yb vs. la ratio, these rocks share the same genesis. geochemical data, including lree enrichment compared to hree, positive pb anomaly, and depletion of nb and ti, reveal the divergent regimes in a back-arc zone, through which the primary magmatic melts are generated in the bakter intrusive complex. the presented geochemical patterns indicate that the parental magma of the studied rock group is derived from an enriched or metasomatized garnet lherzolite with a degree of partial melting within 5% to 15% range..conclusionfield studies, petrography, and geochemical analysis run on the subject region reveal that it is composed of mafic and felsic rocks, with the felsic rocks mainly consisting of syenite, alkaline syenite, and quartz syenite, while the mafic rocks are of a gabbroic composition. spider diagrams normalized to primitive mantle compositions reveal the enrichment in large-ion lithophile elements (lile) and light rare earth elements (lree) compared to high field strength elements (hfse), with negative anomalies observed in niobium (nb) and titanium (ti). the parental magma of the studied plutonic rocks is derived from low degrees of partial melting (5-15%) of a garnet-bearing lherzolite source at depths of 95-110 kilometers. considering that the intrusive rocks of the baetar region are associated with post-collisional activities and the presence of a thin and stretched lithosphere is appropriate for their formation, it is assumed that a secondary subduction with a slab break-off mechanism occurred in the sanandaj-sirjan zone for a relatively long period after the initial collision.