سنگ‌زایی سنگ های نیمه ژرف نئوژن کامو، شمال اصفهان

منطقة کامو از دیدگاه ساختاری در بخش میانی کمان ماگمایی ارومیه-دختر و در فاصلة تقریبی 120 کیلومتری شمال اصفهان جای دارد. در این منطقه، چندین برونزدهای از سنگ‌های آذرین نئوژن با طیف ترکیبی داسیت- آندزیت به شکل گنبدها و توده‌های نیمه‌ژرف رخنمون یافته‌اند. این سنگ‏‌ها بافت‏‌های پورفیروییدی تا پورفیری با خمیرة میکروکریستالین دارند و پلاژیوکلاز، آمفیبول، بیوتیت، کوارتز و کانی‏‌های کدر از کانی‏‌های اصلی سازندة آنها به‌شمار می‌روند. شیمی کل سنگ‏‌ها نشان‌دهندة سرشت ماگمایی متاآلومینه و کالک‌آلکالن با غنی‌شدگی از ‌عنصرهای خاکی کمیاب سبک، روبیدیم، باریم و پتاسیم و آنومالی منفی ‌عنصرهای خاکی کمیاب سنگین، نیوبیم، تیتانیم و تانتالیم است. ‌نمودارهای بهنجارشده به ترکیب کندریت، شیب منفی از ‌عنصرهای خاکی کمیاب سبک به‌سوی سنگین و بدون بی‏‌هنجاری یوروپیم را به نمایش می‌گذارند. بررسی نسبت‌های ‌عنصرهای کمیاب و خاکی کمیاب نشان می‌دهد سنگ‏‌های یادشده ویژگی‏‌های بینابین ماگماهای کالک‌آلکالن و آداکیتی دارند و از ذوب‌بخشی گارنت آمفیبولیت در ژرفای پایداری گارنت پدید آمده‏‌اند. شواهد زمین‌شیمیایی نشان‌دهندة پیدایش و تحول این سنگ‏‌ها در کمان ماگمایی حاشیة قاره‏‌ای هستند. خاستگاه احتمالی ماگمای مادر این سنگ‏‌ها، ذوب‌بخشی گارنت آمفیبولیت دگرنهادشده تحت‌تاثیر سیال‌ها و مذاب‏‌های آزادشده از تختة فرورو است.

petrogenesis of neogene subvolcanic rocks in kamu, north of isfahan

introductionneogene igneous domes, formed by the eurasian and arabian subduction, exposed in the north of kamu in the urumia-dokhtar magmatic arc. the study area is located 120 km north of isfahan, between 31° 05° to 31° 10° north latitude and 51° 10° to 51° 20° east longitude. the purpose of the present paper is to investigate the tectonic setting, petrogenesis, and magmatic evolution processes involved in the formation of studied igneous rocks.analytical methods after analyzing satellite images, maps and the previous studies, outcrops were selected for sampling. following field studies, samples were collected and observed under a polarizing microscope as thin sections. 11 samples with the lowest amount of alteration were selected and analyzed by inductively coupled plasma spectrometry at acme canada laboratory. the article was written after analyzing and combining data from field studies, petrography, geochemistry, and chemical analyses results using gcdkit and coreldraw software.petrography and whole rocks chemistry the studied rocks are mainly dacite and andesite. andesites are porphyritic with phenocrysts of plagioclase, amphibole, biotite and rarely pyroxene. dacites have fine-grained or porphyry textures with aphanitic or trachytic groundmass and phenocrysts of plagioclase, quartz, amphibole and biotite. the mineralogy of dacites is similar to that of the andesitic rocks and are distinguished from andesites by the absence of pyroxene and the presence of quartz microcrystals. sericite, kaolinite, chlorite, calcite, opaque and sphene as secondary minerals are obtained from the alteration of plagioclase, amphibole, biotite and pyroxenethe sio2 content varies from 61.58 to 68.64% wt. the amounts of na2o and k2o range from 3.49-4.95% and 2.33-3.07%wt respectively. the rocks under study are classified as high potassium calc-alkaline and meta-aluminous dacite and andesite in the classification diagram.discussionthe multi-element pattern normalized to the primary mantle as well as to mid-ocean basalt show the enrichment of large ion lithophile elements (lile) such as ba, sr, and rb and the depletion of ti, nb, ta, and p elements. these features are indicators of arc magmatism and are seen in magmas associated with the subducted crust and the mantle wedge. the chondrite normalized pattern shows a steep trend from light rare earth elements lree to heavy rare earth elements hree without a distinct anomaly of europium. the enrichment of lree compared to hree is due to low-grade partial melting or magmatic contamination with crustal materials. the lack of negative anomalies of europium, strontium, and barium indicates that the continental crust did not affect the melting process. also, the lack of europium anomaly is usually attributed to the absence of plagioclase in the origin and its non-involvement in the melting process or the oxidant conditions of the origin. additionally, the presence of garnet or amphibole in the origin is suggested by the small amounts of hrees. the changes of ree in the normalized diagrams with chondrite and primary mantle can also be explained by the metasomatized mantle source.the rocks are classified into a magmatic arc and continental margin arc based on the ternary diagram of nb/8, la/10, y/15, the ta/hf versus th/hf, and also zr versus y diagrams. the magmatism of the subduction zones can be caused by the melting of the slab or mantle. in the th/yb versus nb/yb diagram which is used to detect the origin of magma, the samples also follow the trend of the volcanic arc array. the increase in thorium values and the positive slope of the graphs show the participation of the crustal contamination processes in the evolution of magma.the rocks have higher sio2, sr, sr/y, la/yb and lower mgo, yb, y, and hrees than normal calc-alkaline rocks. the studied samples show adakitic characteristics in the diagrams presented for the separation of adakitic rocks from common magmas of subduction zones. adakitic magmas are produced from the partial melting of metamorphic basaltic compounds in the eclogite-amphibolite facies and slab in subduction zones. according to some researchers, the partial melting of the lower part of the thickened crust or the melting of the metasomatized mantle are other options for the production of adakitic magmas. the rb versus k2o/na2o and cao/al2o3 versus k2o/na2o diagrams indicate that slab melting is more significant in magma formation than lower crust melting. the high values of sr/ce and nb/zr also indicate the role of the slab in magma production. the high and positive correlation of the ba/nb ratio versus niobium is a sign of the direct involvement of the slab in the source melt because in the process of subduction, nb is less mobile and when ba is removed into the mantle. however, the dispersion and negative correlation of the samples observed in the diagrams suggest that the melt was not formed directly from the slab and that the geochemical changes cannot be solely attributed to the differentiation of the oceanic crust. due to the low values of mgo and ni, it does not match with the mantle melts, but it is believed that this magma originated from the mantle, which was metasomatized with slab melts and fluids. metasomatism of the mantle wedge with slab-derived fluids increases the amount of liles (k, rb, ba, sr, th, u) and decreases the amount of hfses (nb, ti, ta). geochemical diagrams show that melts from sediments along with slabs have played a more effective role in mantle deformation. figure 10 represents magma derived from amphibolite origin. at depths greater than 45-50 km, garnet is stable while at depths less than 35 km, pyroxene is stable and at depths less than 30-45 km, amphibole is stable. as a result, magma can be generated from depths of about 45 km. amphibolite is located in the mantle wedge and undergoes metasomatism through slab melts and fluids. melting occurs at the depth of garnet stability originating from an amphibolite source.