پترولوژی و زمین‌شیمی ریولیت‌های نورالدین‌آباد شمال‌باختری ایران

ریولیت‌های نورالدین‌آباد از کوارتز آلکالی‌فلدسپارها و به مقدار کمتر پلاژیوکلاز تشکیل شده است که در زمینه‌ای متوسط تا ریز دانه از کانی‌های کوارتز و فلدسپار قرار گرفته‌اند. از لحاظ زمین‌شیمیایی این سنگ‌ها از نوع ریولیت‌های پرآلومینوس با میزان sio2 بالا هستند. همچنین مقدار na2o+k2o fe2o3t و عنصرهای خاکی کمیاب (به استثنای eu) در ترکیب این سنگ‌ها بالا بوده و در برابر میزان cao mgo p2o5 sr و ba پایین بوده و مشابه با گرانیت‌ها و ریولیت‌های a-type است. از لحاظ رده‌بندی ژنتیکی با در نظر گرفتن مقدار بالای نسبت‌های y/nb rb/sr rb/nb و مقدار نسبتاً پایین nb ریولیت‌های نورالدین‌آباد را می‌توان به ریولیت‌های a-type از نوع a2 نسبت داد. با توجه به خصوصیات زمین‌شیمیایی خاستگاه ریولیت‌های مورد مطالعه پوسته قاره‌ای بوده و احتمالاً در اثر ذوب بخشی پوسته قاره‌ای تشکیل شده‌اند. تبلوربخشی کانی‌هایی همچون پلاژیوکلاز و اکسیدهای fe-ti نقش بارزی در روند تحولات ماگمایی این سنگ‌ها داشته است. استفاده از نمودارهای متمایز کننده محیط زمین‌ساختی نشان‌دهنده‌ این است که ریولیت‌های نورالدین‌آباد از نوع ریولیت‌های غیرکوهزایی بوده و در یک محیط درون صفحه‌ای و وابسته به کافت (احتمالاً کافت نئوتتیس) تشکیل شده‌اند.

petrology and geochemistry of noraldinabad rhyolites nw iran

felsic igneous rocks record crustal evolutions and provide an important tool for determining the composition and modification of the crust (zhang et al. 2018). up to now several classifications have been proposed for granites and rhyolites. alphabetic classification is the most common approach used by petrologists. in this approach granites and rhyolites have been divided into i s m and a-types according to their geochemical mineralogical composition and origin characteristics (loiselle and wones 1979; white 1979). a-type granites and rhyolites include a wide range of felsic rocks. currently the formation of a-type suites has been attributed to both with-in-plate (anorogenic) and postcolision (orogenic) settings (eby 1990; bonin 2007; el dabe 2015).in this study petrological characteristics of noraldinabad rhyolites have been studied. these rhyolites exposed adjacent to gushchi granitoids which petrologically have been studied before (e.g. advay et al. 2010; shafaii moghadam et al. 2015). these studies exhibit that the gushchi granitoides are a-type and formed in an extensional rift-related setting. nevertheless there is not any comprehensive study on the noraldinabad rhyolites and the present study would be the first effort to constrain the geochemical characteristics of these rocks. regional geologythe area of study in aspect of lithological characteristics as well as geological structures has attributed to various zones e.g. khoy-mahabad zone (nabavi 1976) sanandaj-sirjan zone (alavi 1991) and junction of the structural zones of sanandaj-sirjan and central iran (alavi-naini 1972). nabavi (1976) believes that the lithological characteristics of central iran sanandaj-sirjan and alborz-azerbaijan zones are visible in the northwest of iran. in this case the observed geologic successions in the northwest iran are similar to those of the of central iran zone (shafaii moghadam et al. 2015). the exposure of kahar formation (late neoproterozoic) is common in the northwest of iran central iran alborz and sanandaj-sirjan zones (shafaii moghadam et al. 2015). however according to sabzehi and mohammadiha (2003) the main characteristics of the sanandaj-sirjan zone are not visible in this region but could be considered as the northwest termination of the sanandaj-sirjan zone.materials and methodsfollowing the field observations and collecting several rhyolite samples microscopic studies were performed to determine petrographic features. also 10 least altered rhyolite samples were selected for whole rock analyses including xrf and icp-ms performed at the zarazma zanjan laboratories (zanjan iran).petrographythe noraldinabad rhyolites display porphyritic texture with quartz alkali feldspar and subordinate plagioclase phenocrysts. opaque minerals and zircon are the main accessory minerals. altered mafic minerals can be seen in some samples. the matrix is composed of medium- to fine-grained quartz and alkali feldspar. secondary minerals mostly sericite and chlorite are ubiquitous in the matrix of some samples. along with porphyritic texture flow-alignment (in the matrix of some fine-grained samples) spherulitic (in the matrix of medium-grained samples) embayed perthitic and subordinate granophyric textures are the main visible textures.whole rock chemistrythe noraldinabad rhyolites have high sio2 values consistent with “high silica rhyolitic systems” (sio2 > 70 wt%) (gualda and ghiorso 2013; arakawa et al. 2019). the al2o3 content of the studied rhyolites is lower than of this oxide in calc-alkaline rhyolites (> 14 wt%) (philpotts 1990). based on alkali elements content the rhyolites under study can be divided into two groups: na2o- and k2o-rich in concordance with petrographic studies. the k2o-rich samples are dominated by the presence of k-feldspar whereas the na2o-rich samples are characterized by high numbers of anorthoclase. except for alkali elements the contents of other major elements in these two groups are not significantly different. except for one sample (0.54) the feot/(feot+mgo) ratio is high and varies between 0.73 to 0.94. all analyzed samples have high a/cnk ratios indicating their peraluminous characteristics. the studied rocks show shoshonitic magmatic series affinity and are characterized by lrees enrichment and hrees depletion with obvious negative eu anomaly in the chondrite-normalized diagrams. in the nmorb-normalized trace element spider diagram the samples display enrichment in liles ba nb ta sr zr hf eu and ti negative anomalies weakly rb and pb positive anomalies. the eu sr ba and ti depletion indicates plagioclase and titano-magnetite fractionation respectively (shafaii moghadam et al. 2015).discussion and conclusionthe remarkable geochemical features of the noraldinabad rhyolites are high amounts of sio2 na2o+k2o fe2o3t and low abundances of cao mgo and p2o5. furthermore the concentration of rees (except for eu) and liles are high and the contents of sr and rb and compatible elements (i.e. co sc cr and ni) are low. these geochemical features disclose the a-type nature for the studied rhyolites (loiselle and wones 1979; eby 1990; bonin 2007). genetically considering the high ratios of y/nb rb/sr rb/nb and relatively low amounts of nb the investigated rocks can be attributed to the a2-subtype of the a-type rhyolites.the low sr concentration negative eu anomaly and hrees flat- pattern indicate a garnet-absent and plagioclase-bearing as a residue of partial melting origin for studied rhyolites which confirmed these rocks formed by partial melting under shallow depth and low-pressure conditions (norman et al. 1992; petford and atherton 1996; jia et al. 2019). the y/nb ratio is helpful to determine the characteristics of parent magma (eby 1990). granitoids derived from mantle have y/nb<1.2 while crustal granitoids have y/nb > 1.2. the y/nb ratio in the noraldinabad rhyolites varies from 1.30 to 4.72 compatible with crustal origin. additionally the trace element ratios (i.e. th/u nb/u y/nb) are similar to those of the continental crust composition.considering tectonic setting discriminant diagrams the studied rhyolites are anorogenic and formed in within-plate related to rift-related extensional environment. the presence of bimodal magmatism in the studied region along with the exposure of coeval a-type granites and rhyolites in northwest iran can confirm the formation of the studied rhyolites in a continental rift (possibly neotethys rift) setting.