فرورانش و بسته‌شدن پوسته پالئوتتیس تا برخورد قاره‌ای در شمال خردقاره خاور-ایران مرکزی، برپایه شواهد سنگ‌نگاری و ایزوتوپ پایدار اکسیژن در متاگرانیت چاه‌زرد در خاور جندق

این پژوهش شواهد فرورانش پالئوتتیس تا برخورد قاره‌ای در شمال خردقاره خاور ایران مرکزی برپایه سنگ‌نگاری و ایزوتوپ پایدار اکسیژن در کوارتز در متاگرانیت چاه‌زرد در خاور جندق را ارائه می‌دهد. در این منطقه، گرانیتوییدهای ژوراسیک میانی افیولیت جندق و سنگ‌های دگرگونی منطقه را قطع کرده‌اند. بررسی‌های میدانی و سنگ‌نگاری روی نمونه‌های متاگرانیت‌ جندق نشان می‌دهند فرایندهای دگرگونی در ارتباط با فرایند بسته‌شدن پالئوتتیس رخداد دست کم دو فاز دگرگونی ناحیه‌ای در منطقه را به‌دنبال داشته است که یکی از آنها در پی فرورانش پوسته اقیانوسی پالئوتتیس در کربونیفر و پیش از نفوذ گرانیت چاه‌زرد رخ داده است (دگرگونی m1). سپس پس از نفوذ گرانیت چاه‌زرد درون دگرگونه‌ها، دگرگونی دیگری در پی برخورد قاره‌ای رخ داده است (دگرریختی/دگرگونی m2) که این گرانیت را به متاگرانیت تبدیل کرده است. از شواهد فاز دوم دگرریختی/دگرگونی، پیدایش زیردانه و بازتبلور gbm در کوارتزها، پیدایش میرمکیت، پرتیت‌شعله‌ای و بالجینگ در حاشیه فلدسپارها و تبلور گارنت‌های دگرگونی فشار بالا و همزمان با زمین‌ساخت است که با رخداد یک دگرریختی در شرایط فشار بالا و دمای نزدیک به 600 درجه سانتیگراد همخوانی دارد و گویای شرایط دگرگونی ناحیه‌ای در مناطق برخوردی و بسته‌شدن پوسته اقیانوسی پالئوتتیس در شمال بلوک یزد است. افزون بر این، ویژگی‌های زمین‌شیمیایی کانی‌ها (مانند خاستگاه ماگماییِ گارنت‌های شکل‌دار) و مقدار کم δ18o در گرانیت خاور جندق (10.9 ‰) می‌تواند گویای پیدایش مذاب گرانیت چاه‌زرد در پی ذوب اندک بخش‌های فلسیک تخته فرورنده با کمترین آلایش با گوشته در هنگام صعود باشد.

iran, evidence from petrography and stable oxygen isotope in chah zard meta-granite in the east of jandaq

introductionthe jandaq metamorphic complex is a part of the yazd block in the central iran structural zone, and northwest of khur, in the eastern part of the central-east iranian microcontinent (ceim) (figure 1) (heidarianmanesh et al. 2022). the jandaq metamorphic complex (jmc) consists of metamorphic and meta-igneous rocks dated back to late permian- early jurassic periods. the jmc is characterized by the presence of metamorphosed peridotites, schists, amphibolites, migmatites, intruded by granites and pegmatite dikes (romanko et al. 1984; bagheri 2007; tabatabaeimanesh and sharifi, 2011; muttoni et al. 2015; jamshidzaei et al. 2021). thermobarometry studies (heidarianmanesh et al. 2022) showed a regional transformation from amphibolite to granulite facies, attributed to medium p/t burrovian metamorphism during crustal thickening in subduction zones or continental collision due to subduction of the paleo-tethys ocean and subsequent tectonic activity in the region.the primary objective of the present study is to provide evidence of paleo-tethys subduction and the resulting continental collision in the northern ceim. therefore, this study focuses on chah zard meta-granite, intruded the various jmc metamorphic rocks. petrographic observations as well as stable oxygen isotope analysis of quartz and whole-rock samples were conducted to provide deeper insight into the magmatic and metamorphic history of the jmc area.research methodsthe methodology of this study involved extensive fieldwork, petrographic analysis, and stable oxygen isotope studies. field observations were concentrated on the contact between the chah zard granite and the surrounding metamorphic rocks. samples from various parts of the jmc were collected, and thin sections were prepared for petrographic examination. stable oxygen isotope data from quartz crystals and whole-rock samples were analyzed to assess the origin and the evolution of the granitic body and the rocks surrounding it as well.all o-isotope data were carried out at the university of cape town. all the isotope ratios were measured using a finnigan delta xp mass spectrometer in dual-inlet mode. an internal standard (murchison quartz - mq, δ18o=+10.1‰) was analyzed to calibrate the data to smow scale. the long-term variability of mq suggests a 2σ error of 0.16‰.discussionthe petrographic study identified two main phases of regional metamorphism. the first phase (m1) occurred prior to intrusion of the chah zard granite, during the subduction of the paleo-tethys oceanic crust in the carboniferous period. the second phase (m2) happened during/after continental collision. petrographic evidences such as grain boundary migration (gbm) in quartz, the formation of myrmekite, flame perthite in feldspars, and high-pressure garnet crystallization suggests that the granite experienced high-pressure metamorphism in the course of the continental collision.additionally, based on modal ratios and δ18o value of minerals, ∆quartz-magma for the bushveld granite is estimated as 1.11‰ (fourie and harris 2011). in the absence of stable oxygen isotope data for other minerals, it is assumed that the value of ∆quartz-magma is equal to 1.1 ‰, and accordingly, the δ18o value in the chah zard granite is equal to 10.8 ‰ (n: 4; table 1) consistent with its amount in magmatic rocks of mantle origin. moreover, granites with high δ18o values (greater than 10 ‰) are believed to likely contain a significant fraction of mantle-derived melts with δ18o ranging from 5.7 to 6.5 ‰ (e.g., hoefs, 2009). based on the presence of magmatic garnets of i-type granites (figure 7) coupled with the oxygen stable isotopic data, the origin of the rocks of chah zard is possibly the melting of felsic parts of the subducting plate with minimal mantle contamination during ascent. therefore, the petrographic and isotopic evidence from this study suggests that the chah zard granite was generated from a small degree of partial melting of a subducting felsic slab. the low δ18o values further indicate limited interaction with the continental crust as the magma ascended. the presence of high-pressure metamorphic garnets (figure 7) in the meta-granite confirms the occurrence of regional metamorphism associated with the continental collision. moreover, the present data also emphasize the significance of studying the deformed granitic rocks like the chah zard meta-granite to reconstruct the tectonic history of the region.conclusionsin conclusion, the study of chah zard meta-granite offers valuable insights into the subduction of the paleo-tethys ocean and its subsequent closure through continental collision in the ceim. the petrographic evidences (deformation metamorphism of the granite and the magmatic garnets of i-type granites) and stable oxygen isotopic data (10.8 ‰ for the granite) reveal that the region experienced two significant phases of regional metamorphism: the first, related to the subduction of oceanic crust during the carboniferous and the second, linked to the continental collision in the late jurassic. following the intrusion of the chah zard granite into pre-existing metamorphic rocks, it was deformed and metamorphosed into a meta-granite during the final stages of paleo-tethys closure. the magmatic garnet composition and isotopic analysis confirms that this granite originated from a mantle-derived magma with minimal crustal contamination. ultimately, this research contributes to a more comprehensive understanding of the tectonic history of northern ceim and its role in the broader evolution of the paleo-tethys ocean.