شیمی کانی و دمافشارسنجی پریدوتیت‏‌های افیولیت دالامپر (شمال‏‌خاوری اُشنویه): رهیافتی بر تحولات سنگ‏‌زایی و جایگاه زمین‏‌ساختی

افیولیت دالامپر بخش ناشناخته‏‌ای از افیولیت نئوتتیس است و در جنوب شهرستان ارومیه، شمال‏‌باختری ایران درون کمربند دگرگونی سنندج - سیرجان جای گرفته است. واحدهای سنگی منطقه شامل پریدوتیت‏‌های سرپانتینیتی‏‌شده، بازالت بالشی، گابروها، سنگ آهک پلاژیک و رادیولاریت همراه با واحدهای رسوبی-آتشفشانی هستند. این واحدهای سنگی در بیشتر نقاط به‏‌صورت زمین‏‌ساختی و آمیزة رنگین در هم آمیخته‏‌شده‏‌اند. پریدوتیت‏‌ها بیشتر متشکل از هارزبورگیت همراه با عدسی‏‌های کوچکی از دونیت است. پریدوتیت‏‌های مجموعه تحت‏‌تاثیر دگرسانی گرمابی به‏‌صورت کامل و یا بخشی با سرپانتینیت جایگزین شده‏‌اند. بافت‏‌های درپریدوتیت‏‌های دالامپر، مانند جهت‏‌یافتگی، کشیدگی و وجود تیغه‏‌های جدایشی کلینوپیروکسن در اورتوپیروکسن نشان می‏‌دهند این سنگ‏‌ها در شرایط گوشتة بالایی پدید آمده‏‌اند و سپس در محیط پوسته‏‌ای جای گرفته‏‌اند. داده‏‌های به‏‌دست‏‌آمده از تجزیة نقطه‏‌ای الیوین نشان‏‌دهندة میزان بالای فورستریت (fo) برابر با 90-92 درصد است. پیروکسن‏‌ها ترکیب انستاتیت و دیوسید دارند و مقدار عدد منیزیم (mg#) آنها از 90 تا 93 درصد در تغییر است و نشان‏‌دهندة درجة بالای ذوب‌بخشی و ماهیت تهی‏‌شده پریدوتیت‏‌هاست. دربارة اسپینل‏‌ها، عدد منیزیم (mg#) برابر با 64 تا 68 درصد، مقدار عدد کروم (cr#) آنها بالا (77-83 درصد) و محتوای tio2 آنها کم نشان‏‌دهندة خاستگاه گوشته‏‌ای تهی‏‌شده است. ترکیب شیمی کانی‏‌ها نشان‏‌دهندة سنگ‏‌زایی متفاوت برای سنگ‏‌های الترامافیک افیولیت دالامپر است و گویای تکامل از محیط آبیسال به‏‌سوی منطقه فرافرورانش است. گمان می رود این پریدوتیت‏‌ها پیامد درجات بالای ذوب‌بخشی (بیش از % 25) گوشته‏‌ای تهی‏‌شده و زیر اقیانوسی در محیطی فرافرورانش -پیشانی کمان هستند و ویژگی‏‌هایی همانندِ پریدوتیت‏‌های پدیدآمده از تفالة گوشته‏‌ای دارند. دما-فشارسنجی الیوین-اسپینل و ارتوپیروکسن-کلینوپیروکسن در هارزبورگیت‏‌ها دمای تعادل 967 تا 1100 درجة سانتیگراد در فشار 28 کیلوبار را نشان‏‌می‏‌دهد که گویای حضور این سنگ‏‌ها در محدودة اسپینل پریدوتیت است.

mineral chemistry and thermobarometry of peridotitesin dalampar ophiolite (northeast oshnavieh): implication to petrogenetic evolution and tectonomagmatic setting

ophiolites are slices of oceanic lithosphere obducted onto a continental margin and can be classified as mid-oceanic ridge (mor) or suprasubduction zone (ssz) types based on the tectonic setting in which they originally formed (pearce et al., 2000; shervais, 2001). the zagros orogen extends from eastern turkey through northern iraq and northwest of iran to the hormuz strait and oman (alavi 1994; mcquarrie, 2003; homke et al., 2004; agard et al., 2005; shafaii moghadam et al., 2018). the geodynamic evolution of the zagros belt is mainly related to the opening and closure of the neo-tethys oceanic basin. ophiolites, as neotethys oceanic lithosphere remnants, are emplaced along the zagros orogen. these ophiolites are emplaced along two main belts (saccani et al., 2013; shafaii moghadam et al., 2018). the neo-tethys suture zone coincides with the main zagros thrust fault (agard et al., 2005) and ophiolites are exposed scattered along this zone. regional geologythe dalampar ophiolite, located between the piranshahr and salmas ophiolites. the dalampar ophiolites consist mainly of strongly sheared serpentinized ultramafic rocks including harzburgite, dunite overlaid by gabbro and basalt units. the emplacement mechanism of the dalampar ophiolite massifs into the iranian microcontinent and structural evolution is completely unknown. there has been not performed a systematic investigation of these ophiolites and geodynamic and relation with other ophiolites are not well studied.thus, the main purpose of the present paper is to investigate the petrogenetic processes, tectono-magmatic environment, using mineral chemistry and textural evidences of dalampar ophiolite which are fundamental for understanding the tectonic evolution of the neo-tethyan ocean.material and methodsduring the fieldwork, a number of suitable peridotites samples with least alteration effects were selected. folloing the petrographic studies, a thin polished section was prepared from several harzburgites for microprobe analysis using jeol jxa-8600 m model,15 kv accelerator voltage and 2-ray current 10-8 amp, placed at the department of earth sciences and environment of yamagata university, japan.result and discussionharzburgites are the most basic and widespread ultramafic units in dalampar ophiolite complex. they show evidences including the presence of exsolution of clinopyroxene in orthopyroxene, the orientation and elongation of the crystals indicating these rocks were formed in the upper mantle conditions and reached equilibrium in the crustal environment. exsolutions of clinopyroxene lamellae from orthopyroxene are one of the textures observed in the rocks under study and is widespread in both abyssal and ssz peridotites (tamura and arai, 2006). the cr# of spinel in abyssal peridotites is a good indicator of the degree of partial melting for the mantle-derived spinel peridotite. low cr# spinels represent less depleted peridotites, whereas the high cr# spinels highlight more depleted peridotites (dick and bullen, 1984; arai, 1994). in the mid-oceanic ridges, the degrees of partial melting and depletion are generally low. in contrast, the supra-subduction zone peridotites have a high degree of partial melting and depletion (niu and hekinian, 1997). the content of cr# in presented cr-spinels are high (78-83%), and associated with boninitic melts formed by high degrees of partial melting or much more extensive melt-rock reaction (arai, 1994). as the the cr# of spinel versus the fo content of olivine diagram shows the harzburgites fall in ssz domain. furthermore, all rock types fall into olivine-spinel mantle array (osma). this is regarded as the evidence for their residual origin, i.e. they form a trend which was likely caused by partial melting. experimental studies confirm that the al2o3 and tio2 contents, as well as feo/mgo ratios in chromian spinel are directly related to those of the parental melt (rollinson, 2008). according to maurel and maurel (1982), the al2o3 contents of the parental melt range from 10 to 11 wt. % for the studied harzburgites. such values are consistent with boninitic melts, which typically contain 10–14 wt.%, respectively (dilek and thy, 2009). in the cr# versus tio2 diagram, the dalampar peridotites classify as depleted peridotites. in consequence, chemistry of the chromian spinels in the dalampar peridotites are compatible with a genesis in a suprasubduction zone from boninitic or primitive arc magmas. however, forearc regions may contain both ssz and abyssal peridotites, although the former are typically dominant (pearce et al., 2000). olivine-spinel and orthopyroxene-clinopyroxene thermobarometry in the harzburgites shows equilibrium temperatures of 1000-1100 ºc at a pressure of 28 kbar and suggests that they have been equilibrated in spinel peridotite field.conclusionthe dalampar ophiolitic complex as a part of the tethyan ophiolites is exposed in the northwestern part of the iranian-azerbaijan province, extending to the anatolian ophiolites in southeastern turkey. the petrography, geochemistry and microstructural studies of the residual mantle sequence in dalampar ophiolitic complex provide important information regarding the degree of partial melting and deformation in the oceanic mantle lithosphere. mineral chemistry clearly indicate that dalampar ultramafic rocks record an episode of boninitic magmatism occurred within the southern neo-tethys ocean in the late cretaceous. boninitic melts in dalampar ophiolites were formed by partial melting of depleted peridotite, a residue after morb-type melt extraction. mineral chemical data indicate that ssz peridotites are the residues from ∼30–40% of mantle melting. nonetheless, the very low (< 0.25 wt. %) tio2 calculated for the parental melt in equilibrium with chromian spinel are only consistent with boninitic-type parental melts. based on geothermobarometry calibration the temperature and pressure of crystallization of harzburgite rocks were 1000 to 1200 ° c and 24 kbar, corresponding to the ssz tectonic environment.