کانی شناسی رگه های آنورتوزیتی و گابروهای دربرگیرنده در توده نفوذی کالکافی (شمال‌ شرق انارک، استان اصفهان، ایران مرکزی)

منطقه کالکافی در حدود 65 کیلومتری شمال‌شرق شهر انارک (بخش شمال‌شرقی استان اصفهان) واقع‌شده است. در این منطقه توده نفوذی کالکافی با سن ائوسن بالایی، دگرگونه های پالئوزوئیک انارک، سنگ آهک ­های کرتاسه و سنگ ­های آتشفشانی ائوسن زیرین و میانی را قطع‌کرده است. این توده نفوذی طیف گسترده ­ای از سنگ ­ها از گابرو تا آلکالی فلدسپار گرانیت را شامل می شود که بیانگر رخداد تفریق ماگمایی بسیار گسترده است. آنورتوزیت ­ها به صورت رگه ­هایی در درون گابروها دیده می شوند که ضخامت این رگه‌ها متغیر بوده و تا حدود 15 سانتی‌متر هم می رسد. گابروها از پلاژیوکلازهای نوع آنورتیت (%94-84 an=) که برخی از آنها به بیتونیت، آندزین و الیگوکلاز تغییر یافته‌اند، کلینوپیروکسن از نوع دیوپسید (75/0=mg#)، ارتوکلاز (or0.88)، آپاتیت، مگنتیت و پرهنیت تشکیل‌شده است. آنورتوزیت ­ها نیز متشکل از کانی های پلاژیوکلاز از نوع آنورتیت (%9589 an=؛ که گاهی این پلاژیوکلازها به بیتونیت و لابرادوریت تغییر یافته‌اند)، اسفن و زیرکن هستند. بافت ­های اصلی این سنگ ها گرانولار، اینترگرانولار و پوئی­کیلیتیک است. بررسی‌های صحرایی نشان می دهد که آنورتوزیت ها با گابروها همراه بوده و شکستگی های گابروها را پر کرده‌اند. بررسی ترکیب شیمیایی پلاژیوکلازهای درون گابروها و پلاژیوکلازهای موجود در آنورتوزیت ها بیانگر شباهت بسیار زیاد این کانی‌ها از نظر محتوای عناصر اصلی و کمیاب بوده و بیانگر ارتباط ژنتیکی این سنگ ­هاست. این ویژگی نشان می دهد که آنورتوزیت های منطقه کالکافی در حقیقت ادامه تبلور بخش غنی از پلاژیوکلاز ماگمای گابرویی اولیه است.

Mineralogy of anorthosite veins and host gabbros in the Kal-e-Kafi intrusive body (northeast of Anarak, Isfahan Province, Central Iran)

IntroductionIn the northeastern part of the Isfahan province and 65 km northeast of the Anarak city (KaleKafi area), an Itype granitoid pluton cross cut the Paleozoic metamorphic rocks and Eocene volcanic rocks. In the contact of this granitoid body with sourrounding rock units, skarn and hornfels have been formed (Ahmadian, 2012; Ranjbar, 2010). The KaleKafi Eocene intrusive body presents a wide range of mineralogical and petrological compositions, from gabbro to alkalifeldspar granite. Presence of mafic to acidic rocks in this mostlygranitoid body indicates that fractional crystallisation has played an important role during magma evolution. The field and petrographical studies indicate the presence of anorthosite veins within the gabbro section. The mafic and basic parts of this pluton have not been studied yet. The mineralogy and chemistry of rockforming minerals in the anorthosites and gabbros are the subject of this research study. Analytical MethodsThe mineralogical and petrographical studies have been done by using Olympus BH2 polarizing microscope in the mineralogy laboratory of the University of Isfahan. EPMA and LAICPMS analyses were used to obtain chemical characteristics of rockforming minerals. Majorelements composition of minerals were performed by JEOL JXA8800, WDS microprobe electron analyzer with accelerator voltage of 15 kV, current of 15 nmA, diameter of 3 μm, and a counting time of 40 seconds at the Kanazawa University of Japan. Natural and synthetic minerals and compounds were used as standards. The ZAF program was used for data correction.Trace element values of plagioclases and clinopyroxenes were analyzed by LAICPMS (laser ablationinductively coupled plasmamass spectrometry) using an ArF 193 nm Excimer Laser coupled to an Agilent 7500S at the Earth Science Department of the Kanazawa University, Japan. The diameter of the analyzed points was 110 µm at 10 Hz with energy density of 8 J/cm2 per pulse.Mineral abbreviations in tables and photomicrographs are adopted from Whitney and Evans (2010). Results and DiscussionThe Eocene KaleKafi pluton includes a wide range of rocks from gabbro to alkalifeldspar granite, which points to an extensive magmatic differntiation. Field relationships indicate presence of at least 4 magmatic phases, and gabbro is the first and oldest phase. The most predominant rock unit in the KaleKafi intrusive body is granitoid. However,  in the northern parts, the gabbro and anorthosite present substantial exposures. The anorthosites and gabbros are associated with each other in the field. Anorthositic veins with up to 15 cm thickness cut the gabbro.Gabbro is composed of bytownite and anorthite plagioclase (An= 84 – 94 %; some of them have been altered to bytownite, andesine and oligoclase), clinopyroxene (diopside, Mg#= 0.75), orthoclase (Or0.88), apatite, magnetite, and prehnite. Anorthosite rockforming minerals are anorthite plagioclase (An= 89 – 95 %; some anorthite plagioclase have been altered to bytownite and labradorite), sphene and zircon. The main texture of these rocks are granular, intergranular and poikilitic. Field studies suggest that anorthosites are associated with gabbros which have filled the fractures of gabbros.Very simmilar petrography and chemical composition of plagioclases in the anorthosites and gabbros possibly reveal their cogenetic nature. It seems that the primary magma in the magma chamber, first crystallized the clinopyroxene and plagioclase, which caused formation of gabbros. In the next stage, by occurrence of a tectonic activity, the gabbros have broken and the remaining magma which was rich in plagioclase components, crystallized the anorthosites in the fractures. This reveals that the anorthosites of the study area are the plagioclase rich part of the primary basic magma which have formed the gabbros.According to the field relationships, it is generally believed that anorthosites are differentiates of gabbroic magmas. The studied anorthosite veins and gabbros of the KaleKafi area are consanguineous. These anorthosites are perhaps generated by the process of collection of plagioclase crystals from a gabbroic magma under the action of gravity and tectonic activity (filter pressing).Pyroxene is one of the common minerals. The chemical composition of this mineral provides valuable information about the nature of magma, H2O content, Oxygen fugacity, type of magmatic series, tectonic setting, as well as temperature and pressure of crystallisation (Schweitzer et al., 1979; Leterrier et al., 1982; D’Antonio and Kristensen, 2005).Chemistry of clinopyroxens within the gabbros of the KaleKafi  area shows that the parental magma belongs to the subalkaline and calcalkaline magmatic series and these rocks are similar to those of volcanic arcs. The time and place of formation of these plutonic rocks possibly indicate that they are formed by subduction of the CentralEast Iranian Microcontinent (CEIM) – confining oceanic crust beneath the CEIM. AcknowledgmentsThe authors thank the University of Isfahan for financial supports.