سنگ‌های آتشفشانی تفتان: شاهدی از ماگماتیسم شبه آداکیتی در قوس ماگمایی مکران

کوه تفتان در جنوب شرق ایران یکی از مراکز آتشفشانی فعال قوس ماگمایی مکران در محدوده زمانی نئوژن کواترنری است. تفتان از نظر تقسیم بندی ساختاری در زون جوش خورده سیستان قرار گرفته و قاعده آن مجموعه سنگ های افیولیتی، فلیش و سنگ های آتشفشانی(عمدتاً کرتاسه تا ائوسن) است. مجموعه های فنوکریستی سنگ ها شامل پلاژیوکلاز ± کلینوپیروکسن ± هورنبلند ± ارتوپیروکسن ± بیوتیت است. شواهد عدم تعادل در کانیها بویژه در پلاژیوکلاز و هورنبلند بر تغییرات شرایط فیزیکوشیمیایی تبلور همچون دما، فشار و ترکیب شیمیایی دلالت دارد. از نظر تنوع شیمیایی گدازه های ترم بازیک تا اسیدی حضور دارند؛ اگرچه عمده گدازه ها و مواد پیروکلاستیک تفتان ترکیب آندزیتی تا داسیتی دارند. اختصاصات شیمیایی ولکانیسم تفتان همچون بالابودن تمرکز 645ppm<) sr) ، بالا بودن نسبت la/yb >14) و (sr/y (>40 سازگار با مذاب های شبه آداکیتی مرتبط با حواشی فعال قاره ای دارای پوسته ضخیم است. بر اساس شواهد شیمیایی و کانی شناسی دخالت فرایندهای مختلف گوشته ای و پوسته ای در ژنز مذاب تفتان قابل استنباط است. ابتدا ذوب گوه گوشته لیتوسفری نسبتاً غنی زیرقاره ای که تحت تاثیر فرایندهای زون فرورانش قرار گرفته است. سپس تجمع مذاب های گوشته ای در عمق پوسته ضخیم قاره ای که توام با ذوب بخشی پوسته ای، آلایش و همگن شدن بوده است. در نهایت فرایندهای پوسته ای کم عمق تر شامل هضم پوسته ای و تحولات مرتبط با مخزن ماگمایی همچون تغییرات فیزیکوشیمیایی مذاب و تبلور تفریقی که باعث تحول بیشتر مذاب شده است.

Taftan volcanic rocks: implication for adakitic magmatism of Makran magmaticarc

Taftan volcano, located in the southeastern Iran, is one of the active volcanic centers of the Makran magmatic arc during NeogeneQuaternary. The Makran arc is an eastwest trending Neogene Quaternary volcanic edifice (Pang et al., 2014) lying in the north of the Makran accretionary prism. Three major volcanic centers in the north of Makran from west to east are the Bazman, Taftan and KohiSultan volcanoes, respectively, form a ca. 300 km long magmatic arc. Regarding structural zones, the Taftan volcano locates in Sistan suture zone and its basement rocks include ophiolite, flysch and volcanic rocks (mostly Upper Cretaceous to Eocene). In this paper, we present petrographic and whole rock geochemical signatures of the Taftan volcanic rocks with the aims of better understanding of petrogenesis and tectonic implications of magmatism in the Makran arc.Materials and MethodsThe rock samples were crushed into centimetersized chips. After crushing, they handpicked in order to eliminate altered parts and remove any visible impurities. The samples were powdered by Tungsten Carbide in the sample preparation laboratory of the Iranian Mineral Processing Research Center (IMPRC). Wholerock major and trace elements were obtained by Inductively Coupled PlasmaEmission Spectrometry (ICPES) using a Spectro Ciros Vision instrument and Inductively Coupled PlasmaMass Spectrometry (ICPMS) on Perkin Elmer Elan 6000 instruments at Acme Labs. The major elements and also Cr and Ba were determined by ICPES and the remaining trace elements by ICPMS. Powdered rock samples were mixed with LiBO2/Li2B4O7 flux and fused in crucibles in a furnace. The quenched bead was dissolved in ACS grade nitric acid. Lossonignition (LOI) was determined by heating a sample split to 1000 deg;C.Result and Discussion In the Taftan volcanics, phenocrystic assemblage consists of plagioclase + clinopyroxene + hornblende plusmn; orthopyroxene plusmn; biotite. Disequilibrium microtextures of minerals, including sievetexture, resorption surfaces, fine sieve zones and regrowth bands of plagioclase and sieve texture and opacity rim of hornblende suggest changes in physicochemical conditions of the system (T, P, PH2O, melt chemistry). Geochemically, Taftan volcanics comprise basic to acidic compositions, although dominated by intermediate andesitic to dacitic lava and pyroclastics. Based on K2OSiO2 diagram, the samples plot within medium to high K series. Except two samples showing low Mg# [100*Mg/(Mg+Fe2+)] (~17 and 24), the others are characterized by approximately high Mg# ± 50). In Harker variation diagrams, despite scatters of some data points, a continuous and nearly linear trends can be observed suggesting genetic relationships among the samples and melt evolutionary trend. In these diagrams some elements including MgO, TiO2, CaO, FeOT, Mg# and Cr represent negative correlation with SiO2 enhancement. These variations are consistent with fractional crystallization of ferromagnesian phases (clinopyroxene + hornblende), plagioclase and Fe Ti oxides. The volcanics display chondritenormalized fractionated rare earth element (REE) patterns with (La/Yb)N, (Sm/Yb)N, (La/Sm)N ratios in the ranges of 9.9 19.7, 2.4 3.7, 3.6 5.8, respectively. Primitivemantle normalized multielement diagrams are characterized by light ion lithophile element (LILE) such as Rb, K and Ba enrichment and high field strength element (HFSE) such as Ta and Nb depletion which could be interpreted as subduction zone magmatic signatures. Furthermore, geochemical characteristics of the Taftan volcanics are consistent with adakitic signatures. These include high Sr ± 645 ppm), Sr/Y ± 40) and La/Yb ±40). High Sr/Y and La/Yb ratios as well as low Y content of ldquo;adakites rdquo; has been interpreted as melt equilibration with amphibole + garnet and lack of plagioclase in melt source. This mineralogy corresponds to high pressure ±10 kb) condition. Scenarios that match these requirements for adakite generation include partial melting of thickened arc or thickened postcollisional continental crust (e.g. Chung et al., 2003; Petford and Atherton, 1996; Topuz et al., 2011), partial melting of subducted slab (Defant and Drummond, 1990; Drummond and Defant, 1990), low degree partial melting of metasomatized mantle (Gao et al., 2007) or highpressure fractional crystallization involving garnet (e.g. Macpherson et al., 2006). In the case of Taftan volcano, partial melting of subducted slab seems to be impossible because both the subduction zone and subducted slab are too old (pre Cretaceous). Moreover, geochemistry of Taftan volcanics including trace element and isotopic characteristics are different from slab derived adakites. Therefore, it is more realistic to consider Taftan volcanics as ldquo;adakite like rdquo; melts generated in continental arc setting with thickened crust. Consistently, geophysical studies show thick continental crust ± 50 km) beneath Taftan volcano. In arc settings, during interaction of mantle melts with deep lower crust and as a result of complex magmatic processes such as meltingassimilationstoragehomogenization (MASH) and assimilationfractionalcrystallization (AFC), ldquo;adakitic melts rdquo; could be produced (Richards and Kerrich, 2007).ConclusionBringing together all of petrographic and chemical evidence, here we propose following successive processes for the genesis of the Taftan volcanics:1. Partial melting of subcontinental lithospheric mantle wedge. This mantle source is reasonably more enriched than suboceanic depleted mantle. In addition, slabderived materials including fluids and/or sediment melts would increase LILE content and enhancement of LILE/HFSE ratio in melt source.2. Interaction of mantle melts with deep continental crust and assimilation processes. Melt equilibration with high pressure crustal lithology (amphibole plusmn; garnet ndash; plagioclase) would reinforce the adakitic signature of the melt.3. Melt ascent and mid to uppercrustal processes including storage, refilling of magma chamber, magma mixing and fractional crystallization of plagioclase and ferromagnesian minerals.