بررسی رفتار زمین‌شیمیایی عناصر اصلی و کمیاب خاکی در گارنت‌های پگماتیت‌های دره ولی (شمال شرق بروجرد، پهنه سنندج- سیرجان)
محورهای موضوعی :
کلید واژه: بروجرد پگماتيت دره ولی زمین‌شیمی عناصر کمیاب خاکی گارنت ماگمایی,
چکیده مقاله :
پگماتیت‌های منطقه دره ولی در شمال شرق بروجرد و در پهنه ساختاری سنندج- سیرجان واقع شده‌اند. این پگماتیت‌ها به‌صورت دایک‌هایی با روند شمال‌غرب- جنوب‌شرق، واحدهای گرانودیوریتی منطقه مورد مطالعه را قطع کرده‌اند. این سنگ‌ها از نظر کانی‌شناسی شامل کانی‌های کوارتز، فلدسپار‌های آلکالن (ارتوکلاز و میکروکلین)، پلاژیوکلاز، مسکوویت، گارنت (آلماندین- اسپسارتین)، آندالوزیت، تورمالین و آپاتیت هستند. الگو‌های REE بهنجار‌شده نسبت به كندريت در پگماتیت‌های دره ولی، بیانگر غنی‌شدگی اندک LREE نسبت به HREE )04/4-76/1(LaN/YbN=، الگوی نسبتاً مسطح HREE و بی‌هنجاری منفی شدید Eu (54/0-20/0 (Eu/Eu*= است. بررسی شيمي عناصر اصلي گارنت‌هاي درون اين پگماتيت‌ها بيانگر منطقه‌بندی ترکیبی با افزایش FeO و کاهش MnO از مرکز به حاشیه است. مقادیر بسيار بالاي منگنزwt.%) 18/13-27/10 (MnO= و مقدار کم کلسيمwt.%) 29/0-15/0(CaO= گارنت‌هاي موجود در پگماتيت دره ولی، مشابه گارنت‌هاي ماگمايي درون مذاب‌هاي پگماتيتي است. ترکیب بلور‌های گارنت بر روی نمودار MnO+CaO در مقابل FeO+MgO (برحسب درصد وزنی)، بیانگر تبلور آنها در بخش حاشیه‌ای رگه پگماتیتی و از مذاب‌های کم‌تر تفریق‌یافته است. نتایج LA-ICP-MS حاکی از غنی‌شدگی گارنت‌های مورد مطالعه از عناصر کمیاب خاکی سنگین (HREE)، تهی‌شدگی از عناصر کمیاب خاکی سبک (LREE) و بی‌هنجاری منفی شدید Euدر مرکز )41/0-0(Eu/Eu*= و مثبتEu )22/3-0(Eu/Eu*= در حاشیه‌ها است. عناصر Y، HREE، Ti، Zr، Nb، Ta، Hf، U و Mn از مرکز به سمت حاشیه کاهش نشان می‌دهند. این تغییرات از مرکز به حاشیه، به افزایش فاز سیال و اکتیویته H2O در ماگما و افزایش تفریق ماگمایی نسبت داده شده است. الگوی REE و بی‌هنجاری‌های Eu در گارنت‌های دارای منطقه‌بندی، بیانگر تبلور آنها در شرایط احیایی تا اکسیدان است.
The pegmatites of Darreh Vali region is located in the north-east of Boroujerd which is a part of Sanandaj-Sirjan zone. In the Darreh Vali area, granodiorite bodies are cut by small pegmatitic dykes with NW–SE trend. The mineralogy of studied pegmatites consists of quartz, alkali-feldspar (orthoclase and microcline), plagioclase, muscovite, garnet (almandine-spessartin), andalusite, tourmaline, and apatite. Chondrite-normalized patterns of the Darreh Vali pegmatite are characterized by low enrichments of LREE relative to HREE (LaN/YbN=1.76-4.04), with a relatively flat HREE distribution, and a strong negative Eu anomaly (Eu/Eu* =0.20-0.54). Major element chemistry of garnets in these pegmatites indicates a compositional zoning with decreasing MnO and increasing FeO from core towards the rim. In the case of the Darreh Vali pegmatites, all garnet crystals contain low CaO (0.15 to 0.29 wt.%) and high MnO (10.27 to 13.18 wt.%), which are similar to magmatic garnets from pegmatitic melts. On the MnO+CaO versus FeO+MgO (in wt. %) diagram, the composition of garnets shows that they probably crystallised in contact zones of pegmatite vein and from less evolved melts. LA-ICP-MS analyses show that analysed garnets have a high HREE, low LREE contents, and strong negative Eu anomaly (Eu/Eu*=0-0.41) in the core along with positive Eu anomaly (Eu/Eu*=0-3.22) at the rim. Y, HREE, Ti, Zr, Nb, Ta, Hf, U and Mn decrease from core to rim. These core-to-rim elemental variations are attributed to increasing fluid-phase and H2O activity in magma, along with increasing magma fractionation. REE patterns and Eu anomalies in zoned garnets suggest that they probably formed in reducing to oxidizing conditions.
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