مطالعه اثر تتراد در الگوي عناصر كمياب خاكي مجموعه گرانيتوييدي A-Type ميشو در شمال‌غرب ايران

Geochemical modelling of controlling phases on abundances of major and trace elements in the migmatitic amphibolites, Seghalaton, Neyriz, Iran

گرانيتوييدهاي A-Type شرق ميشو،گرانيت‌هاي پرآلومينوس تا ساب‌آلومينوس هستند كه توسط ذوب‌بخشي پوسته تحتاني تشكيل شده‌اند و حاوي كاني‌هاي كوارتز، فلدسپات‌پتاسيك، پلاژيوكلاز، همراه با مقادير اندكي آمفيبول و بيوتيت و كاني‌هاي فرعي آلانيت، زيركن، آپاتيت و اكسيدهاي آهن هستند. الگوي كندريت به‌هنجار شده REE در اين گرانيتوييدها يك الگوي اثر تتراد تيپ M با بي‌هنجاري شديداً منفي Eu نشان مي‌دهد. تلفيق داده‌هاي پتروگرافي و ژيوشيميايي نشان مي‌دهد تفريق بلورهاي غني از REE در طي تبلور تفريقي مذاب و سپس واكنش مذاب با سيالات در مراحل نهايي تبلور، مهم‌ترين عوامل كنترل‌كننده اثر تتراد در گرانيت‌هاي ميشو هستند.

Arc-related regional metamorphic process at c. 147 Ma caused a Barrovian-type metamorphism on the Qori old amphibolites crop outed in the southeast of the Sanandaj-Sirjan zone. As a consequence, the amphibolites from the southern Seghalaton subjected to partial melting under the metamorphic peak 700 to 750 °C and 7 to 9 Kbar. As a result of the process: 1) mafic migmatite, formed by different degrees of partial melting, were expanded in these amphibolites; 2) extracting of small outcrops of trondhjemites were produced by the less than 10 percent of the melt and 3) balanced in situ crystallization and the adhesion force of melt-crystal lead to remain a large part of the partial melt in the mafic migmatites. Additionally, high viscosity of the magma and low temperature of the partial melting in the source were other important factors to be extracting the melt. In situ crystallization of many mafic migmatite from the Seghalaton area caused poikiloblastic texture to be developed. Field and petrographic evidences as well as geochemical modelling using REE, Sr, Ba, Cr, Nb, Y and Ti, show that the protolith (amphibolite) has undergone a partial melting event between 20 to 50 percent, but in situ crystallization and adhesion force of melt-melanosome caused up to 10 percent of the melt (leucosome) to leave these rocks and to produce the trondhjemites.