Mineralogical-Petrographic characteristics of garnet porphyroblasts (Ekinözü-Kahramanmaraş), determination of metamorphism conditions and gemstone potential

Abstract

The studied garnet porphyroblasts occur within metamorphic rocks consisting of schists with variable mineralogical compositions in the Ekinözü (Kahramanmaraş) region. The porphyroblasts crystallized in pentagonal dodecahedron forms, ranging in size from 0.5 to 4 cm and exhibit dark red to reddish-black colors with vitreous luster. Microscopic observations reveal that the garnet porphyroblasts display a highly fractured and cracked texture, containing inclusions of quartz, chlorite and opaque minerals such as ilmenite-magnetite. Confocal Raman spectroscopy analyses indicate that the garnet porphyroblasts are of almandine composition. Their chemical structural formulas were determined as Alm0.80-0.88Prp0.07-0.13Grs0.01-0.07Sps0.00-0.05. Garnet-biotite geothermometry calculations suggest that the garnets crystallized under average pressure conditions of ~4.5 kbar at temperatures of 465.4±41.6 °C. Multi-element variation diagrams normalized to the lower and upper continental crust indicate depletion in LIL elements (Sr, K, Rb and Ba), whereas enrichment is observed in HFS elements (Ta, P, Zr, Hf and Y). Chondrite-normalized multi-element diagrams show up to 100-fold enrichment in Rare Earth Elements (REE) (∑REE: 162.1-284.9; (La/Sm)N = 1.65-3.62; (Sm/Yb)N = 0.43-0.67). Enrichment Factor (EF) calculations from chemical analyses highlight the role of Co, Fe, U and Mn elements in color development. These findings are further supported by EPMA data and optical absorption spectroscopy, both of which confirm the predominant presence of Fe+2 ions. Mineralogical, geochemical and gemological investigations indicate that the Ekinözü garnet porphyroblasts formed under greenschist facies metamorphic conditions. Their euhedral crystal morphology, together with their dark red coloration and vitreous luster, suggests that these garnets have potential for use as gemstones in jewelry.

Keywords

• Garnet porphyroblasts
• Raman spectroscopy
• Mineral chemistry
• Gemology
• Gemstone

Granat porfiroblastlarının (Ekinözü-Kahramanmaraş) mineralojik-petrografik özellikleri, metamorfizma koşullarının belirlenmesi ve süs taşı potansiyeli

Öz

Çalışma konusu olan granat porfiroblastları Ekinözü (Kahramanmaraş) bölgesinde farklı mineralojik bileşime sahip şistlerden oluşan metamorfik kayaçlar içerisinde bulunmaktadır. Pentagonal dodekahedron formda kristallenmiş olan granat porfiroblastlarının boyutları 0.5 - 4 cm arasında değişmekte olup koyu kırmızı, kırmızımsı siyah renkte camsı parlaklığa sahiptirler. Mikroskop incelemelerinde oldukça kırıklı-çatlaklı bir yapıda olan granat porfiroblastları kuvars, klorit ve ilmenit-manyetit türü opak mineral kapanımları içermektedir. Konfokal Raman spektroskopisi analizlerine göre granat porfiroblastları almandin bileşimindedirler. Granat porfiroblastlarının kimyasal yapı formülleri Alm0.80-0.88Prp0.07-0.13Grs0.01-0.07Sps0.00-0.05 olarak belirlenmiştir. Granat-biyotit jeotermometre sonuçları, granatların yaklaşık 4.5 kbar basınç ve 465.4±41.6 °C sıcaklık koşullarında kristalleştiklerini göstermektedir. Alt ve üst kıta kabuğu değerlerine göre normalleştirilmiş çoklu element diyagramları, Ekinözü granat porfiroblastlarının LIL elementleri (Sr, K, Rb ve Ba) açısından tüketildiğini, buna karşılık HFS elementleri (Ta, P, Zr, Hf ve Y) yönünden zenginleştiğini ortaya koymaktadır. Çoklu elementlerin kondrite normalize edilerek değerlendirildiği diyagramlarda, granat porfiroblastlarının Nadir Toprak Element bakımından yaklaşık 100 kata ulaşan zenginleşmeler gösterdiği saptanmıştır (∑NTE: 162.1-284.9; ((La/Sm)N=1.65-3.62); ((Sm/Yb)N=0,43-0,67). Kimyasal analizlerden elde edilen Zenginleşme Faktörü hesaplamaları, özellikle Co, Fe, U ve Mn elementlerinin renk oluşum sürecinde etkili olduğunu göstermektedir. EPMA analizleri ile optik absorpsiyon spektroskopisi de bu bulguları destekleyerek Fe+2 iyonunun baskın varlığını ortaya koymaktadır. Mineralojik, jeokimyasal ve gemolojik incelemeler sonucunda Ekinözü granat porfiroblastlarının yeşilşist fasiyesine karşılık gelen metamorfizma şartlarında oluştukları, öz şekilli kristal yapısına sahip olması ve koyu kırmızı renkte camsı parlaklık göstermesi nedeniyle mücevheratta kullanılabileceği değerlendirilmektedir.

Anahtar Kelimeler

• Granat porfiroblastları
• Raman spektroskopisi
• Mineral kimyası
• Gemoloji
• Süs taşı

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