Petrography and Geochemistry of tourmaline nodules from Aderba leucogranite (northeast of Golpaygan)

The Aderba leucogranite in the Golpayegan metamorphic core complex (GMC), a part of Sanandaj-Sirjan zone, host lentiform small (2*4 cm)(Type 1) and large (7*14 cm)(Type 2)tourmaline nodules. In terms of mineralogical features, the core of these two types tourmaline nodules is different. The Type 1 composed of small blue-green tourmaline, quartz, K-feldspar (microcline) and apatite while the Type 2 is characterized by tourmaline coarse crystals accompanied by quartz. Based on major and trace elements data the tourmalines under discussion are classified as alkaline, schorl (Type 1) and schorl-dravite (Type 2). The mean REE values displays a negative slope and a negative (Type 1) and positive (Type 2) Eu anomalies. The overall petrographic observations and geochemical results indicate that the Type 1 is likely influenced by two mechanisms of liquid immiscibility in the evolved melt followed by biotite breakdown in the final stages of tourmaline crystallization to complete consumption of B (closed system). For nodules, Type 2, breakdown of biotite in equilibrium with the external fluid (open system) is proposed.