Determining the conditions and factors affected partial melting in the Chah-Bazargan migmatitic xenoliths (NE Neyriz) using thermodynamic phase diagrams

Migmatites are silisic rocks form in middle to high degree metamorphic territories, at lower to middle continental crust and through water producing reactions (Makrygina, 1977; Brown, 1979; Ashworth, 1985). During intrusion of Chah-Bazargan batholith in the Barrovian-type Qori-Neyriz regional metamorphic rocks at 170 Ma (Fazlnia, 2017), some pieces of these rocks were trapped in the magma and endured intense metamorphism and anatexis. They show stromatic and nebulitic magmatic structures. In the present study, the stable mineral assemblages, conditions, and factors affected partial melting in the Chah-Bazargan migmatites are investigated using phase diagrams and petrographic evidence.

The nebulitic migmatites representing the highest metamorphic degree and close and gradational relationships with peraluminous granites in the area were sampled. Petrographic studies were performed on the thin sections to determine mineral assemblages of peak metamorphic condition. The phase diagrams were calculated using whole-rock analyses reported in Fazlnia (2017) and TheriakDomino software (de Capitani, 2010).

PetrographyThe main textures of nebulitic migmatites are granoblastic and poikilioblastic, although porphyroblastic texture can be observed in places. Stabe mineral assemblage cordierite + spinel + garnet + prtithic K-feldspar + plagioclase together with dissolved silimanites and biotites inclusions represent the peak metamorphic condition. Corundum + spinel sympetectic around the cordierites also form in high metamorphic degrees (Whittington et al, 1998). With increasing metamorphic degree, the biotites and cordierites become less and the more, respectively.Phase diagramsThe P-T phase diagram was calculated in Na2O-CaO-K2O-FeO-MgO-SiO2-H2O (NCKFMASH) chemical system with fluid as pure water and in excess. According to it, mineral assemblage feldspar + chlorite + biotite + muscovite + quartz was stable at temperatures below 550 °C and represents hornblende hornfels zone of the area metapelitic rocks. Partial melting occurs at 700 that increases up to 750 °C with pressure increasing. The assemblage feldspar + garnet + biotite + cordierite + quartz melt + H2O observed in the xenoliths occurs a pressures below 5 kbar. Although, comparing calculated almandin isopleth for garnet and phlogopite for biotite with chemical compositions of these minerals represented in Fazlnia (2017) indicates that anatexis could onset in lower temperatures.According to the calculated T-XCO2 diagram, the partial melting occurs in lower temperatures with increasing in CO2 content of the fluid. Also, T-XO2 diagram show that presence of sufficient O2 in the environment could decrease the partial melting temperatre to below 700 °C. Moreover, the T-Mg# diagram (Mg# = MgO/MgO+FeO) reveals that low Mg# contents of these rocks (0.2 – 0.3) could trigger anatexis in temperatures below 700 °C.

Falling of metapelitic xenoliths in Chah-Bazargan batholith producing magma caused intense contact metamorphism and partial melting. In the peak condition, the assemblage cordierite + spinel + K-feldspar + garnet + palgiocasle together with corundum + spinel intergrowth on cordierite formed. Based on the calculated phase diagrams, anatexis occured in temperatures 700 – 750 °C and pressures less than 5 kbar, however, chemical compositions of the garnets and biotites from the xenoliths show that it could onset in lower temperatures. The T-XCO2, T-XO2, T-Mg# diagrams reveal that increasing CO2 and O2 and high FeO contents of the protolith probably triggered anatexis in lower temperatures.