Whole rock major element influences on monazite growth: examples from igneous and metamorphic rocks in the Menderes Massif, western Turkey

Abstract

Monazite (LREEPO 4 ) is a radiogenic, rare-earth bearing mineral commonly used for geochronology. Here we examine the control of major element chemistry in influencing the crystallization of monazite in granites (Salihli and Turgutlu bodies) and garnet-bearing metamorphic assemblages (Bozdag and Bayindir nappes) from the Menderes Massif, western Turkey. In S-type granites from the massif, the presence of monazite correlates to the CaO and Al 2 O 3 content of the whole rock. Granites with monazite only are low Ca (0.6-1.8 wt% CaO). As CaO increases (from 2.1-4.6 wt%), allanite [(Ce,Ca,Y) 2 (Al,Fe 3+ ) 3 (SiO 4 ) 3 (OH)] is present. Higher Al 2 O 3 (>15 wt%) rocks contain allanite and/or monazite, whereas those with lower Al 2 O 3 contain monazite only. However, examining data reported elsewhere for A-type granites, the correlation between major element chemistry and presence of monazite is likely restricted to S-type lithologies. Pelitic schists of the Menderes Massif show no correlation between major element chemistry and presence of monazite. One Bayindir nappe sample contains both prograde garnets and those affected significantly by diffusion. These rocks have likely experienced a complicated multi-stage tectonic history, which influenced their current mineral assemblages. The presence of monazite in a metamorphic rock can be influenced by the number, duration, and nature of events that were experienced and the degree to which fluids were involved. The source of monazite in the Bayindir and Bozdag samples was likely reactions that involved allanite. These reactions may not have significantly changed the bulk composition of the rock.

Fluid-mineral interactions and constraints on monazite alteration during metamorphism

“…Besides temperature, bulk-rock and fluid composition also control monazite stability. In particular, the Ca concentration of a rock may increase the stability of epidote over monazite, while also accommodating LREEs and Th (Lee and Bastron, 1967;Bea, 1996;Wing et al, 2003;Janots et al, 2007;Catlos et al, 2008). Several studies have shown that monazite breakdown corresponds with the formation of metamorphic allanite in rocks with moderate to high Ca bulk composition (Ferry, 2000;Wing et al, 2003;Janots et al, 2007;Rasmussen and Muhling, 2009).…”

“…Several studies have shown that monazite breakdown corresponds with the formation of metamorphic allanite in rocks with moderate to high Ca bulk composition (Ferry, 2000;Wing et al, 2003;Janots et al, 2007;Rasmussen and Muhling, 2009). The small Ca concentration in granites favours monazite crystallization (Lee and Bastron, 1967;Bea, 1996;Montel, 1993;Catlos et al, 2008). Detrital monazite in low-Ca sandstones metamorphosed at lower-greenschist facies dissolves, leading to the formation of secondary, low-Th monazite and ThSiO 4 (Rasmussen and Muhling, 2007).…”

Computational modelling studies for high temperature monazite systems

“…These conformations form under differing pressure and temperature environment in sedimentary, metamorphic and igneous rocks (Hoshino et al, 2016). The constituent elements of monazite can be used in a wide variety of applications including catalysis, metallurgy, glass/ polishing and magnetic sector (Catlos et al, 2008).…”