Zircon stability grids in crustal partial melts: implications for zircon inheritance

Abstract

Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into “hot” (TZr > 800 °C, with little or no inherited zircon) and “cold” (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the “hot” and “cold” classification by splitting the “cold” granites (TZr < 800 °C) into two categories, “dry” with little inheritance and “wet” with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis.