What is the relationship between magma chemistry and inclusions in zircon?
Abstract
Efforts to translate the mineral inclusion assemblage of detrital zircons to a reflection on their source rock have been stymied by the general overrepresentation of apatite and bias toward more felsic QAP proportions within zircon compared to the host rock matrix, as observed in several previous studies. Although there is a slight trend toward less abundant apatite for more silicic host magmas, this relationship is ambiguous for zircon suites with >20% apatite. Mineral inclusion assemblages in zircon are almost certainly a complex function of crystallization sequence, magma chemistry, and physical proximity to other crystallizing phases. While the latter may be largely random, the former should be more systematic, and potentially interpretable from the detrital mineral record. To better constrain the effects of crystallization sequence and magma chemistry on inclusion assemblages in zircon, I investigated a set of granitoids from the Peninsular Ranges Batholith (PRB) of southern California, ranging from tonalites to leucogranites and ranging in A/CNK from 0.83 - 1.07. First, I investigated inclusions in multiple accessory minerals within single hand samples to focus on crystallization order effects. Apatite is overrepresented relative to the matrix in all studied minerals. Where crystallization order can be inferred, earlier phases have a higher proportion of apatite. In one granite, plagioclase inclusions also become more albitic in progressively later-crystallizing host phases. In many samples, % apatite decreases with increasing Q/QAP. Second, I compared zircon inclusions and whole rock chemistry in a larger set of PRB granitoids to study magma chemistry effects. Apatite exhibits distinct behavior with respect to whole rock chemistry (e.g., SiO2, relative apatite and zircon saturation temperatures) for metaluminous vs peraluminous granitoids. This likely reflects the suppression of apatite saturation in peraluminous magmas. Overall, inclusion assemblages in detrital zircons are best related to host magma SiO2 and apatite/zircon saturation temperatures by comparing the proportion of apatite to other crystallization sequence markers (e.g., Q/QAP, plagioclase composition) and markers for relative A/CNK (e.g., aluminous inclusion phases, host zircon chemistry).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMV012...05B
- Keywords:
-
- 8499 General or miscellaneous;
- VOLCANOLOGY