Plagioclase textures reveal contrasting magma storage conditions for plutonic versus volcanic rocks

The nature of the relationship between volcanic and plutonic rocks is a topic of ongoing debate despite decades of research. Plutons have been interpreted either as “failed eruptions” equivalent to erupted material; as “crystal graveyards” left behind after melt extraction; or as genetically distinct from volcanic rocks. Extensive geochemical, geochronological, and modelling work has not led to a conclusive resolution; at the same time, comparative textural studies of plutonic and volcanic crystal cargoes are rare despite their potential to reveal petrogenetic differences.

Here, we examine differences in plagioclase textures in volcanic versus purely plutonic rocks across a range of magma compositions and tectonic settings. We target plagioclase, an abundant igneous mineral phase which records significant disequilibrium – caused by, e.g., magma recharge and remobilisation – as prominent resorption horizons. The number of major resorption horizons was counted for ≥100 plagioclase crystals (>100 µm) per sample using BSE images, excluding oscillatory zoning and outermost rims in volcanic crystals. We observe that plagioclase cargoes in volcanic rocks consistently show more major resorption horizons per crystal (mean ≈ 4) than those in plutonic rocks (mean ≈ 1–2). This pattern is reproduced across magma compositions, except basalts, in which plagioclase crystals have a similar number of resorption boundaries (mean ≈ 1) to those in plutonic rocks.

Our results demonstrate that intermediate and silicic volcanic rocks record pronounced disequilibria more often than plutonic rocks of comparable composition, implying fundamental differences in magma storage. Assuming that major resorption horizons record recharge/remobilisation events, the observed textural contrast suggests that plutonic systems experience lower magma recharge rates and limited interaction between distinct batches, whereas higher recharge rates in volcanic systems repeatedly remobilise stored magma and promote the formation of hybridised, eruptible reservoirs. Our results highlight the potential of comparative crystal textural analysis to reassess the plutonic–volcanic connection across tectonic settings.