WHY BULK-ROCK COMPOSITIONS SHOULD NOT BE USED TO DIRECTLY COMPARE GRANITES AND RHYOLITES

Bulk-rock compositions are commonly considered to approximate melt compositions, particularly in silicic plutonic systems. This relationship leads to direct comparisons of granitic and rhyolitic rock compositions in efforts to test genetic relationships between volcanic and plutonic rocks. However, recent studies indicate that crystal accumulation/melt loss is common in granitic magmas, making direct comparisons between volcanic and potentially consanguineous plutons challenging. An alternative approach involves use of hornblende (Hbl)–melt Fe/Mg partitioning and Hbl chemometry to determine whether plutonic rocks approximate melt compositions, and to calculate compositions of melt, and potential volcanic ejecta, in equilibrium with Hbl. We assessed bulk-rock and Hbl compositions from a suite of plutons in CA, OR, and WA along with similar data on volcanic rocks from the literature and found that for calc-alkaline plutons: (1) Hbl zoning, co-variation of fast- and slow-diffusing elements, and thermometry, support crystallization from a melt. (2) Hbl is rarely in Fe-Mg equilibrium with the host’s bulk composition. (3) Over a wide range of bulk-rock SiO₂ contents, Hbl was in equilibrium with dacitic to rhyolitic melts. These observations indicate that bulk-rock compositions of many quartz-bearing plutonic rocks must be viewed as representative of crystal accumulation.

In contrast, Hbl in volcanic rocks is commonly in equilibrium with bulk-rock and glass compositions. In some samples, a wide range of calculated melt compositions provides strong evidence that the bulk-rock compositions represent crystal accumulation and/or magma mixing.

We conclude that assessment of volcanic–plutonic relationships on the basis of bulk-rock compositions is problematic because of melt loss in many plutonic environments. This problem can be addressed by detailed study of individual magmatic systems in which age plus mineral compositions and zoning patterns are utilized.