Plagiogranites from the ocean crust and ophiolites

Abstract

Rock types similar to those found in ophiolites have been recovered from the floors of ocean basins1–3. This is critical evidence for the theory that ophiolites represent the ocean crust4,5. Aumento6 has described diorites, quartz diorites and trondhjemites dredged from the Mid-Atlantic Ridge (MAR) near 45° N, and noted that these were the first rocks analogous to the low-K acid differentiates in ophiolites (collectively referred to as ‘oceanic plagiogranites’7,8) to be recognized amongst those recovered from the modern ocean floor. Only a few plagiogranites from the ocean floor (ocean-crust plagiogranites) have since been described; these were dredged from widely spaced localities on the MAR9–11, the Amani Plateau and the Kyushu-Palau Ridge (north-west Pacific)12. However, the Kyushu-Palau Ridge could be the product of destructive plate-margin magmatism13, as are the potassic granites from the Aves Ridge (Caribbean)14. Plagiogranite xenoliths also occur in extrusives on Surtsey and Askja (Iceland)15. That only some of the petrological compositions of ophiolite plagiogranites are represented in the ocean crust is not surprising in view of the small sample of ocean-crust plagiogranites. It is less obvious, however, if ophiolites represent the ocean crust, why certain acidic rock types from the oceans have not been found in ophiolites. I show here that the difference between the two groups of plagiogranites is evidence that ophiolites do not represent all kinds of ocean crust and that a greater range of magma compositions and/or conditions of magmatic differentiation is represented by the ocean crust.