Plagiogranites from the ocean crust and ophiolites

Abstract

Rock types similar to those found in ophiolites have been recovered from the floors of ocean basins^1–3. This is critical evidence for the theory that ophiolites represent the ocean crust^4,5. 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 MAR^9–11, 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.