Re: Is there a subduction zone between two dense oceanic plates when they meet.

I didn't know the answer to this one, but fortunately a geophysicist friend
did.  Here is her response:

In general, subduction zones can be of two types:
        (1)  oceanic plates subducting underneath continental
             plates
        (2)  oceanic plates subducting underneath oceanic
             plates.

Plates are created at spreading centers.  In the case of
oceanic lithosphere, as the plate moves away from the
spreading center, it cools off, thereby becoming denser.
Therefore, the older the plate, the denser the plate.
When two plates meet at a subduction zone, the denser 
plate always subducts.  Put another way, when two 
oceanic plates converge, the older plate is the one
that is consumed.

There are a number of examples around the world of
ocean-ocean subduction zones.  A few are:

(a)  The Philippine and Pacific plates.  The Pacific
     Plate is subducting underneath the Philippine,
     creating the Mariana Trench.
(b)  The Caribbean Plate and the American Plate, where
     the American is diving underneath the Caribbean.
(c)  The Scotia Plate and the American Plate, where
     Scotia Plate is overriding the American Plate.
     (The Scotia Plate occupies a small area extending from
     the tip of South America eastward.)