how to center in span

[silko]

Hi 

Am still learning,  but cant seem to find a way of centering the anchor 
tags below in the span class, like style="text-align:center; does with text 
. Please can anyone help. 

<div class="container-fluid">
    <div class="row-fluid">
        <div class="span5">
        </div>
       <div class="span7">
            <a href="#" class="twitter"></a>
            <a href="#" class="skype"></a>
       </div>
    </div>
</div>

.twitter{
width:64px;
height:64px;
text-decoration:none;
display:block;
background-image:url(img/NavIcons.png);
background-position: 0 0;
float: left;
margin-left: 20px;
}   .twitter:hover,#logo-link:active  { background-position:192px 0; }
.skype{
width:64px;
height:64px;
text-decoration:none;
display:block;
background-image:url(img/NavIcons.png);
background-position: 128px 0;
float: left;
margin-left: 20px;
}   .skype:hover,#logo-link:active  { background-position:64px 0; }