In XSLT 2.0, use
<xsl:for-each-group select="Field" group-starting-with="Field[@type='Separator']">
For 1.0 it's more difficult but there are well-known solutions: try a search for "XSLT positional grouping".
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference