Enterprise Architecture is a model-based approach to business-oriented IT management. To promote good IT decision making, an Enterprise Architecture framework needs to explicate what kind of analyses it supports. Since creating Enterprise Architecture models is expensive and without intrinsic value, it is desirable to only create Enterprise Architecture models based on metamodels that support well-defined analyses. This paper presents the content and extension of a metamodel which supports creating models containing the information necessary to conduct system quality analyses, specifically with respect to availability, accuracy, confidentiality and integrity. The metamodel is an extension and formalization of the metamodel underlying the ArchiMate modelling language for Enterprise Architecture. The use of the extended metamodel is demonstrated in a case study where the availability, accuracy, confidentiality and integrity of the two Service Oriented Architecture (SOA) platforms Sun JCaps and PrOSeRO were evaluated.