Ophiolitic mélanges are important components of orogenic belts in that they contain fragments of partially disrupted or even disappeared terranes whose understanding is essential in reconstructing paleo-tectonic settings. Both the Round Mountain Serpentinite Mélange (RMSM) in California and the Beimarang Mélange in Tibet are the products of dismemberment of an overlying arc-related ophiolite and underlying units related with oceanic domains of MORB or BABB affinity. Similarities and differences between these chaotic units have implications in understanding the complex processes of ophiolitic mélange formation and collisional belts in general.

Both mélanges are dominated by highly sheared serpentinized peridotites derived from disruption of the juxtaposed ophiolitic upper mantle peridotites. Mafic components enclosed in the mélanges are characterized by low pressure secondary mineral assemblages typical of oceanic metamorphism retrograded from amphibolite down to pumpellyite-prehnite facies. These mafic lithologies have contrasting origins since the RMSM includes composite slivers of basalts and cherts derived from the structurally underlying Franciscan Complex whereas the Beimarang Mélange contains diabase and gabbros which intruded the peridotites prior to the mélange formation. In the latter locality no mafic component derived from the structurally underlying Tethyan units has been accreted to the serpentinite belt despite the fact that in other areas along the suture zone basaltic fragments of Tethyan origin are associated with similar mélanges.

Accretion of basaltic fragments from the oceanic domain to an ophiolitic mélange in a trench setting is not a ubiquitous feature and may require specific conditions to be achieved. These conditions include physical and chemical properties of the subducting lithosphere such as its age, permeability, topography, composition and fracturing. We suggest that different original settings of the paleo-subducting lithospheres, and hence different compositions and ages, might have been responsible for such contrasted behaviors. Basaltic fragments of back-arc composition in the RMSM might be more easily tectonically accreted than MORB-like Tethyan fragments which were probably older, denser and less permeable.