Polymictic Breccia
According to the IUGS Subcommision on the Systematics of Metamorphic Rocks, Study Group for Impactites, a polymict impact breccia is "a breccia with clastic matrix or crystalline matrix (derived from the crystallization of impact melt) containing lithic and mineral clasts of different degree of shock metamorphism excavated by an impact from different regions of the target rock section, transported, mixed, and deposited inside or around an impact crater or injected into the target rocks as dikes".
This IUGS classification and definition seems to be inconsequent. Different from common use to avoid genetically related names, polymictic impact breccias are said to be " ... excavated ... from different regions of the target rock section ...". More inconsistent and questionable, the term "polymict impact breccia" is used also in the case of a crystalline matrix incompatible with the basic definition of a breccia to be a clastic rock. Moreover, the definition contradicts the IUGS own suggestion to discard the term "impact melt breccia".
Here we propose to use the term "polymictic breccia" purely descriptive only. The term "polymictic impact breccia" , a priori genetically related, may be used as a generic term for lithic breccias and suevite breccias .
Polymictic breccia from the Azuara impact structure (Spain).
Breccia-within-breccia; breccia generations
Polymictic breccia; Rubielos de la Cérida impact basin. Note the clast to the right of the coin which itself is a polymictic breccia (breccia-within-breccia).
Close-up 1
Close-up 2
Polished slab of a polymictic limestone breccia (top) and close-ups. Azuara impact structure. The breccia tells a complex story of formation. At least, four breccia generations can be observed. Also note the breccia dikelet cutting through a clast (close-ups 1, 2). After the emplacement of the dikelet, the clast was again fragmented.
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