As brechas de origem ígnea: revisão e proposta para uma classificação geológica

Abstract

The nomenclature and classification of breccias of igneous origin has been, and still is, a subject of considerable debate, which reflects the ongoing discussions about their formation and emplacement. This is especially valid for those breccias emplaced in plutonic or subvolcanic environments. During field studies in the large alkaline massif of Poços de Caldas, southern Brazil, the author was confronted by a seemingly bewildering variety of breccias and other pyroclastic rocks, thus providing the impetus for a revision of the subject and the proposal for simple ways of describing and classifying such breccias. Special emphasis was placed on the important group of breccias emplaced at depth ("subsurface breccias") which are notoriously difficult to catalogue and to interpret; subaqueous types are non-existent, or at least very rare, in Poços de Caldas, and are not dealt with in the present discussion. Breccias should be clearly characterized borrowing terminology and methods from the field of study of fragmental supracrustal (epiclastic and pyroclastic) rocks, both for descriptive purposes (e.g., description of form, sorting, roundness, petrographic variety, etc.) as well as for size ranges; in addition, other textural terms, such as "clast-supported", "clast-matrix supported", and "matrix-supported", should be used to describe the clast-to-matrix relationship. A breccia of igneous origin is defined as a clastic rock with suitable amounts (>;25%) of (angular to rounded) fragments greater than 2 mm which originates as a direct response to igneous phenomena. Genetic aspects of breccia formation are not considered a satisfactory guideline for classification purposes, mostly because breccia-forming processes are still very poorly undeir stood. The mechanisms, which have been proposed in the literature, range from well- researched and qualified hypothesis (such as the "implosion-fluidization" model of Biondi, 1979) to highly unlikely speculations. Still lacking is the ability to take the guesswork out of interpretations of texture and structure of breccias. In the proposed classification, emphasis is placed on descriptive and geologic aspects, in order to define the geologic environment in which the breccia was emplaced. On the one hand, this procedure introduces a standard nomenclature, independent of any genetic meaning (or misconception); additionally, emphasis on detailed description of the geologic environment may ultimately lead to a sounder basis for interpretation. The simplest classification of breccias is therefore threefold, recognizing only "volcanic", " subvolcanic" and "plutonic breccias". The first group comprises all deposits related to volcanic supracrustal activity, including various types of autoclastic flow breccias (including rubble found on advancing block lavas, and talus deposits and collapse fragments shed during cooling and growth of subaerially exposed volcanic domes), tephra deposits, pyroclastic flows and epigenetic sheets formed as a direct response to volcanism (e.g., lahars). The second group, associated with subvolcanic phenomena, includes types found in small, more or less confined bodies (pipes, necks, dikes, sills, irregular bodies: "conduits" in the broadest sense). Among these are autoclastic (e.g.,"friction" breccias) and xenolithic varieties (fragments enclosed within a groundmass of non-fragmented volcanic rocks). The most significant types, the economically important conduit breccias associated with explosive activities, are subdivided, merely for descriptive purposes, into non-intrusive varieties ("shatter" breccias, with angular fragments; "cracked" or "craquelée" rock; collapse breccias, with fragments derived from nearby wall-rocks) and the clearly intrusive types (including both conduit breccias with more or less angular fragments, accompanied by varying but usually important amounts of matrix, and conduit agglomerates, with rounded fragments and little or no matrix). There has been wide speculation as to the proper generating mechanism, fluidization usually being advanced as the main emplacement cause (probably triggered by an initial "implosion"). In well-defined subvolcanic conduits, it is observed that shatter breccias, "cracked" rock, and conduit breccias are distributed in a roughly zoned pattern; the first two types are restricted to wall-rock areas and are probably formed as a response to explosion-related com pressions and decompressions, while the conduit breccias appear as centrally located bodies. The group of "plutonic breccias" are divided into border types (protoclastic and friction breccias marginal to plutons, which are, structurally, cataclasites or milonites), agmatites (fragments in a groundmass of plutonic rock), "metasomatic" substitution varieties, and plutonic conduit breccias (with features similar to those encountered in the equivalent subvolcanic types).