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CLASTIC METAMORPHIC AND IGNEOUS ROCKS Clastic Metamorphic Rocks include Breccia s formed in faults, as well as some Protomylonite and Pseudotachylyte . Occasionally, metamorphic rocks can be brecciated via Hydrothermal fluids, forming a Hydrofracture breccia. Clastic Igneous Rocks include Pyroclastic Volcanic Rocks such as Tuff , Agglomerate and intrusive Breccia s, as well as some marginal Eutaxitic and Taxitic intrusive morphologies. Igneous clastic rocks are broken due to flow, injection or explosive disruption of solid or semi-solid igneous rocks or Lava s. CLASTIC SEDIMENTS Clastic Sedimentary Rocks are rocks composed predominantly of broken pieces or 'clasts' of older Weathered and Eroded rocks. Clastic sediments or sedimentary rocks are classified based on Grain Size , clast and cementing material ( Matrix ) composition, and texture. The classification factors are often useful in determining a sample's environment of deposition. Grain size determines the basic name of a clastic sedimentary rock. Grain size varies from Clay in Shales ; through Silt in Siltstones ; Sand in Sandstones ; and Gravel , Cobble , to Boulder sized fragments in Conglomerates and Breccias . The Krumbein Phi (φ) Scale numerically orders these terms in a logarithmic size scale. Composition includes the Chemical and Mineralogic make-up of the single or varied fragments and the cementing material ( Matrix ) holding the clasts together as a rock. An example clastic environment would be a river system, in which the full range of grains being transported by the moving water consist of pieces Eroded from solid rock upstream. Sedimentary breccias Sedimentary Breccia s are a type of clastic sedimentary rock which are composed of angular to subangular, randomly oriented clasts of other sedimentary rocks. They are formed by either submarine Debris Flow s, Avalanches , mud flow or mass flow in an aqueous medium. Technically, Turbidite s are a form of debris flow deposit and are a fine-grained peripheral deposit to a sedimentary breccia flow. The other derivation of sedimentary breccia is as angular, poorly sorted, very immature fragments of rocks in a finer grained groundmass which are produced by mass wasting. These are, in essence, lithified Colluvium . Thick sequences of sedimentary (colluvial) breccias are generally formed next to fault scarps in Graben s. In the field, it may at times be difficult to distinguish between a debris flow sedimentary breccia and a colluvial breccia, especially if one is working entirely from Drilling information. Sedimentary breccias are an integral host rock for many SEDEX Ore Deposits . IGNEOUS CLASTIC ROCKS breccia, green Groundmass is composed of Epidote .]]Igneous clastic rocks can be divided into two classes
HYDROTHERMAL CLASTIC ROCKS Hydrothermal clastic rocks are generally restricted to those formed by Hydrofracture , the process by which Hydrothermal circulation cracks and brecciates the wall rocks and fills it in with veins. This is particularly prominent in Epithermal Ore deposits and is associated with Alteration Zones around many intrusive rocks, especially Granite s. Many Skarn and Greisen deposits are associated with hydrothermal breccias. IMPACT BRECCIAS A fairly rare form of clastic rock is formed during Meteorite impact. This is composed primarily of ejecta; clasts of country rock, melted rock fragments, Tektite s (glass ejected from the impact crater) and exotic fragments, including fragments derived from the impactor itself. Identifying a clastic rock as an impact breccia requires recognising Shatter Cones , Tektites , Spherulites , and the morphology of an impact Crater , as well as potentially recognising particular chemical and trace element signatures, especialy Osmiridium . SEE ALSO
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