Contribution of enclave studies to the understanding of origin and evolution of granitic magmas

Abstract

Enclaves give important information on the origin and evolution of granitic magmas. The presence in a granite of surmicaceous enclaves (restites) is good evidence of continental crust contribution to magma genesis. The presence of dark microgranular enclaves is an indication of mantle contribution. The joint presence in the same granite of the two types of enclaves indicates that its magma has probably more than one source. Dark microgranular enclaves, and their host granites as well, are hybridized rocks resulting from incomplete mixing (or mingling) of more acidic and basic components. Enclave studies allow one to specify mixing conditions. When mixing occurs relatively early with respect to granite crystallization, only earlier-formed granitic minerals (zircon, apatite ...) may be incorporated in the basic magma. When mixing occurs later all granitic minerals may be involved. When all the dark microgranular enclaves of a granite are similar, the intervention of a single mixing event can be assumed. On the contrary, when several chemically and mineralogically contrasted varieties occur, several mixing events can be assumed. The order of these events may be deduced from the nature of the granitic minerals present in the enclaves (early of late minerals) or sometimes by observation of double enclaves. Enclave shapes bring information on the distance between the place of mixing and the place of observation. Enclaves with irregular or crenulated margins, or with chilled margins as well, always occur near mixing places. The enclave transport in the magma makes these characteristics disappear. In spite of hybridization processes, the primitive composition of the basic component of the mixture may sometimes be recognized. A systematic identification of this component in the different granites from the same orogen should allow to understand the space and time evolution of basic magmatism in an orogen. Magma mixing is a cause of heterogeneity in granites, but it is also an important factor of homogenization as the thermal contribution of the basic magma produces new mingling of the components and sometimes new intrusive processes.