Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level granitoid plutons. They have relatively fine-grained igneous (microgranitoid) microstructures1, and many show evidence of having flowed in a magmatic condition2. These features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3–5 or restite6–11. An alternative interpretation is that the enclaves represent globules of mafic magma that have mingled (‘commingled’) and quenched in the granitoid host magma12–16. A detailed review of the literature, occurrence, morphology and composition of micro-granitoid enclaves strongly supports this view, hut indicates that the enclave magmas range in composition from mafic to felsic, and that magma-mixing may be involved in their formation17. I summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid.

Microgranitoid enclaves in granites—globules of hybrid magma quenched in a plutonic environment

Preface 1. Background 2. Microstructures of sedimentary rocks 3. Microstructures of igneous rocks 4. Microstructures of metamorphic rocks 5. Microstructures of deformed rocks Mineral symbols used in this book Glossary of microstructural terms.

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A Practical Guide to Rock Microstructure

The key to mantled feldspar genesis is epitaxial nucleation of plagioclase on K-feldspar or K-feldspar on plagioclase. Once this nucleation takes place there is a relatively straightforward process of crystal growth yielding “rapakivi” and “antirapikivi” textures. The most common mantling is plagioclase on K-feldspar which occurs in both volcanic and plutonic environments. In the volcanic environment the morphology of the plagioclase overgrowth typically is dendritic, though in subvolcanic and shallow plutonic environments dendritic growth is followed by a more or less continuous non-cellular shell of plagioclase. In the plutonic environment, early stages of plagioclase overgrowth also tend to be dendritic, although with coarser-grained characteristics. Dendritic morphology is thus a common denominator in rapakivi genesis. Since growth of dendritic plagioclase is clearly related to marked undercooling in silicate melt systems its occurrence in many volcanic rocks is to be expected. Equivalent quenching in the plutonic environment requires a cooling mechanism independent of conductive heat transfer to wallrock and also independent of effective cooling related to sudden loss of volatile phases that could only occur late in the crystallization of most magmas and therefore after much dendritic plagioclase had already formed. Internal quenching of portions of magma systems must occur if mafic magma is abruptly mixed with felsic magma. Such magma mixing yields a heterogeneous system at first, one that is in a drastic state of disequilibrium and tending to force nucleation of one feldspar type on the surface of another resulting in epitaxial crystallization of dendritic plagioclase on K-feldspar. Mantling of one feldspar type by another during magma mixing is paralleled by dendritic growth zones in coexisting plagioclase crystals.

The magma mixing origin of mantled feldspars

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

Thermal, mechanical and chemical exchange occurs between felsic and mafic magmas in dynamic magma systems. The occurrence and efficiency of such exchanges are constrained mainly by the intensive parameters, the compositions, and the mass fractions of the coexisting magmas. As these interacting parameters do not change simultaneously during the evolution of the granite systems, the exchanges appear sequentially, and affect magmatic systems at different structural levels, i.e. in magma chambers at depth, in the conduits, or after emplacement

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0263593300007835

Genesis and evolution of mafic microgranular enclaves through various types of interaction between coexisting felsic and mafic magmas

/pdf/relations-between-coexisting-basaltic-and-granitic-magmas-in-t8o71knt6z.pdf

Relations between coexisting basaltic and granitic magmas in a composite dike

Details of plagioclase zoning were correlated throughout a 3-sq mi quartz-monzonite stock on Mount Pilchuk, Wash. A typical zone sequence in all phenocrysts is: andesine cores with oscillatory zoning; core-rim boundaries marked by sharp drops of 8 to 15 percent An, and rims normally zoned from An 28 to An 15 . Discontinuities between rims and cores represent a period of core resorption and non-crystallization caused by pressure decrease during intrusion. Coarse oscillations within cores (3 to 7 percent An) record convection and turbulence in an inhomogeneous magma during the early stages of crystallization. Constant composition of the outermost core zones indicates homogenization of the magma prior to and during intrusion. Zones within the outermost core show low compositional correlation and indicate a high degree of crystal mixing.

Robert A. Wiebe

Papers

Relations between coexisting basaltic and granitic magmas in a composite dike

Plagioclase stratigraphy; a record of magmatic conditions and events in a granite stock