Aplite () is an intrusive igneous rock that has a granitic composition. Aplites are fine-grained to

aphanitic Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geo ...

(without grains visible to the naked eye) and may consist of only quartz and feldspar or the term may refer to any leucocratic (pale-coloured) minor intrusion of that grain size. They are associated with the later stages of many larger intermediate to

felsic In geology, felsic is a grammatical modifier, modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted ...

intrusions.

Occurrence

Aplites have a global distribution and have been described from most areas where there are significant granitic intrusions. They occur in the form of intrusive sheets, both within the associated granitic bodies and in the surrounding

country rock Country rock is a music genre that fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal sty ...

. Trace element analysis shows that there are no volcanic equivalents to aplites. Some aplites form in close association with

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic c ...

s, which are otherwise uncommon in granites. These aplite-pegmatite sheet complexes may show fine-scale banding with alternations of aplite and pegmatite. Syenite-aplites consist mainly of alkali feldspar; the diorite-aplites of

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

; there are

nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatit ...

-bearing aplites, including those containing the elaeolite variety of nepheline. In all cases, they bear the same relation to the parent masses.

Formation

Aplites are intruded at a late stage in the history of the associated intrusion when crystallisation is at an advanced stage. In this state, the parent intrusion is mechanically coherent enough to support tensile stresses, leading to fractures. These fractures may then be filled by the remaining uncrystallised parts of the original magma (the residual fluids) to form the aplites. There may be repeated phases of intrusion, solidification, fracturing and aplite dyke intrusion. This has been observed in the Half Dome

granodiorite Granodiorite ( ) is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gra ...

Yosemite National Park Yosemite National Park ( ) is a List of national parks of the United States, national park of the United States in California. It is bordered on the southeast by Sierra National Forest and on the northwest by Stanislaus National Forest. The p ...

, California, with multiple stages of aplite intrusion recognised, with the earlier aplite sheets becoming deformed by later intrusions into the main granodiorite body, but recognisable from their consistent chemistry.

References

{{Authority control Felsic rocks Phaneritic rocks Plutonic rocks Subvolcanic rocks