Regarding textures, an important point is the different state of matter involved. The process of crustal contamination implies digestion of a solid material (if not consider some ultimate cases) whereas magma mixing is a result of interaction between two more or less viscous liquids. Hence, the  textural evidence for crustal contamination may be the presence of abundant crustal xenoliths which are usually angular in shape. On the contrary, two mixing magmas form spectacular textures with roundish, plumpy or irregular 'amoebiform' outlines (see photo). Round-shape mafic enclaves common of many granitic / granodioritic plutons are also thought to result from injection of mafic magma into a relatively silicic environment.

Dear Petr,

Many thanks for your informative response.

 One of the best places in the central US to see magma mixing is at Tieman Shut-In in eastern Missouri. Here we have Mesoproterozoic granite and basalt mixing.

Petrographically, the two processes seem to be difficult to tell from one another as disequilibrium textures (e.g., coexisting Mg-rich olivine and quartz or calcic and sodic plagioclase, xenocryst of partially dissolved quartz rimmed by clinopyroxene, melt inclusions within phenocrysts that have different composition than the surrounding groundmass etc.) are common in both cases. With regard to partially resorbed and sieve-textured plagioclase phenocrysts, even though there are authors that attribute sieve texture to magma mixing, others argue that the same texture may be formed as a result of changing physical conditions in the magma chamber such as rapid decompression. Similarly, rapakivi texture is attributed by some researchers to magma mixing while others suggest it is produced as a result of decompression. I don’t know of any publication in the literature that links this texture to assimilation. On the other hand, Tsuchiyama (1985) argue that magma mixing and assimilation cannot be distinguished based on dusty-zoned plagioclase alone.

According to Best (2002), a rather useful way to distinguish between assimilated xenocrysts and phenocrysts in mixed magmas is through xenocrysts that show solid-state strain effects, e.g. undulatory extinction under cross-polarized light. In this respect, a quartz grain that is rimmed by clinopyroxene but displays undulatory extinction would be valuable.

Geochemically, ratios of trace elements that are abundant in continental crust (e.g. K, Ba, Rb, U, Th) can be checked out. Importantly, Nb-Ta troughs in spiderdiagrams are considered a good indicator of crustal contamination. At the moment, I don't know of any textbook or research article in which the Nb-Ta troughs are also attributed to magma mixing processes. That being said, isotopic analyses of O, Sr, Nd and Pb would yield satisfying and more precise results, of course.

Below is a list of references to check out:

- Igneous and Metamorphic Petrology (2nd edition) by Myron G. Best, published in 2002

- Tsuchiyama, A. Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite, and origin of dusty plagioclase in andesites. Contr. Mineral. and Petrol. 89, 1–16 (1985).

Sieve-textured plagioclase in volcanic rocks produced by rapid decompression - Nelson and Montana 1992

- Magmatic interactions as recorded in plagioclase phenocrysts of quaternary volcanics in SE Bam (SE Iran) - Yazdi et. al. 2019.

- Andesitic volcanism and crustal evolution - Eichelberger 1978

- F. J. Tepley, III, J. P. Davidson, M. A. Clynne, Magmatic Interactions as Recorded in Plagioclase Phenocrysts of Chaos Crags, Lassen Volcanic Center, California, Journal of Petrology, Volume 40, Issue 5, May 1999, Pages 787–806

- Magma mixing and mingling textures in granitoids: examples from the Galway Granite, Connemara, Ireland - Baxter and Feely 2002

Dear Babaoğlu, Thank you very much for these enlightening and detailed answers.