The Chiquimula pluton is one of a chain of plutons that extends across southern Guatemala and northern Honduras. Its exposed area of about 300 sq km in southeastern Guatemala is highly irregular. Exposed parts of the epizonal pluton are dominantly granodiorite; gabbro may be abundant, but it is exposed only in the deepest canyons. Diorite is predominant along the southern and western margins, and the rock is an intergrading mixture of granodiorite with subordinate adamellite and granite near the northern and eastern margins.
A whole-rock Rb-Sr isochron diagram shows data points falling in two distinct groups. Granite and adamellite form a scattering of points indicating relatively high Rb/Sr (3.3 to 4.6) and high Sr87/Sr86 (0.72 to 0.75). Points from the volumetrically more important gabbro and grand diorite, indicative of low Rb/Sr (1.2 or less) and Sr87/Sr86 (0.706 to 0.708), form a well-defined isochron corresponding to an age of 50 ± 5 m.y. and initial Sr87/Sr86 = 0.7060 ± 0.0002. K-Ar and Rb-Sr biotite ages from granite of 84 m.y. and 95 m.y., respectively, are younger than the whole-rock Rb-Sr age (215 m.y.) of the same specimen, but older than the age of emplacement of the pluton as suggested by the whole-rock isochron from granodiorite and gabbro.
Field relations, petrographic data, and isotopic data are consistent with the interpretation that the Chiquimula pluton is just beginning to be unroofed. The compositional variation probably resulted from a combination of assimilation and differentiation of a granodioritic magma. The more granitic parts may be due to partially assimilated more siliceous exotic blocks of basement rock.