Pegmatite Production by Geochemical Fractionation of a Granodiorite Pluton
Plutons are igneous systems in which magma crystallizes within the crust and does not erupt. Minerals record the chemistry of the magma as they crystallize and they change the composition of the residual liquid in a predictable way as the magma body evolves. Additionally, physical heterogeneity of cooling plutons causes spatial variation in composition and texture. This study examines a hornblende quartz granodiorite from House Mountain, Idaho. This relatively small (100 m2) pluton contains a broad (1 m thick) pegmatite dike. Pegmatites are highly chemically evolved igneous rocks, the presence of which records the late stages of chemical fractionation. I will present field observations, thin section photomicrographs, and trace element data for both the granodiorite and the pegmatite to show how pluton composition and texture changes during magma evolution. Contrary to the idea that such dramatic differentiation requires long, slow cooling, high precision U-Pb series dating by chemical abrasion isotope dilution thermally ionized mass spectrometry CA-ID-TIMS analysis of zircon yields equivalent ~49 Ma ages from the granodiorite and pegmatite, indicating cogenetic crystallization. This study shows that extreme differentiation can occur over relatively short timescales.
Terhaar (Honors), Danielle, "Pegmatite Production by Geochemical Fractionation of a Granodiorite Pluton" (2016). 2016 Undergraduate Research and Scholarship Conference. Paper 33.