229-6 To what extent does deformation impact the geochemistry and radiogenic isotope signatures of accessory minerals and whole-rocks? A case study from the Bergell intrusion, Central Alps

Session: Crustal Petrology (Posters)

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Abstract:

Radiogenic isotope systems are a robust tool used to understand the formation, modification, and evolution of Earth’s crust. However, nearly all ancient crust has experienced episodes of metamorphism and deformation. The extent to which accessory minerals retain their original (i.e., protolithic) isotope compositions following these episodes is poorly known. We aim to provide insights into mineral reactions, geochemical behavior, and (re)distribution trace elements and Sm-Nd, Lu-Hf, Rb-Sr, and U-Pb isotopes during deformation by measuring whole-rock and mineral geochemistry in a sequence of progressive deformation within the same lithology. Here, we investigate the geochemical and isotopic systematics of nine samples along a ~1 km transect through the Bergell intrusion in the Central Alps. The transect starts in tonalite that exhibits weak magmatic deformation and grades into a mylonite near the Insubric line. Field observations indicate that the X:Z aspect ratio of mafic enclaves along the transect increase from ~3:1 to greater than 200:1. As the extent of deformation increases, the whole-rock samples show an increase in the K/Ti and Rb/Sr ratios, but variable differences in the Lu/Hf and Sm/Nd ratios relative to the protolith. The whole-rock ε_Hf(i), ε_Nd(i), and ⁸⁷Sr/⁸⁶Sr_i values trend from -1.7, -4.8, and 0.7098 to -4.5, -6.5, and 0.7131 respectively from the protolith to the mylonitic samples. Zircons from the protolith yield concordant U-Pb dates ranging from 31 Ma to 33 Ma and an average ε_Hf(i) value of -1.5. Zircons from the mylonitic samples yield date populations from 31 Ma to 560 Ma and an average ε_Hf(i) value of -3.9. Results from the whole-rocks and zircons are consistent with fluid alteration and mechanical mixing of country rock into the mylonite. The geochemical compositions as well as the U-Pb and Sm-Nd systematics of titanite, apatite, and allanite, and the Rb-Sr systematics of apatite and plagioclase are currently being measured. Together with the whole-rock data, these analyses may illuminate the trajectory of radiogenic ingrowth into accessory phases and help clarify the conditions under which accessory minerals no longer preserve their protolithic geochemical signatures and instead reflect deformation. Our comprehensive assessment of the reliability of minerals as recorders of protolith composition has implications for interpreting both accessory mineral and whole-rock data in ancient, deformed rocks.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8530

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