Variable zircon, titanite and apatite response during magma hybridization and autometasomatism

Abstract

The U–Pb records of accessory minerals can become difficult to interpret when magmatic crystallization is overprinted by hydrothermal alteration and element redistribution. The Archibarca Pluton, in the shallow portion on the Paleozoic Famatinian Arc in northwestern Argentina, preserves a complex history of magma crystallization, magma mixing (hybridization), and autometasomatism that variably overprints the textures, chemistry and U–Pb record of zircon, titanite and apatite. We found that hydrothermal modification affects zircon U–Pb dates and trace element compositions, producing complex and apparently concordant date distributions. Nevertheless, after filtering analyses affected by metasomatic modification, zircon retains geochronological information that is consistent with the magmatic and hydrothermal history recorded by titanite and apatite. Thus zircon can preserve useful chronological information even in hydrothermally modified systems, but its U–Pb record should be evaluated together with textural and geochemical evidence and complemented by other accessory mineral petrochronometers to distinguish magmatic crystallization from subsequent fluid-mediated changes. Zircon displays a spectrum of preserved and modified domains, in which chemical alteration, expressed as Fe–Ti–P–Y-LREE enrichment or depletion, may be either coupled or decoupled from U–Pb dates. Although zircon yields apparently concordant LA–ICP–MS dates spanning from ~ 500 to 420 Ma, these dates should be interpreted with caution, as metasomatic modification can disturb the U–Pb system without producing evident chemical alteration. Therefore, trace-element-based filtering is insufficient to discriminate disturbed zircon dates. A SHRIMP date of 441 ± 5 Ma (MSWD = 0.5) obtained from a composite magmatic-metasomatic zircon overlaps with its LA–ICP–MS date (445 ± 1 Ma, MSWD = 11); however, the latter exhibits substantial excess scatter, indicating that part of the date dispersion reflects analytical sensitivity to disturbed domains rather than geological complexity alone. In contrast, titanite preserves a more robust magmatic U–Pb record, with most samples yielding consistent crystallization dates of ~ 460 to 450 Ma, overprinted by fluid-related Pb loss at ~ 420 to 390 Ma. Additional complexities include inheritance at ~ 611 Ma and partial Permian (~ 280 Ma) resetting linked to regional Choiyoi magmatism. Titanite exhibits dual petrochronological behaviour, recording both magmatic dates and hydrothermal overprint, whereas magmatic and hydrothermal apatite yield indistinguishable U–Pb dates (439 ± 13 Ma and 436 ± 13 Ma), interpreted to record cooling of the pluton below ~ 450 °C. In summary, integrating the textural, chemical and isotopic data from these three accessory phases help constrain the nature and timing of the magmatic-hydrothermal transition in plutonic rocks.