Bimodal magmatism produced by progressively inhibited crustal assimilation

Area of Science:

• Geoscience
• Volcanology
• Igneous Petrology

Background:

• Bimodal rock suites (mafic and felsic) are common but their origin remains a key question in volcanology.
• Understanding bimodal magmatism is crucial for interpreting the evolution of volcanic centers and crustal processes.

Purpose of the Study:

• To investigate the mechanisms driving bimodal magmatism at the Carlingford Igneous Centre, Ireland.
• To elucidate the role of crustal assimilation in generating mafic-felsic rock suites.

Main Methods:

• Analysis of major and trace elements and high-resolution Sr, Nd, and Pb isotopes.
• Conducting experimental petrology and thermodynamic modeling.
• Investigating the Carlingford Igneous Centre, a well-known example of bimodal magmatism.

Main Results:

• Early microgranites formed via assimilation of metasiltstone partial melts into mafic magmas.
• Crustal melting experiments showed high fusibility, but thermodynamic modeling revealed rapid decreases in melt production capacity with repeated heating.
• Granite generation ceased as enriched melts diminished, leading to subsequent magmatism incorporating less fertile restite and a compositional gap.

Conclusions:

• Progressively inhibited crustal assimilation (PICA) is proposed as a primary driver of bimodality in continental volcanism.
• The PICA model explains the cessation of granite formation and the development of compositional gaps in bimodal suites.
• This mechanism is likely significant in the North Atlantic Igneous Province and other continental volcanic settings.