The origin of the Daly gap by fractional crystallization in a transcrustal plumbing system in the Passa Quatro alkaline complex (SE Brazil)

ABSTRACT

The surface gap of tephriphonolite compositions in the Passa Quatro alkaline complex, in SE Brazil, is attributed to non-buoyancy of these high viscosity intermediate compositions during magmatic fractional crystallization at different depths. Thermodynamic and Rayleigh models for three alkaline suites of a strongly silica-undersaturated series were integrated to demonstrate that parental (basanite, nephelinite and alkaline basalt) and residual (phonolites) magmas are cogenetic by fractional crystallization at pressures of 10 kbar and 5 kbar, i.e. at magma chambers located above and below the Conrad discontinuity within the Upper Cretaceous-Palaeogene 40 km-thick continental crust in SE Brazil. The fractional crystallization of the parental magmas produced an ultramafic residual solid (wehrlite, olivine-bearing clinopyroxenite and clinopyroxenite) and residual liquids (evolved basanite and alkaline basalt, and phonotephrite). The latter moved up and built magma chambers at lower depths in the continental crust where those magmas initially crystallized clinopyroxenite extracts and high viscosity residual liquid that was entrapped and prevented to ascent towards the surface. Then, an abrupt change in the extract mineralogy produced a feldspathic-rich extract (leucocratic leucite monzosyenite, mesocratic syenite or leucocratic monzodiorite) and a residual phonolitic liquid with low viscosity. Following the feldspar crystallization, the magma viscosity decreased, acquiring adequate buoyancy that resulted in shallow-crust magma chambers in which phonolite diversification took place. These processes occurred due to the existence of a transcrustal, plumbing architecture where the high viscosity intermediate tephriphonolites were prevented to reach shallower depths, resulting in the Daly gap observed in the Passa Quatro Alkaline Complex.

KEYWORDS:

• Fractional crystallization modelling
• thermodynamic model, Rayleigh model

Acknowledgments

We gratefully thank Mark Ghiorso, Steffi Burchardt and Vincenza Guarino for revisions of this paper. We also thank João Pedro Lopes, Graciano Freitas, Mayara Reis, Rodrigo Garcia and Bárbara Queiroz for their help in field geology. We also gratefully thank Julio Mendes and Gaston Enrich for improve the text, Marcos Mansueto and Rogério Azzone for assistance in EMP, Actalabs for lithogeochemistry analysis, and Einari Suikkanen for development of easyMelts. This research was supported by Agência Nacional do Petróleo, Gás Natural e Biocombustíveis do Brasil, PETROMAGMATISMO project (FAPUR 03/19 and SAP 4600581919).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2269441

Additional information

Funding

The work was supported by the Agência Nacional do Petróleo, Gás Natural e Biocombustíveis .