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Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaia Kimberlite, Alto Paranaiba Province, southeast Brazil

Nicholas M. Lima, Rogerio G. Azzone, Luanna Chmyz, Vincenza Guarino, Excelso Ruberti, Simone Silva and Darcy P. Svisero
Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaia Kimberlite, Alto Paranaiba Province, southeast Brazil
The Canadian Mineralogist (November 2020) 58 (5): 563-585

Abstract

The Indaia-I and Indaia-II intrusions are hypabyssal, small-sized ultrabasic bodies belonging to the Cretaceous magmatism of the Alto Paranaiba Alkaline Province (southeast-central western Brazil). While Indaia-I is classified as an archetypal group-I kimberlite, Indaia-II (its satellite intrusion) presents several petrographic and chemical distinctions: (1) an ultrapotassic composition (similar to kamafugites), (2) lower volumes of olivine macrocrysts, (3) diopside as the main matrix phase (in contrast with the presence of monticellite in Indaia-I), (4) high amounts of phlogopite, and (5) abundant felsic boudinaged and stretched microenclaves and crustal xenoliths. Disequilibrium features, such as embayment and sieve textures in olivine and clinopyroxene grains, are indicative of open-system processes in Indaia-II. Mineral reactions observed in Indaia-II (e.g., diopside formed at the expense of monticellite and olivine; phlogopite nearby crustal enclaves and close to olivine macrocrysts) point to an increase in the silica activity of the kimberlite magma; otherwise partially melted crustal xenoliths present kalsilite, generated by desilification reactions. The high Contamination Index (2.12-2.25) and the large amounts of crustal xenoliths (most of them totally transformed or with evidence of partial melting) indicate a high degree of crustal assimilation in the Indaia-II intrusion. Calculated melts (after removal of olivine xenocrysts) of Indaia-II have higher amounts of SiO2, Al2O3, K2O, slightly higher Rb/Sr ratios, lower Ce/Pb and Gd/Lu ratios, higher 87Sr/86Sr, and lower 143Nd/144Nd than those calculated for Indaia-I. Crustal contamination models were developed considering mixing between the calculated melts of Indaia-I and partial melts modeled from the granitoid country rocks. Mixing-model curves using major and trace elements and isotopic compositions are consistent with crustal assimilation processes with amounts of crustal contribution of ca. 30%. We conclude that (1) Indaia-II is representative of a highly contaminated kimberlitic intrusion, (2) this contamination occurred by the assimilation of anatectic melts from the main crustal country rocks of this area, and (3) Indaia-I and Indaia-II could have had the same parent melt, but with different degrees of crustal contamination. Our petrological model also indicates that Indaia-II is a satellite blind pipe linked to the main occurrence of Indaia-I.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 58
Serial Issue: 5
Title: Petrographic, geochemical, and isotopic evidence of crustal assimilation processes in the Indaia Kimberlite, Alto Paranaiba Province, southeast Brazil
Affiliation: Universidade de Sao Paulo, Instituto de Geociencias, Sao Paulo, SP, Brazil
Pages: 563-585
Published: 20201124
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 95
Accession Number: 2021-003295
Categories: Igneous and metamorphic petrologyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch map
S19°00'00" - S18°00'00", W48°00'00" - W46°00'00"
Secondary Affiliation: Universita di Napoli Federico II, ITA, Italy
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202103
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