Magmatic inheritance vs. UHT metamorphism: Zircon petrochronology of granulites and petrogenesis of charnockitic leucosomes of the Socorro-Guaxupe nappe, SE Brazil

Author(s)

Rocha, Brenda Chung ^{[1, 2]} ; Moraes, Renato ^[1] ; Moller, Andreas ^[2] ; Cioffi, Caue Rodrigues ^{[1, 2]}

Affiliation

^[1] Univ Sao Paulo, Inst Geociencias, Dept Mineral & Geotecton, Rua Lago 562, BR-05508080 Sao Paulo, SP - Brazil

^[2] Univ Kansas, Dept Geol, 1475 Jayhawk Blvd, Lindley Hall, Lawrence, KS 66045 - USA

• Document type: Journal article
• Source: LITHOS; v. 314, p. 16-39, AUG 2018.
• Web of Science Citations: 7

Abstract

We present a detailed study of zircon petrochronology of UHT granulites combined with chamockite petrogenesis, using geochemical and Sr-Nd isotopic constraints for the Socorro-Guaxupe Nappe (SGN) in southeast Brazil. This multi-technique approach is used to refine the timescales of pre-collisional arc magmatism and syn-collisional UHT metamorphism and to constrain the P-T-t evolution of granulites. The SGN features the transition from a subduction-related to a continental-collision setting, involving the formation of in-situ orthopyroxene- and hornblende-bearing leucosome. The application of the REE array plot approach enabled a clear distinction between pre-collisional (prior to garnet growth) and syn-collisional (during or after garnet growth) zircon crystallization events in the SGN. A period of pre-collisional arc magmatism at ca. 730-640 Ma is identified. Al-in-orthopyroxene thermometry yields minimum temperatures of ca. 900 degrees C, 12 kbar in the garnet-rich granulite, consistent with UHT metamorphism, followed by cooling with re-equilibration to ca. 770 degrees C at 10 kbar. Garnet growth near peak conditions prior to zircon, and small-scale equilibrium between later garnet cores and ca. 615 Ma zircon rims are revealed by the array plot approach. Zircon growth during melt crystallization is documented in well-developed sector- and oscillatory-zoned overgrowths and ``soccer ball{''} zircon at ca. 613 Ma and 608 Ma. Additionally, the origin and sources of both granulites and leucosomes are investigated. Orthopyroxene- and hornblende-bearing leucosomes are present both as fractionated melts and cumulates. Leucosomes and their granulite residues belong to the shoshonitic series, characterized by high K2O contents, and they display negative epsilon Nd-(t) values (-11.7 to -5.1) and high initial Sr-87/Sr-86 ratios (0.7055-0.7080). The combination of geochemical and isotopic evidence, in addition to zircon inheritance patterns, point to enriched-mantle sources in their origin rather than crustal recycling involving either Archean or Paleoproterozoic older crust. The enrichment of subcontinental lithospheric mantle is interpreted to be related to the subduction stage prior to collision. (C) 2018 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 16/22627-3 - Metamorphism and partial melting in ancient orogens II
• Grantee: Renato de Moraes
• Support Opportunities: Regular Research Grants
• FAPESP's process: 13/04007-0 - Metamorphism and partial melting in ancient orogens
• Grantee: Renato de Moraes
• Support Opportunities: Regular Research Grants
• FAPESP's process: 12/22380-7 - Origin and evolution of melting in the migmatites and granulites of the Socorro-Guaxupé Nappe, in the Alfenas region (MG)
• Grantee: Brenda Chung da Rocha
• Support Opportunities: Scholarships in Brazil - Doctorate
• FAPESP's process: 14/05563-6 - Linking in-situ U-Pb dating in accessory minerals using LA-ICP-MS with P-T data in high-grade metamorphic rocks of the Socorro-Guaxupé nappe
• Grantee: Brenda Chung da Rocha
• Support Opportunities: Scholarships abroad - Research Internship - Doctorate