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Contributions to Mineralogy and Petrology

, Volume 92, Issue 3, pp 341–350 | Cite as

Origin of granite at Cabo de Santo Agostinho, Northeast Brazil

  • Leon E. Long
  • Alcides N. Sial
  • Hanna Nekvasil
  • Glicia S. Borba
Article

Abstract

A 4 km2 exposure of shallowly-emplaced leucogranite on the Atlantic coast at Cabo de Santo Agostinho, 30 km south of Recife, Brazil has been extensively studied chemically and isotopically. Twenty-three major-element analyses indicate that the Cabo granite ranges from peralkaline to peraluminous; Na2O+K2O is very high (7.4 to 10.4 wt.%), with CaO low (∼0.3%) and MgO vanishing (<0.06%). Microprobe analyses confirm the presence of arfvedsonite (biotite absent), and nearly total absence of plagioclase. The rocks are moderately to highly enriched in LREE (La 45 to 350 times chondritic), with extremely pronounced negative Eu anomalies (Eu/Eu*=0.02 to 0.07). Whole-rockδ18O is consistent at +8.5±0.3%.oSMOW A Rb-Sr isochron age of isotopically slightly disturbed samples is 104.8±1.8 Ma, with initial87Sr/86Sr=0.7084±0.0011. Sr is depleted (2–20 ppm) but Ba is 200–750 ppm.

Crystallization path calculations and petrographic observations suggest that magma formed at a pressure close to 6 kbar but rose to a crustal level equivalent to roughly 1 kbar. Quartz, the liquidus phase at moderate H2O concentrations and pressures above 2 kbar, was resorbed during decompression as the magma moved upwards. Ultimately, quartz and alkali feldspar coprecipitated. Feldspar was not retained in the source rock nor removed early from the fractionating magma. Therefore the strong negative Eu anomaly and low Sr abundance are characteristics inherited from the source. A high H2O concentration necessary for a large degree of melting was lacking, hence the Cabo magma composition must reflect a small degree of partial melting of a rather quartz-rich rock such as a feldspathic arenite.

In a pre-drift reconstruction of Gondwanaland, the Cabo granite fits on the southernmost and youngest end of the trend of the Niger-Nigerian igneous centers with which it has close affinity. The Cabo granite occupies the western end of the trace of the ancestral Ascension mantle plume which presumably served as the heat source.

Keywords

Source Rock Mantle Plume Arenite Isochron Crystallization Path 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Springer-Verlag 1986

Authors and Affiliations

  • Leon E. Long
    • 1
  • Alcides N. Sial
    • 2
  • Hanna Nekvasil
    • 3
  • Glicia S. Borba
    • 2
  1. 1.Department of Geological SciencesUniversity of TexasAustinUSA
  2. 2.Geology DepartmentFederal University of PernambucoRecifeBrazil
  3. 3.Department of GeosciencesPennsylvania State UniversityUniversity ParkUSA

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