Petrogenesis of the Cabreúva and Indaiatuba plutons, Itu Granitic Batolith, SP: ages, source areas and conditions of crystallization

Author(s)

Giovanna de Souza Pereira

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2019-10-08

Examining board members

Valdecir de Assis Janasi; Leonardo Fadel Cury; Ignez de Pinho Guimarães; Silvio Roberto Farias Vlach

• Advisor: Valdecir de Assis Janasi

Abstract

The Itu Granitic Batolith is the most voluminous occurrence of the Itu Granitic Province, post-orogenic to the Brasiliano orogenesis (~660-600 Ma) in the Socorro-Guaxupé Nappe. It is formed by the amalgamation of several magmatic chambers emplaced at the shallow levels of the continental crust, which formed four main plutons: Cabreúva, Indaiatuba, Itupeva and Salto, of which the first two were studied in greater detail in this work. The Indaiatuba Pluton is defined by a main unit of coarse porphyritic (hornblende)-biotite rapakivi granite, which hosts microgranular enclaves of felsic and mafic nature; mediumgrained equigranular hololeucocratic biotite granite varieties invade this main unit at solid state, as indicated by the presence of angulous xenoliths. Contemporary basic manifestations occur as a monzogabbro (locally clinopyroxene-bearing) and small quartz monzonitic hybrid bodies. The Cabreúva Pluton is characterized by a main unit of coarse-grained (hornblende)-biotite inequigranular syenogranites, with frequent mafic and felsic microgranular enclaves, and border varieties of medium-grained hololeucocratic biotite syenogranites whose chemical composition indicates that they are more differentiated varieties (~ 75-77% SiO2, < 0,1% MgO, < 0,8% CaO). In the central portion of the pluton, formed by porphyritic granites, occur small mafic bodies and blocks of slightly oriented coarse quartz monzodiorite, whose composition and texture suggest cumulative origin. The magmatic crystallization ages obtained in this work by dating seven samples of the four plutons that make up the Itu Batholith by U-Pb zircon in situ dating methods (SHRIMP and LA-MC-ICPMS) are all similar, and define the main episode of generation of the whole batholith around 578 ± 4 Ma. A sample of syenogranite from Cabreúva provided an age significantly younger than the others (561 ± 6 Ma); this age, if it does not reflect a hydrothermal post-magmatic event (suggested by the chemistry of zircon crystals), would imply that the formation of the batholith occurred over a period of up to 20 Ma. Inherited zircon crystals are very rare in the Itu samples reflecting the relatively high temperatures of these magmas. Clearly inherited nuclei were identified only in one sample of Itupeva, that have paleoproterozoic (~ 2.2 Ga) and neoproterozoic (~ 620 Ma) ages. The Itu Batholith granites exhibit characteristics of aluminous A-type granites derived from H2O-poor magmas with relatively high liquidus temperatures. The temperature values obtained by zircon saturation calculation vary between 880 and 800°C among less and more evolved granites from Cabreúva (SiO2 contents between 69 and 77 wt%); slightly lower temperatures (between 800 and 730°C) are observed in the more evolved equigranular granites (> 74 wt% SiO2) considered differentiated from Indaiatuba. The pressures of crystallization estimated in this work using Al in hornblende geobarometer calibrations are in the 2-3 kbar range, confirming shallow crustal emplacement. In the context of the evolution of shallow magmatic chambers, recurrent recharge events by felsic (predominant) and mafic magma pulses are recorded in the Cabreúva and Indaiatuba plutons in the form of abundant microgranular enclaves. Structural and textural evidences of magma interaction processes (mingling and mixing) are frequent, and in part confirmed by geochemical and isotopic data. Shallow-chamber crystal-liquid separation processes are evidenced by continuous compositional variations and, in Cabreúva, by the presence of a cumulative rock. Whole-rock Sr and Nd and zircon Hf isotopic data obtained for the different plutons of the Itu Batolith show signatures characteristic of crustal granites, with strongly negative values of \'épsilon\'Nd(t) (-10 to -12) and \'épsilon\'Hf(t), but with relatively low 87Sr /86Sr(t) (0.706-0.708), indicative of sources with low Rb/Sr ratio. Among the potential crustal sources for the granite-generating magmas can be recognized orthogneisses or granulites from Paleoproterozoic basement, or calc-alkaline granites generated during the preceding orogenic cycle. The presence of a mantle component is identified in all four Itu plutons (in the form of mafic bodies and mafic microgranular enclaves); this component must correspond to an enriched mantle, similar to that from which the mafic rocks of the monzodioritic Piracaia massif, contemporary to the Itu Province, were derived (\'épsilon\'Nd(t) -7.3 and -8.5 and 87Sr /86Sr(t) ~ 0.705). (AU)

• FAPESP's process: 14/03725-9 - Combined use of structural and geochemical-isotopic tools in the study of the Cabreuva and Indaiatuba plutons, Itu Batholith, SP: implications to the dynamics of shallow granitic magma chambers
• Grantee: Giovanna de Souza Pereira
• Support Opportunities: Scholarships in Brazil - Doctorate