Petrogênese dos plútons Cabreúva e Indaiatuba, Batólito Graní­tico Itu, SP: idade, áreas-fonte e condições de cristalização

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 shallo...

Granite Weathering, Denudation Processes, and Associated Landforms in the Itu-Cabreúva Region, São Paulo, Brazil

“…The landforms have been sculpted into intrusive acid igneous rocks with a wide range of textures and a mineralogical assemblage that includes, in lesser or greater proportion, k-feldspar, plagioclase, quartz, biotite, and/or amphibole [33], [35].…”

“…These were initially referred to as the Rapakivi Itu Province [32] but were later named the Itu Granite Province [15]. The dominant facies of the province are the medium to coarse-grained granitoid, partly with a rapakivi texture, massive structure, and reddish to red-brown color [15], [33]- [35]. The large-scale removal of sedimentary cover has exposed granite plutons in the landscape, forming flattened hills and discrete clustered blocks commonly known as "stone seas" [33], [34].…”

“…Although considerable effort has been devoted over the past three decades to characterize the occurrence of the Itu Granite Province plutons, detailed studies remain limited. Among the most notable results are the individualization of the granitic bodies into four distinct plutons [36], the distinction of several petrographic facies [33], [34], the inference on the emplacement mechanism in the context of regional tectonic evolution [15], as well as the characterization of petrogenetic evolution based on mineral chemistry, geochemistry, and isotopic data [35]. The main lithological units comprise extensive exposure of granitic rocks and, to a lesser extent, migmatite rocks, and Paleozoic sedimentary cover.…”

“…A careful examination of the texture and mineralogical association reveals three different types of lithology, ranging from monzogranite, syenogranite, to granite [33], [34]. In another recent paper, the pluton in the Cabreúva region is described as an irregular outcrop, mostly formed by coarse-grained syenogranite, with k-feldspar crystals reaching up to 2 cm, and porphyritic monzogranite, hosting fine-grained xenoliths with rounded edges [35].…”

“…Fig.2. Schematic geological map showing the distribution of lithological units in the region according to Pereira[35].…”