SiO 2-saturated potassic alkaline magmatism in the central Amazonian Craton , southernmost Uatumã-Anauá Domain , NE Amazonas , Brazil

This paper approaches the record of SiO2 -saturated potassic alkaline magmatism of Castanhal Quartz Monzonite, Mapuera Suite, and Ladeira da Vovo Quartz Syenite. These samples are located near the Northern border of the Amazon Basin. Such rocks show K2 O + 2 > Na2 O and K2 O/Na2 O < 2 values that confirm the potassic or shoshonitic character of these rocks. The Castanhal Quartz Monzonite contains less than 20% volume of quartz, which is also a characteristic of the shoshonitic or SiO2 -satured potassic alkaline A-type magma signature observed on geochemical plots. Listric faults, representing the rifting phase of Amazon Basin formation, emplaced and reworked Ladeira da Vovo Quartz Syenite, which caused its granophyric texture, probably during the Tonian period. A group of 21 zircon crystals was extracted from a hornblende quartz monzonite and yields an average age of 1872  ±  6  Ma (MSWD  = 2.4). However, an additional zircon crystal yielded a Trans-Amazonian age of 2062 ± 17 Ma. These potassic alkaline rocks of Orosirian (1872  Ma) age may correspond to a post-collisional setting. Dominantly negative eHft values and Hf TDM ages reveal a large contribution of a mafic crustal component from Mesoarchean to Neoarchean age (2.95 – 2.66 Ga), and a felsic crustal component from Neoarchean to later Siderian ages (2.51 – 2.34 Ga).


INTRODUCTION
There are still uncertainties about the origin and evolution of alkaline granitic magmas.The origin of alkaline magmas may be derived from various sources: mantle; lower crust; and magmas mixing from crustal and mantle sources.
However, some authors have concluded that alkaline granites are only formed in a continental rift setting (Barbarin 1999).Moreover, other authors have suggested that SiO 2 -saturated alkaline granitoids were derived from a crustal source with pieces of evidence of a mantle component, largely rich in alkalis, that migrates along detachment faults, extending to the peridotitic mantle and to alkalis-enriched mantle plumes (Bonin 1990, Brown et al. 1992).
Moreover, this paper includes petrographic, whole-rock geochemical, in-situ U-Pb zircon geochronological and Hf isotope composition data upon the Castanhal Quartz Monzonite firstly, and field and petrographic features of the Ladeira da Vovó Quartz Syenite, secondly.

RESULTS AND DISCUSSION
The previously proposed models and geological observations have instigated the investigation of these Orosirian alkaline granitic rocks through this short communication paper.This paper includes Sar/Sipam radar and airborne geophysical images, because the access to the Castanhal stock was very difficult due to the dense forest and thick regolith mantle covering those rocks.Nevertheless, the samples of hornblende quartz monzonite display seriated texture and interpenetrative contacts between quartz and plagioclases (normally zoned).These rocks present less than 20% wt. of quartz or SiO 2 -satured potassic alkaline A-type signatures.Another and intricate observation is related to a quartz syenite that shows a pervasive granophyric texture (granophyre).This sample was discovered along the listric faults, near the detachment fault.These listric faults served as conduit for emplacement and probably reworked this rock, which was observed in micro-faults, in the rifting stage of formation of the Amazon Basin (likely generated at Tonian period).According to Hibbard (1987), granophyric feature is originated by the simultaneous crystallization of the quartz, and K-feldspar started from a volatile-enriched eutectic mixture, mainly in shallow deep low-pressure areas and it is commonly associated with post-magmatism.Fenn (1986) declared that this texture is the result of premature cooling related to kinetic processes in borders of the feldspar crystals.However, Paterson et al. (1989) admitted that this type of texture could be produced in deformed granitoids in the solid state, instead of felsic liquids presence.As the quartz syenite recorded at least one pulse of the brittle deformation is cleaner to accept the Paterson's et al. (1989) model.
On the R1-R2 geochemical classification plot, the samples are falling in the quartz monzonite field (De la Roche et al. 1980, Fig. 2A).These hornblende quartz monzonites are alkaline and have intermediate to acid composition on the total alkalis-silica (TAS) plot (Cox et al. 1979, Fig. 2B), meta-luminous on the A/CNK-A/NK plot (Shand 1943, Fig. 2C) and are shoshonitic on the SiO 2 wt.% versus K 2 O wt.% plot (Peccerilo & Taylor 1976, Fig. 2D).By magmatic differentiation, these rocks have evolved to SiO 2 -supersaturated composition (quartz-alkali feldspar-plagioclase-feldspatoid -QAPF and TAS fields of Nardi 1991).These rocks show K 2 O + 2 > Na 2 O and K 2 O/Na 2 O < 2 values that confirm their potassic alkaline or shoshonitic characteristics.Potassic SiO 2 -saturated alkaline A-type granitic rocks are meta-luminous, and mainly occur in post-collisional, anorogenic and mature arc setting (Bonin 1990).However, their temporal and geographical associations with the SiO 2 -supersatured subalkaline granites of the same area suggest a similar tectonic setting (post-collisional), which is confirmed on the Nb-Y and Ta-Yb plots (Pearce et al. 1984, Fig. 2E and 2F).
A group of 22 zircon crystals was selected for in-situ U-Pb zircon dating and extracted from a hornblende quartz monzonite of the Castanhal Quartz Monzonite, Mapuera Suite (sample CC-42).Zircon crystals show zoning, are Tectonic setting plots by Pearce et al. (1984), Y-Nb (E) and Yb-Ta (F) plots; see field descriptions in Pearce et al. (1984).

CONCLUSIONS
In conclusion, SiO 2 -saturated potassic alkaline A-type granites (hornblende quartz monzonite), composing the Castanhal Quartz Monzonite, Mapuera Suite, are meta-luminous and of Orosirian age (1872 Ma) generated in a post-collisional setting.The occurrence of the Ladeira da Vovó Quartz Syenite suggests it has been emplaced and reworked along listric faults, during the Tonian period, possibly ( rifting phase of the Amazon Basin).Dominantly negative εHf t values and Hf T DM ages in zircon crystals from a hornblende quartz monzonite reveal a significant contribution of crustal material, including Mesoarchean to Neoarchean amphibolite and Neoarchean to later Siderian granulite.