Roberto Dall’Agnol

1.88 Ga Oxidized A‐Type Granites of the Rio Maria Region, Eastern Amazonian Craton, Brazil: Positively Anorogenic!

1.88 Ga A‐type granites in the Archean Rio Maria granite‐greenstone terrane of eastern Amazonian craton, Brazil, have much in common with the oxidized ∼1.4 Ga “anorogenic” granites of the western United States. Nd isotopic data on these granites and their Archean country rocks show that (1) the Rio Maria crust was differentiated from depleted mantle at 3.0 Ga, (2) the Paleoproterozoic granites were derived from deep parts of this crust, and (3) the Paleoproterozoic granites postdate their protolith by at least 1 Ga. No convergent processes are known to have affected the eastern Amazonian craton at ∼1.9 Ga and, therefore, there is little doubt as to the anorogenic origin of the oxidized A‐type granites of Rio Maria. Recognition of this genuinely anorogenic Proterozoic granite suite in Amazonia adds cymatogenic flavor to the current debate on the origin of the mid‐Proterozoic “anorogenic” granites of the western United States.

Four decades of land-cover, land-use and hydroclimatology changes in the Itacaiúnas River watershed, southeastern Amazon

Long-term human-induced impacts have significantly changed the Amazonian landscape. The most dramatic land cover and land use (LCLU) changes began in the early 1970s with the establishment of the Trans-Amazon Highway and large government projects associated with the expansion of agricultural settlement and cattle ranching, which cleared significant tropical forest cover in the areas of new and accelerated human development. Taking the changes in the LCLU over the past four decades as a basis, this study aims to determine the consequences of land cover (forest and savanna) and land use (pasturelands, mining and urban) changes on the hydroclimatology of the Itacaiúnas River watershed area of the located in the southeastern Amazon region. We analyzed a multi-decadal Landsat dataset from 1973, 1984, 1994, 2004 and 2013 and a 40-yr time series of water discharge from the Itacaiúnas River, as well as air temperature and relative humidity data over this drainage area for the same period. We employed standard Landsat image processing techniques in conjunction with a geographic object-based image analysis and multi-resolution classification approach. With the goal of detecting possible long-term trends, non-parametric Mann-Kendall test was applied, based on a Sen slope estimator on a 40-yr annual PREC, TMED and RH time series, considering the spatial average of the entire watershed. In the 1970s, the region was entirely covered by forest (99%) and savanna (∼0.3%). Four decades later, only ∼48% of the tropical forest remains, while pasturelands occupy approximately 50% of the watershed area. Moreover, in protected areas, nearly 97% of the tropical forest remains conserved, while the forest cover of non-protected areas is quite fragmented and, consequently, unevenly distributed, covering an area of only 30%. Based on observational data analysis, there is evidence that the conversion of forest cover to extensive and homogeneous pasturelands was accompanied by systematic modifications to the hydroclimatology cycle of the Itacaiúnas watershed, thus highlighting drier environmental conditions due to a rise in the regions air temperature, a decrease in the relative humidity, and an increase in river discharge.

Mineral chemistry and crystallization parameters of the A-type Paleoproterozoic Bannach Granite, Carajás Province, Pará, Brazil

The A-type Paleoproterozoic Bannach Granite belongs to the Jamon Suite and cross-cut the Archean units of the Rio Maria Domain in the Carajas Province. It comprises eight facies with varied mafic content and texture: even-grained cumulate granite (CG), even coarse-grained biotite- amphibole monzogranite (cBAMz), even coarse-grained amphibole-biotite monzogranite (cABMz), porphyritic biotite monzogranite (pBMz), even coarse-grained leucomonzogranite (cLMz), early even medium-grained leucomonzogranite (EmLMz), late even medium-grained leucomonzogranite (LmLMz) and even fine-grained leucomonzogranite (fLMz). In the less evolved facies, the dominant amphibole is Fe-hornblende passing to Fe-edenite or hastingsite, with associated cummingtonite/grunerite originated from the destabilization of clinopyroxene. Biotite has ferroan composition and approaches annite in the late-emplaced leucomonzogranite. Plagioclase varies from andesine to oligoclase or from oligoclase to albite. Titanite, magnetite, and ilmenite are found in all granite facies showing magmatic origin. The near liquidus temperatures vary between 943oC and 795oC and the estimated emplacement pressure is 300 ± 50 MPa (~11.1 ± 1.9 km). Amphibole Fe/(Fe + Mg) ratios together with magmatic magnetite and titanite indicate that these granites crystallized at moderately oxidizing conditions, similar to those admitted for the Jamon granite and magnetite series granites of Laurentia. However, the composition of biotite suggests a more reduced character close to NNO to NNO -0.5. The late leucomonzogranite facies is an exception because it displays high Fe/(Fe + Mg) ratios in biotite and approaches to the reduced granites of Carajas, Laurentia and Fennoscandia in this regard. It derived from a more reduced source than the other facies of Bannach Granite.