Granito Porquinho, Domínio Tapajós: exemplo de magmatismo alcalino-peralcalino e considerações metalogenéticas preliminares

Abstract

Porquinho Granite (PG) is a subcircular batholith located at the Tapajós Domain, Amazonian Craton, petrographically composed of isotropic hypersolvus alkali feldspar granites, grouped In: Biotite alkali feldspar granite (BAFG), Amphibole alkali feldspar granite (AAFG) and greisenized rocks. Biotites are classified as annite, while amphiboles vary in composition, from calcic (hastingsite, ferro-edenite, ferro-actinolite and ferro-ferri-hornblende) to sodic-calcic (ferro-ferri-winchite, ferro-ferri-katophorite, ferro-richterite, potassic-ferro-ferri-katophorite and ferro-katophorite) to sodic (riebeckite). Geochemically, PG is metaluminous to peraluminous, locally peralkaline, shows K 2 O/ Na 2 O ratios between 1 and 2, and FeO t /(FeO t + MgO) very close to 1, which led it to be classified as a ferroan granite. It is a reduced granite, with geochemical characteristics of A-type granites of the A 2 -subtype and is similar to the Phanerozoic intraplate granites. AAFG rocks are slightly more enriched in REE than BAFG, but they all present similar comportment, and strong negative Eu anomaly, forming the typical gullwing-shaped pattern of the A-type and the Sn-Ta-Nb mineralized granites. BAFG also shows similar behavior with Europa Granite (EG, Madeira Intrusive Suite; Pitinga) and Velho Guilherme Intrusive Suite (VGIS; Xingu), both mineralized in Sn-Ta-Nb, while AAFG approaches, geochemically, more to Maloquinha Intrusive Suite (MIS; Tapajós), non-mineralized. In this work, PG presented a geochronological age of 1,889 ± 2 Ma, interpreted as the age of crystallization, which is about 100 Ma older than the former one (1,786 ± 14 Ma). The older geochronological age fits better with geological context in which Porquinho Granite is inserted, but also means more detailed investigations need to be done in this area in order to eliminate possible doubts about its crystallization age and find out if PG has a more complex history of evolution.