U-Pb (LA-ICP-MS) zircon age of the La Paz Granite (Pando Belt, Uruguay): an Upper Neoproterozoic magmatic event in the Rio de la Plata craton

Abstract

Granitic rocks assigned to the La Paz Granite crop out 20 km to north of Montevideo city in Uruguay (34° 50’S-56°10’W), it is considered to crosscutting gneisses of 2.2 Ga from the ‘Pando belt’ in the Piedra Alta block. It is non-deformed coarse-grained, homogenous, pink granite, made up of 70% perthitic-microcline, 29% quartz (mainly idiomorphic), 1% hornblende and biotite. Chemical analysis show SiO2: 70%; K2O: 5.3%, Na2O: 3.9 % and CaO: 1.3%. The geochemical affinity points to subalkaline, meta-aluminous and high-K signature, with negative Eu anomaly. Spider multielement diagram present Pb and P negative anomalies and in the petrotectonic diagram this granite plots in the WPG field. During 70’s a Rb-Sr isochron of 565±15 Ma was reported for this pluton. New LA-ICP-MS U-Pb data on 16 concordant zircon spots show a 206Pb/238U weighted average mean age of 587.1±7.9 Ma. This data place this anorogenic magmatic event in the Ediacaran (Upper Neoproterozoic). The existence of an important sinistral shear zone striking N60E, associated with 1.2 Ga age muscovite rich peraluminous granites demonstrated by other authors, may have provided a weakness zone for the intrusion of La Paz Granite as an intraplate event in the Uruguayan sector of the Río de la Plata craton. 2: 70%; K2O: 5.3%, Na2O: 3.9 % and CaO: 1.3%. The geochemical affinity points to subalkaline, meta-aluminous and high-K signature, with negative Eu anomaly. Spider multielement diagram present Pb and P negative anomalies and in the petrotectonic diagram this granite plots in the WPG field. During 70’s a Rb-Sr isochron of 565±15 Ma was reported for this pluton. New LA-ICP-MS U-Pb data on 16 concordant zircon spots show a 206Pb/238U weighted average mean age of 587.1±7.9 Ma. This data place this anorogenic magmatic event in the Ediacaran (Upper Neoproterozoic). The existence of an important sinistral shear zone striking N60E, associated with 1.2 Ga age muscovite rich peraluminous granites demonstrated by other authors, may have provided a weakness zone for the intrusion of La Paz Granite as an intraplate event in the Uruguayan sector of the Río de la Plata craton.