The geochemistry and geochronology of the northeastern opx-bearing granitoids in relation to UHT metamorphism in the Bakhuis Granulite Belt, Suriname

The geochemistry and geochronology of the northeastern opx-bearing granitoids in relation to UHT metamorphism in the Bakhuis Granulite Belt, Suriname

van der Steeg, W.

Molengraaff Fonds

2016

The Bakhuis Granulite Belt of 30-40 x 100 km transects a large Paleoproterozoic TTG-greenstone belt in the Guiana Shield and is surrounded in its southern part by younger metavolcanics and associated granites. The granulite belt has experienced ultra high temperature metamorphism. The metamorphism was previously dated at 2072-2055 Ma. In the SW part of the belt, near Kabalebo, charnockites predominate. These charnockites crystallized at 960-990 ⁰C, a UHT-like temperature, but their age (1.98-1.99 Ga) indicates a gap of at least 60 Ma between UHT metamorphism and charnockite magmatism. A small anhydrous granitic body in the northeastern part of the belt was thought to be contemporaneous with metamorphism. The first objective of this study is to establish the age and the chemical composition of this northeastern charnockite. Secondly the age and chemical composition of another, larger and hydrous, granitic body in the most northeastern point of the Bakhuis Granulite Belt will be determined. The third objective is to accurately determine the age gap between ultra high temperature metamorphism and magmatism. The northeastern charnockite and granite ages have been accurately determined by dating zircons with LA-ICP-MS at 1968.5 ±2.9 and 1982.5 ±4.2 Ma respectively. The two different ages outside of the margin of error suggest two different Orosirian magmatic pulses, supported by recent data of other magmatic bodies. The charnockite has a low abundance of hornblende and biotite. Its petrography indicates a high crystallisation temperature by the presence of antiperthitic plagioclase and of inverted pigeonite, which indicates a minimum temperature of 950 C⁰. This temperature is supported by the P2O5 and TiO2 content which indicate temperatures of 1000-1100 ⁰C and 950-1000⁰C respectively. The charnockites are magnesian, calc-alkalic, metaluminous and have an SiO2 content of 49-61 wt% SiO2. The parental magma is estimated at approximately 52 wt% SiO2. This disagrees with the C-type magma concept, the charnockites are interpreted as anhydrous high temperature I-type Cordilleran charnockites. The northeastern granites are magnesian, calc-alkalic and metaluminous to peraluminous. They range from 62-68 wt% SiO2, which is more comparable to the range of the Kabalabo charnockites but are they are hydrous and thus contain very low amounts of pyroxene and its pseudomorphs. Petrography and chemical analysis indicate a high crystallisation temperature with the presence of antiperthitic plagioclase and P2O5 and TiO2 between the 1000-1100 ⁰C and 900-950 ⁰C isotherms respectively. These are high temperature I-type Cordilleran-like granites which suggest partial melting of the deep crust or mantle as the source. The duration of metamorphism was determined more accurately using SHRIMP on metamorphic zircons and monazites from several samples. The ultra high temperature metamorphism is now dated at 2088-2030 Ma which is longer than previously thought. However a gap of almost 40 Myr still remains between UHT metamorphism and the magmatism starting at 1.99 Ga.

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Utrecht University

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