Building Orogens

Anatexis of the continental crust with the efficient extraction of granitoid melts is a fundamental process that governs the internal architecture of a number of orogenic belts around the world. The melt extracts are produced via breakdown of hydrous minerals and – for a variety of orogenic belts – they give rise to voluminous intrusive and extrusive granitoids.

Sugar-Loaf Granitoids From the Aracuai Orogen by Fabiana Ligeirinha Richter

Melt composition changes due to several factors including rock sources, depth of melting and amount of water in the system. An important question that remains for a number of recent and ancient orogenic systems is how much melt is produced from older crustal material and how much comes from fractionation of mantle-derived magmas.
In any dynamically evolving orogen, the volumes of such crust-derived melts can be as important as those produced by magma fractionation during convergence; it all depends on the fertility of the crust and the heat source involed.
For average geothermal conditions, dehydration melting of metapelites and metavolcanics would generate approximately 25% and 10% granite melt, respectively. The products of such partial melting may range in scale from centimetre-sized migmatitic segregations to km-scale granitoid intrusive complexes cropping out over large areas of exhumed orogens. However, since the link between anatectic rocks and peraluminousgranitoid plutons has been well established in the literature, there has been many questions about the melting behavior of various crustal protoliths during production of vast seas of S-type granitoids. Foremost among these are the duration of the melting process and the heat source responsible for widespread melting of mid-crustal protoliths.

S-type Granitoid exposure along a giant quarry near Aracuai

The type of orogen will have some bearing on the possible heat source. There are two main types of orogeny. Collisional orogens are the result of formation and then closure of an oceanic basin while accretionary orogens occur on active subduction margin when fragments of continental crust collide (see figure on the left). Accretionary orogens are generally much more long-lived than collisional with episodic activity. In collisional orogens, radiogenic heating may have a more dominant role, while in both settings conducted or advected heat could also be dominant.
The Araçuaí Belt
The Araçuaí Belt was formed by the collision of the Sao Francisco and Congo Cratons. The belt is an ideal place to apply tools for understanding the magmatic and metamorphic evolution of different levels of the crust through time. In addition, the Araçuaí orogen records several episodes of I-type magmatism and crustal anatexis associated with metamorphism and deformation. This indicates that the crust in this area stayed at elevated temperatures for a prolonged duration. Thus an understanding of the metamorphic evolution of the Araçuaí belt could provide insights into many crustal processes including crustal melting and cooling processes. While dating has been done on the various suites of granitic plutons, only limited dating of metamorphism has been conducted. Thus the relationship between the granite plutons, their host rocks and the potential sources rocks is very difficult to evaluate.
This project involves primarily the development of Lu-Hf  dating of metamorphic minerals on a recently acquired, top of the range Neptune high-precision multi collector ICPMS system at Universidade Federal of OuroPreto (UFOP). In addition, monazite U-Pb dating and Sm/Nd isotope tracing, garnet Sm/Nd isotope tracing and diffusional speedometry will be developed to constrain the age, style and duration of the main tectonometamorphic events that were responsible for the construction of the Araçuai Belt.
The techniques will be applied to a suite of representative samples from the belt that occurred at different points in geological time in order to look at the evolution of the rates of the orogenesis through time.
This project will be done in collaboration with masters and PhD students at both UFOP/UFMG, Brazil and Stellenbosch University, South Africa.

 

 
 

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