Petrologic and thermal constraints on the origin of leucogranites in collisional orogens

Abstract

Leucogranites are typical products of collisional orogenies. They are found in orogenic terranes of different ages, including the Proterozoic Trans-Hudson orogen, as exemplified in the Black Hills, South Dakota, and the Appalachian orogen in Maine, both in the USA, and the ongoing Himalayan orogen. Characteristics of these collisional leucogranites show that they were derived from predominantly pelitic sources at the veining stages of deformation and metamorphism in upper plates of thickened crusts. Once generated, the leucogranite magmas ascended as dykes and were emplaced within shallower parts of their source sequences. In these orogenic belts, there was a strong connection between deformation, metamorphism and granite generation. However, the heat sources needed for partial melting of the source rocks remain controversial. Lack of evidence for significant intrusion of mafic magmas necessary to cause melting of upper plate source rocks suggests that leucogranite generation in collisional orogens is mainly a crustal process.The present authors evaluate five types of thermal models which have previously been proposed for generating leucogranites in collisional orogens. The first, a thickened crust with exponentially decaying distribution of heat-producing radioactive isotopes with depth, has been shown to be insufficient for heating the upper crust to melting conditions. Four other models capable of raising the crustal temperatures sufficiently to initiate partial melting of metapelites in thickened crust include: (1) thick sequences of sedimentary rocks with high amounts of internal radioactive heat production; (2) decompression melting; (3) thinning of mantle lithosphere; and (4) shear-heating. The authors show that, for reasonable boundary conditions, shear-heating along crustal-scale shear zones is the most viable process to induce melting in upper plates of collisional orogens where pelitic source lithologies are usually located. The shear-heating model directly links partial melting to the deformation and metamorphism that typically precede leucogranite generation.

Geochronology, geochemistry, and isotope compositions of Piaochi S-type granitic intrusion in the Qinling orogen, central China: Petrogenesis and tectonic significance

“…climax of collision (Barbarin, 1999;Nabelek and Liu, 2004), backarc extension (Collins and Richards, 2008), ridge subduction (Cai et al, 2011), above mantle plume , or post-collisional setting (Han, 2007;Sylvester, 1998 and references therein). Two-mica monzonite granites (muscovite-bearing granites) are further considered to form in crustal thickening zones resulting from the convergence of two continental lithospheres (Barbarin, 1990(Barbarin, , 1996(Barbarin, , 1999.…”

“…Barbarin, 1999;Collins and Richards, 2008;Kemp et al, 2009;Phillips et al, 2011). Previously, some researchers have preferred continent-continent collision/syn-collision to achieve anatexis for their formation (Barbarin, 1999;Nabelek and Liu, 2004, and references therein). However, it has been shown in the past decade that S-type peraluminous granites can form in various geodynamic processes e.g.…”

“…However, it has been shown in the past decade that S-type peraluminous granites can form in various geodynamic processes e.g. the climax of collisional orogens (Barbarin, 1999;Nabelek and Liu, 2004), backarc extension (Collins and Richards, 2008;Kemp et al, 2009), ridge subduction (Cai et al, 2011), mantle plumes , or more probably late-stage exhumation (Han, 2007;Sylvester, 1998 and references therein). It is worthy to note that S-type peraluminous granites without a significant depleted component appear to be emplaced in the early stage of orogenic extension in thick orogens (Phillips et al, 2011;Sylvester, 1998).…”

Geochemistry of Ferriferous, High-K Calc-Alkaline Granitoids from the Banefo-Mvoutsaha Massif (NE Bafoussam), Central Domain of the Pan- African Fold Belt, Cameroon~!2009-11-12~!2010-01-05~!2010-03-11~!

“…[1,2]). There is general agreement that highly differentiated leucogranitic magmas (leucogranites) could be mostly related to anatectic products of metapelitic crustal sources [3][4][5] although the heat sources for their production have remained controversial [6]. Associated intermediate to basic plutonic rocks are typically lacking.…”

Production of carbonic fluids during metamorphism of graphitic pelites in a collisional orogen—An assessment from fluid inclusions

“…We also interpreted the data in the context of microstructures of the host quartz. Because the conditions of metamorphism and magmatism that occurred in this region during the Proterozoic were similar to those in other collisional orogens, including the Himalayas and portions of the Appalachians (Nabelek and Liu, 2004), this study bears on identifying the potential sources and phase relationships that affect the composition of C-O-H fluids in metapelites undergoing metamorphism during continental collisions.…”