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.

The metamorphism and exhumation of the Himalayan metamorphic core, eastern Garhwal region, India

“…The relatively evolved bulk chemical composition, high Rb/Sr ratio, and normative corundum of the Malari leucogranite indicate melting of a sedimentary source, likely the GHS [Sachan et al, 2010]. The advection of heat associated with emplacement of leucogranites in the upper GHS (Badrinath Formation) can explain the elevated metamorphic temperatures we observe which is consistent with leucogranites found throughout the range [Vidal et al, 1982;Inger and Harris, 1993;Guillot and Le Fort, 1995;Searle et al, 1997;Searle, 1999;Searle and Godin, 2003;Singh et al, 2003;Nabelek and Liu, 2004]. Compositional transects across representative garnets from each of the garnet-bearing formations show increasingly flat profiles (Ca, Mn, Mg, Fe) upsection in the Badrinath Formation (Figure 3).…”

Temperature-dependent thermal transport properties of carbonate minerals and rocks

“…Lithoheat uses boundary conditions of surface temperature (298 K) and basal temperature (1573 K). Constant basal heat flux was not used because it implies temperatures increase (or decrease) in the asthenosphere in response to elevated (or reduced) temperatures at the base of the lithosphere to keep heat flux into the lithosphere constant (Nabelek and Liu, 2004). Heat production within the lithosphere is thus the major driving force behind the thermal structure of the lithosphere, and in our models was concentrated in the middle tonalite layer, with minor heat production in the granulite layer.…”

Strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens: Implications of temperature‐dependent thermal diffusivity and rheology

“…The lithosphere in the Bohemian Massif consists of ∼30 km of mostly granulite crust and 70 km or more of lithospheric mantle [ Babuška and Plomerová , 2001]. Figure 3a shows two models with exponentially decreasing radiogenic heat production in the crust from 2 μ W/m 3 at the surface, which is a typical heat production in shales and schists [ Nabelek and Liu , 2004]. Constant D of 1 mm 2 /s (but variable C p and ρ ) gives a curved geotherm with a significantly larger d T /d z near the surface than in the lithospheric mantle.…”

“…Many processes within the continental lithosphere, including partial melting and high‐temperature metamorphism, are driven by the transfer of heat. In thickened collisional orogens, where evidence for heat advection by mantle‐derived basaltic magmas is often lacking, the temperatures necessary to obtain partial melting and granulite or ultrahigh temperature (UHT) metamorphism are difficult to achieve without a large amount of internal heat generation [ Royden , 1993; Thompson and Connolly , 1995; Nabelek and Liu , 2004]. Most thermotectonic models for large‐scale orogens, such as the Himalayas, have relied on deep accretion of material with upper crustal concentrations of radioactive elements to provide the necessary heat to induce melting and ductile flow of the middle crust [ Huerta et al , 1998; Beaumont et al , 2001; Jamieson et al , 2002].…”

“…However, in the ductile middle to lower crust, strain heating may dominate the internal heat production. Strain heating within ductile shear zones has been proposed as a potential mechanism for leucogranite generation in the Proterozoic Black Hills Orogen, South Dakota, and the Himalayas, because episodes of granite magmatism and metamorphism in these orogens were coeval with periods of deformation [ Treloar , 1997; Harrison et al , 1998; Nabelek et al , 2001; Nabelek and Liu , 2004]. Leloup et al [1999] also examined strain heating along continental strike‐slip shear zones.…”