The sources of granitic melt in Deep Hot Zones

Abstract

A Deep Hot Zone develops when numerous mafic sills are repeatedly injected at Moho depth or are scattered in the lower crust. The melt generation is numerically modelled for mafic sill emplacement geometries by overaccretion, underaccretion or random emplacement, and for intrusion rates of 2, 5 and 10 mm/yr. After an incubation period, melts are generated by incomplete crystallisation of the mafic magma and by partial melting of the crust. The first melts generated are residual from the mafic magmas that have low solidi due to concentration of H 2 O in the residual liquids. Once the solidus of the crust is reached, the ratio of crustal partial melt to residual melt increases to a maximum. If wet mafic magma, typical of arc environments, is injected in an amphibolitic crust, the residual melt is dominant over the partial melt, which implies that the generation of I-type granites is dominated by the crystallisation of mafic magma originated from the mantle and not by the partial melting of earlier underplated material. High ratios of crustal partial melt over residual melt are reached when sills are scattered in a metasedimentary crust, allowing the generation of S-type granites. The partial melting of a refractory granulitic crust intruded by dry, high-T mafic magma is limited and subordinate to the production of larger amount of residual melt in the mafic sills. Thus the generation of A-type granites by partial melting of a refractory crust would require a mechanism of selective extraction of the A-type melt.

Comparative petrogenesis and tectonics of Paleoproterozoic Malanjkhand and Dongargarh granitoids, Central India

“…In summary, origin of granitic magma either from partial melting of the crust or mafic magma differentiation (Annen et al, 2008) is a dynamic processes. Previous study based on geochemistry of microgranular enclaves in the Malanjkhand granitoid (Kumar et al, 2004;Kumar and Rino, 2006), whole rock geochemistry and field evidences of microgranular enclaves in DG indicates that both granitoids were originated by magma mixing processes.…”

The petrologic evolution and pre-eruptive conditions of the rhyolitic Kos Plateau Tuff (Aegean arc)

“…Its geochemical characteristics attest the importance of crystal fractionation in producing evolved magma composition the Aegean arc, which represent a relatively clean "magmatic distillation column" (very little assimilation from surrounding crust). The lack of involvement of pre-existing crust in the Aegean subduction zone (which is characterized by slow convergence rate, low magma productivity, and thin overriding continental crust) is expected from recent numerical models of basalt interaction with the continental crust [102,103,114].…”

Construction and evolution of igneous bodies: Towards an integrated perspective of crustal magmatism

“…Transferring heat from basalt to melt the crust is an inefficient process, so that large volumes of mafic magmas are required to produce significant volumes of crustal melt (Annen et al, 2008a;Leeman, et al, 2008;Petford and Gallagher, 2001). Generating felsic melts by partial melting of the upper crust poses the same volume ratio problem as basalt fractionation; if it were the case, the upper crust would be mafic rather than granodioritic.…”

“…Generating felsic melts by partial melting of the upper crust poses the same volume ratio problem as basalt fractionation; if it were the case, the upper crust would be mafic rather than granodioritic. Even at depth, where the crust is hotter and thus more inclined to melt, heat transfer calculations indicate that felsic melt produced by crustal melting is often subordinate to that residual from basalt fractionation (Annen, et al, 2008a;Dufek and Bergantz, 2005). The efficiency problem is exacerbated by the fact that crustal rocks contain relatively…”