Simulations of crustal anatexis: Implications for the growth and differentiation of continental crust

Abstract

Abstract. Them is overwhelming and incontrovertible petrological and geophysical evidence for the significant role played by mantle-derived mafic magma in the generation and growth of continental crust. Likewise, intrusion of mafic magmas beneath or into continental crust is very likely the major source of enthalpy that drives intracrustal differentiation. A simple dynamical model has been constructed to examine the critical factors that govern the evolution and timescale of crustal anatexic events when driven by basaltic magma underplating or injection into crust. Critical factors include the intensity of the enthalpy input (i.e., the power dissipated) and the rheological propertie.s, bulk composition and compositional structure of the source rocks undergoing partial fusion. We have performed numerical simulations to evaluate these factors using phase equilibria and thermochemical and transport property data applicable to the binary eutectic system CaA12Si2Oa-CaMgSi20 6 as a rough analog to study anatexis of mafic lower crust. In distinction, forfscrit small, more enthalpy is stored in the solid and less melt is produced. However, the compositional spectrum of liquids generated within the anatexic region is significantly larger for smallfscrit. Because of the sensitive dependence of solidi and liquidi on bulk composition, melt productivity is related to bulk composition given a particular enthalpy power input and rheological model. There is a factor of 2 difference in volume of melt generated when the bulk composition of the source is 10 modal percent less refractory. Compositionally zoned melts form by melting of either homogeneous or layered sources, although compositionfrequency relations are sensitive to the initial (subsolidus) compositional structure. The effects of anatexic events are to fundamentally reorganize the pattern of compositional structure within the crust. Intracrustal differentiation is a long-term inevitable process associated with the underplating or injection of mafic magma into preexisting crust.

Early Cretaceous low-Mg adakitic granites from the Dabie orogen, eastern China: Petrogenesis and implications for destruction of the over-thickened lower continental crust

“…Furthermore, the NDC, which are mainly composed of the Neoproterozoic (700-800 Ma) orthogneisses (>90% felsic gneisses; Liou et al, 2009), has been identified as the main composition of the lower continental crust of the Dabie orogen before it was thinned or destroyed . Because the gneisses are widely migmatited in the NDC, which is considered as an anatexis of the lower continental crust (e.g., Raia and Spera, 1997). Thus the gneisses in the NDC seem the most likely source rocks for the Tiantangzhai adakitic granites.…”

The sources of granitic melt in Deep Hot Zones

“…Models of heat transfer between mafic magma and crust have typically involved a single mafic sill instantaneously emplaced in contact with crustal rocks (Barboza & Bergantz 1996;Bergantz 1989;Huppert & Sparks 1988;Jackson et al 2003;Raia & Spera 1997). The amount of crustal melt generated in these models depends on the thickness of the mafic sill, and on the initial temperatures and physical properties of the mafic magma and crust.…”

Thermal Constraints on the Emplacement Rate of a Large Intrusive Complex: The Manaslu Leucogranite, Nepal Himalaya

“…The melting of continental crust can be achieved via two fundamental mechanisms: either by advective heating resulting from basalt underplating or intrusion (Huppert & Sparks, 1988;Bergantz, 1989;Raia & Spera, 1997) or via thickening of crust rich in heat-producing elements (England & Thompson, 1984), assisted or not by shear heating (Le Fort, 1975;England et al, 1992;Harrison et al, 1998) or isothermal decompression (Harris & Massey, 1994). The Himalaya range is a collisional belt characterized by the production of crustal melts without the involvement of basalt .…”