Crustal Growth

Laminar natural convection has been studied in a laterally heated vertical cylindrical enclosure with a free insulated surface and a centrally located constant temperature wall at the top. These conditions are a simplification of the conditions existing in a Czochralski crystal pulling system. The laminar, axisymmetric flow of a Newtonian, constant physical properties fluid under Boussinesq’s approximation has been considered. Governing equations in primitive variable form are solved numerically by control volume method. SIMPLE algorithm due to Patankar has been used for the numerical simulation. The effects of the constant wall heat flux boundary condition at the side wall have been investigated whereas the bottom wall is considered to be insulated. Streamlines and isotherms are presented for various Rayleigh numbers and Prandtl numbers. Heat flux vectors through the melt are plotted for selected cases. The axial velocity and temperature variations at different horizontal sections of the crucible have been presented graphically to explain the transport processes inside the crucible. It has been observed that in case of low Pr and high Ra, flow separation occurs at the vertical wall of the crucible which leads to an oscillatory flow as Ra increases. The investigation has been extended to the oscillatory regime of flow in the zone of supercritical Rayleigh numbers and some unsteady results are also presented. Finally a heat transfer correlation has been developed for steady-state case.

The generation of the continental crust may be connected to mantle plume activity. However, the nature of this link, and the processes involved, are not well constrained. An obstacle to understanding relationships between plume-related mafic material and associated silicic rocks is that later tectonic movements are liable to obscure the original relationships, particularly in ancient greenstone belts. Studies of younger analogous regions may help to clarify these relationships. On the island of Aruba in the southern Caribbean, a sequence of partly deformed mafic volcanic rocks intruded by a predominantly tonalitic batholith is exposed. The mafic lavas show geochemical and isotopic affinities with other basaltic, picritic and komatiitic rocks that crop out elsewhere in the Caribbean—these are well documented as belonging to an 88–91 Ma plume-related oceanic plateau, which is allochthonous with respect to the Americas, and is thought to have been formed in the Pacific region. The ∼85 to ∼82 Ma tonalitic rocks share some geochemical characteristics (high Sr and Ba, low Nb and Y) with Archaean tonalite–trondhjemite–granodiorite (TTG) suites. Field relationships suggest that deformation of the plateau sequence, possibly related to collision with a subduction zone, was synchronous with intrusion of the Aruba batholith. New incremental heating dates, combined with existing palaeontological evidence, show that cooling of the batholith occurred shortly after eruption of the plateau basalt sequence. Sr–Nd isotopic data for both rock suites are uniform (, εNdi≈+7), whereas Pb isotopes are more variable (Plateau sequence: , , ; Aruba batholith: , , ). This suggests that there has been a minor sedimentary input into the source region of the batholith. However, the limited time interval between basaltic and tonalitic magmatism makes a normal subduction-related origin for the tonalites improbable. Instead, models involving derivation of tonalite from partial melting of the plateau sequence, or alternatively, genesis in an unusual subduction zone environment, are investigated.

A suite of metaplutonic rocks representing a mafic to intermediate calc-alkaline batholith has been identified in the Voisey's Bay area of northern Labrador. The suite represents the southern extension of similar metaplutonic rocks occurring along some 500 km of the Paleoproterozoic Torngat Orogen. The rocks have been metamorphosed at upper amphibolite to granulite facies conditions that only mildly disturbed their igneous characteristics. Two magma series are present, one comprising gabbros, diorites and quartz diorites, and another composed of tonalites. The gabbro to quartz diorite series evolved by fractional crystallization of a mantle-derived mafic magma. Two samples of the metaplutonic suite yield U-Pb zircon thermal ionization mass spectrometry (TIMS) ages of 1893 ± 1 and 1890 ± 2 Ma. Based on U-Pb zircon laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) geochronology, one of these samples contains inherited cores, some 50-100 Ma older than the igneous population, but no Archean cores were found. Metatonalite rocks formed from a separate tonalitic magma during a slightly younger magmatic event dated at 1883 ± 5 Ma. Both the gabbro to quartz diorite and tonalite series rocks are enriched in Rb, Ba, Sr, Zr and light rare earth elements relative to primitive mantle; and have negative Nb-Ta anomalies, which are thought to reflect subduction processes. They have fractionated REE patterns, with the extent of fractionation greatest for the tonalites (chondrite normalized La/Yb = 30-50), suggesting that their parent magmas left more residual garnet in their source regions than did the gabbro to quartz diorite magmas. Sr-Nd-Pb isotope compositions of the metaplutonic suite indicate significant involvement of a lower crustal component in their genesis. When combined with previous data for the suite, and excluding data thought to be disturbed by metamorphism, 87 Sr/ 86 Sr (1890 Ma) = 0.7036-0.7048, εNd (1890 Ma) = +4.7 to −6.9, and µ * (time-integrated 238 U/ 204 Pb of the mantle and crustal sources) = 7.66-7.98. A model for the metaplutonic suite is presented in which the gabbro to quartz diorite series formed by partial melting of the mantle wedge in a subduction zone and interacted with a lower crustal contaminant, possibly in the source region. The tonalites were derived by partial melting of a mixture of subducting oceanic crust and lower crustal detritus. An important role for lower crustal recycling is thus indicated. Crustal genesis by both mantle wedge and slab melting in the Paleoproterozoic may mark the transition from Archean to post-Archean subduction processes.