Journal of Conference Abstracts

Deformation Enhanced Melt Segregation: Migmatites from the Gander Zone, NE Newfoundland

D. I. Schofield (dischofield@brookes.ac.uk) & R. S. D'Lemos (rsd'lemos@brookes.ac.uk)

Geology and Cartography Division, Oxford Brookes University, OX3 0BP.

It is fundamentally important to understand how small volumes of melt are segregated from their source regions, accumulate into larger magma bodies and ultimately ascend through the crust to be emplaced as plutons. Taken alongside theoretical and experimental insights, it is necessary to correlate with processes preserved in real crustal sections, in particular to understand the role of deformation in segregation and transfer processes. Study of structures related to melting in the lower crust is hampered by poor preservation. Although they probably do not represent a major granite magma source, mid-crustal migmatites may provide useful analogues for these processes.

The Gander Zone of NE Newfoundland underwent high temperature, low pressure metamorphism during Silurian-Devonian terminal collision of Laurentia and the peri-Gondwanan Avalon Zone. Biotite dehydration melting and migmatite formation accompanied deformation, exhumation and emplacement of syn-tectonic granite plutons.

Outcrop-scale structural observations indicate that migmatites formed as a flat lying pile controlled by the orientation of early deformation fabrics. E-W shortening of the migmatite pile caused layer parallel melt re-distribution in response to pore- pressure gradients and strain partitioning, and accumulation of small melt increments in dilational fractures and fold hinges. Accumulation of larger volumes of melt caused weakening and local disruption of the folded and sheared migmatite package. With the resulting increased permeability, larger bodies of melt became mobile and formed intrusive schlieric granite which become the focus for subsequent sinistral shearing.

The significant observation from this study is that deformation and partial melting are apparently closely linked and form a positive feedback loop where deformation facilitates melt accumulation, allowing further deformation to be partitioned into weak zones of high melt proportion. Consequently large bodies of melt can accumulate in structurally weak zones which may then aid their ascent through the crust and emplacement as granite plutons.

Powder Neutron Diffraction Studies of the Thermal Expansion and Compressibility of Deuterated Gypsum (CaSO_4.2D₂O)

P. F. Schofield (P.Schofield@nhm.ac.uk), I. C. Stretton & K. S. Knight

Department of Mineralogy, Natural History Museum, Cromwell Road, London, SW7 5BD.

Structural parameters of deuterated gypsum, an industrially and geologically important mineral, have been extracted from Rietveld analysis of powder neutron diffraction data within the temperature range 4.2 to 320 K and up to 5.4 GPa pressure, at room temperature. From such data we have measured the thermal expansivity, the static compressibility and monitored the structural and hydrogen bond controls on the observed behaviour.

The thermal expansion of gypsum is highly anisotropic along the b-axis due principally to the effect of the H2^....O1 hydrogen bond. Single crystal diffraction studies at 50, 115 and 200 K have allowed us to adopt an anisotropic thermal parameter model confirming the structural behaviour of, and associated with, the hydrous component. The high temperature limits for the expansion coefficients of the cell edges a, b and c are 3.98 x 10^-6, 4.36 x 10^-5 and 2.53 x 10^-5 K^-1, respectively, and for the cell volume it is 6.96 x 10^-5 K^-1. The beta angle displays oscillatory variation reflecting a change in the influence of at least two coupled processes. Empirical data analysis, of the data set prior to dehydration, results in (alpha)_b =1.251 x 10^-6 sin(0.0116T + 0.311) K^-1.

The static compressibility measurements of powdered gypsum, made using the Paris-Edinburgh cell, when fitted to a Birch-Murnaghan equation revealed values of 42 GPa for K and 0.48 for K´, the pressure derivative of the bulk modulus.