Occurrence of low-Ca clinopyroxene and the role of deformation in the formation of pyroxene-Fe−Ti oxide symplectites

Pyroxene-Fe−Ti oxide symplectites in a norite from the leuconoritic phase of the Bjerkreim-Sokndal lopolith, SW Norway, have been studied using EMPA (electron microprobe analysis) and TEM (transmission electron microscopy) techniques. Textural and mineralchemical data indicate that the symplectites formed under subsolidus conditions at T=720–736°C and f_{o 2}=10^-17 bars. Solidus temperatures are estimated as 981–1060° C. Two models of formation are proposed: non-isochemical replacement of olivine, and growth at boundaries between exsolving grains of orthomagmatic high-Ca pyroxene and Fe−Ti oxide. TEM reveals the presence of low-Ca clinopyroxene and this formed in response to strain and/or shear stress. Comparison with published experimental data indicates that strain rates of up to 10^-12 s^-1 are necessary to explain the occurrence of low-Ca clinopyroxene. The transition from orthopyroxene to low-Ca clinopyroxene may be related to deformation which accompanied intrusion of the leuconoritic phase of the Bjerkreim-Sokndal lopolith or to local post-intrusive faulting and/or shearing. Selected-area diffraction shows that the Fe−Ti oxide lamellae are oriented with respect to the pyroxene such that (111)_oxide/(100)_pyroxene. the planes of closest oxygen packing in the constituent phases are thus adjacent to one another which leads to minimal misfit between the structures and to low interfacial strain energy. The same topotactic relationship exists in both lamellar and vermicular parts of symplectites (but is not continuous along the total length of the interface), indicating that the form of the intergrowths is a primary growth feature that to some extent can be explained in terms of growth models for duplex cells. Deformation during growth is probably necessary to account for the lattice distortion observed in the symplectites. Supercooling is necessary for nucleation and growth of the symplectites. A period of cooling under static, fluid-absent conditions allows substantial overstepping of equilibrium reaction boundaries. Nucleation and growth is triggered by local deformation accompanied by influx of fluid or, possibly, melt. Under these conditions growth can be rapid (≈1 year), and low-Ca clinopyroxene forms during or after growth. The proportions of pyroxene and Fe−Ti oxide in symplectites from a variety of rocks fall in a narrow range (70.5:29.5–74.8:25.3) which suggests a common growth mechanism. The interlamellar spacing is related to the amount of supercooling prior to growth, but the effects of fluid on growth rate must be studied before this relationship can be quqntified. Symplectites provide evidence for the development of retrograde mineral assemblages in relatively short time intervals under transitory fluid-present conditions. The models for symplectite formation in the intrusives of southwestern Norway may be applicable to pyroxene-Fe−Ti oxide intergrowths in other slowly cooled basic igneous rocks.