Melt organisation and strain partitioning in the lower crust

Abstract

Partial melts can form as a result of crustal thickening due to orogenesis. Even small melt fractions weaken the crust, so that partially molten volumes should accumulate significant amounts of strain. However, relatively little is known of how strain partitions in partial melts, and how effective the melt expulsion processes from the partially molten crust are. Using examples from the Western Gneiss Region (WGR), Norway, we consider a case of co-existing migmatites and shear zones. Field, image analysis, and microanalytical methods allow (semi)quantification of melt volume, rock mineralogy and mineral chemistry, and microstructures. Integration of these analyses implies effective syn-melt strain partitioning and subsequent freezing of both the shear zone and migmatite texture. We propose a mechanism that allows i) syn-melt strain localisation at an outcrop scale through stress-driven melt organisation, resulting in significant relative competence differences in a partially molten rock volume; and ii) formation of fine-grained rocks at outcrop

Dynamics, EPMA Th‐U‐Total Pb Monazite Geochronology and Tectonic Implications of Deformational Fabric in the Lower‐Middle Crustal Rocks: A Case Study of Ambaji Granulite, NW India

“…Occurrence of garnet corona around spinel (Figure S1e) indicates isobaric cooling following peak metamorphism. Minerals in leucosomes show irregular grain boundaries and low dihedral angle characteristic of partial melting origin (e.g., Lee et al, , and references therein). Melting produced G 1 granite at circa 0.86 Ga (Singh et al, ).…”

How does orogenic crust deform? Evidence of crustal-scale competent behaviour within the partially molten middle crust during orogenic compression

“…This does not, of course, apply to shear zones which typically have 1-3 orders of magnitude higher strain rates than large-scale orogenic deformation: if anatectic melts formed within or intruded into a highstrain zone they do accumulate significant deformation. While melts were present in such shear zones they would, therefore, be weak; this weakness is likely to be enhanced by the high degree of fabric organization in the shear zones (Lee et al, 2018). A 'sideeffect' of this strain localisation, on the other hand, would be to further reduce the strain accumulation in and weak behaviour of the partially molten volumes outside the shear zones.…”

“…For example, ubiquitous observations globally show that significant amounts of melts are retained more or less in situ, even in the vicinity of shear zones (e.g. up to 20% in Lee et al, 2018).…”

The Competition Between Rates of Deformation and Solidification in Syn-Kinematic Granitic Intrusions: Resolving the Pegmatite Paradox

“…Further work is now needed to explore the rheological impacts of the causes and the relationships between the rates of crystallization, strain rates, grain size of (partly) crystallized melts vs. host rocks, and deformation in various tectono-magmatic systems, together with how deformation can influence the sites of fractional crystallization and the fate of residual fluids and fluxes in these systems. Indeed, recent work by Lee et al (2018) suggests that 'freezing' of partial melts within migmatitic, non-pegmatite-bearing syn-melt shear zones also occurs. The 'melt-strengthening' behaviour may be more common and more important to the behaviour of orogenic crust than previously realized.…”