Petrogenesis of zoned and unzoned mafic pegmatites: An insight from the Palaeoproterozoic mafic-ultramafic Hamn intrusion, Northern Norway

• DOI: 10.1016/j.lithos.2022.106818
• Publication Date: 2022-08-03T06:04:28Z
• Authors: Paul Heckmann; Sabina Strmić Pal...; Harald Hansen; Giada Iacono-Marz...; Kristijan Rajič; Mélanie Forien; Steffen G. Bergh
• Keywords: Pegmatite; Petrogenesis; Ultramafic rock; Amphibole; Pyroxene; Fractional crystallization (geology)

Mafic pegmatites have been reported from various geological environments, including ophiolites, layered magmatic intrusions, and volcanic arcs, but their petrogenesis stayed poorly constrained. This study brings new mineralogical geochemical data obtained unzoned zoned gabbroic hosted by the 1.8 Ga mafic-ultramafic Hamn intrusion, Northern Norway, with an aim to improve our understanding of primary physicochemical factors that control development pegmatitic textures in mafic rocks. The intrusion are gabbronorite gabbro. differ macroscopically only grain size color feldspars. bulk chemical compositions, REE concentrations rock-forming mineral assemblages both rocks, identical. composition pegmatite host gabbro overlap. However, experienced epidotization scapolitization, suggesting H2O- chlorine were essential formation. Furthermore, pyroxene shows a more distinct negative Eu anomaly than gabbronorite. Amphibole dyklets associated pockets consist Cl-rich pargasite Cl-scapolite, indicating fluid pressure during crystallization exceeded confining resulted fracturing rocks expulsion Cl- H2O-rich residual melt subsequently formed amphibole veins. internally differentiated terms size, texture, composition. A characteristic comb-like diopside layering change numerous small fewer larger grains rim towards core pocket. textural suggests pegmatite-forming transition high nucleation rate (N) vs. growth (G) ratio low N/G ratio. decreasing degree undercooling continuous can explain this transition. Apart layering, bifurcating texture some is another evidence undercooling. could remobilized intercumulus or fractionated was emplaced as into colder rock. Fast heat diffusion rock caused melt, which led formation partly extremely coarse-grained texture. In contrast pegmatites, lack significant involvement chlorine- H2O. Instead, inclusion revealed enriched CO2. Microthermometry CO2-bearing inclusions indicates minimum 647 734 MPa, titanium-in-quartz geothermometer yields temperature 753 ± 34 °C for quartz segregation.