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Abstract

The 465 Ma Svarthopen pluton in northcentral Norway was emplaced under middle-crustal conditions (~700 MPa) into metasedimentary rocks of the Helgeland Nappe Complex. The pluton is characterized by zones of mingling and mixing of gabbro/ diorite with peraluminous, garnet-bearing biotite granite. Variation in bulk-rock Sr and Nd isotope ratios are consistent with simple mixing; however, nonuniform enrichment of Zr and the rare earth elements (REEs) suggests that individual magma batches underwent postmixing fractionation. Hybrid intermediate rocks are characterized by Ca-rich garnet. Such garnet is absent in possible mafi c end members, and garnet in felsic end members is Ca poor. Evidently, the ferroan , peraluminous hybrid rocks promoted garnet stability, and we interpret these garnets to be igneous in origin. Garnets in the hybrids have low Zr contents, positive light REE slopes, and fl at to negative heavy REE slopes with lower REE abundances than in typical igneous garnet. These trace-element data combined with textural evidence suggest that garnet formed near the solidus, after fractionation of zircon, allanite, and possibly xenotime. The Svarthopen pluton is not unique: similar intermediate rocks with Ca-rich garnets crop out adjacent to three other plutons in the region.Formation of garnet-bearing hybrid rocks in the Svarthopen pluton provides an analog for mixing of peraluminous and ferroan endmember magmas in the deep crust, where such mixing should be widespread, particularly in continental arcs and zones of continental collision. Postmixing fractionation of hybrid magmas could greatly increase the diversity of major-and trace-element abundances yet retain an isotopic signature of mixing. More-over, formation of garnet-rich hybrids could result in lower-crustal rocks dense enough to delaminate from the arc crust.

Insights from mineral trace chemistry on the origin of NYF and mixed LCT + NYF pegmatites and their mineralization at Mangodara, SW Burkina Faso

“…Petrogenetic studies of LCT and NYF pegmatites are mostly based on the chemical composition of their minerals, because these reflect the physical conditions of crystallization (pressure, temperature) and can record the chemical evolution of the pegmatite-forming melt (Barnes et al 2012;Breiter et al 2018;Deveaud et al 2015;Müller et al 2018;Novák et al 2011;Pichavant et al 2016;Roda-Robles et al 1999;Simmons et al 1987;Thomas et al 2000;Van Lichtervelde et al 2008). The composition of micas is used to define enrichment trends in trace elements for evolved granites and pegmatites (Hulsbosch et al 2016;Martins et al 2012;Stepanov et al 2014;Stepanov and Hermann 2013;Van Lichtervelde et al 2008).…”