Origin of serpentinite-related nephrites from Gogołów-Jordanów Massif, Poland

Abstract

The Gogołów-Jordanów Massif (GJM) in the Fore-Sudetic Block, SW Poland, hosts nephrites traditionally interpreted as serpentinite-related (ortho-nephrite). This contribution confirms the serpentinite-related origin of the nephrites on the basis of mineralogy, bulk-rock chemistry, and O and H isotopes. Rock-forming amphiboles from nephrites of the GJM have 7.73–7.99 Si apfu, comparable to 7.76–8.03 Si apfu of serpentinite-related Crooks Mountain nephrite amphibole (Granite Mountains, Wyoming, USA). The GJM amphiboles also have Mg/(Mg + Fe2+) values ranging from 0.82 to 0.94, similar to serpentinite-related Crooks Mountain and New Zealand nephrites amphiboles with Mg/(Mg + Fe2+) values of 0.86–0.90 and 0.91 to 0.92, respectively. The GJM nephrite amphiboles differ from the Val Malenco dolomite-related nephrite (Italy) amphibole, e.g., Val Malenco has a higher Si content (~8.0 Si apfu), although it overlaps with some of the GJM nephrite samples, and ~1.0 Mg/(Mg + Fe2+), also higher than the GJM samples. Also, apatite in the nephrite studied from the GJM has a slightly higher Ca content than apatite from dolomite-related nephrite. Chlorites found in the Jordanów nephrite have similar compositions to that of chlorites in the serpentinite-related nephrites and also to chlorites associated with serpentinisation/rodingitisation. The bulk-rock FeO vs. Fe/(Fe + Mg), Cr, Ni, and Co are also typical of the serpentinite-related nephrites. The d18O values range from +6.1 to +6.7‰ (±0.1‰), and the average dD values = –61‰, corresponding with the serpentinite-related nephrites range. Based on petrographic observations, we suggest four crystallisation stages (including rodingitisation prior to nephrite formation): 1 – leucogranite rodingitisation and black-wall formation; 2 – tremolite formation at the expense of rodingite diopside and black-wall chlorite (nephritisation) and garnet break-down, with spinel and chlorite formation (chlorite can be a product of garnet break-down or spinel with serpentine reaction); 3 – prehnite vein formation; 4 – tremolite formation at the expense of prehnite veins and actinolite veins formation. Spinels composed of 0.29–1.96 wt.% MgO, 24.87–29.67 wt.% FeO, 8.72–22.82 wt.% Fe2O3, 3.11–4.36 wt.% Al2O3, and 39.07–54.46 wt.% Cr2O3 suggest nephritisation in the greenschist to lower-amphibolite-facies conditions.