Chemistry and Sr-Nd isotope signature of amphiboles of the magnesio-hastingsite–pargasite–kaersutite series in Cenozoic volcanic rocks: Insight into lithospheric mantle beneath the Bohemian Massif

Abstract

Amphibole phenocrysts, xenocrysts and cumulate xenoliths from Cenozoic volcanic rocks of the Bohemian Massif (BM) belong to the magnesio-hastingsite-pargasite-kaersutite series. Their host rocks are mostly basaltic lavas, stocks, dykes and breccia pipe fills, less commonly also felsic rocks. Felsic rocks with amphibole cumulate xenoliths represent differentiated magmas which have undergone polybaric fractionation of the mafic minerals. The calculated p-T conditions suggest that almost all amphiboles crystallized in a relatively narrow temperature range (1020-1100°C) at depths of~20-45 km (0.7-1.2 GPa) during the magma ascent. These p-T estimates are compatible with the published experimental data on the stability of kaersutite. We therefore suggest the presence of a deep magma chamber situated close to the crust-mantle boundary where amphibole xenoliths to megacrysts could have formed. Nevertheless, crystallization of rare amphibole rims during the magma ascent was observed in a hornblendite cumulate in sodalite syenite from "Giegelberg". The lowest concentration of incompatible elements in the amphiboles was found in xenocrysts in alkaline basaltic rocks and mantle xenoliths and megacrysts, followed by phenocrysts/xenocrysts in lamprophyric rocks, xenocrysts of cumulates in felsic rocks, and phenocrysts in subvolcanic rocks. Amphibole compositional and Sr-Nd isotope characteristics resemble those of amphiboles from metasomatic clinopyroxene/amphibole veins in mantle peridotites. The initial 143 Nd/ 144 Nd and 87 Sr/ 86 Sr ratios of amphiboles (0.51266-0.51281 and 0.70328-0.70407, respectively) are similar to those of their whole rocks (0.51266-0.51288 and 0.70341-0.70462, respectively). Amphiboles of the magnesio-hastingsite-pargasite-kaersutite series of the BM are mostly chemically homogeneous, with no pronounced Mg-Fe fractionation and zoning. The amphiboles are characterized by relatively homogeneous εNd (i) = +1.4 to +3.8 values: only a single sample from the České Středohoří Volcanic Complex (CSVC) yielded a negative εNd (i) (−0.6). This testifies to locally elevated proportions of recycled Variscan crustal material during melting of mantle peridotites rich in clinopyroxene-amphibole veins. These veins were formed by metasomatic fluids enriched in High Field Strength Elements (HFSE) and are isotopically similar to Enriched Mantle1 (EM1)-type mantle. Amphibole host rocks occur in areas with a significant concentration of basaltic magmas in rift zones along lithospheric block boundaries of the BM. Lithospheric mantle beneath such zones was probably strongly influenced by metasomatic fluids during the formation of clinopyroxene-amphibole veins in mantle peridotite that facilitated the generation of basaltic magma with amphibole.

Pyroxenites and Megacrysts From Alkaline Melts of the Calatrava Volcanic Field (Central Spain): Inferences From Trace Element Geochemistry and Sr-Nd Isotope Composition

“…Nevertheless, amphibole primitive mantle-normalized patterns are very different to those of clinopyroxene due to its higher LILE (K, Rb, Ba > Sr), Nb-Ta and Pb contents (Figure 8B). The trace element chemistry of the Calatrava amphibole megacrysts (Supplementary Table S3) is very similar to that described in megacrysts from the Eifel (Shaw and Eyzaguirre, 2000) or from the Bohemian Massif (Mayer et al, 2014;Ulrych et al, 2018), as was the case of clinopyroxenes. Moreover, the D amp/cpx (REE,Th,U, Zr and Hf) estimated for both mineral megacrysts are in the range of 0.6-2, which is very similar to the values obtained from natural rocks and experimental data (Chazot et al, 1996;Grégoire et al, 2000;Raffone et al, 2009;Ubide et al, 2014), indicating the approach of chemical equilibrium between them.…”

40Ar/39Ar step-heating dating of phlogopite and kaersutite megacrysts from the Železná hůrka (Eisenbühl) Pleistocene scoria cone, Czech Republic

“…Their cores crystallized in thinned lithosphere beneath the OR at a depth of ca. 25 km (Ulrych et al 2018), in a magma chamber that received repeated injections of a hydrous magma that mixed with the residual magma, causing the evolved crystallization of the mafic macrocrysts. Hence, phlogopite macrocrysts are likely to be products of the late hydrous magma.…”

O and H isotopic evidence for a mantle source of water in appinite magma: An example from the late Neoproterozoic Greendale Complex, Nova Scotia

“…Although the thermobarometer of Ridolfi and Renzulli (2012) is suitable for the host rock composition, we deem it unsuitable for the purposes of this study as its sensitivity to compositional variations in amphibole may introduce large uncertainties in estimations of crystallization conditions (Ulrych et al, 2018). Although the Al content of hornblende indicates crystallization at 5-7 kbar and between 650-750 °C (Anderson and Smith, 1995;Pe-Piper and Piper, 2018), Figure 10a suggests that temperature conditions could have been closer to 650 °C which supports crystallization between 5-8 kbar.…”

“…When compared to mineral and fluid isotope signatures, these two data sources may be used to derive if a given mineral precipitated at the fluid source or away from it (e.g. Ulrych et al, 2018). The Greendale Complex is interpreted to have been emplaced at shallow structural levels during dextral motion along strike-slip faults .…”