Amphibole-rich xenoliths from Devonian igneous rocks of the Pripyat rift, Southeastern Belarus: a window into cratonic lower-crust–upper-mantle boundary

Abstract

To obtain new insights into the composition and geological history of the crust–mantle boundary beneath a continental rift, we have investigated a suite of amphibole-rich xenoliths in comparison with amphibole-bearing granulite xenoliths, and amphibole megacrysts from Devonian volcanic rocks of the Pripyat rift in Belarus, East European Craton (EEC), for mineralogy, geochemistry and Sr–Nd isotope composition. Hornblendite, clinopyroxene hornblendite and hornblende melagabbro xenoliths are characterised by cumulate textures. Amphibole is mostly zoned magnesio-hastingsite. Two types of melts in equilibrium with this amphibole were calculated: those having more primitive composition than the host rocks of the hornblendite xenolith and those having composition similar to the host rocks of the clinopyroxene hornblendite and hornblende melagabbro xenoliths. Sr–Nd-isotopic compositions of these xenoliths confirm their affinity with the Devonian igneous rocks of the Pripyat rift. Temperature and pressure conditions were estimated as 1056 °C–1083 °C and 12–14 kbars for the clinopyroxene hornblendite xenolith, 1022 °C–1039 °C and 12–15 kbars for the hornblende melagabbro xenolith, and 1050 °C–1065 °C and 13–14 kbars for the cores of the amphibole megacrysts. The hornblendite xenolith represents a cumulate from an earlier batch of ultramafic alkaline magma, the clinopyroxene hornblendite xenolith could be a cumulate derived from the erupted Devonian alkaline magmas, and the hornblende melagabbro probably was crystallised from an unerupted batch of more-evolved magma. A granoblastic garnet–biotite–amphibole xenolith is composed mostly of unzoned ferrisadanagaite amphibole. Its bulk rock multielement pattern has positive large-ion lithophile element (LILE) and negative high field strength element (HFSE) anomalies. Mineral chemistry and bulk rock Sr–Nd isotope compositions indicate a Palaeoproterozoic granulite protolith, similar to suprasubduction magmatic rocks of the nearby\u2009~\u20092.0–1.95-Ga old Osnitsk–Mikashevichi Igneous belt (OMIB). Based on our results, we proposed a model for the evolution of the crust–mantle boundary beneath the Pripyat rift. The crust–mantle boundary beneath the Pripyat rift was formed during the Palaeoproterozoic (2.0–1.95-Ga) orogeny by emplacement of a mafic underplate and was reworked by subsequent post-orogenic events. Magmatism during Devonian rifting introduced hornblendite veins and amphibole-rich bodies, while amphibole-producing metasomatism developed from intraplate rift-related magmatism.