Petrogenises of Triassic gabbroic and basaltic rocks from Chukotka, NE Russia: Eastern end of the ‘arc–type’ Siberian LIP?

Abstract

The Triassic gabbroic intrusions and associated basaltic lavas from Chukotka are mainly tholeiitic with both ocean island basalt (OIB) and island arc basalt (IAB)–type geochemical signatures. Mg–number [Mg# = 100 × Mg/(Mg + Fe²⁺)] is around 40 for OIB–type gabbros, ranges from 48 to 66 for IAB–type hornblende–rich gabbros, and 43 to 65 for IAB–type basaltic rocks (ankaramites, lamprophyres, pyroxene–phyric basalts and hornblende–phyric basaltic andesite). TiO₂ contents of the IAB–type gabbros and basaltic rocks are low (<2 wt%), but are high in OIB–type gabbros (4.3–5.3 wt%). OIB–type gabbros are typically enriched in FeO^* (16–18 wt%) as compared to IAB–type gabbros (10–14 wt%) and IAB–type lavas (ankaramites, ~ 10 wt%; lamprophyres, ~ 14; pyroxene–phyric basalt, 11 wt% and basaltic andesite, 9–10 wt. %). In the primitive mantle normalized trace element patterns, IAB–type basalts and gabbros are characterized by depletion in HFSE (Nb, Ta, Zr and Hf) and enrichment in LILE. OIB–type gabbros can be distinguished from the rests by the absence of HFSE depletion, with strong negative Sr anomaly. The positive Ti anomaly in the OIB–type gabbros can be attributed to high content of ilmenite in these rocks. Trace element characteristics of IAB–type gabbroic rocks and basalts are compatible with their magmas derived from subduction influenced melts, whereas OIB–type gabbros show within–plate geochemical characteristics. IAB–type gabbros and basaltic rocks display similar geochemical features to the low–Ti Nadezhdinsky suit (Noril’sk region) and Bel’kov dolerite (New Siberian Islands) of the Siberian large igneous province (LIP) in view of HFSE depletion and high H₂O content of the magma to crystallize abundant hornblende not only in gabbros but also as phenocrysts in basalts. The Triassic gabbroic and basaltic rocks of both OIB and IAB types may as a whole represents the eastern end of the Siberian LIP.