Petrogenesis of late Paleozoic tholeiitic, Nb-enriched, calc-alkaline and adakitic rocks in southwestern Mongolia: Implications for intra-oceanic arc evolution

Abstract

Intra-oceanic subduction has been taken as one of the most important contributors to the crustal growth of the immense Central Asian Orogenic Belt (CAOB), however, the detailed geological processes of the development and growth of numerous island arcs still remain unclear. In order to explore the development history of representative ancient island arcs, we have conducted geochronological and geochemical studies on an intrusive rock association in southwestern Mongolia. The studied rock association comprises a mafic intrusive complex (consisting of norite and diabase-porphyry), diabase and diorite dykes. New zircon geochronological data and whole-rock geochemical compositions reveal sequential magmatic events of an island arc development.

The mafic intrusive complex has an emplacement age of 356 Ma old. Geochemically, its two components of norite and diabase-porphyry have a common “tholeiitic” character, but show striking differences in elemental contents. The norites are characterized by relatively higher MgO (7.25–16.1 wt.%), lower TiO₂ (0.54–0.93 wt.%) and relatively lower Nb contents (0.99–6.39 ppm) than those of the Nb-enriched (Nb > 9 ppm) diabase-porphyry. These characters, together with variable Nb/La ratios, suggest that the parental magma of the norites was derived from a refractory mantle source metasomatized by slab-derived fluids and/or melts, whereas, the generation of the Nb-enriched diabase-porphyry was associated with the mantle metasomatism by slab-derived melts. The 335 Ma diabase dykes have negative Nb–Ta–Ti anomalies and elevated LREE and LILE contents, similar to calc-alkaline subduction-related magmas, which together with high zircon ε_Hf(t) values (+ 10.6 to + 15.8) indicate a derivation from a mantle wedge metasomatized by slab-related fluids. The 290 Ma dioritic dykes show the highest SiO₂ (53.1–58.3 wt.%) and total alkali (K₂O + Na₂O = 5.84–7.77 wt.%) contents, and their distinctively high Sr/Y (56.1–97.9) ratios and low Yb (< 1.9 ppm) contents are comparable to adakitic rocks. Taking into account the regional geological facts, we attribute the formation of the dioritic dykes to partial melting of the lower crust caused by an extensional regime in the Permian.

The timing and petrogenesis of the basic–intermediate rock association may supply valuable information on tectonic evolution of an intra-oceanic subduction–accretion system of the CAOB. The coexistence of the early Carboniferous tholeiitic, Nb-enriched mafic intrusive complex, and boninite as well as picrite may represent an immature intra-oceanic island arc. The middle Carboniferous calc-alkaline diabase dykes were probably associated with the relatively mature island arc. The Permian adakitic rocks was likely produced by partial melting of a delaminated lower crust in an extensional regime, when numerous subduction–accretion complexes were likely amalgamated together and docked on the Siberian active margin.