Cambrian–Ordovician magmatism of the Ikh-Mongol Arc System exemplified by the Khantaishir Magmatic Complex (Lake Zone, south–central Mongolia)

Abstract

The Khantaishir Magmatic Complex (KMC) (south–central Mongolia) exposes a section of a magmatic system consisting of deep crustal, ultramafic cumulates (coarse-grained Amp gabbros and hornblendites; c. 0.35–0.5GPa) to shallower crustal levels dominated by Amp–Bt tonalites (c. 0.1–0.2GPa). The magmatic rocks were emplaced during most of the Cambrian (c. 538–495Ma) and are mostly geochemically primitive (Mg#~50), Na-rich and metaluminous. The (normal-) calc-alkaline signature and characteristic trace-element enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE) relative to high-field strength elements (HFSE) suggest an origin within a magmatic arc. Multiple intrusions of basic magma derived from a subduction-modified depleted mantle developed by fractional crystallization and/or accumulation of (Ol, Cpx) Amp+Bt, later joined by Pl. Magma mixing with, or without, exchange of xenocrysts between compositionally dissimilar melt batches was also important. Over time, partial melting of older, lower crustal metabasic rocks became increasingly significant, again with a strong subduction signature. The lack of zircon inheritance in the magmatic products and rather high zircon εHft values (all >+3, but for most samples>+8) as well as whole-rock Sr–Nd isotopic compositions imply that the arc was not founded on mature continental crust. It was probably located at the margin of the Baydrag microcontinent, dominated by accreted metabasic rocks of an older (early Tonian?) island arc covered by a thin layer of subordinate metasediments containing detrital zircons with Tonian and ill-defined Palaeoproterozoic ages. The KMC represents a small vestige of an extensive Cambrian–Ordovician subduction system (termed here the Ikh-Mongol Arc System), bordering the western margin of a chain of Precambrian microcontinents (Tuva-Mongolia, Zabkhan and Baydrag) that, together with accreted Neoproterozoic marginal basins (the Lake Zone), defines the external part of the Mongolian orocline.