Geophysical and geochemical nature of relaminated arc‐derived lower crust underneath oceanic domain in southern Mongolia

Abstract

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trendingNeoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low-to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the "Khantaishir" arc (520-495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite-and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m 3 ) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.

Petrogenesis of the Late Carboniferous Sagsai Pluton in the SE Mongolian Altai

“…The analysis of magnetic lineaments in SW Mongolia also demonstrates that the zones of magnetic highs coincide with other E-W trending Late Paleozoic volcano-plutonic provinces previously defined by Kovalenko et al (2006) as continental rifts related to Pacific-type geodynamic processes and protracted activity of mantle plumes (Yarmolyuk et al 2014). Indeed the geophysical lineaments and magmatic zones can be interpreted as tectono-magmatic deformation zones deeply rooted in the lithosphere that probably channelled mantle-derived magmas (Guy et al 2015). Our data confirm the idea that Late Carboniferous was a period of elevated thermal structure at the lithospheric scale, but the crust was molten heterogeneously leading to localized emplacement of magmas along E-W oriented lineaments.…”

“…Geophysical study of Guy et al (2014b) has shown that this tectono-magmatic zone spatially coincides with a strong, c. 15-20 km deep magnetic lineament. The analysis of magnetic lineaments in SW Mongolia also demonstrates that the zones of magnetic highs coincide with other E-W trending Late Paleozoic volcano-plutonic provinces previously defined by Kovalenko et al (2006) as continental rifts related to Pacific-type geodynamic processes and protracted activity of mantle plumes (Yarmolyuk et al 2014).…”

“…The formation of the Main Mongolian Lineament and the Gobi-Tien Shan rift zones corresponded to early evolution of this rift system (Kuzmin et al 2011). Recent geophysical study (Guy et al 2014a(Guy et al , 2015 showed that the presumed rift systems coincide with intense magnetic lineaments suggesting existence of deep-seated tectonomagmatic zones. These zones are probably connected to a giant source located deep in the mantle as indicated by the gravity signal.…”

Folded Basinal Compartments of the Southern Mongolian Borderland: A Structural Archive of the Final Consolidation of the Central Asian Orogenic Belt

“…Terrane maps of Mongolia with accompanying stratigraphic columns and geochronological data provide a regional lithotectonic framework for understanding the region's crustal evolution [4,21]. More recently, new maps, structural and geochronological data, and gravity and magnetic models from different areas in southern Mongolia further refine the polyphase metamorphic, deformational, and magmatic evolution of the different terranes and call into question the locations of some previously mapped terrane boundaries [8,20,22].…”

“…The continental crust of southern Mongolia and the Beishan region consists of a Paleoproterozoic basement complex in southern Mongolia surrounded by various lower through upper Paleozoic arc, accretionary prism/melange, and ophiolitic complexes [4,9,19,20]. Terrane maps of Mongolia with accompanying stratigraphic columns and geochronological data provide a regional lithotectonic framework for understanding the region's crustal evolution [4,21].…”