Geochemical signature of Paleozoic accretionary complexes of the Central Asian Orogenic Belt in South Mongolia: Constraints on arc environments and crustal growth

Abstract

We report a reconnaissance geochemical and Nd-isotopic study for 33 metaigneous and metavolcaniclastic rocks from the ca. 470 to 290 Ma old Gurvan Sayhan, Zoolen, Nemegt Uul, Bayanleg-Hatuu, and Tseel mountain ranges in southern Mongolia, in order to infer tectonic environments, test a recently proposed multiple terrane model, and explore the late-stage development of the southern Central Asian Orogenic Belt (CAOB). Samples from the Gurvan Sayhan and the Zoolen ranges have a predominantly intermediate, calc-alkaline composition; they display LREE enrichment, excess abundances of fluid-soluble elements, and low concentrations of high-field strength elements. These data, in combination with high initial ε Nd-values of ca. + 5 to + 9, are consistent with a juvenile intra-oceanic arc. Some of the samples are chemically similar to adakites and high-Mg andesites and thus predictably evolved in forearc settings. The Gurvan Sayhan and Zoolen ranges have previously been considered as an island arc and accretionary prism, respectively. However, our data show indistinguishable, mutual, geochemical characteristics suggesting that both areas were part of a single intra-oceanic island arc-forearc. Samples from Nemegt Uul exhibit calc-alkaline, LREE-enriched island arc as well as tholeiitic LREE-depleted backarc-basin signatures of predominantly juvenile composition (initial ε Nd-values + 6 to + 10). We suggest that Nemegt Uul belongs to the Gurvan Sayhan and Zoolen island arc terrane rather than an accretionary prism, as previously proposed. Samples from Bayanleg-Hatuu and Tseel reveal LREE-enriched calc-alkaline arc as well as tholeiitic LREE-depleted to slightly enriched backarc-basin signatures. The remarkably wide range of initial ε Nd-values of ca. + 8 to − 6 indicates juvenile as well as old crustal material with mean residence times up to 1.5 Ga. Nd isotopic data for Bayanleg-Hatuu and Tseel suggest a variably rejuvenated margin of a Paleoproterozoic microcontinent with an associated backarc basin. Our Nd-isotopic data for the CAOB in southern Mongolia indicate a highly heterogeneous composition, and we conclude that a juvenile arc-backarc segment in the eastern study area was juxtaposed with a rejuvenated Paleoproterozoic microcontinent.