Petrogenesis of a Triassic Magmatic Suite in the West Kunlun, northwest Tibetan Plateau: Implications for the Geodynamic Evolution of the Paleo-Tethys Ocean

Abstract

Mafic-to-felsic magmatic suites commonly occur in convergent margins, providing a unique opportunity to reveal the geodynamic evolution and mantle-crust interactions during oceanic subduction. This study presents new zircon U-Pb geochronology, whole-rock geochemistry (major and trace elements, Sr-Nd-Pb isotopes), mineral chemistry, and zircon Hf-O isotope data from the Muztaga and Taer plutons in the West Kunlun Orogenic Belt (WKOB), northwest Tibetan Plateau. The Muztaga-Taer plutonic complex consists of mafic-intermediate rocks (including microgranular mafic enclaves (MMEs), mafic dikes, and gabbroic diorites) and granites. Zircon U-Pb dating reveals crystallization ages of ca. 233 Ma for the mafic dikes, ca. 226 Ma for the MMEs, ca. 223 Ma for the gabbroic diorites and ca. 244–216 Ma for the granitoids. The mafic-to-intermediate rocks (SiO2=48.7–58.2 wt.%; Cr=19–474 ppm; Ni=11–182 ppm) exhibit high large-ion lithophile elements (LILEs, e.g. Rb, Ba and K) and low high field-strength elements (HFSEs, e.g. Nb, Ta, P and Ti), as well as high whole-rock (87Sr/86Sr)i ratios (0.7058 to 0.7071) and negative whole-rock εNd(t) values (–3.5 to –3.0), indicating derivation from an enriched lithospheric mantle source. Temporal variations in the in-situ zircon Hf-O isotopic ratios suggest that the mantle source was modified by subducted sediment-derived melts over a protracted period of time. Estimates derived from mineral geobarometers and trace element modeling suggest generation by variable degrees partial melting of spinel- to garnet-facies mantle sources at different depths. The Muztaga and Taer granitoids display characteristics of I-type granites with elevated SiO2 (66.4–74.7 wt.%), Na2O (2.96–4.44 wt.%) and δ18Ozrn (6.28‰–7.51‰). Together with enriched (86Sr/87Sr)i (0.7070 to 0.7085), εNd(t) (–4.31 to –3.53) and εHf(t) (–5.27 to +1.51), these features indicate a likely derivation via remelting of an ancient crustal source. The temporally and spatially associated mantle-derived mafic-intermediate rocks and crust-derived granites collectively constitute a Triassic intrusive suite within the WKOB. To constrain the driving forces and evolutionary pathways of this magmatic system controlled by Paleo-Tethys subduction, we establish a comprehensive petrogenetic model as follows: (1) Northward subduction of the Paleo-Tethys slab induced partial melting of the heterogeneous hydrous lithospheric mantle, generating primitive mafic magmas; (2) Processes operating within magmatic recharge zones modified mantle-derived magmas through magma storage and recharge, producing diverse mafic-intermediate lithologies; (3) Mafic magma underplating provided thermal and aqueous fluxes for lower crustal melting and granite generation. Regional geochronological and geochemical data indicate that the Pamir-WKOB formed an active continental margin during the Triassic-Early Jurassic, driven by sustained northward Paleo-Tethys slab subduction. When considered together with regional metamorphic-sedimentary records, these data reveal a two-stage geodynamic evolution of the Paleo-Tethys oceanic subduction system as follows: (1) Early Triassic flat–slab subduction promoted back-arc crustal thickening and the generation of granites; and (2) the onset of slab roll-back at ca. 223 Ma triggered voluminous continental arc magmatism, accompanied by crustal thinning and extension, which ultimately led to the final closure of the Paleo-Tethys Ocean during the Late Triassic-Early Jurassic.