Cambrian intermediate-felsic rocks in Central Pamir: Insights into source heterogeneity and subduction initiation in the Proto-Tethys Ocean

Abstract

The Early Paleozoic magmatic rocks in the Central Pamir provide insights into the tectonic evolution of the Proto-Tethys Ocean. This study integrates zircon UPb ages, in situ HfO isotopes, whole-rock elemental and SrNd isotopic data of the intermediate and felsic rocks from the Zorburulyuk, Panghazjilga, and Taxkorgan complexes in the eastern segment of the Central Pamir to constrain their magma sources and tectonic implications. Zircon UPb ages reveal these magmatic rocks formed at Early Cambrian (pegmatite: 527.0 ± 5.4 Ma; gneiss-granite: 524.3 ± 3.6 Ma; monzodiorite mafic microgranular enclaves (MMEs): 524.1 ± 1.6 Ma; quartz diorite: 517.3 ± 1.1 and 520.3 ± 2.2 Ma and biotite granite: 512.1 ± 3.5 Ma). The intermediate rocks exhibit enriched isotopic compositions (ε_Hf(t) = −5.78 to +6.93; ε_Nd(t) = −5.56 to −4.02; ⁸⁷Sr/⁸⁶Sr = 0.70212–0.70844), indicating mainly derivation from a metasomatized subcontinental lithospheric mantle source that was enriched by subduction-derived sediments, and involved magma fractionation of mantle-derived mafic melts to produce intermediate rocks. The felsic rocks exhibit variable isotopic compositions: pegmatites (ε_Hf(t) = −2.41 to +0.95; ε_Nd(t) = −4.02 to −5.15), biotite granites (ε_Nd(t) = −1.54 to −0.94; δ¹⁸O = 7.87–9.17 ‰), which are similar to coeval mafic-intermediate rocks, indicating that they were derived from melting of hybrid lower crustal sources represented by the latter. Our results propose that the Cambrian intermediate-felsic rocks record the early stages of southward subduction of the Proto-Tethys Ocean with arc magmatism peaking in the Early Cambrian (ca. 524–527 Ma). In this process, slab rollback facilitated magma underplating and crustal hybridization, leading to the formation of intermediate-felsic rocks through melting of metasomatized subcontinental lithospheric mantle and lower crustal sources.