Formation of deep arc root cumulates and implications for crustal growth in the southern Central Asian Orogenic Belt.

Formation of ultramafic cumulate rocks at the arc root serves as a key process to understanding the deep evolution of arc magma and crustal growth. However, the arc root cumulates are easy to delaminate due to their high density, making limited field exposure. Here, we conducted comprehensive petrographic observations and geochemical analysis of typical ultramafic rocks (peridotites and pyroxenites) in an ophiolitic mélange from the central Beishan orogenic belt in the southern Central Asian Orogenic Belt (CAOB). Petrographic evidence shows that these rocks exhibit characteristic adcumulate and mesocumulate textures with intercumulus minerals such as olivine, orthopyroxene, and clinopyroxene. The mineral characteristics of ultramafic rocks, including olivine and orthopyroxene with lower NiO (0.10–0.33 wt% and 0.03–0.09 wt%, respectively) and Mg# (81–87 and 86–88, respectively), clinopyroxene with higher CaO (19.0–24.7 wt%), and spinel with lower Mg# (0–49), are distinct from those of mantle rocks but resemble cumulates. Both whole rock and calculated equilibrium melt exhibit the compositions of island arc magma characterized by enrichment in large ion lithophile elements (e.g., Cs, U, Pb, and Sr) and depletion in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti). The covariation of high Mg# and low SiO 2 in whole rocks is akin to typical arc root cumulates from classic island arcs (e.g., Kohistan and Talkeetna). The positive zircon ε Hf (t) values (+13 to +16) and calculated crystallization pressure of 0.9–1.1 GPa (∼30–40 km) support the formation of ultramafic cumulates in a juvenile island arc setting. These findings imply that most ultramafic rocks in the central Beishan orogenic belt are arc root cumulates instead of the mantle portion of an ophiolitic mélange. Fractional crystallization models suggest that arc root cumulate formation may have influenced the evolution of intermediate-felsic rocks in this area, and that the differentiation of hydrous arc magma played a crucial role in large-scale crustal growth in the southern CAOB during the early Paleozoic. • Ultramafic rocks show cumulate textures and island arc geochemical features. • These rocks are arc-root cumulates, not the mantle portion of an ophiolitic complex. • The crystallization of cumulates promotes intermediate-felsic rock formation. • Hydrous arc magma differentiation has a critical role in crustal growth in the CAOB. [ABSTRACT FROM AUTHOR]