Late Carboniferous intrusions along the Kalamaili suture zone, southwestern Central Asian Orogenic Belt (CAOB): implications for a tectonic switch from subduction to collision

Abstract

ABSTRACT A tectonic switch from subduction to collision and/or arc–arc collision is a key issue for understanding accretionary orogenic processes. We present new U-Pb ages, geochemistry, whole-rock Nd and zircon Hf isotope data for a suite of late Carboniferous intrusive rocks along the Kalamaili suture zone of the southwestern Central Asian Orogenic Belt (CAOB) and demonstrate their petrogenesis and the transition of their tectonic settings. Zircon U-Pb dating for the gabbros, diorites, granodiorites, and monzogranites yields ages of 317–301 Ma. The gabbros (311 Ma) were likely derived from partial melting of a mixed source of depleted MORB-like asthenospheric mantle with minor old crustal material; the diorites (301 Ma) from the mixed depleted lithospheric mantle and crustal components; the granodiorites (303 Ma) from the mixed mantle- and crust-derived magmas; and the monzogranites (317 Ma) from melting of the newly underplated basaltic lower crust. Combined with previous data, we have grouped all the intrusive rocks along the Kalamaili suture zone into three phases: 350–340 Ma, 330–320 Ma, and 320–300 Ma. The first phase consists of tholeiitic and high-K calc-alkaline granites, the second phase is composed of high-K calc-alkaline I-type granites with strong deformation, and the third phase comprises voluminous and various rock types (mainly mafic and felsic, including many A-type granites), revealing a magmatic flare-up. The magmatic associations and evolution from the first to the third phase, together with regional geology, suggest a tectonic transition from subduction/accretion (350–340 Ma) through syn-collision (330–320 Ma) to post-collision (or post-accretionary, 320–300 Ma), respectively. This study provides new magmatic evidence for the switch from subduction to collision (arc-arc collision) along the Kalamaili suture zone in the southwestern CAOB.