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

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.

俯冲向碰撞（或弧-弧碰撞）的构造转换，是认识增生造山作用的关键科学问题。本文针对中亚造山带（Central Asian Orogenic Belt, CAOB）西南缘卡拉麦里缝合带沿线的一套晚石炭世侵入岩，报道了新的U-Pb年代学、地球化学、全岩Nd同位素及锆石Hf同位素数据，并阐明了其岩石成因与构造环境的演化过程。对辉长岩、闪长岩、花岗闪长岩及二长花岗岩的锆石U-Pb定年结果显示，其形成时代介于317~301 Ma之间。其中，形成于311 Ma的辉长岩可能起源于亏损MORB型软流圈地幔与少量古老地壳物质的混合源区的部分熔融；301 Ma的闪长岩则源自亏损岩石圈地幔与地壳组分的混合熔融；303 Ma的花岗闪长岩由幔源与壳源岩浆混合形成；317 Ma的二长花岗岩则来源于新生底侵玄武质下地壳的熔融。结合已有研究数据，我们将卡拉麦里缝合带沿线的侵入岩划分为三个期次：350~340 Ma、330~320 Ma以及320~300 Ma。第一期以拉斑质与高钾钙碱性花岗岩为主；第二期为强变形的高钾钙碱性I型花岗岩；第三期则发育大量岩性多样的岩石（以镁铁质与长英质岩石为主，包含大量A型花岗岩），指示一次大规模岩浆爆发事件。结合区域地质背景，从第一期到第三期的岩浆组合与演化序列，表明构造环境依次从俯冲/增生阶段（350~340 Ma），历经同碰撞阶段（330~320 Ma），最终转换至后碰撞（或后增生）阶段（320~300 Ma）。本研究为中亚造山带西南缘卡拉麦里缝合带的俯冲向碰撞（弧-弧碰撞）构造转换提供了新的岩浆学证据。