Identification of subduction to post-collisional tectonic transitions is critical to the study of orogenic belts. To characterize such a transition in the Tianshan Orogenic Belt, a systematic study was conducted on the late Carboniferous (305–301 Ma) Hongshankou dolerite and Dikan’er basalt of Eastern Tianshan. The Hongshankou dolerites have relatively high Ti and Nb contents, akin to Nb-enriched arc basalts. Based on the Nb/La ratios, these dolerites can be divided into low-Nb/La (0.35–0.40) and high-Nb/La (0.67–1.4) groups, which were likely derived respectively from slab melt-metasomatized mantle wedge and a mixed mantle source involving depleted super-slab and enriched sub-slab asthenospheric components. Like the low-Nb/La dolerites, the Dikan’er basalts possess low Nb/La (0.42–0.46) ratios, suggesting a mantle source previously modified by slab components. In addition, the Dikan’er basalts have variable Nb contents and can be grouped into normal arc basalts and Nb-rich basalts that can be attributed to a common mantle source with different degrees of mantle melting, as demonstrated by the positive correlations of La/Sm with La and Nb. By integrating available data, two late Carboniferous belts of Nb-enriched mafic magmatism are recognized in the Eastern Tianshan, with one in the Yamansu arc (336–301 Ma) and the other in the Bogda Mountains (305–301 Ma). The former is characterized by low Nb/La (<0.6) ratios, reflecting derivation from mantle metasomatized by slab-derived melt during a subduction process; the latter exhibits an abrupt Nb/La increase from 0.6 to 1.4, indicating significant input of sub-slab asthenospheric mantle that was probably induced by slab break off. Accordingly, we propose that the tectonic transition from subduction to post-collision in the Eastern Tianshan occurred in the latest Carboniferous (305–301 Ma) and was marked by the abrupt input of deep and enriched asthenospheric mantle.

中文翻译：

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天山东部含Nb的镁铁质岩浆作用的两个晚石炭纪带：地幔来源不同和地球动力学意义

碰撞后构造过渡带俯冲的识别对于造山带的研究至关重要。为了表征天山造山带的这种过渡，对天山东部晚石炭世（305-301 Ma）的红山口白云岩和迪肯尔玄武岩进行了系统的研究。洪山口白云岩具有较高的Ti和Nb含量，类似于富含Nb的弧形玄武岩。根据Nb / La比率，这些白云母可分为低Nb / La（0.35–0.40）和高Nb / La（0.67–1.4）组，它们可能分别来自板状熔融变质的地幔楔和混合的地幔源，包括耗尽的超平板和富集的亚平板软流圈成分。像低Nb / La白云母一样，Dikan'er玄武岩具有低Nb / La（0.42-0.46）比 建议先前由板坯组件修改的地幔源。此外，Dikan'er玄武岩具有可变的Nb含量，可以分为普通弧形玄武岩和富Nb玄武岩，这可以归因于具有不同地幔融化程度的常见地幔源，如La /的正相关性所示。 Sm与La和Nb。通过整合现有数据，在东天山发现了两条晚晚的富含铌的镁铁质岩浆作用的石炭纪带，一条在Yamansu弧（336-301 Ma）中，另一条在博格达山（305-301 Ma）中。前者的特征是Nb / La（<0.6）比率低，反映了俯冲过程中由板状熔体交代的地幔的衍生；后者的Nb / La从0.6突然增加到1.4，表明可能由板块破裂引起的板下软流圈地幔的大量输入。因此，我们认为东天山由俯冲到碰撞后的构造过渡发生在最新的石炭纪（305-301 Ma），其特征是深部和富集的软流圈地幔的突然输入。