Despite numerous years and avenues of investigation, the timing, evolution, and mechanism of lithospheric thinning of the North China Craton (NCC) are still highly debated. Late Triassic igneous rocks especially mantle derived mafic rocks could provide critical information related to the post-collisional lithospheric thinning of the Liaodong Peninsula in the eastern NCC. Using newly acquired geochronological data points, we determined that the intrusive rocks have emplacement ages of 219–209 Ma, which are typical of Late Triassic magmatism. The studied Qinghekou gabbros have low SiO₂ (49.43–51.86 wt.%), Na₂O+K₂O (3.59–7.20 wt.%), high MgO (3.70–8.54 wt.%) and transition metal element contents, with Mg^# values ranging from 49.2 to 69.3. While these samples are enriched in large ion lithophile elements such as Rb, Ba, and K and light rare earth elements, they are depleted in high field strength elements such as Nb, Ta, Ti, and P and heavy rare earth elements. We conclude that these gabbros samples were formed in a subduction-related tectonic setting, where the primary magmas were derived from partial melting of the lithospheric mantle that experienced metasomatism via slab-related fluids. Our Tongyuanpu granite samples have I-type granite affinity, as indicated by their high SiO₂ (73.60–75.30 wt.%) and low MgO (0.08–0.32 wt.%) contents, as well as the A/CNK values of 0.99–1.25 and the zircon saturated temperature values of 757–832°C, the parental magmas for these granites were generated by partial melting of the lower crust. Combined with the spatio-temporal distribution and rock-assemblages of the Late Triassic intrusive rocks, we conclude that these rocks were formed in an extensional environment characterized by lithospheric thinning caused by lithospheric delamination after the collision between the NCC and the Yangtze Craton. Furthermore, we infer that the initial lithospheric thinning of the eastern NCC occurred during the Late Triassic. Graphical Abstract

A previously unidentified overthrust system has recently been discovered in the Ondor Sum area at the boundary between the Solonker – Xar Moron Suture Zone (SXSZ) and the Early to Mid – Paleozoic Subduction – accretion Belt to the south. The overthrusting occurred during the middle Permian and was overprinted by late Permian to Middle Triassic dextral shearing. The overthrusting (D1) was characterized by the development of WSW – ENE-trending folds and reverse faults, implying NNW – SSE shortening. The later deformation (D2) involved WSW – ENE dextral strike-slip shearing with a minor reverse component, as indicated by slightly plunging striae on slickensides. A comparison of geochronological and structural data for ophiolites in the SXSZ suggests that the overthrust system formed in response to the closure of the Solonker Ocean and the collision along the SXSZ. The earlier NNW – SSE shortening and later WSW – ENE dextral shearing indicate that a tectonic transformation took place along the SXSZ from the middle Permian to the Late Triassic. Dextral and sinistral strike-slip faults are, respectively, distributed to the south and northwest of the Songliao – Xilinhot Block to the north of the SXSZ, implying eastward extrusion of the Songliao – Xilinhot Block after the closure of Solonker Ocean. The tectonic transformation and extrusion might have been a part of the intracontinental tectonic transformation of the eastern segment of the southern Central Asian Orogenic Belt.

中国西南地区在早古生代期间位于冈瓦纳大陆北缘,该地区保存有早古生代岩浆岩记录,它们的形成与原特提斯洋的演化密切相关。本文报道了滇西南地区保山地块平达乡附近的早古生代花岗岩的岩石学、全岩主-微量地球化学、LA-ICP-MS锆石U-Pb年龄、锆石Hf同位素和磷灰石Nd同位素组成,以探讨保山地块早古生代花岗岩的岩石成因、岩浆源区性质以及岩体形成的地球动力学机制。通过对保山地块平达花岗岩中岩浆锆石开展LA-ICP-MS U-Pb同位素测试,结果显示花岗岩形成于465.9~454.9Ma,是中-晚奥陶世岩浆作用的产物。平达岩体主体岩性为二长花岗岩,主要矿物组成为斜长石、石英和碱性长石(条纹长石和正长石),次要矿物为黑云母和白云母。本文报道的花岗岩样品显示出高的SiO₂(72.16%~76.87%)和Al₂O₃(12.50%~14.36%)含量,低的MgO(0.12%~0.64%)、Mg^#(20.9~35.9)、Cr和Ni含量以及较高的Rb/Sr和Rb/Ba比值;结合花岗岩较高的A/CNK值(1.06~1.45),显示弱过铝质至强过铝质的特征,表明其属于S型花岗岩。此外,平达岩体二长花岗岩样品具有低的Al₂O₃/TiO₂比值和变化的CaO/Na₂O、Rb/Sr、Rb/Ba比值,表明其岩浆源区包含泥质岩石和砂质岩石,显示混合源区的特征。结合滇西南早古生代二长花岗岩变化较大的锆石ε_Hf(t)值(-11.10~-4.05)、磷灰石ε_Nd(t)值(-8.76~-6.96)和Hf同位素二阶段模式年龄(2140~1704Ma),表明它们的岩浆源区为古元古代地壳中泥质岩石和砂质岩石混合源区。综合滇西南地区广泛发育的早古生代岩浆岩以及同时期的蛇绿混杂岩记录,本文认为保山地块早古生代S型花岗岩是地壳沉积组分部分熔融的产物,形成于原特提斯洋西向俯冲相关的活动大陆边缘背景。

滇西保山地块是东特提斯构造域的主要微陆块之一，但对其物源和古地理位置仍存在较大争议。本文通过对保山地块西缘早古生代地层进行碎屑锆石U-Pb定年来约束其物源及古地理位置，并进一步探讨原特提斯洋早古生代构造演化模式。保山地块西缘早古生代地层具有相似的年龄分布模式，主年龄峰期为~0.95Ga、次级年龄峰期为~1.2Ga和~2.5Ga。寒武系公养河群最小锆石年龄为526Ma，结合其上部年龄为499.2Ma的火山岩夹层，约束其沉积时代为早寒武世早期。对比保山地块不同区域早古生代地层的碎屑锆石年龄数据，它们都具有相似的锆石年龄分布模式和年龄峰值。~0.95Ga主年龄峰期和~2.5Ga的次级年龄峰期指示保山地块早古生代的沉积物主要来自于印度大陆，而~1.2Ga的次级年龄峰期表明有部分沉积物来自于西澳大利亚，其早古生代古地理位置位于印度和西澳大利亚之间。结合沉积学证据及滇西地区广泛发育的早古生代岩浆作用，本文认为早古生代冈瓦纳大陆北缘为活动大陆边缘。