Petrogenesis of the Middle Devonian syenites in the Miyun–Huairou Region of Beijing and its tectonic implications

Abstract

Three Devonian magmatic rocks have been identified from the mid‐western part of the Yanshan Thrust‐and‐Fold Belt (YTFB). Here, we present new geochronological, Sr–Nd–Hf isotopic, and whole‐rock chemical data. Our new LA‐ICP‐MS U–Pb dating recognized syenite plutons of ca. 385 Ma, which include Laogongying, Shicheng, and Qiaomaiyu, respectively, around the Yunmeng Mountain in the Miyun–Huairou area of Beijing. Rocks from three plutons are characterized by low SiO2 and high alkali contents, especially high K2O, and they show syenite and foid syenite compositions. They display slight–middle light REE enrichment, no negative or positive Eu anomalies, strong depletion in Rb, Th, U, Nb, Ta, P, Zr, Hf, and Ti, enrichment in Ba, K, and Sr, low contents of Y and Yb, and high Sr/Y ratios. They have a relatively narrow range of isotopic compositions with initial 87Sr/86Sr ratios from 0.708203 to 0.710477, εNd(t) values from −12.26 to −13.28, and εHf(t) values from −19.92 to −17.96. This study shows the Middle Devonian intrusions were produced by fractional crystallization of a parental magma, which was derived by interaction between mantle‐derived alkaline magmas of a type I‐enriched mantle and pre‐existing continental crust. Tectonic background was likely emplaced in a back‐arc extension environment related to southward subduction of the Palaeo‐Asian oceanic plate or during the cessation of the subduction. Feldspar geothermometer and aluminium‐in‐hornblende barometry studies of the Middle Devonian plutons yielded emplacement temperature of ~700°C and pressure of ~6.5 kbar, indicating the emplacement depths of about 24 km. Combined with previous geobarometry results on the Middle Devonian plutons within the Inner Mongolia Palaeo‐uplift (IMPU) on the northern margin of the NCC, the strong uplift and exhumation the IMPU were identified during Middle Devonian to Late Triassic times. The exhumation was more than 4–5 km than those of the IMPU, resulting in widely absent of Palaeozoic strata within the YTFB.

Devonian bimodal volcanic rocks from the northeastern margin of the North China Block: Implications for post‐collisional extension and orogen‐craton boundary

“…In recent years, a number of Devonian alkaline intrusions (alkaline rocks and A‐type granites) and minor mafic‐ultramafic intrusions emplaced during 409 ± 2 Ma to 373 ± 6 Ma have been identified from Guyang to Chifeng in the northern margin of the NCB (Gong et al, ; He et al, ; Huang & Hou, ; Liu, Zhao, et al, ; Luo et al, ; Ni, ; Shi et al, ; Teng et al, ; Wang, Xiang, et al, ; Xu et al, ; Zeng et al, ; Zhang et al, ; Zhang, Zhang, et al, ; Zhang, Zhao, Liu, et al, ; Zhang, Zhao, Song, & Liu, ), these alkaline rocks constitute an east‐west‐trending alkaline magmatic belt along the northern margin of the NCB (Zhang et al, ). Compared with the Devonian intrusive rocks, the volcanic rocks have only been identified from the Chifeng and Aohan areas in the northeastern margin of the NCB, including the early‐middle Devonian rhyolitic tuff (404 ± 1 Ma; Liu et al, ) and meta‐rhyolite (388 ± 3 Ma; Wu et al, ), Late Devonian rhyolitic tuff, meta‐rhyolite, and meta‐basalt (366–359 Ma; Sun et al, ; Ye et al, ).…”

“…Controversy remains on the tectonic environment of the northern margin of the NCB at the early‐middle Devonian period due to minimal records of magmatic activities and deposition. Several diverse models have been proposed to interpreting the tectonic environment of the northern NCB in the Devonian, including back‐arc extension related to southward subduction of the Paleo‐Asian oceanic plate or cessation of the subduction (He et al, ; Xiao et al, ), post‐collisional extension attributed to slab break‐off (Zhang, Zhang, et al, ) and post‐collisional extension related to arc‐continent collision between the Bainaimiao island arc and the northern NCC in the period of late Silurian to early Devonian (Gong et al, ; Liu et al, ; Teng et al, ; Zhang et al, ; Zhang et al, ; Zhang, Zhao, Liu, et al, ). Some other researchers argued that closure of the Paleo‐Asian Ocean occurred during the Devonian; in other words, the collision between the Mongolian arc terranes and the northern NCB occurred at this period (Shao, ; Tang, ; Xu, Charvet, Chen, Zhao, & Shi, ).…”

“…It is noteworthy that numerous Devonian intrusive rocks have been identified recently along the northern margin of the NCB, and they are mainly distributed in the IMPU that is situated between the Bayan Obo‐Duolun‐Chifeng‐Kaiyuan Fault and Jining‐Shangyi‐Chicheng‐Pingquan Fault, with minor in the Yanshan fold‐and‐thrust belt (YFTB) south to the Jining‐Shangyi‐Chicheng‐Pingquan Fault (Gong et al, ; He et al, ; Huang & Hou, ; Liu, Zhao, et al, ; Luo et al, ; Ni, ; Shi et al, ; Teng et al, ; Wang, Liu, et al, ; Wang, Xiang, et al, ; Xu et al, ; Zeng et al, ; Zhang et al, ; Zhang, Zhang, et al, ; Zhang, Zhao, Liu, et al, ; Zhang, Zhao, Song, & Liu, ). The Devonian intrusive rocks consist predominantly of syenites, monzonites, and alkaline granites with some mafic‐ultramafic rocks and minor monzodiorites ranging in ages from 400 to 380 Ma, which consists of a ca.…”

“…Although most researchers agreed that the Devonian intrusive and volcanic rocks formed in an extensional environment (Gong et al, ; He et al, ; Liu et al, ; Teng et al, ; Zhang et al, ; Zhang et al, ; Zhang, Zhang, et al, ; Zhang, Zhao, Liu, et al, ; Zhang, Zhao, Song, & Liu, ), the reasons for the Devonian extension are still highly controversial. Some researches attributed the extension to back‐arc extension or slab break‐off during southward subduction of the Paleo‐Asian oceanic plate beneath the NCB (He et al, ; Zhang, Zhang, et al, ); other considered it as a result of arc‐continent collision between the Bainaimiao arc and the NCC (Gong et al, ; Liu et al, ; Teng et al, ; Zhang et al, ; Zhang et al, ; Zhang, Zhao, Liu, et al, ). Resolving the above controversies requires a better understanding on the tectonic affinity of the Bainaimiao arc belt and the nature of the continental margin in the northern NCB during the early Palaeozoic.…”

“…In recent years, some early‐middle Devonian alkaline intrusive rocks (syenites, monzonites), A‐type granites, and minor mafic‐ultramafic rocks (pyroxenites and gabbros) with emplacement ages of 400–380 Ma have been discovered in the northern margin of the NCB (Gong, Cai, Yan, Liu, & Liu, ; He et al, ; Huang & Hou, ; Liu et al, ; Luo et al, ; Ni, ; Shi et al, ; Teng, Yang, & Santosh, ; Wang et al, ; Wang, Xiang et al, ; Xu et al, ; Zeng et al, ; Zhang, Zhang, Jiang, Zhai, & Zhang, ; Zhang, Zhang, Zhao, & Liu, ; Zhang, Zhao, Liu, et al, ; Zhang, Zhao, Song, & Liu, ). Rock assemblage and geochemical compositions of these intrusive rocks suggest an extensional environment in the northern NCB during this period.…”