A mid-Palaeozoic ocean–continent transition in the Mazongshan subduction–accretion complex, Beishan, NW China: new structural, chemical and age data constrain the petrogenesis and tectonic evolution

Abstract

Accretionary orogens contain key evidence for the conversion of oceanic to continental crust. The late tectonic history and closure time of the Palaeo-Asian Ocean are recorded in the Mazongshan subduction–accretion complex in the southern Beishan margin of the Central Asian Orogenic Belt. We present new data on the structure, petrology, geochemistry and zircon U–Pb isotope ages of the Mazongshan subduction–accretion complex, which is a tectonic mélange with a block-in-matrix structure. The blocks are of serpentinized peridotite, basalt, gabbro, basaltic andesite, chert and seamount sediments within a matrix that is mainly composed of fore-arc-trench turbidites. U–Pb zircon ages of two gabbros are 454.6 ± 2.5 Ma and 434.1 ± 3.6 Ma, an andesite has a U–Pb zircon age of 451.3 ± 3.5 Ma and a tuffaceous slate has the youngest U–Pb zircon age of 353.6 ± 5.1 Ma. These new isotopic ages, combined with published data on ophiolitic mélanges from central Beishan, indicate that the subduction–accretion of Beishan in the southernmost Central Asian Orogenic Belt lasted until Late Ordovician – Early Carboniferous time. Structure and age data demonstrate that the younging direction of accretion was southwards and that the subduction zone dipped continuously to the north. Accordingly, these results record the conversion of oceanic to continental crust in the southern Beishan accretionary collage.

Early Paleozoic Ocean Plate Stratigraphy of the Beishan Orogenic Zone, NW China: Implications for Regional Tectonic Evolution

“…Within the four mainly ophiolite zones, there are numerous ophiolitic blocks with variable ages, from north to south, namely the Hongshishan ophiolite zone, the Jijitaizi-Xiaohuangshan ophiolite zone, the Hongliuhe-Niujuanzi-Xichangjing tectonic mé lange zone and the Liuyuan ophiolite zone ( Fig. 1c; Xiao et al, 2010;Zhang et al, 2018;Wang et al, 2020).…”

“…The longest, E-W trending Hongliuhe-Niujuanzi-Mazongshan-Xichangjing (HNMX) tectonic mé lange zone is exposed in the central part of the Beishan orogenic zone, which is fault-bounded with two neighboring tectonic units: the Gongpoquan arc unit to the north (Wang X Y et al, 2018;Zheng et al, 2018Zheng et al, , 2019 and the Huaniushan arc unit to the south (Liu et al, 2011;Mao et al, 2012;Guo et al, 2014Guo et al, , 2017. The HNMX is composed mainly of several geological units, such as fragments of island arcs, crustal debris, trench slope turbidites, and pelagic and forearc sediments (Zhang et al, 2008;Hu et al, Ao et al, 2012;Tian et al, 2014;Sun et al, 2017;Wang S D et al, 2017Wang S D et al, , 2018Wang et al, 2020). All these units were likely formed from Late Neoproterozoic to Paleozoic, through convergent tectonics that took place off the southern margin of the southern Altaids.…”

“…(a) Field photographs of the radiolarian siliceous mudstone from the southern Dahulukou Section (S15); (b) photomicrographs of the radiolarian siliceous mudstone from the southern Dahulukou Section (S15); (c, d) the recrystallization radiolarian fossils from the bedded cherts in the southern Dahulukou Section (S15). zone in the central portion (Wang et al, 2020). In this paper, we reconstruct the Xichangjing-Yueyashan ophiolite mé lange (data from Hu et al, 2016), the Mazongshan accretion complex (data from Wang et al, 2020; shown in Fig.…”

“…The Mazongshan accretion complex occurs widely to the south of Mazongshan town, composed mostly of unmetamorphozed to weakly metamorphosed ultramaficbasic rocks, andesites, marbles, sandstones with amount of tuffaceous slates (Wang et al, 2020). At outcrop scale, these blocks occur as several fault-bounded slices.…”

“…The Beishan orogenic zone, as the distal segments of the CAOB, encompasses the final attachment of the Tarim and North China cratons to the southern Altaids (Zuo et al, 1991;Liu et al, 1995;Gong et al, 2002;Windley et al, 2007;Xiao et al, 2010;Yang et al, 2010;Wilhem et al, 2012;Zhang et al, 2018). The complexity of the evolution and tectonic settings within the Beishan orogenic zone has stimulated much debate recently, especially in the areas of high-resolution geochronology and geochemistry (Zhang et al, 2008;Liu et al, 2011;Mao et al, 2010Mao et al, , 2011Mao et al, , 2012Qu et al, 2011;Ao et al, 2012Ao et al, , 2016Song et al, 2013Song et al, , 2014Song et al, , 2015Song et al, , 2016Zheng et al, 2013Zheng et al, , 2018Zheng et al, , 2019Guo et al, 2014;Tian et al, , 2016Wang et al, 2014Wang et al, , 2016Hu et al, 2015;Sun et al, 2017;Chen et al, 2017a, b;Wang S D et al, 2017Wang S D et al, , 2018Shi et al, 2018;Wang X Y et al, 2018;Wang et al, 2020).…”