Petrogenesis and tectonic implication of the lower Silurian high-Sr/Y subvolcanic rocks from the South Qilian suture zone in the Qilian Orogen, NW China

Abstract

As the remnant of the South Qilian Ocean, the South Qilian suture zone recorded abundant information on the Cambrian–Ordovician subduction history of the southern branch of the Proto-Tethyan Ocean. However, the closure timing of the South Qilian Ocean and subsequent collision are poorly constrained. In this study, we report early Silurian (433–435 Ma) U–Pb ages of felsic subvolcanic rocks from Lianhuashan, Ayishan and Shihuiyao of the Lajishan district within the South Qilian suture zone. They intruded the Late Ordovician – Silurian sedimentary or Late Ordovician volcanic rocks and have high SiO2 (61.43–73.06 wt%), Sr/Y ratios with significant different rare earth elements (REEs) and trace-element spider diagrams, and Sr–Nd isotopic compositions, probably implying that they were formed through distinctly different generation mechanisms. Geochemistry of the Lianhuashan dacites reveals compositions typical of adakitic rocks derived from partial melting of lower crust in a thickened setting. The Ayishan dacites were derived from partial melting of crustal materials with the involvement of minor peridotite mantle, and the Shihuiyao rhyolites were derived from partial melting of felsic crust. The similar geochemical characteristics of coeval post-collisional igneous rocks in the Central Qilian and South Qilian blocks indicates that the lower Silurian subvolcanic rocks were generated in a thickened crust of post-collisional setting. Considering their intrusive contacts with Late Ordovician – Silurian retro-foreland basin and Late Ordovician collisional volcanic rocks, we propose that the South Qilian suture zone was at a transitional stage from collisional to post-collisional during the early Silurian Period.

An Overview on the Composition and Age of Upper Crust of Proto‐Tethyan Lajishan Intra‐oceanic Arc, NE Tibet Plateau

“…It can be used to distinguish between fractional crystallization and partial melting (Fig. 10c; He et al, 2021). Felsic rocks exhibit an increase trend in La/Sm ratios in the La versus La/Sm diagram (Fig.…”

Geochemical and Isotopic Compositions of Fluorites from the Yama Fluorite Deposit in the Qilian Orogen in Northwest China, and Their Metallogenic Implications

“…The Qinghai-Tibet Plateau's northeastern edge hosts the QLO, which is bordered by the Alxa Block to the north and the Quanji massif (QM) to the south (Figure 1a) [4,6]. The QLO, the northernmost orogenic collage of the Tethyan domain, has an excellent record of the Early to Middle Paleozoic subduction-accretion that resulted from the convergence of the Alax and Qaidam blocks [1,6,9,25]. The North Qilian and South Qilian suture zones normally split the OLO into three tectonic belts, i.e., the NQB, CQB, and SQB (Figure 1a) [1,3,4,6,7,9,10,26].…”

“…The Qilian Orogen (QLO) is traditionally subdivided from north to south into the North Qilian Belt (NQB), the Central Qilian Block (CQB), and the South Qilian Belt (SQB) (Figure 1a) [1][2][3][4][5][6]. It is generally believed that the QLO has developed as a result of the closure of the Proto-Tethys Ocean [1,2,[6][7][8][9][10]. Because numerous granitic rocks in the QLO are thought to be associated with subducted, collisional, or post-collisional magmatism that was connected to the Proto-Tethys Ocean, these rocks have been studied in the past [1,4,6,[10][11][12][13][14][15][16][17][18][19][20].…”