Geochemistry and Geochronology of the Accreted Mafic Rocks From the Hengchun Peninsula, Southern Taiwan: Origin and Tectonic Implications

Abstract

The accretionary prism of the Hengchun Peninsula at the southernmost tip of the active Taiwan orogen has newly emerged due to the transition from a subduction zone to an arc‐continent collision zone. The mafic rocks within the Hengchun accretionary prism, whose provenance has long been controversial, are highly indicative of the mantle nature and tectonic evolution around the northern Manila Trench. Based on geochemical analyses, the accreted basalts are classified as normal and enriched mid‐ocean ridge basalts (N‐MORB and E‐MORB) and alkaline ocean island basalts. The N‐MORBs, which exhibit geochemical signatures indicative of virtually unmetasomatized depleted MORB mantle and yield zircon U‐Pb ages of 22.8 ± 0.3 and 24.2 ± 1.1 Ma, were off‐scraped from the subducted oceanic crust of the South China Sea (SCS). The alkaline ocean island basalts are considered fragments of subducted seamount due to their geochemical similarity with seamount basalts from the SCS. The Hf‐Nd isotopic compositions of basalts from the north SCS may represent the contributions of recycled oceanic crust and the upwelling Hainan Plume. The E‐MORBs, which exhibit similar geochemical compositions as rocks from the Huatung Basin and yield an 40Ar/39Ar age of 135.0 ± 5.6 Ma, were removed from the overriding plate due to the excessive shear stress between the converging plates and were laterally transported by strike‐slip movement; alternatively, they were off‐scraped from the subducted Luzon forearc slices. The temporary uplift and collapse of the accretionary prism caused by seamount subduction may have been responsible for the genesis of the lenticular Shihmen Conglomerate.

Formation of Hengchun Accretionary Prism Turbidites and Implications for Deep‐water Transport Processes in the Northern South China Sea

“…Page and Lan, 1983;Pelletier and Stephan, 1986;Zhang et al, 2014). These basic gravels commonly show geochemical characteristics of N-MORBs, E-MORBs and OIBs, which confirm their affinities to normal oceanic crust or seamounts (Chen H Y et al, 2018;Yu et al, 2018;Tian et al, 2019). It has been speculated that they were possibly sourced from the West PSP over the Manila Trench (Chen H Y et al, 2018;Tian et al, 2019), or the SCS downward plate (Yu et al, 2018;Tian et al, 2019).…”

“…It has been speculated that they were possibly sourced from the West PSP over the Manila Trench (Chen H Y et al, 2018;Tian et al, 2019), or the SCS downward plate (Yu et al, 2018;Tian et al, 2019). The ages of these basic rocks range mainly between the late Oligocene and early Miocene (Zhang et al, 2016;Tian et al, 2019). Therefore, they were more likely from the SCS subducting oceanic crust, with lesser possibility from the Huatung Basin of the PSP (Tian et al, 2019).…”

Ages of ophiolitic rocks along plate suture in Taiwan orogen: Fate of the South China Sea from subduction to collision

“…The incorporation of the South China Sea ophiolitic materials in the Kenting Mélange occurred during plate boundary shearing that brought footwall rock into the mega-thrust fault zone of the Manila subduction zone (Lu and Hsu 1992;Zhang et al 2016;Tian et al 2019), since the Kenting Mélange is a relict subduction fault zone uplifted within the accretionary prism as the Hengchun Ridge/Peninsula (Chang et al 2009). Thus ophiolitic materials were fed into the matrix of the Kenting tectonic mélange from the onset of South China Sea subduction, which should be prior than the earliest known volcanism in the Luzon Arc of the Taiwan region (~15 Ma; Yang et al 1995;Lai et al 2017;or ~18 Ma;Huang et al 2018), until the trench jump (Plio-Pleistocene; Chang et al 2009).…”

“…The destruction of the ~100 km-wide Luzon forearc crust is essential in bringing the Luzon Arc to collisional contact with the exhumed continental margin (Lu and Hsu 1992;Shyu et al 2005); tectonic erosion through underthrusting beneath the arc is the currently favored mechanism (Chang et al 2001;Shyu et al 2011;Chen et al 2017a). Of the Taiwan ophiolite-bearing belts, only the Kenting Mélange was the direct product of South China Sea subduction and the ophiolitic rocks within are mostly of South China Sea affinity (e.g., Lu and Hsu 1992;Chang et al 2003;Zhang et al 2016;Tian et al 2019); debates remain for the origin of the ophiolitic rocks in the Lichi Mélange as either Philippine Sea Plate/Luzon forearc (e.g., Juan et al 1980;Malavieille et al 2002;Chang et al 2009;Huang et al 2018) or South China Sea (e.g., Chung and Sun 1992;Hsieh et al 2017); the mafic blocks within the Yuli Belt have been inferred from the South China Sea (Yui et al 2014;Chen et al 2017b;Beyssac et al 2008) but lack further constraints. Such ambiguities made tectonic interpretation and reconstruction difficult and fraught with uncertainties, especially for the Lichi-Yuli pair on the opposing sides of the active plate boundary, the Longitudinal Valley (Fig.…”

Fingerprinting subducted oceanic crust and Hainan Plume in the melt sources of Cenozoic Basalts from the South China Sea Region

“…What is the exact emplacement time of the Hainan Plume? To address these challenging issues, we conduct a systematic study on a comprehensive compilation of the elemental and isotopic data of regional Cenozoic basalts (Tian, Yan & Yu, 2019, and references therein), including those of accreted basalts from the Hengchun Peninsula, Taiwan (Tian, Yan, Huang, et al, 2019).…”