Geochemical and Geochronological Constraints on the Origin and Emplacement of the East Taiwan Ophiolite

Abstract

The East Taiwan Ophiolite (ETO) occurs as blocks and thrust sheets associated with the Lichi Mélange in the Coastal Range of eastern Taiwan. The blocks consist of serpentinized harzburgite, serpentinite breccia, gabbro, dikes of dolerite and plagiogranite, pillow basalts, and red clay within a mud‐ and serpentinite‐rich mélange matrix. New U‐Pb zircon dating of a pegmatite gabbro yields a weighted mean age of 16.65 ± 0.20 Ma. This age is earlier than the North Luzon Arc but overlaps with the late‐stage spreading of the South China Sea. ETO glassy basalt has low K2O, MgO and high CaO contents, similar to MORB. REE and trace element patterns show both N‐MORB patterns with LREE depletion and E‐MORB patterns with slight LREE enrichment. A few samples show slight depletion in Nb‐Ta and Ti and enrichment in Rb, Ba, U and Sr, indicating a hint of subduction influence. Most ETO basalt plots within the overlapping fields of N‐MORB and BABB on Ti‐V, Cr‐Y, Nb/Yb‐Th/Yb, and Hf/3‐Th‐Ta discrimination diagrams. These geochemical compositions are emblematic of mid‐ocean ridge or back‐arc lava, like South China Sea basalt. We interpret ETO basalt and gabbro as fragments of the subducted South China Sea basement that were scrapped off and accreted to the Luzon forearc during the process of subduction initiation along the Manila Trench. Blocks of mantle material in the mélange may originate from the upper plate of the arc‐continent collision and were mixed with lower plate crustal material in a subduction channel now represented by the Lichi Mélange.

The Yuli Belt in Taiwan: Part of the suture zone separating Eurasian and Philippine Sea plates

“…This is supported by more recently obtained U-Pb zircon ages of c. 14.2 Ma in the Chimei complex (Shao et al 2015). Geochronological studies on the East Taiwan Ophiolite (ETO) contained Lichi Mélange yielded U-Pb magmatic zircon ages of 14 -17 Ma (Shao et al 2015;Hsieh et al 2017;Huang et al 2018;Lin et al 2019). Similar ages of 15.6 ± 0.3 and 16.0 Ma were also obtained by U-Pb dating of zircons from blueschists in the Yuli Belt (Chen et al 2017), suggesting very similar early Middle Miocene protolith ages as for the mafic rocks in the ETO ( 9,10,11,12,13,14 tuff, basaltic andesite, Mn-rich pelagic clay?~1 5 Ma 16,18 Lichi Mélange…”

“…2). The geochemical composition of basalts and gabbros in the ETO has affinities towards a mid-ocean ridge or back-arc basin setting (Lin et al 2019), although the debate about the origin of the Lichi Mélange is ongoing (e.g., Chang et al 2000;Huang et al 2018). Based on these geochemical data and ages, some of the metamorphic rocks in the Yuli Belt probably have affinities with the ETO at the base of the Lichi Mélange.…”

“…(1) Available protolith ages for the blueschists, constrained from U-Pb LA-ICP-MS dating of zircons, are around 15.4 -16.0 Ma (Chen et al 2017;Lo et al 2020), mak-ing them well comparable in age with gabbros from the easterly adjacent, unmetamorphosed ETO accreted to the base of the Lichi Mélange (Hsieh et al 2017;Huang et al 2018;Lin et al 2019; Table 1 and Fig. 2).…”

P‐wave Tomography and Azimuthal Anisotropy of the Manila‐Taiwan‐Southern Ryukyu Region

“…Apart from the slab breakoff beneath northern Taiwan, the subducted EP rollback may cause horizontal subslab mantle flow beneath southwest Taiwan and the northeast SCS at depths of 200–600 km as revealed by our AZA tomography (Figures 5e–5g), which was also suggested by analogue experiments (Strak & Schellart, 2014). The newest geochemical and geochronological data suggest that the EP subduction along the northern Manila Trench may start from ∼17 Ma (C. T. Lin et al., 2019). Due to trench retreat and slab rollback during subduction, the mantle material beneath the subducted EP would be squeezed and escape to adjacent areas (Strak & Schellart, 2014), forming subslab horizontal mantle flow from the Taiwan Strait to the northeast SCS.…”

Strain Partitioning and Exhumation in Oblique Taiwan Collision: Role of Rift Architecture and Plate Kinematics

“…The Coastal Range to the east represents the northern Luzon Arc accreted to the collided margin. The Longitudinal Valley Fault is the plate boundary fault between the Coastal Range and the Central Range, along which the Lichi mélange is exposed (Chang et al, 2000;Ho, 1986;Huang et al, 2018;Jahn, 1986;Lin et al, 2019). The metamorphic core of the Central Range comprises two metamorphic units: the Tailuko Belt (TB) and the Yuli Belt (YB).…”