Metasomatism between serpentinite and pelitic schist in the Yuli belt, eastern Taiwan: fluid-rock interactions during subduction metamorphism

Metasomatic rocks in orogenic mélanges bear critical information about fluid-rock interactions and element mobilities during subduction processes. The Yuli belt contains a few mélange units that crop out high-pressure blocks of metaigneous rocks and serpentinites enclosed in metasedimentary rocks. Metasomatic rocks are found along contacts between the serpentinites and metasedimentary rocks. However, the protolith and formation of those metasomatic rocks are largely unknown. Meter-scale metasomatic zones occur at the contact between pelitic schists (PS) and serpentinites (SP) in the Tsunkuanshan area. Five zones from PS to SP are newly identified: (I) chlorite-albite schist, (II) amphibole-albite rock, (III) albite-chlorite schist, (IV) epidote-chlorite schist, and (V) chlorite-talc schist. Minor garnet and amphibole (glaucophane core - barroisite mantle - actinolite rim) are present in the zone I and II, respectively. Field and petrographic observations combined with whole-rock major elements data suggest that this rock association likely was formed by chemical exchanges between the SP and PS. However, the zone II shows enrichment of Si, Na, and Ca, but Al depletion relative to the other metasomatic rocks. This anomaly might be due to infiltration of external fluids. Rare earth element patterns of the PS, zone I, II, III, and IV are similar, indicating a similar protolith origin. Hence, the original boundary between the PS and SP is likely between the zone IV and V. We estimate the chemical mass balance from the PS to the metasomatic rocks (zone I, II, III, and IV) using the sparse isocon method (Kuwatani et al., 2020). The result shows that the chemical components in zone I, III, and IV are gained relative to the PS, whereas those in zone II are of loss. We interpret that the zone I, III, IV, and V were produced by diffusive exchanges of components between the PS and SP, whereas formation of the zone II was likely created by Na-Ca rich fluid infiltrations. The newly-found occurrence of glaucophane within the zone II indicates fluid-rock interactions during subduction metamorphism.

Keywords: Chemical mass balance, sparse isocon method, Na-Ca rich fluids, Yuli belt.