Identification of ca. 520 Ma mid-ocean-ridge–type ophiolite suite in the inner Cathaysia block, South China: Evidence from shearing-type oceanic plagiogranite
Authors: Li Longming , Lin Shoufa , Xing Guangfu , Xiao Fan , Xiao Wenjiao 
Published: October 2021

An ophiolite suite, predominantly composed of residual mantle peridotites, mid-ocean-ridge basalt (MORB)–like ultramafic rocks, and oceanic plagiogranites, has been identified in the Zhenghe-Dapu fault zone, Cathaysia block, South China. The peridotites experienced strong serpentinization and are characterized by low 187Os/188Os ratios of 0.11621–0.12008 and very low 187Re/188Os values of 0.031–0.129, similar to those from highly refractory mantle residues. The meta-ultramafic rocks, mainly amphibolites, can be classified into two groups. Group I is characterized by high Ni and Cr and low K2O contents. Their spoon-like rare earth element (REE) patterns, along with lower concentrations of highly incompatible elements, indicate that the protolith was of cumulate origin. Group II displays depleted REE patterns and low Nb/Yb, Th/Yb, and Ti/V ratios, which are geochemically similar to normal (N) MORB. Both groups exhibit positive εNd(t) values (1.2–4.1) and relatively high (87Sr/86Sr)i ratios (0.7046–0.7096), suggesting their origin from partial melting of depleted mantle sources in a mid-ocean-ridge setting that experienced a greater extent of fluid-rock alteration. The meta-plagiogranites intercalated with the mylonitic amphibolites are characterized by low K2O (0.09–0.21 wt%) and total REE contents along with low K2O/Na2O and Rb/Sr ratios, which are consistent with those of typical oceanic plagiogranite. They exhibit strongly positive zircon εHf(t) values (+9.5 to +15.1) and positive whole-rock εNd(t) values (+2.8 to +3.6). Their extremely low MgO (0.6–1.65 wt%), Cr (0.22–6.26 ppm), and Ni (0.77–4.74 ppm) compositions and low Mg# (22.4–31.9) preclude their origination from mantle-derived primary magma but favor oceanic crust. Low zircon δ18O values (4.02‰–5.4‰) and decoupled Sr-Nd isotope features imply the involvement of high-temperature seawater alteration in their source region. The enriched light rare earth element (LREE) patterns with strongly positive Eu anomalies, similar to the East Karmøy–type plagiogranite in western Norway, imply that the plagiogranites were derived from anatexis of amphibolite in an active shear zone near the mid-ocean ridge. The magmatic and metamorphic zircons from the meta-plagiogranites yield nearly identical secondary ion mass spectrometry (SIMS) U-Pb ages ranging from 523 to 521 Ma and from 522 to 518 Ma, respectively. The simultaneous magmatism and metamorphism also signify an active high-temperature shear zone, where the plagiogranites were formed slightly later than the oceanic crust. The age of ca. 520 Ma represents the formation age of the oceanic crust rather than its emplacement age. The identification of the ca. 520 Ma ophiolite suite along the Zhenghe-Dapu fault zone indicates that the Cathaysia block consisted of at least two different terranes rather than a single tectonic unit in the Cambrian, and the final amalgamation of the eastern and western Cathaysia block may have occurred far later than ca. 520 Ma, most likely during the late early Paleozoic.