Early Paleozoic subduction-collision orogeny in South China: Evidence from Silurian meta-mafic rocks in the Cathaysia block

To better comprehend the nature and geodynamic processes of the early Paleozoic orogeny in the South China block, comprehensive geochronological, geochemical, and isotopic studies were conducted on meta-mafic rocks, specifically the Dongyue, Shangyan, and Jiumu amphibolites, from the NW Fujian region within the Cathaysia block. Secondary ion mass spectrometry and laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating revealed that these mafic rocks crystallized between 440 Ma and 432 Ma and underwent metamorphism between 426 Ma and 421 Ma. Geochemical assessments highlighted distinct characteristics among the amphibolites. Dongyue amphibolites exhibit SiO2 contents of 50.6−53.3 wt%, TiO2 of 0.47−0.88 wt%, alkalis (Na2O + K2O) of 1.71−3.9 wt%, Mg# of 71.5−78.2, Ni of 110−159 ppm, and Cr of 799−1309 ppm, suggesting primary magma compositions. In contrast, Shangyan amphibolites display variable SiO2 (47.6−55.6 wt%), higher alkalis (3.77−7.4 wt%), and reduced Mg# (44.2−58.5), Ni (16.1−106 ppm), and Cr (21−371 ppm), along with enriched Nb contents (8.19−25.2 ppm), indicating evolved magma sources. Jiumu amphibolites exhibit intermediate compositions, with SiO2 of 51.1−59.4 wt%, alkalis of 2.29−3.73 wt%, Mg# of 63−69.1, Ni of 17−68.6 ppm, and Cr of 47.1−507 ppm. These variations imply different fractionation processes involving olivine, clinopyroxene, plagioclase, apatite, and Fe-Ti oxides during magma evolution of the protoliths. All amphibolites display right-inclined chondrite-normalized rare earth element patterns with negative Eu anomalies, large ion lithophile element enrichment, and high field strength element depletion with pronounced negative Nb-Ta anomalies. Geochemical data suggest that the sources of the Dongyue and Jiumu mafic rocks were influenced by subduction-related fluid metasomatism, whereas Shangyan mafic rocks experienced fluid metasomatism with minimal melt metasomatism. They share similar initial Sr-Nd isotopic compositions with low εNd(t) values (−12.7 to −4.9), negative εHf(t) values (−24.4 to −1.96), and old single-stage depleted mantle model ages (TDM1) of 2007−1121 Ma, indicating derivation from an enriched lithospheric mantle formed during the Paleoproterozoic to Mesoproterozoic. The presence of these Silurian meta-mafic rocks supports the interpretation that the early Paleozoic orogeny was a subduction-collision event. Given the subdivision of the Cathaysia block into the Eastern and Western Cathaysia blocks along the Zhenghe-Dapu fault zone, coupled with the accretion of Cambrian mid-ocean-ridge−type ophiolites (ca. 520 Ma) and Ordovician continental margin arc volcanic rocks (ca.470 Ma) within this fault, we propose that the early Paleozoic orogeny in South China resulted from westward oceanic subduction from the Middle Ordovician to the early Silurian, followed by collision between the West and East Cathaysia blocks by the middle Silurian, with the Zhenghe-Dapu fault zone acting as the suture zone.