Earliest Neoproterozoic (ca. 1.0 Ga) arc–back-arc basin nature along the northern Yunkai Domain of the Cathaysia Block: Geochronological and geochemical evidence from the metabasite

The tectonic nature of the Cathaysia Block during earliest Neoproterozoic period has been long disputed and attracted considerable attentions. One reason for this problem is rare exposure and few data for the igneous rocks of Grenvillian age in this Block. In this paper, a set of new U–Pb zircon geochronological, elemental and Nd isotopic data are presented for the metabasites in the Yunkai Group which shows WNW-trending extension with a length of more than 100 km along the northern Yunkai Domain of the Cathaysia Block. The plagioclase amphibolite and metabasite from Shiwo (Beiliu) and Tantu (Xinyi) from the northern Yunkai Domain, representative of Groups 1 and 2, yielded the 206Pb/238U weighted mean ages of 997 ± 21 Ma and 978 ± 19 Ma, respectively, evidencing the presence of the Grenvillian mafic rocks within the Cathaysia Block. Based on the Nb, Nb/La and Nb/U, these samples can be geochemically subdivided into two groups (Groups 1 and 2). Group 1 has Nb/La = 0.33–0.54 and Nb = 1.71–2.85 ppm, and exhibits similar geochemical signatures to those of Saunders island-arc. Group 2 gives Nb/La of 0.57–0.86 and Nb of 7.58–12.70 ppm, resembling to those of Phanerozoic Nb-enriched basalts, and shows the geochemical affinity to the Okinawa back-arc basin basalt. Both groups have positive ɛNd(t) values, ranging from +3.84 to +5.36 and +3.93 to +6.55, respectively. These geochemical and isotopic characteristics suggest that Group 1 might have been originated from a MORB-like source modified by subduction-derived fluids. The source of Group 2 Nb-enriched basalt might be metasomatized by melt released from a newly subducted slab. In conjunction with other geological observations, it is identified for the Grenville-aged (ca. 1.0 Ga) arc and back-arc basin along the northern Yunkai Domain of the Cathaysia Block, analog to the modern West Pacific. The South China Block probably located at the margin of Rodinia supercontinent between west Australia and east India.

► Two metabasites from the northern Yunkai gave the weighted mean ages of 978–997 Ma.
► Group 1 is similar geochemically to Saunders island-arc and Group 2 to Nb-enriched basalt.
► Both groups originated from MORB source newly modified by subduction-derived fluid/melt.
► A Grenvillian arc–back-arc basin was developed along the southern Cathaysia Block.