SYN-convergent extension in the early Paleozoic orogen, South China Block : insights from granitoids

Abstract

The nature and tectonic evolution of the early Paleozoic orogen in South China (i.e., the Wuyi-Yunkai orogen, WYO) remains ambiguous. this study provides significant new constraints on these issues via studying some Tonalite-Trondhjemite- Granodiorite (TTG) suite, mafic microgranular enclave (MME)-bearing granite and gneissic granites in the WYO. Host and MME samples from the Heping pluton yield LA-ICP-MS zircon U-Pb ages of ca. 445 Ma. The host granodiorite is metaluminous (A/CNK = 0.68-0.91), belonging to I-type granite, most host and MME samples share similar (87Sr/86Sr)i ratios of 0.7089-0.7112 and εNd(t) values of -6.52 to -7.12, except one MME sample with a lower (87Sr/86Sr)i of 0.70758 and a higher εNd(t) of -4.33. The zircon εHf(t) values exhibit a wide range from -13.68 to -0.87. Petrological, geochemical, and chronological data suggest that the Heping pluton were generated by mingling of mafic and felsic magmas. The mafic magma was originated mainly from the enriched subcontinental lithosphere mantle at the spinel-garnet transitional zone (~70 km), subsequently underplated/intraplated into the lower-middle crust resulting in the melting of the intermediate rock at pressure <8 kbar to produce the felsic endmember magma. TTGs are distributed in the northern and eastern margin of the Yunkai domain, composed mainly of biotite tonalite, biotite-hornblende tonalite, biotite granodiorite and minor trondhjemite, with a formation age of 451.4-440.6 Ma, low Sr contents and La/Yb ratios, moderate to low εNd(t) ( -2.57 to -9.3), and variable zircon εHf(t) (mainly of +7.5 to -7.5), are considered to be generated by fluid-fluxed partial melting or/and fluid-absent melting of newly underplated basaltic crust, within an intracontinental post-collision environment (related to syn-convergent extension and asthenosphere upwelling) rather than a subduction setting. The partial melting progressed at temperatures ca. 750-820°C and at pressure of around 8 or 11 kbar (corresponding to the northern and eastern margin of Yunkai domain, respectively). Eight gneissic granite plutons/veins in the Wuyi and Wugong domains yield ages of 453-438Ma, and have A/CNK, εNd(t) and zircon εHf(t) values of 1.09 to 1.27, -6.79 to -13.45 and -2.0 to -12.4, respectively, belonging to S-type granite, and generated mainly by partial melting of reworked late Paleoproterozoic basement. The partial melting is primarily via dehydration of biotite at temperatures ca. 745 to 790°C, and at pressure about 11 kbar in the Wuyi domain and <8 kbar in the Wugong domain. The gneissic and associated massive granites in the Wuyi-Yunkai orogen were all formed in the intracontinental post collisional environment with alike age ranges and without any obvious temporal evolutionary order, but distributed in different tectonic domains, i.e., the gneissic granites distributed in compressional areas with syn-emplacement gneissosity, and the massive granites distributed in syn-convergent extensional zones. A new tectonic model “syn-convergent extension and asthenosphere upwelling during the intracontinental orogeny” is proposed. The syn-convergent extension zones, which including a series of NW-trending transverse faults and the NE-trending reactivated pre-existing suture and rift zone (i.e., Shaoxing-Jiangshan-Pingxiang- Chenzhou fault) within the WYO, are in favor of asthenosphere upwelling leading to intensive crust-mantle interaction.