Different response to middle-Palaeozoic magmatism during intracontinental orogenic processes: evidence from southeastern South China Block

Intracontinental orogens are major sites to better understand tectonic regime of plate interior. However, deep response to those processes remains poorly constrained due to the rare exposure of magmatic rocks. Middle-Palaeozoic magmatic rocks from the eastern Yangtze Block (EYB) and Cathaysia Block (CB) of the southeastern South China Block (SCB) have markedly unimodal age distribution with peaks at ~424 and ~438 Ma, respectively. The zircon saturation temperatures (T–Zr) range from 699 to 920°C, and increase in granitic rocks of younger age in the CB; however, most of the granitic T–Zr values in EYB are low and relatively uniform (643–855°C, most of them lower than 800°C). In CB, the Nd-Hf isotopic ratios of the granitic rocks (εNd(t) = – 13.2 to + 1.0; magmatic zircon εHf(t) = – 36 to + 12) are variable, nonetheless they clearly show the presence of a juvenile component. On the contrary, Nd isotopic ratios (εNd(t) = – 7.8 to + 1.6) of mafic rocks in EYB abruptly increase in rocks of younger age. Within the long-lived intracontinental orogenic processes of EYB and CB, consistent age peaks between metamorphism and magmatism and the abrupt increase in contributions of juvenile materials suggest that the thickened lithospheric roots were delaminated into asthenospheric mantle as many independent parts below the metamorphic domes in CB. The data suggest that the deep response to the intracontinental orogenic processes was strong non-uniform between EYB and CB in terms of magmatic period and contribution of mantle derived material to granitic magmas (related to the crustal reworking). We deduced that the combined influence of far-field stress transmission (derived from the convergence of Australia and the SCB), local delamination event in CB and different degree of reworking of the crustal sources, is the main reason to the different responses between EYB and CB.