Geochronological and geochemical constraints on the petrogenesis of the Ailaoshan granitic and migmatite rocks and its implications on Neoproterozoic subduction along the SW Yangtze Block

Abstract

The Neoproterozoic tectonic pattern of the South China Block (SCB) is considered to be a key element in the Rodinia reconstruction and subsequent Gondwana assembly. Numerous studies have been done on the Neoproterozoic igneous rocks along the SE and NW Yangtze Block of the SCB, but the contemporaneous igneous rocks in the SW Yangtze Block has poorly been known so far. This paper documents a comprehensive set of U–Pb geochronological, elemental and Sr–Nd–Hf isotopic data for the granitic gneiss and migmatite from the Ailaoshan tectonic zone of the SW Yangtze Block on the basis of field investigations. These granitic rocks are mainly granodiorite, monzogranite and quartz diorite and spatially associated with the synchronous mafic and dioritic rocks. On the basis of the A/CNK values and CIPW-normative corundum contents, these samples are subdivided into Group 1 with A/CNK>1.1 and corundum of 1.5–6.5vol.% and Group 2 with A/CNK=0.87–1.09 and corundum=0–1.5vol.%, generally similar to the S- and I-type granitic rocks, respectively. The representative samples from Group 1 and 2 give the weighted mean ages of 810–785Ma and 790–764Ma, respectively, interpreted as the crystallization ages. They show strong Nb–Ta and P–Ti negative anomalies and variable Eu/Eu∗ (0.10–0.92). The εNd(t) and εHf(t) values are in the ranges of −6.5∼−4.2 and −3.8∼+3.1 for Group 1 and of −6.2∼+1.8 and −6.3∼+5.7 for Group 2, respectively. These Neoproterozoic granitic rocks might originate from a hybridization source of metagreywacke with proportionally “ancient” mafic component for Group 1 but with abundant juvenile mafic-derived rocks for Group 2. Our data, along with the available geological observations, demonstrate the extensive occurrence of the Neoproterozoic (761–833Ma) igneous rocks in the Ailaoshan tectonic zone. The previously-mapped Ailaoshan and Yaoshan groups might be the Complexes including the Neoproterozoic, Triassic and Cenozoic igneous rocks. A persistent and long-lived Neoproterozoic (∼830–760Ma) subduction zone along the southwestern and northwestern margins of the Yangtze Block from Ailaoshan to Panxi and then to Hannan is proposed. The Yangtze Block might locate at the periphery of Rodinia during the early Neoproterozoic.