To better reveal the tectonic process of the Jiangnan Orogen in creating the united South China Block (SCB), a set of new geochronological, geochemical, and Sr-Nd-Hf-O isotopic data is herein presented for the coeval early Neoproterozoic mafic to acid rocks in the Lengjiaxi Group and its equivalents. Our data show that these rocks belong to subalkaline basalt, basaltic andesite, andesite, and dacite in composition, and can be geochemically classified into N-MORB- and arc-like mafic (Group 1 and Group 2, respectively), along with high-Mg andesitic-dacitic (Group 3) rocks. They yield zircon U-Pb ages of 837 ± 4 Ma, 832 ± 11 Ma, and 835 ± 7 Ma, respectively, suggesting their early Neoproterozoic (ca. 835 Ma) origin. Group 1 has SiO2 = 47.21−52.45 wt%, MgO = 7.36−10.07 wt% with Mg-number of 56−68, 87Sr/86Sr(t) = 0.7062−0.7136 and εNd(t) = +1.3−+9.4, along with N-MORB-like REE- (rare earth element) and PM (primitive mantle)-normalized patterns, suggesting its derivation from a refractory MORB-like source newly introduced by slab-derived fluid. The Group 2 samples are classified as tholeiitic basalt and basaltic andesite with SiO2 = 48.78−55.75 wt%, MgO = 7.70−12.74 wt%, Mg-number = 63−77, 87Sr/86Sr(t) = 0.7046−0.7098 and εNd(t) = −1.6 to −0.4. They exhibit similar “spiky” REE- and PM-normalized patterns to typical arc basalt, with pronounced enrichment in (large ion lithophile elements) and sharp depletion in (high field strength elements), probably inheriting from a sub-arc source modified by fluids and/or melts released from recycled sedimentary component. The Group 3 rocks show SiO2 = 57.74−66.30 wt%, FeOt = 6.22−8.59 wt%, Mg-number = 43−66, K2O+Na2O = 3.18−6.17 wt% with K2O/Na2O of 0.59−3.28, classified as high-Mg andesites and dacites. Diagnostic elemental and isotopic ratios of Group 3 include (La/Yb)cn = 7.7−11.4, (Gd/Yb)cn = 1.44−1.87, Eu/Eu* = 0.49−0.80, Nb/La = 0.27−0.42 and εNd(t) = −7.3 to −5.4, resembling to those of other early Neoproterozoic high-Mg intermediate rocks in the SCB, originating from a mantle wedge source with proportional addition of the recycled SCB sediment-derived components. In combination with the available data and geological observations, it is concluded that the N-MORB- and arc-like mafic, along with high-Mg andesitic rocks are coeval in the Lengjiaxi Group and its equivalents along the central Jiangnan Orogen and formed at ca. 847−832 Ma of the early Neoproterozoic period. The synthesis of these observations suggests the development of an early Neoproterozoic continental arc-basin setting. The arc-basin system westerly subducted and finally closed at ca. 830 Ma and the assembly of the Yangtze and Cathaysia blocks terminated at ca. 810 Ma along the central Jiangnan Orogen.
Early Neoproterozoic continental arc system at the central Jiangnan Orogen, South China: Geochronological and geochemical constraints on the key igneous rock-association
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Yuzhi Zhang, Yuejun Wang; Early Neoproterozoic continental arc system at the central Jiangnan Orogen, South China: Geochronological and geochemical constraints on the key igneous rock-association. GSA Bulletin doi: https://doi.org/10.1130/B35221.1
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