Abstract:

Based on geological survey of the Early Indosinian granites in southern Guangxi, two types of granites are recognized in the southwestern segment of the Qinfang tectonic belt, namely, metamorphic minerals-bearing granite and not metamorphic minerals-bearing granite. The petrological, geochemical and Sr-Nd isotope data reveal that these granites chemically belong to high K calc-alkaline series, and are characterized by enrichment in large ion lithophile elements (LILEs, e.g., Rb, Th, U, K and Pb) and relative depletion in high field strength elements (HFSEs, e.g., Nb, Ta, P and Ti). They have high values of (⁸⁷Sr/⁸⁶Sr)_i=0.715 25-0.716 32 and low values of ε_Nd(t)=-10.80--9.92. Such signatures display geochemical characteristics of typical subduction-related arc magmatic rocks. Zircon SHRIMP U-Pb dating shows that the ²⁰⁶Pb/²³⁸U weighted average ages of zircons from hypersthene-bearing granitic porphyry and biotite monzogranite are (248.8±1.9) Ma (MSWD=1.5) and (246.0±3.4) Ma (MSWD=2.2), respectively. Together with the Early Paleozoic MORB-type metabasic volcanic rocks and Permian E-MORB type basalts formed in a back-arc spreading center setting in the region, we conclude that a Paleozoic oceanic basin could occur within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks (so-called Qinzhou-Hangzhou joint belt), and that the Early Indosinian granites in the region were most likely generated in the setting associated with northward subduction of the oceanic plate between the Yangtze and Cathaysian blocks, where the mantle-derived magma caused the remelting of the ancient crust within subducted slab, and the melts was subjected to modification of the mantlederived magma in different proportions.

Key words: Early Indosinian granite, chronology, geochemistry, petrogenesis, Qinfang tectonic belt, southern Guangxi