Geochemistry and Geochronology of Diorite in Pengshan Area of Jiangxi Province: Implications for Magmatic Source and Tectonic Evolution of Jiangnan Orogenic Belt

Abstract

Magmatic activities associated with tectonic events play a significant role in understanding the evolution of an orogenic belt. The Jiangnan orogenic belt has been regarded as the collisional suture zone between the Yangtze Block and the Cathaysia Block. Although the magmatic activities during the period of intra-plate extension after the collision have been well studied in recent years, some remaining issues, including source nature and geodynamic mechanism, need to be further addressed. In this paper, based on a detailed field geological, petrological, geochemical and geochronological study, we focus our work on diorites in the Pengshan area located at the northwestern margin of the Jiangnan orogenic belt. The mineral assemblages are mainly composed of plagioclase (55 vol.%-65 vol.%) and hornblende (35 vol.%-45 vol.%). One diorite sample yields zircon ²⁰⁶Pb/²³⁸U mean age of 768±8 Ma (MSWD=0.29). The diorites have enriched large ion lithophile elements (Ba, K and Rb) and incompatible elements (Th and U), and are depleted in high field-strength elements including Ta, Ti and Nb. Diorites in this study have relatively high MgO content (6.56 wt.%-7.58 wt.%, 7.07 wt.% on average) and Mg number values (65-67, 65.8 on average). The diorites are metaluminous, high K calc-alkaline series rocks with high contents of K₂O (1.59 wt.%-1.97 wt.%) and total alkali (Na₂O+K₂O=5.56 wt.%-6.05 wt.%). The Nd/Th ratio (4.34-5.27) is higher than that of crust-derived rocks and lower than mantle-derived rocks. The Rb/Sr ratio (0.19-0.22) is slightly lower than crust, but significantly higher than upper mantle. Based on the above geochemical and geochronological analyses, we suggest that the diorites in the Pengshan area were mainly derived from crustal materials with a small amount of mantle-originated materials involved, and possibly produced from an extensional tectonic setting after the collision between the Yangtze Block and Cathaysia Block.

Keywords

Neoproterozoic / diorite / geochemistry / Pengshan area / Jiangnan orogenic belt

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