Geochemical Characteristics and Genesis of Hua’an Jade from Fujian Province

Hua'an jade originates from Xinci hornfels member of the early Triassic Xikou formation, which is endemic jade of Fujian Province.The locality of Hua'an jade is mainly in Hua'an area where a large outcrop exposes.The colours of Hua'an jade from different river sections are different, such as lotus red, brown black, navy blue, dark blue, light green, golden yellow and beige and so on.The navy blue, dark blue and light green of Hua'an jade are most common.Electron microprobe and X-ray powder diffraction analysis show that the different colour streaks of Hua'an jade are formed by different mineral components.Green streak is mainly composed of Fe-diopside with high iron-bearing, the colour of which could change from light green to dark green by content changes of Fe and Mg.Red streak is mainly composed of potassium feldspar and white streak is composed of quartz.Geochemical analysis of Hua'an jade shows that the content of SiO₂ ranges from 57.55%to 66.46%, with an average of 60.69%, and the content of CaO ranges from 9.88%to 15.95%, with an average of 12.54%, which display the characteristics of relatively low Si, Al and being rich in Ca, Mg. It is intensive depletion of large ion lithophile elements (LILE) such as Sr and Ba and high strength field elements (HFSE) such as Ti and Nb, with obvious negative anomalies, while it is weak positive anomalies of Th、U、Nd、Hf, with evident positive anomaly of Pb.The range ofΣREE is relatively narrow, from 196.19×10^-6 to 241.77×10^-6, with an average of 222.57×10^-6.The ratio of ΣLREE and ΣHREE (ΣLREE/ΣHREE) ranges from 3.60to 4.59, with an average of 4.32.The value of δEu ranges from 0.63 to 1.00, with an average of 0.81.The characteristic curves of trace elements and rare earth elements are consistent with the compositions of continental crust.Xinci hornfels formed in the tidal flat lagoon environment of passive continental margin.The original rock was mainly calcareous siltstone and mudstone, which the source of sediments was from terrigenous transformational granitic rocks in mountainous and moutain area of northern Fujian Province.The intensive tectonic and magmatic activities of Yanshanian had significant role in the formation of Hua'an jade. The high quality Hua'an jade was from Hua'an, Nanjing and Changtai area, etc, which benefited from multiple magmatic intrusive activities of Jinshan composite granite body.The periphery and depth of Xinci hornfels had close relationship with Jinshan rock masses and different period pulsating intrusions provide sufficient heat source, which resulted in calcareous siltstone and mudstone metamorphosing into diopside hornstone, and formed the high quality deposits of Hua'an jade.