福建“华安玉”岩石地球化学特征及成因研究

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

"华安玉"是福建省特有的玉石矿种,产于早三叠世溪口组新祠角岩段中,主要矿物为铁次透辉石,次要矿物为钾长石、石英和方解石等。地球化学分析表明,w(SiO2)变化范围为57.55%～66.46%,平均为60.69%,w(CaO)变化范围为9.88%～15.95%,平均为12.54%,相对低Si,Al而富Ca,Mg等;大离子亲石元素Sr,Ba及高场强元素Ti,Nb强烈亏损而具有明显的负异常,而Th、U、Nd、Hf表现为弱正异常,Pb正异常明显;ΣREE变化范围较小,一般196.19×10-6～241.77×10-6,平均222.57×10-6,轻重稀土比值(ΣLREE/ΣHREE)为3.60～4.59,平均为4.32,δEu=0.63～1.00,平均为0.81。微量元素和稀土元素特征曲线显示与大陆上地壳组成一致。新祠角岩形成于被动大陆边缘的潮坪泻湖环境,原岩主要是钙质粉砂岩、泥岩等,物源主要来自于闽北山地丘陵区的陆源改造型花岗质岩石。燕山期强烈的构造岩浆活动对其进行了多期热接触变质或交代作用,钙质粉砂岩、泥岩等经变质为透辉石角岩,形成优质的"华安玉"矿体。

Hua＇an jade originates from Xinci hornfels member of the early Triassic Xikou for- mation, 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 60.69G, 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, A1 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 of EREE is relatively narrow , from 196. i9x10 6 to 241. 77 X10 6, with an average of 222.57X 10-6. The ratio of Y.LREE and EHREE （Y, LREE/Y, HREE） ranges from 3.60 to 4.59, with an average of 4.32. The value of 8Eu 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 en- vironment of passive continental margin. The original rock was mainly calcareous sihstone 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 calcare- ous sihstone and muds＇rune metamorphosing into diopside hornstone, and formed the high quality deposits of Hua＇an jade.