南岭富重稀土花岗岩类的特征和意义

CHARACTERISTICS AND SIGNIFICANCE OF HREE-RICH GRANITOIDS OF THE NANLING MOUNTAIN AREA

南岭地区广泛发育钙碱—次碱性的富重稀土花岗岩类,它们主要呈复式岩体的最晚期相或边缘相产出,常伴随钨、锡、铌钽和稀土成矿作用。岩体的形成时代主要为雪峰期和燕山期,岩性以富硅的钾长花岗岩、碱长花岗岩和钠长花岗岩为主,LREE/HREE和Y/∑REE值分别变化于0.20—0.99和0.29—0.6,Sm/Nd比值高达0.26—0.55,微量元素以富Rb、Th、F和贫Sr、Ba、Co、Ni、V为特征,副矿物组合以稀有、放射性矿物为主,而缺少钛铁氧化物。岩体具高的锶初始比值(0.709—0.732),长石富含放射性成因铅同位素。上述特征一致表明该类岩石是高成熟度的地壳所派生的岩浆经强烈分异演化而形成的。

Calca-alkaline to subalkaline HREE-rich granitoids, refering to granitic rocks with LREE/HREE ratio<1, are widely distributed in the Nanling Mountain area, South China. They occur as late or marginal phases of milti-phased intrusive complexes with ages ranging from 909 to 82 Ma, and on various tectonic backgrounds including the Jiangnan geoanticline, post-Caledonian uplifts and Hercynian-Indosinian depressions of the South China fold belt. Either hypergene, such as granitic or quartz pophyry, or hypogene diversities, including two-feldspar granite, K-feldspar granite and alkaline feldspar granite, are characterized by high silica and alkali, ;aluminia-ove-rsaturation and depletion of basic components. The main minerals are quartz, K-feldspar (orthoclase, microcline or perthitic microcline), plagioclase(An 0-30), small amounts of lepidomelane and siderophyllite, and/or muscovite.Among trace elements (ppm),Rb (340-1991) and F (340-6700) are significantly enriched, Sr (10-30) and Ba (20-70) greatly depleted, and Li varies widely from 8 to 373. REE contents range from 100 to 427 ppm, but those with strong albitization which is often related to Nb-Ta mineralization have a very low range between 35 and 50 ppm. In most cases, δEu values are <0.1, the LREE/HREE and Y/REE ratios are 0.23-0.99 and 0.3-0.6, respectively.Most of HREE-rich granites are characterized by high (87Sr/86Sr)i ratios(0.709-0.732) and enrichment of radiogenic lead isotopes for K'-feldspars, which generally cannot be discriminated from those of the early coexisting LREE-rich phases. Combined with lack of temporal and spatial constraints, HREE concentration in granitic magma is suggested as local processes including ( 1 ) fractional crystallization; ( 2 ) thermogravitational diffusion; ( 3 ) fluid metasomatism and ( 4 ) remelting, which are cotrolled by local physi-cochemical environments.