CHARACTERISATION OF MAGMATIC-HYDROTHERMAL TOURMALINE FROM THE LAND'S END GRANITE - GOING FROM METRES TO MICROMETRES

摘要

Tourmaline from the St. Byron lobe of the Land's End granite shows a great variety in colour and mineral chemistry (Fig. 1). By combining optical properties, major and trace element chemistry and field relations, late-magmatic tourmaline can be distinguished from closed and open system hydrothermal tourmaline. In some localities a transition from disseminated, brown magmatic tourmaline to hydrothermal, blue tourmaline can be directly observed. In other localities, an external, or at least less local, fluid precipitated colourless-green pleochroic tourmaline. The Sn-mineralisation in the area is unequivocally related to this type of tourmaline [1,2]. The data set includes 680 laser ablation inductively coupled mass spectrometry (LA-ICP-MS) and 900 electron probe microanalysis (EPMA) spot analyses of 16 samples from six localities along the SW coast of the Land's End area. The compositional evolution of the major elements of the three main tourmaline types is well established; brown tourmaline plot in the middle of the schorl field, blue tourmaline are more Fe-rich, and green tourmaline plot in the dravite field[1, 3, 4] (Fig. 2). Some trace elements show a correlation with colour and plot as distinct groups in binary plots. This is particularly evident in the Co versus Ni and Zn plots, where blue (～0.2 - 2 μg/g Co, ～150 - 300 μg/g Zn, ～0.5 - 2 μg/g Ni), brown (～10-40 μg/g Co; ～250 - 350 μg/g Zn; ～1 - 40 μg/g Ni) and green (～2 - 20 μg/g Co; ～25 - 60 μg/g Zn; ～30 - 100 μg/g Ni) tourmaline form separate clusters (Fig. 2). Other general trends including groups based on colour are the positive correlations between Sn and Sr and Y. In both cases, all three elements increase from brown to blue to green. Brown and blue tourmaline may also be distinguished in the Sc versus V and Nb versus Ta plots. Growth zoning is commonly observed, typically in the form of a brown core overgrown by a section of oscillatory zoned brown-blue tourmaline, a blue zone, and in some samples an oscillatory zoned pale blue-dark blue zone. This is interpreted to represent the transition from a melt-dominated to a fluid-dominated stage without a significant influence of external fluids. The green tourmaline may crosscut and overprint former tourmaline generations, and is often observed together with quartz as mineral replacements. This is interpreted to be the result of sub-solidus fluid-rock interaction.