北関東山地,単斜輝石かんらん岩の金雲母とパーガス閃石

Coexisting interstitial phlogopites and pargasites in iron rich wehrlite from the northern Kanto mountains were studied. The textural relationｓ between those interstitial hydrous minerals and major constituent minerals, such as olivines and clinopyroxenes suggest the crystallization of the former from the intergranular residual liquids. The modal amounts of phlogopites in wehrlite are less than 1%, and that of pargasites range from 2 to 5%. Microprobe analyses have been carried for those hydrous minerals and the major constituent minerals. namely, olivine, clinopyroxene and idiomorphic zoned chromite. Olivines in wehrlite from the northern Kanto mountains have rather iron-rich nature and range from Foso to Fo(80) and those coexisting with the hydrous minerals are Fo(82.5) in average. Clinopyroxenes from mica bearing wehrlite contain about 2.5% Al(2)O(3). 17% MgO and 5.5% FeO. Tentative cation per cent of Ca, Mg and Fe are calculated as Ca(42.6) Mg(48.1) Fe(9.3) in average. The present probe analyses show high magnesian nature of the interstitial phlogopites. In addition, these phlogopites are conspicuous by their high TiO(2) content ranging from 4.6 to 5.2%. Mg/Mg + Fe versus Ti relation plotted in Fig. 4 revealed that the present phlogopite are similar to that of the alkaline pyroxenite nodules from the Lashaine volcano, northern Tanzania. Pargasites in mica bearing wehrlite are also rich in TiO(2) (3.0～4.7%), whereas pargasitic common hornblende from hornblendite or gabbro are rather low in TiO(2) (0.9～1.7%). Mg/Mg+Fe+Mn versus Ti relations of amphiboles illustrate that Ti contents increase with decrease of Mg/Mg+Fe +Mn. Two different increasing trend for Ti conten t are also noticed. Idiomorphic chromites show distinct compositional variation from core to rim. Total FeO increases about 16% at rim, whereas TiO(2), Al(2)O(3) and MgO decrease at the rim. Possible crystallization scheme of those hydrous minersls is proposed in Fig. 10, where the maximum stability limits of phlogopite and hornblende are shown with the curve for begining of melting of basalt. Separation of liquid fraction from cumulate phase with a small trapped intergranular liquid might occur at point A. The liquid fraction then might move upward through the course II. In cumulate phase, intergranular liquid might crystallize pargasitic amphibole by the reaction with olivine and clinopyroxene through the course I in Fig. 10. Intergranular liquid might decrease with production of amphiboles and K(2)O might concentrate in residual liquid. At final stage of crystallization, residual liquid rich in K(2)O might crystallize as phlogopite by the reactions with olivine. Decrease of Ti02 content at the nm of idiomorphic chromite might be related to the formations of Ti-rich hydrous minerals.

原著論文 (Original Papers)