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Porphyrites and Other Mafic Abyssal Masses. the Felsic to Intermediate

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Porphyrites and Other Mafic Abyssal Masses. the Felsic to Intermediate J. Japan. Assoc. Min. Petr. Econ. Geol. 74, 235-244, 1979. THULITE AND ASSOCIATED MINERALS FROM THE NAKADORI ISLAND, GOTO, NAGASAKI PREFECTURE HISASHI TAKESHITA Moji High School, Fukuoka Prefecture and YUKIO MATSUMOTO Geological Institute, Faculty of Liberal Arts, Nagasaki University The association of thulite-scolectei-prehnite in felsic volcanic pyroclastics of the Neogene Tertiary developed at the Nakadori Island, Goto, is reproted here as a peculiar case of the Green-Tuff alteration. The thulite is mangan-clinozoisite, (Ca2 .039Mg0.034Mn0.058)2.131 (Fe0.172Al2.828)3.000(Al0.023Si2.913)2.936(OH)O12. Thulite in porphyrite dikes which intrude into the pyroclastics, however, ranges from mangan-clinozoisite ((Na0 .005Ca1.939Mg0.097Mn0.044)2.082 (Fe0.049Al2.936)2.984Si2.972(OH)O12 to withermite ((Na0.006Ca2.033Mg0.019Mn0.038)2.095(Fe0.546Al2.389) 2.930Si3.000(OH)O12-(Ca1.999Mg0.002Mn0.034)2.036(Fe0.696Al2.302)2.998Si2.893(OH)O12). Scolecite associated with thulite has the chemical composition of (K0 .001Na0.012Ca0.989Mg0.004)1.006Al1.933 Si3.049O10•E3H2O. Prehnite associated with thulite and scolecite has the composition of (Na0 .001 Ca1.988Mg0 .001Fe0.006Mn0.002)1.998Al1.015(Al1.036Si2.964)4.000(OH)2O10. porphyrites and other mafic abyssal masses. INTRODUCTION AND OCCURRENCE The felsic to intermediate pyroclastics, Thulite comprises pink zoisite as well as partially welded in their lower part, belong pink clinozoisitie. In the present study the to the Goto Volcanic Complex (Ueda, 1961), clinozoisite, light pinkish color in appearance, and are correlated to the Arikawa Forma is easily recognized under microscope by tion (Tsukahara et al., 1975) underlaid with high relief, rather low birefringence, pinkish the Goto Formation (Ueda, 1961), composed weak pleochroisms and optically positive of sandstone and mudstone. Of these, character. It becomes withermite toward volcanic pyroclastic rocks are regionally the small cores which show relatively high altered. They carry montmorillonitic clay birefringence and optically negative minerals, sericite, chlorite, and epidote in character. cases. The sedimentary basin filled with The locality is situated at the Narao large volumes of these pyroclastics has been Port, south of the Nakadori Island of Goto. recognized as a part of the Green-Tuff Region The thulite-bearing rocks are exposed in Southwest Japan (Matsumoto, 1973; within a quite restricted area, while the 1977). greenish epidote-bearing ones rather widely The leucccratic thulite-bearing rocks occur in pyroclastic formations, where they appear to grade into light greenish epidote- are intruded by many dikes and sheets of bearing rocks within a sh ??t distance (Fig. (Manuscript received February 21, 1979) 236 Hisashi Takeshita and Yukio Matsumoto Thulite and associated minerals from the Nakadori Island, Goto, Nagasaki Prefecture 237 1). A dike rock which is not so leucocratic 74073007A) or porphyritic texture. Phe as compared with the thulite-bearing tuff nocrysts are plagioclase, An34-30, dusty breccia of dacitic composition also has kali-feldspar, and variable amounts of thulite in its silicified and pyritized part. primary quartz. Primary mafic minerals In such a part the dike rock has lost its are not observed. Epidote frequently original chilled texture (Fig. 2-1) by intense replaces plagioclase and often constructs a alteration. The chilled margin can clearly beautiful radial aggregate. Zeolite is rarely be seen (Fig. 2-2) against the boundary with found in cavities. Its features (2V(-)= epidote-bearing tuff breccia. Epidote does 41-42•‹) are as same as those of scolecite of not always develop throughout the whole the sample No. N-51. As a subordinate pyroclastic formations, but is sporadically component augite-andesite (No. 74073008) found where abyssal or hypabyssal masses is found as debris in the pyroclastics. Zonal have intruded. structure is well-developed in the pheno crysts of plagioclase, ranging from An71 to An54, and hour-glass structure is often DESCRIPTION OF ROCKS observed in the augite phenocrysts. The As far as surveyed by the present groundmass minerals are lath-shaped plagio- authors, the Green-Tuff of the Nakadori clase, anhedral alkali feldspar, considerable Island are chiefly composed of felsic or amounts of augite and magnetite. Xeno- pumiceous tuff, siliceous shale, quartzose crystic quartz is surrounded by coronas of sandstone, and their mixed pyroclastic clinopyroxene rods or shows corroded form. assemblage. Replacement by montmoril A dusty plagioclase is also considered as a lonitic clay minerals is distinct in the Green- xenocryst which is rimmed by clear plagio- Tuff. Locally epidote is associated with clase. Chlorite, light green in color and secondary quartz in the altered pyroclastics very low in birefringence, forms pseudomorph of rhyolitic to dacitic composition. Their after olivine, associated with augite, magne component debris show eutaxitic (No. tite, and epidote replacing plagioclase. Fig. 1 Distribution of various rocks of the Green-Tuff, developed near the environs of the Narao Port, southern part of the Nakadori Island. Legend: 1. Porphyrite dike, characterized by epidote. 2. Porphyrite dike, characterized by thulite, withermite, scolecite, wairakite and rarely prehnite. 3. Dacite tuff and tuff breccia, containing montmorillonitic clay minerals. 4. Dacitic tuff breccia, characterized by epidote and chlorite. 5. Dacite tuff breccia, characterized by thulite, scolecite and wairakite. 6. Shale of the Goto Formation. 7. Dip and strike of the boundary between dike and country rocks. 8. Dip and strike of stratum. No. 74073001•@ Well-stratified dacite tuft, greenish in appearance No. 74073007•@ A. Eutaxitic dacite debris B. Dacite debris No. 74073008•@ Mafic andes ite debris No. 74073009•@ Augite-hypersthene-hornblende-porphyrite No. 74072902•@ Dacite tuff breccia No. 74072903•@ Ditto, collected from a brecciated part close to the dike. No. 74073002•@ Thulite-bearing part of the dike No. 74073005•@ Porphyrite, characterized by greenish epidote. 238 Hisashi Takeshita and Yukio Matsumoto Fig. 2. 1. Dike II, intruding into dacitic pyroclastic bed (left), shows rather melanocratic appearance on its bulk core, whereas its margin appears to have been grown dull in color (a portion where a hammer is placed). 2. Elongation of the Dike II, showing rather melanocratic chilled margin against light gray dacitic pyroclastic bed (top). 3. Pyrite (Py) enclosing thulite (Th). Scolecite (Sc) is seen in the matrix of the tuff. Crosse nicols. 4. Scolecite showing radial aggregate and rimmed by deep yellowish brown antigorite (A). 5. Thulite-prehnite assemblage. In the light portion, scolecite is present. Prehnite (Pr) encloses fine grains of thulite. Thulite and associated minerals from the Nakadori Island, Goto, Nagasaki Prefecture 239 Dike rocks are intermediate in composition. Table 1 Acute sides of Kohler's angle, An mol.% and ordering degree (o.d.) of porphyritc A thulite-bearing rock (e.g. Dike II in Fig. 1) plagioclase in propyritized thulite-bear becomes a thulite-free and epidote-bearing ing porphyrite, No. N-51, collected from rock on the elongation of the same dike. Narao Another thulite-bearing dike (e.g. Dike I) has thulite-free core. Texture of these dike rocks is porphyritic and pilotaxitic, characterized by anhedral quartz and alkali-feldspar. Porphyritic plagioclase is rather fresh and is calcic as high as the An content from 64 to 90%. Original mafic minerals have been replaced by chlorite and epidote. The marginal part of Dike I (No. 74072904) carries small amounts of thulite Its porphyritic plagioclase is very dusty and calcic (Ann in the core and An,, on average). important metamorphic mineral, whereas It is intruded by many strings of opal and prehnite is rare in the pyroclastics. Prehnite sodic plagioclase. The terminal part of associated with thulite is shown in Fig. 2-5. Dike II is more or less brecciated to produce The only phenocryst preserved well is pockets filled by quartz, calcite, thulite, plagioclase, though it is considerably scolecite, and pyrite in cubic or pyritehedral altered by intrusion of strings of opal and form. Sometimes thulite and scolecite are sodic plagioclase. Measurements on acute apparently enclosed in pyrite (Fig. 2-3). sides of Kohler's angle reveal that there may The thulite-bearing volcanic pyroclastic be two types of plagioclase (Table 1); one is rock (Nos. 74072902, 03) contains essential the ordered type, where the An mol.% fragments of dacite, pumice, and accidental ranges from 75 to 84, and another is the fragments of quartzose sandstone and tuff disordered type with the composition, aceous mudstone. Porphyritic crystals An68-74. are clear oligoclase to andesine (nx(min.)= OPTICS AND CHEMICAL COMPOSITIONS 1.542 nz(max)=1.557, An27-42), quartz OF THULITE AND ASSOCIATED MIN angular to subangular in form, and very ERALS dusty alkali-feldspar. Thulite, almost color- less to light pinkish, is sporadically found, Refractive indices were measured by and shows high relief against the felsic the immersion method, using immersion oil whose indices have been measured by groundmass or matrix of scolecite. The scolecite occurs as feather-like radial ag microrefractometer. The error of the in dices may reach •}0.002. The electron- gregates (Fig. 2-4). Its optic axial angle varies from 30•‹ to 53•‹ in negative sign. probe analysis has been carried out by Dr. Wairakite is rarely recognized by much M. Hayashi of Research Institute of Science larger optic axial angles than the largest and Industry, Kyushu University. In limit of those measured on the scolecite. every case the H2O content was regarded as A sample, No. N-51, collected from a the difference between 100% and the total obtained. large boulder of a dike, has prehnite as an 240 Hisashi Takeshita and Yukio Matsumoto Thulite From the refractive indices the presence of Analyses Nos. 1 and 2 in Table 2 the crystals with the composition inter represent a chnozoisite whose composition is mediate between Nos. 2 and 3 should be close to the end member, Ca2Al3Si3(OH)O12, expected. Really the thulite should be of the clinoziosite-epidote series.
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