Metamorphose: Togatake Subgroup stones found in Tokawa Park 塔ヶ岳亜層群

 

Geologically, the west of Oyama Subgroup is Togatake Subgroup which spreads deep down of the main ridge of Tanzawa mountains. From north, Mt. Sodehira 袖平山 (ASL 1432m), Mt. Hiru 蛭ヶ岳 (ASL 1673m), Mt. Tanzawa 丹沢山 (ASL 1567m), and Mt. Nabewari 鍋割山 (ASL 1272m) stand on the layer. Basically, the subgroup was for pyroclastic rock spewed out from the undersea volcanos on Philippines Plate 17 million years ago. Not only taller but also qualitatively, the rocks for the old Togatake Subgroup is different from younger Oyama and Susugaya Subgroups. The young ones are mainly tuffs that were piled up volcanic ashes over the seabeds. The old Togatake Subgroup was very near to the undersea volcanic craters. The ejacta included larger mass such as violently spewed lapilli and magma. Sometimes they were mixed all together with volcanic ashes and become tuff breccia. Here is a gabbro I found in Tokawa Park.

It’s plutonic rock from slowly cooled magma in deep underground. Here we can recognize large crystals typical for slowly cooled magma. The characteristics for Togatake Subgroup is not only this. About 4 to 5 million years ago magma intruded from beneath and the mass received uber high heat and pressure. There are several theories why this happened. One sure thing is it was due to the crash of Honshu Island Arc on Eurasian Plate and Izu-Ogasawara Island Arc on Philippines Plate. Such collision of island arcs can induce magma quickly rising and cooling from the beneath. It’s like a huge dome-shaped volcanic mountain was formulated beneath the crustal-shortened layers on Philippines Plate. “Foothills” of this volcano spreads to 10km beneath Mt. Oyama (ASL 1252m) and Miyagase Dam on Oyama Subgroup. In contrast, the main ridge of Tanzawa was nearer to the “peak” of the underground “volcano” and pushed not only from the collision of plates but also from beneath for the longest period of time. The mountains get taller and taller for 17 million or so years with heat and pressure from below. It metamorphosed the ash-lapilli-magma mixture. Here is the altered one I found in Tokawa Park.

It is hornfels metamorphosed from tuff. When the mass was cracked by humongous pressure of the planet, a very hot quarts-solution seeped in, cooled and crystalized to make a white band. Next one, with its microscopic photo, from Tokawa Park is another transformed green tuff.

It’s epidote. Long prismatic crystal is clear in microscopic view. Don’t you think it’s shockingly beautiful … The next one is also typical from the area.

It’s a metamorphosed basalt. The microscopic view show still angled white grains = crystals of plagioclase which can be formed from slowly cooling magma deep underground. The overall greenish impression is due to the influence of heat and pressure on rapidly cooled magma = basalt. It surely is different from the rocks made of slowly piling up volcanic ashes. And today’s final is this hornblende hornfels.

At the beginning, it was cooling magma from which lots of gas went out from the holes covering the entire surface of a rock. It then received extremely high heat and pressure which turned the color of basalt into greenish. As this has a white streak with yellowish grains, the heat was brought from hot water containing quarts, plagioclase and pyrite. Metamorphose was happening on the basalt from the underground volcano. Well, this “volcano” becomes over the surface in Yadoriki. Next week, let’s return Yadoriki Water Source Forest.

At the entrance of Tokawa Park, there is Tanzawa Visitor Centre. The contact address is

Hadano Visitor Center for Tanzawa-Oyama Quasi-national Park
丹沢大山国定公園・自然公園秦野ビジターセンター

1513 Horiyamashita, Hadano, Kanagawa Prefecture, 259-1304
〒259-1304 秦野市堀山下1513

Phone: 0463-87-9300
https://www.kanagawa-park.or.jp/tanzawavc/