| 概要 |
Prismatic zircons and rounded and anhedral sphenes in gneissic and granitic rocks make a linear array in 206^Pb/204^Pb-288^U/204^Pb, 207^Pb/204^Pb, 208^Pb, -332^Th/ 204^Pb, 207^Pb/204^Pb-206^Pb/204^Pb..., and modified (206^Pb/238^U)*-(207^Pb/235^U)* isochron diagrams. The average initial Pb isotopic compositions determined on these isochrons are 206^Pb/204^Pb =16. 93±0.88, 207^Pb/204^Pb =15. 71±0.15, 208＾Pb/204＾Pb=39. 60±0.87. The ages calculated from the slope of these isochrons are 239±3m.y.~228±3m. y. which signify an early stage of metamorphism probably related to granite activity. Three rounded zircons, two are from para-gneisses of the Hida area and one from para-gneiss of Kimigano, Kinki in the Ryoke metamorphic area, which indicated 207P^b/206^Pb ages older than 1500 m. y., also show a loosely correlated pattern in the modified (206^Pb/238^nU)*-(207^Pb/236^U)* diagram suggesting a metamorphic event around 485 m. y. ago. The initial Pb isotopic compositions are distinctly less radiogenic than the isotopic composition of K-feldspar of the Hida gneissic and granitic rocks. Since the present 288^U/204^Pb of the whole rocks are low (μ=~3. 0) and the Pb・ 1sotop1c compos1t10ns of Kfeldspars and the whole rocks are close to each other, the Pb isotopic compositions of K-feldspar recorded the last phase of Hida metamorphism around 180 m. y. as indicated by many mineral ages in the metamorphic rocks. To produce the Pb isotopic compositions of K-feldspar and whole rocks from the initial Pb of zircons and sphenes during the time interval of 240-180 m. y., μ=46~137, distinctly higher than theμvalue of common silicic volcanic rocks. The whole rocks and K-feldspars of the Hida gneissic and granitic rocks show a linear correlation in the 207^Pb/24^Pb-206^Pb/204^Pb diagram with a slope of 3.66±0.20 b. y. if calculated by the two stage evolution model. The slope and the isoLopic compositions coincide with the lead isotopic compositions of Cenozoic basaltic rocks from Fuji-Hakone-Oshima area. A Sr isotope evolution diagram is constructed on these metamorphic and granitic rocks from the Hida area. The biotite hornblendeplagioclase gneiss, highly radiogenic in Pb isotopic composition, show high Sr isotope ratio and the ratio lies on the extension of the growth line of Bushveld Complex, Stillwater Complex or Amitsoq gneiss, early crustal rocks separated around 3. 7 b. y. ago. On the other hand, the initial Sr isotope ratios of granitic and some acid volcanic rocks are low and are close to the ratios of hornblende schist or garnet-hornblende-diopsideplagioclase gneiss derived from basic volcano-sedimentary materials. The multi-stage evolution calculation (up to five stages) of the lead isotopic compositions based on these Pb and Sr isotope data support the idea that the ultimate source materials of granitic rocks are basic volcano-sedimentary materials (μ=5. 7). The remelting of once formed crustal materials is also a possible mechanism to produce graniti rocks. The drastic change of u value observed in the Hida area (~46 to 3) might be interpreted to have been due to the partial remelting of the source material, separation of lead followed by the release of uranium and lead at a later stage続きを見る
|
Mendeley出力
