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| Abstract |
Two rock samples were collected, one from the Itoshima granodiorite and the other from the Haki granodiorite, to see a size-dependent variation of Rb and Sr contents, and a Rb-Sr radiometric age of bi...otite. To do this, biotite crystal grains separated from these rock samples were split into size fractions by sieving and then analyzed. Biotite samples from the Itoshima and Haki granodiorites show the same sizedependent variation of Rb with a marked break at about 500μ,m (Fig. 1). Going from coarse to fine grains, Rb gently decreases first, then jumps up to high values at the break and again decreases smoothly. This break is interpreted to represent the injection of basic magma into a cooling granodiorite magma and the resultant heating of it. The size-dependent variation of Rb is assumed to come from a temperature-dependent variation of the partition coefficient of Rb between biotite and coexisting magma, because it is not likely for a magma to become gradually depleted in Rb during the crystallization. Size fractions of the biotite sample from the Itoshima granodiorite define a single isochron which gives an age of 87. 3土0.5Ma and an initial 87Sr/86Sr ratio of 0.70629±0. 00013. Size fractions of the biotite sample from the Haki granodiorite do not define a single isochron, but show a size-dependent variation of age (Table 2). The size fractions larger than 24 mesh give ages such as 93. 4Ma and 92. 2Ma. These ages may represent the result of integration of events occurred during the crystallization. The size fractions less than 32 mesh give the younger same age of 87. 6土o.4Ma. This age may probably be close to the time of completion of the crystallization. The size fraction between 24 and 32 meshes gives a curiously young age of 82. 6Ma.show more
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