Leached apatite samples were first separated from the whole rock powder as a heavy mineral fraction by heavy liquid technique, and the heavy mineral fraction was then leached by 3 M HNO3. In this study, we examine the effects of secondary apatite on the initial 87 Sr/86 Sr ratio determination of granitic rocks by using acid leaching technique. Secondary post-magmatic event such as hydrothermal alteration may also crystallize secondary apatite, which may inhibit the estimation of SrI of igneous rocks. Because Rb contents in apatite are very low, 87 Sr/86 Sr ratios of magmatic apatite are useful to estimate the initial 87 Sr/86 Sr ratio ( SrI) of igneous rocks. Ishikawa, T.Īpatite is a major accessory mineral in igneous rocks. The effect of secondary apatite on the initial 87 Sr/86 Sr ratio determination in granitic rocks: a case study of the Tadamigawa pluton, northeastern Japan The coherent resetting of the RbSr isochron suggests large-scale pervasive fluid movement during thrust-related burial metamorphism. Consequently, such reset isochrons may date large-scale events such as regional thrusting and metamorphism. Pressure during low-grade metamorphism of the volcanic rocks, estimated from phengite composition to have been in the range of 4 to 6 kbar, points to thrust-related burial as the main cause of resetting. Phengite is the K-bearing product of the breakdown of biotite and K-feldspar. The complete breakdown of all major minerals that contain Rb and Sr resulted in an alteration mineral assemblage consisting of phengite, albite, secondary quartz, and minor amounts of chlorite and epidote. The rocks range in composition from rhyodacite tuffs to andesite lavas. The data demonstrate that whole-rock RbSr ages of volcanic rocks, even lava flows with SiO2 content as low as 57 wt.%, are susceptible to complete resetting. The same rocks have also been dated by the UThPb method on zircon, giving a crystallization age of 166 ? 11 Ma, over twice as old as the RbSr age. We report a nine-point RbSr whole-rock isochron age of 70?3 Ma (MSWD 3.97) for Mid-Jurassic volcanic rocks. Resetting of RbSr ages of volcanic rocks by low-grade burial metamorphismĪsmeroma, Y.
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