Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand
Uranium-lead ages from the Dun Mountain ophiolite belt and Brook Street terrane, South Island, New Zealand
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新西兰南岛邓山蛇绿岩带和布鲁克街地体的铀铅年龄
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发表时间：
1992
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通讯作者：
M. Johnston
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作者：
D. Kimbrough;J. Mattinson;D. Coombs;C. Landis;M. Johnston
在新西兰南岛，盾山蛇绿岩及其上覆麦台群不连续出露480公里。斜长花岗岩中的锆石U/Pb年龄为相对完整的蛇绿岩、蛇绿混杂岩，以及缺乏典型蛇绿岩组合的南部较多的硅质火山-深成岩组合。对略微不协调的斜长花岗岩锆石进行了逐步溶解实验，得到了更多的一致性残留物，表明锆石最近失去了1%至5%以上的放射性成因铅(∼)。高精度的207Pb/206Pb测年确定了蛇绿岩形成的年龄，沿走向至少350公里，形成于约275 Ma至285 Ma之间的狭窄区间。锆石U/Pb数据证实了盾山蛇绿岩不同岩段的岩石学对比，并表明镁铁质-超镁铁质蛇绿岩组合和中钾质高级花岗岩类在短时间间隔内发育，可能是在火山-弧缘盆地的伸展期间。麦台群多晶角砾岩和生物碎屑灰岩的厚透镜体局部位于相对完整的蛇绿岩上的沉积接触部位。将石灰岩的生物地层年龄与斜长花岗岩的∼年龄进行对比，发现∼年龄相差20 Ma。在蛇绿岩形成之后。麦台群上部砾岩中的花岗岩碎屑获得了265 Ma的接近一致的U/Pb年龄，为这部分层序提供了最大年龄。盾山蛇绿岩可能在蛇绿岩形成和麦台群沉积之间发生了伸展断裂作用和剥蚀作用的衰减作用。盾山蛇绿岩及其上覆的麦台群以西为三叠纪和侏罗纪的Murihiku地体火山成因沉积岩，又以Brook Street地体为界，被解释为早二叠世洋弧的残留物。与布鲁克街地体中的层状镁铁质-超镁铁质侵入体有关的角闪辉长岩的年龄为265 Ma，明显晚于盾山蛇绿岩。这种侵入作用可能代表了深成火山岩的根源，这些火山岩构成了Brook Street地体的一个显著组成部分，但在麦台群中没有被碎屑所代表。黑云母花岗岩赋存于Brook Street地体中，年龄为260 Ma。Brook Street地体和Dun Mountain-Maitai地体之间没有任何明显的地层对比或物源联系，这表明在介于两者之间的地体边界断层上存在走滑位移。
The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.